-<!-- $PostgreSQL: pgsql/doc/src/sgml/advanced.sgml,v 1.52 2006/10/21 23:12:57 tgl Exp $ -->
+<!-- $PostgreSQL: pgsql/doc/src/sgml/advanced.sgml,v 1.53 2007/01/31 20:56:16 momjian Exp $ -->
<chapter id="tutorial-advanced">
<title>Advanced Features</title>
<para>
Making liberal use of views is a key aspect of good SQL database
design. Views allow you to encapsulate the details of the
- structure of your tables, which may change as your application
+ structure of your tables, which might change as your application
evolves, behind consistent interfaces.
</para>
<note>
<para>
Some client libraries issue <command>BEGIN</> and <command>COMMIT</>
- commands automatically, so that you may get the effect of transaction
+ commands automatically, so that you might get the effect of transaction
blocks without asking. Check the documentation for the interface
you are using.
</para>
-<!-- $PostgreSQL: pgsql/doc/src/sgml/arch-dev.sgml,v 2.28 2006/09/16 00:30:11 momjian Exp $ -->
+<!-- $PostgreSQL: pgsql/doc/src/sgml/arch-dev.sgml,v 2.29 2007/01/31 20:56:16 momjian Exp $ -->
<chapter id="overview">
<title>Overview of PostgreSQL Internals</title>
similar to the raw parse tree in most places, but it has many differences
in detail. For example, a <structname>FuncCall</> node in the
parse tree represents something that looks syntactically like a function
- call. This may be transformed to either a <structname>FuncExpr</>
+ call. This might be transformed to either a <structname>FuncExpr</>
or <structname>Aggref</> node depending on whether the referenced
name turns out to be an ordinary function or an aggregate function.
Also, information about the actual data types of columns and expression
<note>
<para>
In some situations, examining each possible way in which a query
- may be executed would take an excessive amount of time and memory
+ can be executed would take an excessive amount of time and memory
space. In particular, this occurs when executing queries
involving large numbers of join operations. In order to determine
a reasonable (not optimal) query plan in a reasonable amount of
scanned in parallel, and matching rows are combined to form
join rows. This kind of join is more
attractive because each relation has to be scanned only once.
- The required sorting may be achieved either by an explicit sort
+ The required sorting might be achieved either by an explicit sort
step, or by scanning the relation in the proper order using an
index on the join key.
</para>
</para>
<para>
- Complex queries may involve many levels of plan nodes, but the general
+ Complex queries can involve many levels of plan nodes, but the general
approach is the same: each node computes and returns its next output
row each time it is called. Each node is also responsible for applying
any selection or projection expressions that were assigned to it by
into the target table specified for the <command>INSERT</>. (A simple
<command>INSERT ... VALUES</> command creates a trivial plan tree
consisting of a single <literal>Result</> node, which computes just one
- result row. But <command>INSERT ... SELECT</> may demand the full power
+ result row. But <command>INSERT ... SELECT</> can demand the full power
of the executor mechanism.) For <command>UPDATE</>, the planner arranges
that each computed row includes all the updated column values, plus
the <firstterm>TID</> (tuple ID, or row ID) of the original target row;
-<!-- $PostgreSQL: pgsql/doc/src/sgml/array.sgml,v 1.54 2007/01/31 04:12:01 momjian Exp $ -->
+<!-- $PostgreSQL: pgsql/doc/src/sgml/array.sgml,v 1.55 2007/01/31 20:56:16 momjian Exp $ -->
<sect1 id="arrays">
<title>Arrays</title>
</para>
<para>
- An alternative syntax, which conforms to the SQL standard, may
+ An alternative syntax, which conforms to the SQL standard, can
be used for one-dimensional arrays.
<structfield>pay_by_quarter</structfield> could have been defined
as:
To write an array value as a literal constant, enclose the element
values within curly braces and separate them by commas. (If you
know C, this is not unlike the C syntax for initializing
- structures.) You may put double quotes around any element value,
+ structures.) You can put double quotes around any element value,
and must do so if it contains commas or curly braces. (More
details appear below.) Thus, the general format of an array
constant is the following:
</para>
<para>
- The <literal>ARRAY</> constructor syntax may also be used:
+ The <literal>ARRAY</> constructor syntax can also be used:
<programlisting>
INSERT INTO sal_emp
VALUES ('Bill',
WHERE name = 'Carol';
</programlisting>
- An array may also be updated at a single element:
+ An array can also be updated at a single element:
<programlisting>
UPDATE sal_emp SET pay_by_quarter[4] = 15000
Note that the concatenation operator discussed above is preferred over
direct use of these functions. In fact, the functions exist primarily for use
- in implementing the concatenation operator. However, they may be directly
+ in implementing the concatenation operator. However, they might be directly
useful in the creation of user-defined aggregates. Some examples:
<programlisting>
<tip>
<para>
Arrays are not sets; searching for specific array elements
- may be a sign of database misdesign. Consider
+ can be a sign of database misdesign. Consider
using a separate table with a row for each item that would be an
array element. This will be easier to search, and is likely to
scale up better to large numbers of elements.
or backslashes disables this and allows the literal string value
<quote>NULL</> to be entered. Also, for backwards compatibility with
pre-8.2 versions of <productname>PostgreSQL</>, the <xref
- linkend="guc-array-nulls"> configuration parameter may be turned
+ linkend="guc-array-nulls"> configuration parameter might be turned
<literal>off</> to suppress recognition of <literal>NULL</> as a NULL.
</para>
</para>
<para>
- You may write whitespace before a left brace or after a right
- brace. You may also write whitespace before or after any individual item
+ You can write whitespace before a left brace or after a right
+ brace. You can also write whitespace before or after any individual item
string. In all of these cases the whitespace will be ignored. However,
whitespace within double-quoted elements, or surrounded on both sides by
non-whitespace characters of an element, is not ignored.
-<!-- $PostgreSQL: pgsql/doc/src/sgml/backup.sgml,v 2.95 2006/12/01 03:29:15 tgl Exp $ -->
+<!-- $PostgreSQL: pgsql/doc/src/sgml/backup.sgml,v 2.96 2007/01/31 20:56:16 momjian Exp $ -->
<chapter id="backup">
<title>Backup and Restore</title>
<para>
By default, the <application>psql</> script will continue to
- execute after an SQL error is encountered. You may wish to use the
+ execute after an SQL error is encountered. You might wish to use the
following command at the top of the script to alter that
behaviour and have <application>psql</application> exit with an
exit status of 3 if an SQL error occurs:
passing the <option>-1</> or <option>--single-transaction</>
command-line options to <application>psql</>. When using this
mode, be aware that even the smallest of errors can rollback a
- restore that has already run for many hours. However, that may
+ restore that has already run for many hours. However, that might
still be preferable to manually cleaning up a complex database
after a partially restored dump.
</para>
<listitem>
<para>
If you have dug into the details of the file system layout of the
- database, you may be tempted to try to back up or restore only certain
+ database, you might be tempted to try to back up or restore only certain
individual tables or databases from their respective files or
directories. This will <emphasis>not</> work because the
information contained in these files contains only half the
</para>
<para>
- If your database is spread across multiple file systems, there may not
+ If your database is spread across multiple file systems, there might not
be any way to obtain exactly-simultaneous frozen snapshots of all
the volumes. For example, if your data files and WAL log are on different
disks, or if tablespaces are on different file systems, it might
Since we can string together an indefinitely long sequence of WAL files
for replay, continuous backup can be achieved simply by continuing to archive
the WAL files. This is particularly valuable for large databases, where
- it may not be convenient to take a full backup frequently.
+ it might not be convenient to take a full backup frequently.
</para>
</listitem>
<listitem>
<para>
As with the plain file-system-backup technique, this method can only
support restoration of an entire database cluster, not a subset.
- Also, it requires a lot of archival storage: the base backup may be bulky,
+ Also, it requires a lot of archival storage: the base backup might be bulky,
and a busy system will generate many megabytes of WAL traffic that
have to be archived. Still, it is the preferred backup technique in
many situations where high reliability is needed.
</programlisting>
which will copy archivable WAL segments to the directory
<filename>/mnt/server/archivedir</>. (This is an example, not a
- recommendation, and may not work on all platforms.)
+ recommendation, and might not work on all platforms.)
</para>
<para>
<para>
In writing your archive command, you should assume that the file names to
- be archived may be up to 64 characters long and may contain any
+ be archived can be up to 64 characters long and can contain any
combination of ASCII letters, digits, and dots. It is not necessary to
remember the original relative path (<literal>%p</>) but it is necessary to
remember the file name (<literal>%f</>).
<filename>postgresql.conf</>, <filename>pg_hba.conf</> and
<filename>pg_ident.conf</>), since those are edited manually rather
than through SQL operations.
- You may wish to keep the configuration files in a location that will
+ You might wish to keep the configuration files in a location that will
be backed up by your regular file system backup procedures. See
<xref linkend="runtime-config-file-locations"> for how to relocate the
configuration files.
between <function>pg_start_backup</> and the start of the actual backup,
nor between the end of the backup and <function>pg_stop_backup</>; a
few minutes' delay won't hurt anything. (However, if you normally run the
- server with <varname>full_page_writes</> disabled, you may notice a drop
+ server with <varname>full_page_writes</> disabled, you might notice a drop
in performance between <function>pg_start_backup</> and
<function>pg_stop_backup</>, since <varname>full_page_writes</> is
effectively forced on during backup mode.) You must ensure that these
</para>
<para>
- You may, however, omit from the backup dump the files within the
+ You can, however, omit from the backup dump the files within the
<filename>pg_xlog/</> subdirectory of the cluster directory. This
slight complication is worthwhile because it reduces the risk
of mistakes when restoring. This is easy to arrange if
the file system backup and the WAL segment files used during the
backup (as specified in the backup history file), all archived WAL
segments with names numerically less are no longer needed to recover
- the file system backup and may be deleted. However, you should
+ the file system backup and can be deleted. However, you should
consider keeping several backup sets to be absolutely certain that
you can recover your data.
</para>
require that you have enough free space on your system to hold two
copies of your existing database. If you do not have enough space,
you need at the least to copy the contents of the <filename>pg_xlog</>
- subdirectory of the cluster data directory, as it may contain logs which
+ subdirectory of the cluster data directory, as it might contain logs which
were not archived before the system went down.
</para>
</listitem>
<listitem>
<para>
Create a recovery command file <filename>recovery.conf</> in the cluster
- data directory (see <xref linkend="recovery-config-settings">). You may
+ data directory (see <xref linkend="recovery-config-settings">). You might
also want to temporarily modify <filename>pg_hba.conf</> to prevent
ordinary users from connecting until you are sure the recovery has worked.
</para>
<filename>recovery.conf</> is the <varname>restore_command</>,
which tells <productname>PostgreSQL</> how to get back archived
WAL file segments. Like the <varname>archive_command</>, this is
- a shell command string. It may contain <literal>%f</>, which is
+ a shell command string. It can contain <literal>%f</>, which is
replaced by the name of the desired log file, and <literal>%p</>,
which is replaced by the path name to copy the log file to.
(The path name is relative to the working directory of the server,
It should also be noted that the default <acronym>WAL</acronym>
format is fairly bulky since it includes many disk page snapshots.
These page snapshots are designed to support crash recovery, since
- we may need to fix partially-written disk pages. Depending on
- your system hardware and software, the risk of partial writes may
+ we might need to fix partially-written disk pages. Depending on
+ your system hardware and software, the risk of partial writes might
be small enough to ignore, in which case you can significantly
reduce the total volume of archived logs by turning off page
snapshots using the <xref linkend="guc-full-page-writes">
use of the logs for PITR operations. An area for future
development is to compress archived WAL data by removing
unnecessary page copies even when <varname>full_page_writes</> is
- on. In the meantime, administrators may wish to reduce the number
+ on. In the meantime, administrators might wish to reduce the number
of page snapshots included in WAL by increasing the checkpoint
interval parameters as much as feasible.
</para>
connectivity between the two and the viability of the primary. It is
also possible to use a third system (called a witness server) to avoid
some problems of inappropriate failover, but the additional complexity
- may not be worthwhile unless it is set-up with sufficient care and
+ might not be worthwhile unless it is set-up with sufficient care and
rigorous testing.
</para>
Once failover to the standby occurs, we have only a
single server in operation. This is known as a degenerate state.
The former standby is now the primary, but the former primary is down
- and may stay down. To return to normal operation we must
+ and might stay down. To return to normal operation we must
fully recreate a standby server,
either on the former primary system when it comes up, or on a third,
possibly new, system. Once complete the primary and standby can be
It is recommended that you use the <application>pg_dump</> and
<application>pg_dumpall</> programs from the newer version of
<productname>PostgreSQL</>, to take advantage of any enhancements
- that may have been made in these programs. Current releases of the
+ that might have been made in these programs. Current releases of the
dump programs can read data from any server version back to 7.0.
</para>
<note>
<para>
When you <quote>move the old installation out of the way</quote>
- it may no longer be perfectly usable. Some of the executable programs
+ it might no longer be perfectly usable. Some of the executable programs
contain absolute paths to various installed programs and data files.
This is usually not a big problem but if you plan on using two
installations in parallel for a while you should assign them
-<!-- $PostgreSQL: pgsql/doc/src/sgml/bki.sgml,v 1.19 2006/09/16 00:30:11 momjian Exp $ -->
+<!-- $PostgreSQL: pgsql/doc/src/sgml/bki.sgml,v 1.20 2007/01/31 20:56:16 momjian Exp $ -->
<chapter id="bki">
<title><acronym>BKI</acronym> Backend Interface</title>
</para>
<para>
- Related information may be found in the documentation for
+ Related information can be found in the documentation for
<application>initdb</application>.
</para>
-<!-- $PostgreSQL: pgsql/doc/src/sgml/catalogs.sgml,v 2.143 2007/01/22 01:35:19 tgl Exp $ -->
+<!-- $PostgreSQL: pgsql/doc/src/sgml/catalogs.sgml,v 2.144 2007/01/31 20:56:16 momjian Exp $ -->
<!--
Documentation of the system catalogs, directed toward PostgreSQL developers
-->
The catalog <structname>pg_amop</structname> stores information about
operators associated with access method operator families. There is one
row for each operator that is a member of an operator family. An operator
- can appear in more than one family, but may not appear in more than one
+ can appear in more than one family, but cannot appear in more than one
position within a family.
</para>
<entry></entry>
<entry>
Always -1 in storage, but when loaded into a row descriptor
- in memory this may be updated to cache the offset of the attribute
+ in memory this might be updated to cache the offset of the attribute
within the row
</entry>
</row>
<entry><type>bool</type></entry>
<entry></entry>
<entry>
- This column is defined locally in the relation. Note that a column may
+ This column is defined locally in the relation. Note that a column can
be locally defined and inherited simultaneously
</entry>
</row>
database authorization identifiers (roles). A role subsumes the concepts
of <quote>users</> and <quote>groups</>. A user is essentially just a
role with the <structfield>rolcanlogin</> flag set. Any role (with or
- without <structfield>rolcanlogin</>) may have other roles as members; see
+ without <structfield>rolcanlogin</>) can have other roles as members; see
<link linkend="catalog-pg-auth-members"><structname>pg_auth_members</structname></link>.
</para>
<row>
<entry><structfield>rolcreaterole</structfield></entry>
<entry><type>bool</type></entry>
- <entry>Role may create more roles</entry>
+ <entry>Role can create more roles</entry>
</row>
<row>
<entry><structfield>rolcreatedb</structfield></entry>
<entry><type>bool</type></entry>
- <entry>Role may create databases</entry>
+ <entry>Role can create databases</entry>
</row>
<row>
<entry><structfield>rolcatupdate</structfield></entry>
<entry><type>bool</type></entry>
<entry>
- Role may update system catalogs directly. (Even a superuser may not do
+ Role can update system catalogs directly. (Even a superuser cannot do
this unless this column is true)
</entry>
</row>
<entry><structfield>rolcanlogin</structfield></entry>
<entry><type>bool</type></entry>
<entry>
- Role may log in. That is, this role can be given as the initial
+ Role can log in. That is, this role can be given as the initial
session authorization identifier
</entry>
</row>
<entry><structfield>admin_option</structfield></entry>
<entry><type>bool</type></entry>
<entry></entry>
- <entry>True if <structfield>member</> may grant membership in
+ <entry>True if <structfield>member</> can grant membership in
<structfield>roleid</> to others</entry>
</row>
</tbody>
<entry><type>char</type></entry>
<entry></entry>
<entry>
- Indicates what contexts the cast may be invoked in.
+ Indicates what contexts the cast can be invoked in.
<literal>e</> means only as an explicit cast (using
<literal>CAST</> or <literal>::</> syntax).
<literal>a</> means implicitly in assignment
<para>
In all cases, a <structname>pg_depend</structname> entry indicates that the
- referenced object may not be dropped without also dropping the dependent
+ referenced object cannot be dropped without also dropping the dependent
object. However, there are several subflavors identified by
<structfield>deptype</>:
<listitem>
<para>
A normal relationship between separately-created objects. The
- dependent object may be dropped without affecting the
- referenced object. The referenced object may only be dropped
+ dependent object can be dropped without affecting the
+ referenced object. The referenced object can only be dropped
by specifying <literal>CASCADE</>, in which case the dependent
object is dropped, too. Example: a table column has a normal
dependency on its data type.
</varlistentry>
</variablelist>
- Other dependency flavors may be needed in future.
+ Other dependency flavors might be needed in future.
</para>
</sect1>
This is false for internal languages (such as
<acronym>SQL</acronym>) and true for user-defined languages.
Currently, <application>pg_dump</application> still uses this
- to determine which languages need to be dumped, but this may be
+ to determine which languages need to be dumped, but this might be
replaced by a different mechanism in the future
</entry>
</row>
<entry>
True if this is a trusted language, which means that it is believed
not to grant access to anything outside the normal SQL execution
- environment. Only superusers may create functions in untrusted
+ environment. Only superusers can create functions in untrusted
languages
</entry>
</row>
<entry><type>bytea</type></entry>
<entry>
Actual data stored in the large object.
- This will never be more than <symbol>LOBLKSIZE</> bytes and may be less
+ This will never be more than <symbol>LOBLKSIZE</> bytes and might be less
</entry>
</row>
</tbody>
Each row of <structname>pg_largeobject</structname> holds data
for one page of a large object, beginning at
byte offset (<literal>pageno * LOBLKSIZE</>) within the object. The implementation
- allows sparse storage: pages may be missing, and may be shorter than
+ allows sparse storage: pages might be missing, and might be shorter than
<literal>LOBLKSIZE</> bytes even if they are not the last page of the object.
Missing regions within a large object read as zeroes.
</para>
It is <literal>s</literal> for <quote>stable</> functions,
whose results (for fixed inputs) do not change within a scan.
It is <literal>v</literal> for <quote>volatile</> functions,
- whose results may change at any time. (Use <literal>v</literal> also
+ whose results might change at any time. (Use <literal>v</literal> also
for functions with side-effects, so that calls to them cannot get
optimized away.)
</entry>
<para>
In all cases, a <structname>pg_shdepend</structname> entry indicates that
- the referenced object may not be dropped without also dropping the dependent
+ the referenced object cannot be dropped without also dropping the dependent
object. However, there are several subflavors identified by
<structfield>deptype</>:
</varlistentry>
</variablelist>
- Other dependency flavors may be needed in future. Note in particular
+ Other dependency flavors might be needed in future. Note in particular
that the current definition only supports roles as referenced objects.
</para>
</para>
<para>
- Since different kinds of statistics may be appropriate for different
+ Since different kinds of statistics might be appropriate for different
kinds of data, <structname>pg_statistic</structname> is designed not
to assume very much about what sort of statistics it stores. Only
extremely general statistics (such as nullness) are given dedicated
<para>
<structname>pg_statistic</structname> should not be readable by the
public, since even statistical information about a table's contents
- may be considered sensitive. (Example: minimum and maximum values
+ might be considered sensitive. (Example: minimum and maximum values
of a salary column might be quite interesting.)
<link linkend="view-pg-stats"><structname>pg_stats</structname></link>
is a publicly readable view on
default expression represented by <structfield>typdefaultbin</>. If
<structfield>typdefaultbin</> is null and <structfield>typdefault</> is
not, then <structfield>typdefault</> is the external representation of
- the type's default value, which may be fed to the type's input
+ the type's default value, which might be fed to the type's input
converter to produce a constant
</para></entry>
</row>
<para>
Cursors are used internally to implement some of the components
of <productname>PostgreSQL</>, such as procedural languages.
- Therefore, the <structname>pg_cursors</> view may include cursors
+ Therefore, the <structname>pg_cursors</> view might include cursors
that have not been explicitly created by the user.
</para>
</note>
<para>
<structname>pg_locks</structname> contains one row per active lockable
object, requested lock mode, and relevant transaction. Thus, the same
- lockable object may
+ lockable object might
appear many times, if multiple transactions are holding or waiting
for locks on it. However, an object that currently has no locks on it
will not appear at all.
<entry><structfield>rolcreaterole</structfield></entry>
<entry><type>bool</type></entry>
<entry></entry>
- <entry>Role may create more roles</entry>
+ <entry>Role can create more roles</entry>
</row>
<row>
<entry><structfield>rolcreatedb</structfield></entry>
<entry><type>bool</type></entry>
<entry></entry>
- <entry>Role may create databases</entry>
+ <entry>Role can create databases</entry>
</row>
<row>
<entry><type>bool</type></entry>
<entry></entry>
<entry>
- Role may update system catalogs directly. (Even a superuser may not do
+ Role can update system catalogs directly. (Even a superuser cannot do
this unless this column is true.)
</entry>
</row>
<entry><type>bool</type></entry>
<entry></entry>
<entry>
- Role may log in. That is, this role can be given as the initial
+ Role can log in. That is, this role can be given as the initial
session authorization identifier
</entry>
</row>
<entry><structfield>usecreatedb</structfield></entry>
<entry><type>bool</type></entry>
<entry></entry>
- <entry>User may create databases</entry>
+ <entry>User can create databases</entry>
</row>
<row>
<entry><type>bool</type></entry>
<entry></entry>
<entry>
- User may update system catalogs. (Even a superuser may not do
+ User can update system catalogs. (Even a superuser cannot do
this unless this column is true.)
</entry>
</row>
<row>
<entry><structfield>usecreatedb</structfield></entry>
<entry><type>bool</type></entry>
- <entry>User may create databases</entry>
+ <entry>User can create databases</entry>
</row>
<row>
<entry><structfield>usecatupd</structfield></entry>
<entry><type>bool</type></entry>
<entry>
- User may update system catalogs. (Even a superuser may not do
+ User can update system catalogs. (Even a superuser cannot do
this unless this column is true.)
</entry>
</row>
-<!-- $PostgreSQL: pgsql/doc/src/sgml/charset.sgml,v 2.80 2007/01/09 22:22:55 momjian Exp $ -->
+<!-- $PostgreSQL: pgsql/doc/src/sgml/charset.sgml,v 2.81 2007/01/31 20:56:16 momjian Exp $ -->
<chapter id="charset">
<title>Localization</>
environment variables seen by the server, not by the environment
of any client. Therefore, be careful to configure the correct locale settings
before starting the server. A consequence of this is that if
- client and server are set up in different locales, messages may
+ client and server are set up in different locales, messages might
appear in different languages depending on where they originated.
</para>
If locale support doesn't work in spite of the explanation above,
check that the locale support in your operating system is
correctly configured. To check what locales are installed on your
- system, you may use the command <literal>locale -a</literal> if
+ system, you can use the command <literal>locale -a</literal> if
your operating system provides it.
</para>
-<!-- $PostgreSQL: pgsql/doc/src/sgml/client-auth.sgml,v 1.96 2006/11/23 05:39:17 momjian Exp $ -->
+<!-- $PostgreSQL: pgsql/doc/src/sgml/client-auth.sgml,v 1.97 2007/01/31 20:56:16 momjian Exp $ -->
<chapter id="client-authentication">
<title>Client Authentication</title>
runs. If all the users of a particular server also have accounts on
the server's machine, it makes sense to assign database user names
that match their operating system user names. However, a server that
- accepts remote connections may have many database users who have no local operating system
+ accepts remote connections might have many database users who have no local operating system
account, and in such cases there need be no connection between
database user names and OS user names.
</para>
</para>
<para>
- A record may have one of the seven formats
+ A record can have one of the seven formats
<synopsis>
local <replaceable>database</replaceable> <replaceable>user</replaceable> <replaceable>auth-method</replaceable> <optional><replaceable>auth-option</replaceable></optional>
host <replaceable>database</replaceable> <replaceable>user</replaceable> <replaceable>CIDR-address</replaceable> <replaceable>auth-method</replaceable> <optional><replaceable>auth-option</replaceable></optional>
<term><replaceable>IP-mask</replaceable></term>
<listitem>
<para>
- These fields may be used as an alternative to the
+ These fields can be used as an alternative to the
<replaceable>CIDR-address</replaceable> notation. Instead of
specifying the mask length, the actual mask is specified in a
separate column. For example, <literal>255.0.0.0</> represents an IPv4
# If these are the only three lines for local connections, they will
# allow local users to connect only to their own databases (databases
# with the same name as their database user name) except for administrators
-# and members of role "support", who may connect to all databases. The file
+# and members of role "support", who can connect to all databases. The file
# $PGDATA/admins contains a list of names of administrators. Passwords
# are required in all cases.
#
<literal>trust</> authentication is appropriate and very
convenient for local connections on a single-user workstation. It
is usually <emphasis>not</> appropriate by itself on a multiuser
- machine. However, you may be able to use <literal>trust</> even
+ machine. However, you might be able to use <literal>trust</> even
on a multiuser machine, if you restrict access to the server's
Unix-domain socket file using file-system permissions. To do this, set the
<varname>unix_socket_permissions</varname> (and possibly
<literal>./configure --with-krb-srvnam=whatever</>. In most environments,
this parameter never needs to be changed. However, to support multiple
<productname>PostgreSQL</> installations on the same host it is necessary.
- Some Kerberos implementations may also require a different service name,
+ Some Kerberos implementations might also require a different service name,
such as Microsoft Active Directory which requires the service name
to be in uppercase (<literal>POSTGRES</literal>).
</para>
as which database user. The same <replaceable>map-name</> can be
used repeatedly to specify more user-mappings within a single map.
There is no restriction regarding how many database users a given
- operating system user may correspond to, nor vice versa.
+ operating system user can correspond to, nor vice versa.
</para>
<para>
will encrypt only the connection between the PostgreSQL server
and the LDAP server. The connection between the client and the
PostgreSQL server is not affected by this setting. To make use of
- TLS encryption, you may need to configure the LDAP library prior
+ TLS encryption, you might need to configure the LDAP library prior
to configuring PostgreSQL. Note that encrypted LDAP is available only
if the platform's LDAP library supports it.
</para>
</programlisting>
The database you are trying to connect to does not exist. Note that
if you do not specify a database name, it defaults to the database
- user name, which may or may not be the right thing.
+ user name, which might or might not be the right thing.
</para>
<tip>
<para>
- The server log may contain more information about an
+ The server log might contain more information about an
authentication failure than is reported to the client. If you are
confused about the reason for a failure, check the log.
</para>
-<!-- $PostgreSQL: pgsql/doc/src/sgml/config.sgml,v 1.106 2007/01/25 11:53:50 petere Exp $ -->
+<!-- $PostgreSQL: pgsql/doc/src/sgml/config.sgml,v 1.107 2007/01/31 20:56:16 momjian Exp $ -->
<chapter Id="runtime-config">
<title>Server Configuration</title>
<para>
All parameter names are case-insensitive. Every parameter takes a
value of one of four types: Boolean, integer, floating point,
- or string. Boolean values may be written as <literal>ON</literal>,
+ or string. Boolean values can be written as <literal>ON</literal>,
<literal>OFF</literal>, <literal>TRUE</literal>,
<literal>FALSE</literal>, <literal>YES</literal>,
<literal>NO</literal>, <literal>1</literal>, <literal>0</literal>
<filename>postgresql.conf</filename>. Note that this means you won't
be able to change the value on-the-fly by editing
<filename>postgresql.conf</filename>, so while the command-line
- method may be convenient, it can cost you flexibility later.
+ method might be convenient, it can cost you flexibility later.
</para>
<para>
<para>
Determines the maximum number of concurrent connections to the
database server. The default is typically 100 connections, but
- may be less if your kernel settings will not support it (as
+ might be less if your kernel settings will not support it (as
determined during <application>initdb</>). This parameter can
only be set at server start.
</para>
<para>
- Increasing this parameter m
ay cause <productname>PostgreSQL</>
+ Increasing this parameter m
ight cause <productname>PostgreSQL</>
to request more <systemitem class="osname">System V</> shared
memory or semaphores than your operating system's default configuration
allows. See <xref linkend="sysvipc"> for information on how to
<listitem>
<para>
On systems that support the <symbol>TCP_KEEPCNT</symbol> socket option, specifies how
- many keepalives may be lost before the connection is considered dead.
+ many keepalives can be lost before the connection is considered dead.
A value of zero uses the system default. If <symbol>TCP_KEEPCNT</symbol> is not
supported, this parameter must be zero. This parameter is ignored
for connections made via a Unix-domain socket.
<para>
Sets the amount of memory the database server uses for shared
memory buffers. The default is typically 32 megabytes
- (<literal>32MB</>), but may be less if your kernel settings will
+ (<literal>32MB</>), but might be less if your kernel settings will
not support it (as determined during <application>initdb</>).
This setting must be at least 128 kilobytes and at least 16
kilobytes times <xref linkend="guc-max-connections">. (Non-default
</para>
<para>
- Increasing this parameter m
ay cause <productname>PostgreSQL</>
+ Increasing this parameter m
ight cause <productname>PostgreSQL</>
to request more <systemitem class="osname">System V</> shared
memory than your operating system's default configuration
allows. See <xref linkend="sysvipc"> for information on how to
</para>
<para>
- Increasing this parameter m
ay cause <productname>PostgreSQL</>
+ Increasing this parameter m
ight cause <productname>PostgreSQL</>
to request more <systemitem class="osname">System V</> shared
memory than your operating system's default configuration
allows. See <xref linkend="sysvipc"> for information on how to
operations can be executed at a time by a database session, and
an installation normally doesn't have many of them running
concurrently, it's safe to set this value significantly larger
- than <varname>work_mem</varname>. Larger settings may improve
+ than <varname>work_mem</varname>. Larger settings might improve
performance for vacuuming and for restoring database dumps.
</para>
</listitem>
routine in the server, but only in key potentially-recursive routines
such as expression evaluation. The default setting is two
megabytes (<literal>2MB</>), which is conservatively small and
- unlikely to risk crashes. However, it may be too small to allow
+ unlikely to risk crashes. However, it might be too small to allow
execution of complex functions. Only superusers can change this
setting.
</para>
<para>
These parameters control the size of the shared <firstterm>free space
map</>, which tracks the locations of unused space in the database.
- An undersized free space map may cause the database to consume
+ An undersized free space map can cause the database to consume
increasing amounts of disk space over time, because free space that
is not in the map cannot be re-used; instead <productname>PostgreSQL</>
will request more disk space from the operating system when it needs
</para>
<para>
- Increasing these parameters m
ay cause <productname>PostgreSQL</>
+ Increasing these parameters m
ight cause <productname>PostgreSQL</>
to request more <systemitem class="osname">System V</> shared
memory than your operating system's default configuration
allows. See <xref linkend="sysvipc"> for information on how to
<para>
By preloading a shared library, the library startup time is avoided
when the library is first used. However, the time to start each new
- server process may increase slightly, even if that process never
+ server process might increase slightly, even if that process never
uses the library. So this parameter is recommended only for
libraries that will be used in most sessions.
</para>
Note that on many systems, the effective resolution
of sleep delays is 10 milliseconds; setting
<varname>vacuum_cost_delay</varname> to a value that is
- not a multiple of 10 may have the same results as setting it
+ not a multiple of 10 might have the same results as setting it
to the next higher multiple of 10.
</para>
</listitem>
(<literal>200ms</>). Note that on many systems, the effective
resolution of sleep delays is 10 milliseconds; setting
<varname>bgwriter_delay</> to a value that is not a multiple of
- 10 may have the same results as setting it to the next higher
+ 10 might have the same results as setting it to the next higher
multiple of 10. This parameter can only be set in the
<filename>postgresql.conf</> file or on the server command line.
</para>
allowed to do its best in buffering, ordering, and delaying
writes. This can result in significantly improved performance.
However, if the system crashes, the results of the last few
- committed transactions may be lost in part or whole. In the
- worst case, unrecoverable data corruption may occur.
+ committed transactions might be lost in part or whole. In the
+ worst case, unrecoverable data corruption might occur.
(Crashes of the database software itself are <emphasis>not</>
a risk factor here. Only an operating-system-level crash
creates a risk of corruption.)
Turning this parameter off speeds normal operation, but
might lead to a corrupt database after an operating system crash
or power failure. The risks are similar to turning off
- <varname>fsync</>, though smaller. It may be safe to turn off
+ <varname>fsync</>, though smaller. It might be safe to turn off
this parameter if you have hardware (such as a battery-backed disk
controller) or file-system software that reduces
the risk of partial page writes to an acceptably low level (e.g., ReiserFS 4).
</para>
<para>
- Increasing this parameter m
ay cause <productname>PostgreSQL</>
+ Increasing this parameter m
ight cause <productname>PostgreSQL</>
to request more <systemitem class="osname">System V</> shared
memory than your operating system's default configuration
allows. See <xref linkend="sysvipc"> for information on how to
These configuration parameters provide a crude method of
influencing the query plans chosen by the query optimizer. If
the default plan chosen by the optimizer for a particular query
- is not optimal, a temporary solution may be found by using one
+ is not optimal, a temporary solution can be found by using one
of these configuration parameters to force the optimizer to
choose a different plan. Turning one of these settings off
permanently is seldom a good idea, however.
Sets the default statistics target for table columns that have
not had a column-specific target set via <command>ALTER TABLE
SET STATISTICS</>. Larger values increase the time needed to
- do <command>ANALYZE</>, but may improve the quality of the
+ do <command>ANALYZE</>, but might improve the quality of the
planner's estimates. The default is 10. For more information
on the use of statistics by the <productname>PostgreSQL</>
query planner, refer to <xref linkend="planner-stats">.
<para>
The planner will merge sub-queries into upper queries if the
resulting <literal>FROM</literal> list would have no more than
- this many items. Smaller values reduce planning time but may
+ this many items. Smaller values reduce planning time but might
yield inferior query plans. The default is eight. It is usually
wise to keep this less than <xref linkend="guc-geqo-threshold">.
For more information see <xref linkend="explicit-joins">.
The planner will rewrite explicit <literal>JOIN</>
constructs (except <literal>FULL JOIN</>s) into lists of
<literal>FROM</> items whenever a list of no more than this many items
- would result. Smaller values reduce planning time but may
+ would result. Smaller values reduce planning time but might
yield inferior query plans.
</para>
explicit <literal>JOIN</>s. Thus, the explicit join order
specified in the query will be the actual order in which the
relations are joined. The query planner does not always choose
- the optimal join order; advanced users may elect to
+ the optimal join order; advanced users can elect to
temporarily set this variable to 1, and then specify the join
order they desire explicitly.
For more information see <xref linkend="explicit-joins">.
This method, in combination with logging to <application>stderr</>,
is often more useful than
logging to <application>syslog</>, since some types of messages
- m
ay not appear in <application>syslog</> output (a common example
+ m
ight not appear in <application>syslog</> output (a common example
is dynamic-linker failure messages).
This parameter can only be set at server start.
</para>
<para>
When <varname>redirect_stderr</> is enabled, this parameter
determines the directory in which log files will be created.
- It may be specified as an absolute path, or relative to the
+ It can be specified as an absolute path, or relative to the
cluster data directory.
This parameter can only be set in the <filename>postgresql.conf</>
file or on the server command line.
<varname>log_rotation_age</varname> to <literal>60</literal>, and
<varname>log_rotation_size</varname> to <literal>1000000</literal>.
Including <literal>%M</> in <varname>log_filename</varname> allows
- any size-driven rotations that may occur to select a file name
+ any size-driven rotations that might occur to select a file name
different from the hour's initial file name.
</para>
</listitem>
<para>
When logging to <application>syslog</> is enabled, this parameter
determines the <application>syslog</application>
- <quote>facility</quote> to be used. You may choose
+ <quote>facility</quote> to be used. You can choose
from <literal>LOCAL0</>, <literal>LOCAL1</>,
<literal>LOCAL2</>, <literal>LOCAL3</>, <literal>LOCAL4</>,
<literal>LOCAL5</>, <literal>LOCAL6</>, <literal>LOCAL7</>;
<term><literal>NOTICE</literal></term>
<listitem>
<para>
- Provides information that may be helpful to users, e.g.,
+ Provides information that might be helpful to users, e.g.,
truncation of long identifiers and the creation of indexes as part
of primary keys.
</para>
<para>
Controls whether the server should start the
statistics-collection subprocess. This is on by default, but
- may be turned off if you know you have no interest in
+ can be turned off if you know you have no interest in
collecting statistics or running autovacuum.
This parameter can only be set at server start, because the collection
subprocess cannot be started or stopped on-the-fly. (However, the
<listitem>
<para>
Sets the locale to use for formatting date and time values.
- (Currently, this setting does nothing, but it may in the
+ (Currently, this setting does nothing, but it might in the
future.) Acceptable values are system-dependent; see <xref
linkend="locale"> for more information. If this variable is
set to the empty string (which is the default) then the value
</para>
<para>
- Increasing this parameter m
ay cause <productname>PostgreSQL</>
+ Increasing this parameter m
ight cause <productname>PostgreSQL</>
to request more <systemitem class="osname">System V</> shared
memory than your operating system's default configuration
allows. See <xref linkend="sysvipc"> for information on how to
The regular expression <quote>flavor</> can be set to
<literal>advanced</>, <literal>extended</>, or <literal>basic</>.
The default is <literal>advanced</>. The <literal>extended</>
- setting may be useful for exact backwards compatibility with
+ setting might be useful for exact backwards compatibility with
pre-7.4 releases of <productname>PostgreSQL</>. See
<xref linkend="posix-syntax-details"> for details.
</para>
behavior of treating backslashes as escape characters.
The default will change to <literal>on</> in a future release
to improve compatibility with the standard.
- Applications may check this
+ Applications can check this
parameter to determine how string literals will be processed.
The presence of this parameter can also be taken as an indication
that the escape string syntax (<literal>E'...'</>) is supported.
installed. As such, they have been excluded from the sample
<filename>postgresql.conf</> file. These options report
various aspects of <productname>PostgreSQL</productname> behavior
- that may be of interest to certain applications, particularly
+ that might be of interest to certain applications, particularly
administrative front-ends.
</para>
the system to instead report a warning, zero out the damaged page,
and continue processing. This behavior <emphasis>will destroy data</>,
namely all the rows on the damaged page. But it allows you to get
- past the error and retrieve rows from any undamaged pages that may
+ past the error and retrieve rows from any undamaged pages that might
be present in the table. So it is useful for recovering data if
corruption has occurred due to hardware or software error. You should
generally not set this on until you have given up hope of recovering
-<!-- $PostgreSQL: pgsql/doc/src/sgml/cvs.sgml,v 1.39 2006/11/17 05:29:46 neilc Exp $ -->
+<!-- $PostgreSQL: pgsql/doc/src/sgml/cvs.sgml,v 1.40 2007/01/31 20:56:16 momjian Exp $ -->
<appendix id="cvs">
<appendixinfo>
<note>
<para>
- If you have a fast link to the Internet, you may not need
+ If you have a fast link to the Internet, you might not need
<option>-z3</option>, which instructs
<productname>CVS</productname> to use <command>gzip</command> compression for transferred data. But
on a modem-speed link, it's a very substantial win.
-<!-- $PostgreSQL: pgsql/doc/src/sgml/datatype.sgml,v 1.188 2007/01/30 22:29:22 momjian Exp $ -->
+<!-- $PostgreSQL: pgsql/doc/src/sgml/datatype.sgml,v 1.189 2007/01/31 20:56:16 momjian Exp $ -->
<chapter id="datatype">
<title id="datatype-title">Data Types</title>
<para>
<productname>PostgreSQL</productname> has a rich set of native data
- types available to users. Users may add new types to
+ types available to users. Users can add new types to
<productname>PostgreSQL</productname> using the <xref
linkend="sql-createtype" endterm="sql-createtype-title"> command.
</para>
paths, or have several possibilities for formats, such as the date
and time types.
Some of the input and output functions are not invertible. That is,
- the result of an output function may lose accuracy when compared to
+ the result of an output function might lose accuracy when compared to
the original input.
</para>
</para>
<para>
- The <type>bigint</type> type may not function correctly on all
+ The <type>bigint</type> type might not function correctly on all
platforms, since it relies on compiler support for eight-byte
integers. On a machine without such support, <type>bigint</type>
acts the same as <type>integer</type> (but still takes up eight
<para>
Inexact means that some values cannot be converted exactly to the
internal format and are stored as approximations, so that storing
- and printing back out a value may show slight discrepancies.
+ and printing back out a value might show slight discrepancies.
Managing these errors and how they propagate through calculations
is the subject of an entire branch of mathematics and computer
science and will not be discussed further here, except for the
<listitem>
<para>
- Comparing two floating-point values for equality may or may
+ Comparing two floating-point values for equality might or might
not work as expected.
</para>
</listitem>
1E-37 to 1E+37 with a precision of at least 6 decimal digits. The
<type>double precision</type> type typically has a range of around
1E-307 to 1E+308 with a precision of at least 15 digits. Values that
- are too large or too small will cause an error. Rounding may
+ are too large or too small will cause an error. Rounding might
take place if the precision of an input number is too high.
Numbers too close to zero that are not representable as distinct
from zero will cause an underflow error.
digits. The assumption that <type>real</type> and
<type>double precision</type> have exactly 24 and 53 bits in the
mantissa respectively is correct for IEEE-standard floating point
- implementations. On non-IEEE platforms it may be off a little, but
+ implementations. On non-IEEE platforms it might be off a little, but
for simplicity the same ranges of <replaceable>p</replaceable> are used
on all platforms.
</para>
The storage requirement for data of these types is 4 bytes plus the
actual string, and in case of <type>character</type> plus the
padding. Long strings are compressed by the system automatically, so
- the physical requirement on disk may be less. Long values are also
+ the physical requirement on disk might be less. Long values are also
stored in background tables so they do not interfere with rapid
access to the shorter column values. In any case, the longest
possible character string that can be stored is about 1 GB. (The
terminator) but should be referenced using the constant
<symbol>NAMEDATALEN</symbol>. The length is set at compile time (and
is therefore adjustable for special uses); the default maximum
- length may change in a future release. The type <type>"char"</type>
+ length might change in a future release. The type <type>"char"</type>
(note the quotes) is different from <type>char(1)</type> in that it
only uses one byte of storage. It is internally used in the system
catalogs as a poor-man's enumeration type.
<para>
Depending on the front end to <productname>PostgreSQL</> you use,
- you may have additional work to do in terms of escaping and
- unescaping <type>bytea</type> strings. For example, you may also
+ you might have additional work to do in terms of escaping and
+ unescaping <type>bytea</type> strings. For example, you might also
have to escape line feeds and carriage returns if your interface
automatically translates these.
</para>
<para>
When <type>timestamp</> values are stored as double precision floating-point
numbers (currently the default), the effective limit of precision
- may be less than 6. <type>timestamp</type> values are stored as seconds
+ might be less than 6. <type>timestamp</type> values are stored as seconds
before or after midnight 2000-01-01. Microsecond precision is achieved for
dates within a few years of 2000-01-01, but the precision degrades for
dates further away. When <type>timestamp</type> values are stored as
<type>time</type> type can.
Time zones in the real world have little meaning unless
associated with a date as well as a time,
- since the offset may vary through the year with daylight-saving
+ since the offset can vary through the year with daylight-saving
time boundaries.
</para>
</listitem>
<para>
A time zone abbreviation, for example <literal>PST</>. Such a
specification merely defines a particular offset from UTC, in
- contrast to full time zone names which may imply a set of daylight
+ contrast to full time zone names which might imply a set of daylight
savings transition-date rules as well. The recognized abbreviations
are listed in the <literal>pg_timezone_abbrevs</> view (see <xref
linkend="view-pg-timezone-abbrevs">). You cannot set the
</table>
<para>
- Functions coded in C (whether built-in or dynamically loaded) may be
+ Functions coded in C (whether built-in or dynamically loaded) can be
declared to accept or return any of these pseudo data types. It is up to
the function author to ensure that the function will behave safely
when a pseudo-type is used as an argument type.
</para>
<para>
- Functions coded in procedural languages may use pseudo-types only as
+ Functions coded in procedural languages can use pseudo-types only as
allowed by their implementation languages. At present the procedural
languages all forbid use of a pseudo-type as argument type, and allow
only <type>void</> and <type>record</> as a result type (plus
-<!-- $PostgreSQL: pgsql/doc/src/sgml/datetime.sgml,v 2.55 2006/10/17 21:03:20 tgl Exp $ -->
+<!-- $PostgreSQL: pgsql/doc/src/sgml/datetime.sgml,v 2.56 2007/01/31 20:56:16 momjian Exp $ -->
<appendix id="datetime-appendix">
<title>Date/Time Support</title>
<productname>PostgreSQL</productname> uses an internal heuristic
parser for all date/time input support. Dates and times are input as
strings, and are broken up into distinct fields with a preliminary
- determination of what kind of information may be in the
+ determination of what kind of information can be in the
field. Each field is interpreted and either assigned a numeric
value, ignored, or rejected.
The parser contains internal lookup tables for all textual fields,
<para>
If the numeric token contains a dash (<literal>-</>), slash
(<literal>/</>), or two or more dots (<literal>.</>), this is
- a date string which may have a text month. If a date token has
+ a date string which might have a text month. If a date token has
already been seen, it is instead interpreted as a time zone
name (e.g., <literal>America/New_York</>).
</para>
<tip>
<para>
- Gregorian years AD 1-99 may be entered by using 4 digits with leading
+ Gregorian years AD 1-99 can be entered by using 4 digits with leading
zeros (e.g., <literal>0099</> is AD 99).
</para>
</tip>
</para>
<para>
- A timezone abbreviation file may contain blank lines and comments
+ A timezone abbreviation file can contain blank lines and comments
beginning with <literal>#</>. Non-comment lines must have one of
these formats:
<para>
The <literal>@OVERRIDE</> syntax indicates that subsequent entries in the
- file may override previous entries (i.e., entries obtained from included
+ file can override previous entries (i.e., entries obtained from included
files). Without this, conflicting definitions of the same timezone
abbreviation are considered an error.
</para>
-<!-- $PostgreSQL: pgsql/doc/src/sgml/ddl.sgml,v 1.71 2006/12/30 20:31:11 momjian Exp $ -->
+<!-- $PostgreSQL: pgsql/doc/src/sgml/ddl.sgml,v 1.72 2007/01/31 20:56:16 momjian Exp $ -->
<chapter id="ddl">
<title>Data Definition</title>
</para>
<para>
- The default value may be an expression, which will be
+ The default value can be an expression, which will be
evaluated whenever the default value is inserted
(<emphasis>not</emphasis> when the table is created). A common example
- is that a <type>timestamp</type> column may have a default of <literal>now()</>,
+ is that a <type>timestamp</type> column can have a default of <literal>now()</>,
so that it gets set to the time of row insertion. Another common
example is generating a <quote>serial number</> for each row.
In <productname>PostgreSQL</productname> this is typically done by
The <literal>NOT NULL</literal> constraint has an inverse: the
<literal>NULL</literal> constraint. This does not mean that the
column must be null, which would surely be useless. Instead, this
- simply selects the default behavior that the column may be null.
+ simply selects the default behavior that the column might be null.
The <literal>NULL</literal> constraint is not present in the SQL
standard and should not be used in portable applications. (It was
only added to <productname>PostgreSQL</productname> to be
unique constraint it is possible to store duplicate
rows that contain a null value in at least one of the constrained
columns. This behavior conforms to the SQL standard, but we have
- heard that other SQL databases may not follow this rule. So be
+ heard that other SQL databases might not follow this rule. So be
careful when developing applications that are intended to be
portable.
</para>
(Of course, this is only possible if the table contains fewer
than 2<superscript>32</> (4 billion) rows, and in practice the
table size had better be much less than that, or performance
- may suffer.)
+ might suffer.)
</para>
</listitem>
<listitem>
<para>
<productname>PostgreSQL</> will attempt to convert the column's
default value (if any) to the new type, as well as any constraints
- that involve the column. But these conversions may fail, or may
+ that involve the column. But these conversions might fail, or might
produce surprising results. It's often best to drop any constraints
on the column before altering its type, and then add back suitably
modified constraints afterwards.
in turn contain tables. Schemas also contain other kinds of named
objects, including data types, functions, and operators. The same
object name can be used in different schemas without conflict; for
- example, both <literal>schema1</> and <literal>myschema</> may
+ example, both <literal>schema1</> and <literal>myschema</> can
contain tables named <literal>mytable</>. Unlike databases,
- schemas are not rigidly separated: a user may access objects in any
+ schemas are not rigidly separated: a user can access objects in any
of the schemas in the database he is connected to, if he has
privileges to do so.
</para>
<para>
Schema names beginning with <literal>pg_</> are reserved for
- system purposes and may not be created by users.
+ system purposes and cannot be created by users.
</para>
</sect2>
own. To allow that, the owner of the schema needs to grant the
<literal>USAGE</literal> privilege on the schema. To allow users
to make use of the objects in the schema, additional privileges
- may need to be granted, as appropriate for the object.
+ might need to be granted, as appropriate for the object.
</para>
<para>
the search path. If it is not named explicitly in the path then
it is implicitly searched <emphasis>before</> searching the path's
schemas. This ensures that built-in names will always be
- findable. However, you may explicitly place
+ findable. However, you can explicitly place
<literal>pg_catalog</> at the end of your search path if you
prefer to have user-defined names override built-in names.
</para>
<para>
In <productname>PostgreSQL</productname> versions before 7.3,
table names beginning with <literal>pg_</> were reserved. This is
- no longer true: you may create such a table name if you wish, in
+ no longer true: you can create such a table name if you wish, in
any non-system schema. However, it's best to continue to avoid
such names, to ensure that you won't suffer a conflict if some
future version defines a system table named the same as your
</para>
<para>
- In some cases you may wish to know which table a particular row
+ In some cases you might wish to know which table a particular row
originated from. There is a system column called
<structfield>tableoid</structfield> in each table which can tell you the
originating table:
<listitem>
<para>
- Bulk loads and deletes may be accomplished by adding or removing
+ Bulk loads and deletes can be accomplished by adding or removing
partitions, if that requirement is planned into the partitioning design.
<command>ALTER TABLE</> is far faster than a bulk operation.
It also entirely avoids the <command>VACUUM</command>
<para>
As we can see, a complex partitioning scheme could require a
substantial amount of DDL. In the above example we would be
- creating a new partition each month, so it may be wise to write a
+ creating a new partition each month, so it might be wise to write a
script that generates the required DDL automatically.
</para>
This allows further operations to be performed on the data before
it is dropped. For example, this is often a useful time to back up
the data using <command>COPY</>, <application>pg_dump</>, or
- similar tools. It can also be a useful time to aggregate data
+ similar tools. It might also be a useful time to aggregate data
into smaller formats, perform other data manipulations, or run
reports.
</para>
-<!-- $PostgreSQL: pgsql/doc/src/sgml/diskusage.sgml,v 1.17 2006/09/16 00:30:12 momjian Exp $ -->
+<!-- $PostgreSQL: pgsql/doc/src/sgml/diskusage.sgml,v 1.18 2007/01/31 20:56:16 momjian Exp $ -->
<chapter id="diskusage">
<title>Monitoring Disk Usage</title>
there is also a <acronym>TOAST</> file associated with the table,
which is used to store values too wide to fit comfortably in the main
table (see <xref linkend="storage-toast">). There will be one index on the
- <acronym>TOAST</> table, if present. There may also be indexes associated
+ <acronym>TOAST</> table, if present. There might also be indexes associated
with the base table. Each table and index is stored in a separate disk
file — possibly more than one file, if the file would exceed one
gigabyte. Naming conventions for these files are described in <xref
<para>
The most important disk monitoring task of a database administrator
is to make sure the disk doesn't grow full. A filled data disk will
- not result in data corruption, but it may well prevent useful activity
+ not result in data corruption, but it might prevent useful activity
from occurring. If the disk holding the WAL files grows full, database
- server panic and consequent shutdown may occur.
+ server panic and consequent shutdown might occur.
</para>
<para>
-<!-- $PostgreSQL: pgsql/doc/src/sgml/dml.sgml,v 1.14 2006/09/18 19:54:01 tgl Exp $ -->
+<!-- $PostgreSQL: pgsql/doc/src/sgml/dml.sgml,v 1.15 2007/01/31 20:56:16 momjian Exp $ -->
<chapter id="dml">
<title>Data Manipulation</title>
<programlisting>
UPDATE products SET price = 10 WHERE price = 5;
</programlisting>
- This may cause zero, one, or many rows to be updated. It is not
+ This might cause zero, one, or many rows to be updated. It is not
an error to attempt an update that does not match any rows.
</para>
<para>
Let's look at that command in detail. First is the key word
<literal>UPDATE</literal> followed by the table name. As usual,
- the table name may be schema-qualified, otherwise it is looked up
+ the table name can be schema-qualified, otherwise it is looked up
in the path. Next is the key word <literal>SET</literal> followed
by the column name, an equals sign and the new column value. The
new column value can be any scalar expression, not just a constant.
-<!-- $PostgreSQL: pgsql/doc/src/sgml/docguide.sgml,v 1.68 2007/01/31 15:23:28 teodor Exp $ -->
+<!-- $PostgreSQL: pgsql/doc/src/sgml/docguide.sgml,v 1.69 2007/01/31 20:56:17 momjian Exp $ -->
<appendix id="docguide">
<title>Documentation</title>
<para>
The following tools are used to process the documentation. Some
- may be optional, as noted.
+ might be optional, as noted.
<variablelist>
<varlistentry>
<para>
We have documented experience with several installation methods for
the various tools that are needed to process the documentation.
- These will be described below. There may be some other packaged
+ These will be described below. There might be some other packaged
distributions for these tools. Please report package status to the
documentation mailing list, and we will include that information
here.
It appears that current versions of the <productname>PostgreSQL</productname> documentation
trigger some bug in or exceed the size limit of OpenJade. If the
build process of the <acronym>RTF</acronym> version hangs for a
- long time and the output file still has size 0, then you may have
+ long time and the output file still has size 0, then you might have
hit that problem. (But keep in mind that a normal build takes 5
to 10 minutes, so don't abort too soon.)
</para>
<para>
The <productname>PostgreSQL</productname> distribution includes a
parsed DTD definitions file <filename>reference.ced</filename>.
- You m
ay find that when using <productname>PSGML</productname>, a
+ You m
ight find that when using <productname>PSGML</productname>, a
comfortable way of working with these separate files of book
parts is to insert a proper <literal>DOCTYPE</literal>
declaration while you're editing them. If you are working on
<para>
Reference pages that describe executable commands should contain
the following sections, in this order. Sections that do not apply
- may be omitted. Additional top-level sections should only be used
+ can be omitted. Additional top-level sections should only be used
in special circumstances; often that information belongs in the
<quote>Usage</quote> section.
<listitem>
<para>
A list describing each command-line option. If there are a
- lot of options, subsections may be used.
+ lot of options, subsections can be used.
</para>
</listitem>
</varlistentry>
-<!-- $PostgreSQL: pgsql/doc/src/sgml/ecpg.sgml,v 1.77 2006/10/23 18:10:31 petere Exp $ -->
+<!-- $PostgreSQL: pgsql/doc/src/sgml/ecpg.sgml,v 1.78 2007/01/31 20:56:17 momjian Exp $ -->
<chapter id="ecpg">
<title><application>ECPG</application> - Embedded <acronym>SQL</acronym> in C</title>
EXEC SQL ...;
</programlisting>
These statements syntactically take the place of a C statement.
- Depending on the particular statement, they may appear at the
+ Depending on the particular statement, they can appear at the
global level or within a function. Embedded
<acronym>SQL</acronym> statements follow the case-sensitivity rules
of normal <acronym>SQL</acronym> code, and not those of C.
(single-quoted) string literal or a variable reference. The
connection target <literal>DEFAULT</literal> initiates a connection
to the default database under the default user name. No separate
- user name or connection name may be specified in that case.
+ user name or connection name can be specified in that case.
</para>
<para>
</itemizedlist>
As above, the parameters <replaceable>username</replaceable> and
- <replaceable>password</replaceable> may be an SQL identifier, an
+ <replaceable>password</replaceable> can be an SQL identifier, an
SQL string literal, or a reference to a character variable.
</para>
EXEC SQL EXECUTE IMMEDIATE :stmt;
</programlisting>
- You may not execute statements that retrieve data (e.g.,
+ You cannot execute statements that retrieve data (e.g.,
<command>SELECT</command>) this way.
</para>
...
EXEC SQL EXECUTE mystmt INTO v1, v2, v3 USING 37;
</programlisting>
- An <command>EXECUTE</command> command may have an
+ An <command>EXECUTE</command> command can have an
<literal>INTO</literal> clause, a <literal>USING</literal> clause,
both, or neither.
</para>
<command>FETCH</command> statement. An SQL descriptor area groups
the data of one row of data together with metadata items into one
data structure. The metadata is particularly useful when executing
- dynamic SQL statements, where the nature of the result columns may
+ dynamic SQL statements, where the nature of the result columns might
not be known ahead of time.
</para>
<para>
The statement sent to the <productname>PostgreSQL</productname>
server was empty. (This cannot normally happen in an embedded
- SQL program, so it may point to an internal error.) (SQLSTATE
+ SQL program, so it might point to an internal error.) (SQLSTATE
YE002)
</para>
</listitem>
<para>
If you manage the build process of a larger project using
- <application>make</application>, it may be convenient to include
+ <application>make</application>, it might be convenient to include
the following implicit rule to your makefiles:
<programlisting>
ECPG = ecpg
<para>
Here is a complete example describing the output of the
- preprocessor of a file <filename>foo.pgc</filename> (details may
+ preprocessor of a file <filename>foo.pgc</filename> (details might
change with each particular version of the preprocessor):
<programlisting>
EXEC SQL BEGIN DECLARE SECTION;
-<!-- $PostgreSQL: pgsql/doc/src/sgml/errcodes.sgml,v 1.21 2006/12/24 00:29:17 tgl Exp $ -->
+<!-- $PostgreSQL: pgsql/doc/src/sgml/errcodes.sgml,v 1.22 2007/01/31 20:56:17 momjian Exp $ -->
<appendix id="errcodes-appendix">
<title><productname>PostgreSQL</productname> Error Codes</title>
According to the standard, the first two characters of an error code
denote a class of errors, while the last three characters indicate
a specific condition within that class. Thus, an application that
- does not recognize the specific error code may still be able to infer
+ does not recognize the specific error code can still be able to infer
what to do from the error class.
</para>
-<!-- $PostgreSQL: pgsql/doc/src/sgml/extend.sgml,v 1.32 2006/09/16 00:30:13 momjian Exp $ -->
+<!-- $PostgreSQL: pgsql/doc/src/sgml/extend.sgml,v 1.33 2007/01/31 20:56:17 momjian Exp $ -->
<chapter id="extend">
<title>Extending <acronym>SQL</acronym></title>
<para>
A domain is based on a particular base type and for many purposes
- is interchangeable with its base type. However, a domain may
+ is interchangeable with its base type. However, a domain can
have constraints that restrict its valid values to a subset of
what the underlying base type would allow.
</para>
-<!-- $PostgreSQL: pgsql/doc/src/sgml/external-projects.sgml,v 1.14 2006/11/20 17:42:16 neilc Exp $ -->
+<!-- $PostgreSQL: pgsql/doc/src/sgml/external-projects.sgml,v 1.15 2007/01/31 20:56:17 momjian Exp $ -->
<appendix id="external-projects">
<title>External Projects</title>
All other language interfaces are external projects and are distributed
separately. <xref linkend="language-interface-table"> includes a list of
- some of these projects. Note that some of these packages may not be
+ some of these projects. Note that some of these packages might not be
released under the same license as <productname>PostgreSQL</>. For more
information on each language interface, including licensing terms, refer to
its website and documentation.
In addition, there are a number of procedural languages that are developed
and maintained outside the core <productname>PostgreSQL</productname>
distribution. <xref linkend="pl-language-table"> lists some of these
- packages. Note that some of these projects may not be released under the same
+ packages. Note that some of these projects might not be released under the same
license as <productname>PostgreSQL</>. For more information on each
procedural language, including licensing information, refer to its website
and documentation.
-<!-- $PostgreSQL: pgsql/doc/src/sgml/features.sgml,v 2.25 2006/09/16 00:30:13 momjian Exp $ -->
+<!-- $PostgreSQL: pgsql/doc/src/sgml/features.sgml,v 2.26 2007/01/31 20:56:17 momjian Exp $ -->
<appendix id="features">
<title>SQL Conformance</title>
9075 Working Group during the preparation of SQL:2003. Even so,
many of the features required by SQL:2003 are already supported,
though sometimes with slightly differing syntax or function.
- Further moves towards conformance may be expected in later releases.
+ Further moves towards conformance should be expected in later releases.
</para>
<para>
PostgreSQL supports most of the major features of SQL:2003. Out of
164 mandatory features required for full Core conformance,
PostgreSQL conforms to at least 150. In addition, there is a long
- list of supported optional features. It may be worth noting that at
+ list of supported optional features. It might be worth noting that at
the time of writing, no current version of any database management
system claims full conformance to Core SQL:2003.
</para>
that <productname>PostgreSQL</productname> supports, followed by a
list of the features defined in <acronym>SQL:2003</acronym> which
are not yet supported in <productname>PostgreSQL</productname>.
- Both of these lists are approximate: There may be minor details that
+ Both of these lists are approximate: There might be minor details that
are nonconforming for a feature that is listed as supported, and
- large parts of an unsupported feature may in fact be implemented.
+ large parts of an unsupported feature might in fact be implemented.
The main body of the documentation always contains the most accurate
information about what does and does not work.
</para>
-<!-- $PostgreSQL: pgsql/doc/src/sgml/func.sgml,v 1.355 2007/01/30 22:29:22 momjian Exp $ -->
+<!-- $PostgreSQL: pgsql/doc/src/sgml/func.sgml,v 1.356 2007/01/31 20:56:17 momjian Exp $ -->
<chapter id="functions">
<title>Functions and Operators</title>
<tip>
<para>
- Some applications may expect that
+ Some applications might expect that
<literal><replaceable>expression</replaceable> = NULL</literal>
returns true if <replaceable>expression</replaceable> evaluates to
the null value. It is highly recommended that these applications
data type as its argument.
The functions working with <type>double precision</type> data are mostly
implemented on top of the host system's C library; accuracy and behavior in
- boundary cases may therefore vary depending on the host system.
+ boundary cases can therefore vary depending on the host system.
</para>
<indexterm>
other characters, the respective character in
<replaceable>pattern</replaceable> must be
preceded by the escape character. The default escape
- character is the backslash but a different one may be selected by
+ character is the backslash but a different one can be selected by
using the <literal>ESCAPE</literal> clause. To match the escape
character itself, write two escape characters.
</para>
Like <function>LIKE</function>, the <function>SIMILAR TO</function>
operator succeeds only if its pattern matches the entire string;
this is unlike common regular expression practice, wherein the pattern
- may match any part of the string.
+ can match any part of the string.
Also like
<function>LIKE</function>, <function>SIMILAR TO</function> uses
<literal>_</> and <literal>%</> as wildcard characters denoting
</listitem>
<listitem>
<para>
- Parentheses <literal>()</literal> may be used to group items into
+ Parentheses <literal>()</literal> can be used to group items into
a single logical item.
</para>
</listitem>
<para>
A <firstterm>constraint</> matches an empty string, but matches only when
specific conditions are met. A constraint can be used where an atom
- could be used, except it may not be followed by a quantifier.
+ could be used, except it cannot be followed by a quantifier.
The simple constraints are shown in
<xref linkend="posix-constraints-table">;
some more constraints are described later.
</table>
<para>
- An RE may not end with <literal>\</>.
+ An RE cannot end with <literal>\</>.
</para>
<note>
<entry>
<literal>{</><replaceable>m</><literal>,</><replaceable>n</><literal>}</> </entry>
<entry> a sequence of <replaceable>m</> through <replaceable>n</>
- (inclusive) matches of the atom; <replaceable>m</> may not exceed
+ (inclusive) matches of the atom; <replaceable>m</> cannot exceed
<replaceable>n</> </entry>
</row>
</table>
<para>
- Lookahead constraints may not contain <firstterm>back references</>
+ Lookahead constraints cannot contain <firstterm>back references</>
(see <xref linkend="posix-escape-sequences">),
and all parentheses within them are considered non-capturing.
</para>
<literal>^</literal> are the members of an equivalence class, then
<literal>[[=o=]]</literal>, <literal>[[=^=]]</literal>, and
<literal>[o^]</literal> are all synonymous. An equivalence class
- may not be an endpoint of a range.
+ cannot be an endpoint of a range.
</para>
<para>
<literal>xdigit</literal>. These stand for the character classes
defined in
<citerefentry><refentrytitle>ctype</refentrytitle><manvolnum>3</manvolnum></citerefentry>.
- A locale may provide others. A character class may not be used as
+ A locale can provide others. A character class cannot be used as
an endpoint of a range.
</para>
</para>
<para>
- An ARE may begin with <firstterm>embedded options</>:
+ An ARE can begin with <firstterm>embedded options</>:
a sequence <literal>(?</><replaceable>xyz</><literal>)</>
(where <replaceable>xyz</> is one or more alphabetic characters)
specifies options affecting the rest of the RE.
<para>
Embedded options take effect at the <literal>)</> terminating the sequence.
- They may appear only at the start of an ARE (after the
+ They can appear only at the start of an ARE (after the
<literal>***:</> director if any).
</para>
</table>
<para>
- Certain modifiers may be applied to any template pattern to alter its
+ Certain modifiers can be applied to any template pattern to alter its
behavior. For example, <literal>FMMonth</literal>
is the <literal>Month</literal> pattern with the
<literal>FM</literal> modifier.
In these expressions, the desired time zone <replaceable>zone</> can be
specified either as a text string (e.g., <literal>'PST'</literal>)
or as an interval (e.g., <literal>INTERVAL '-08:00'</literal>).
- In the text case, a time zone name may be specified in any of the ways
+ In the text case, a time zone name can be specified in any of the ways
described in <xref linkend="datatype-timezones">.
</para>
<note>
<para>
- Other database systems may advance these values more
+ Other database systems might advance these values more
frequently.
</para>
</note>
statement (more specifically, the time of receipt of the latest command
message from the client).
<function>statement_timestamp()</> and <function>transaction_timestamp()</>
- return the same value during the first command of a transaction, but may
+ return the same value during the first command of a transaction, but might
differ during subsequent commands.
<function>clock_timestamp()</> returns the actual current time, and
therefore its value changes even within a single SQL command.
<para>
The effective resolution of the sleep interval is platform-specific;
0.01 seconds is a common value. The sleep delay will be at least as long
- as specified. It may be longer depending on factors such as server load.
+ as specified. It might be longer depending on factors such as server load.
</para>
</note>
<literal>t.p</> is a <type>point</> column then
<literal>SELECT p[0] FROM t</> retrieves the X coordinate and
<literal>UPDATE t SET p[1] = ...</> changes the Y coordinate.
- In the same way, a value of type <type>box</> or <type>lseg</> may be treated
+ In the same way, a value of type <type>box</> or <type>lseg</> can be treated
as an array of two <type>point</> values.
</para>
</programlisting>
Note that late binding was the only behavior supported in
<productname>PostgreSQL</productname> releases before 8.1, so you
- may need to do this to preserve the semantics of old applications.
+ might need to do this to preserve the semantics of old applications.
</para>
<para>
value and sets its <literal>is_called</literal> field to <literal>true</literal>,
meaning that the next <function>nextval</function> will advance the sequence
before returning a value. In the three-parameter form,
- <literal>is_called</literal> may be set either <literal>true</literal> or
+ <literal>is_called</literal> can be set either <literal>true</literal> or
<literal>false</literal>. If it's set to <literal>false</literal>,
the next <function>nextval</function> will return exactly the specified
value, and sequence advancement commences with the following
same sequence, a <function>nextval</function> operation is never rolled back;
that is, once a value has been fetched it is considered used, even if the
transaction that did the <function>nextval</function> later aborts. This means
- that aborted transactions may leave unused <quote>holes</quote> in the
+ that aborted transactions might leave unused <quote>holes</quote> in the
sequence of assigned values. <function>setval</function> operations are never
rolled back, either.
</para>
It should be noted that except for <function>count</function>,
these functions return a null value when no rows are selected. In
particular, <function>sum</function> of no rows returns null, not
- zero as one might expect. The <function>coalesce</function> function may be
+ zero as one might expect. The <function>coalesce</function> function can be
used to substitute zero for null when necessary.
</para>
<note>
<para>
Users accustomed to working with other SQL database management
- systems may be surprised by the performance of the
+ systems might be surprised by the performance of the
<function>count</function> aggregate when it is applied to the
entire table. A query like:
<programlisting>
whether at least one row is returned, not all the way to completion.
It is unwise to write a subquery that has any side effects (such as
calling sequence functions); whether the side effects occur or not
- may be difficult to predict.
+ might be difficult to predict.
</para>
<para>
<note>
<para>
- The search path may be altered at run time. The command is:
+ The search path can be altered at run time. The command is:
<programlisting>
SET search_path TO <replaceable>schema</> <optional>, <replaceable>schema</>, ...</optional>
</programlisting>
creating command for a constraint, index, rule, or trigger. (Note that this
is a decompiled reconstruction, not the original text of the command.)
<function>pg_get_expr</function> decompiles the internal form of an
- individual expression, such as the default value for a column. It may be
+ individual expression, such as the default value for a column. It can be
useful when examining the contents of system catalogs.
<function>pg_get_viewdef</function> reconstructs the <command>SELECT</>
query that defines a view. Most of these functions come in two variants,
the transaction log archive area. The history file includes the label given to
<function>pg_start_backup</>, the starting and ending transaction log locations for
the backup, and the starting and ending times of the backup. The return
- value is the backup's ending transaction log location (which again may be of little
+ value is the backup's ending transaction log location (which again might be of little
interest). After noting the ending location, the current transaction log insertion
point is automatically advanced to the next transaction log file, so that the
ending transaction log file can be archived immediately to complete the backup.
<entry><type>bigint</type></entry>
<entry>
Disk space used by the table or index with the specified name.
- The table name may be qualified with a schema name
+ The table name can be qualified with a schema name
</entry>
</row>
<row>
<entry><type>bigint</type></entry>
<entry>
Total disk space used by the table with the specified name,
- including indexes and toasted data. The table name may be
+ including indexes and toasted data. The table name can be
qualified with a schema name
</entry>
</row>
The functions shown in <xref
linkend="functions-admin-genfile"> provide native file access to
files on the machine hosting the server. Only files within the
- database cluster directory and the <varname>log_directory</> may be
+ database cluster directory and the <varname>log_directory</> can be
accessed. Use a relative path for files within the cluster directory,
and a path matching the <varname>log_directory</> configuration setting
for log files. Use of these functions is restricted to superusers.
</indexterm>
<para>
<function>pg_advisory_lock</> locks an application-defined resource,
- which may be identified either by a single 64-bit key value or two
+ which can be identified either by a single 64-bit key value or two
32-bit key values (note that these two key spaces do not overlap). If
another session already holds a lock on the same resource, the
function will wait until the resource becomes available. The lock
<para>
The function <function>xmlcomment</function> creates an XML value
containing an XML comment with the specified text as content.
- The text may not contain <literal>--</literal> or end with a
+ The text cannot contain <literal>--</literal> or end with a
<literal>-</literal> so that the resulting construct is a valid
XML comment. If the argument is null, the result is null.
</para>
-<!-- $PostgreSQL: pgsql/doc/src/sgml/gin.sgml,v 2.8 2007/01/31 15:09:45 teodor Exp $ -->
+<!-- $PostgreSQL: pgsql/doc/src/sgml/gin.sgml,v 2.9 2007/01/31 20:56:17 momjian Exp $ -->
<chapter id="GIN">
<title>GIN Indexes</title>
<acronym>GIN</acronym> stands for Generalized Inverted Index. It is
an index structure storing a set of (key, posting list) pairs, where
a <quote>posting list</> is a set of rows in which the key occurs. Each
- indexed value may contain many keys, so the same row ID may appear in
+ indexed value can contain many keys, so the same row ID can appear in
multiple posting lists.
</para>
<listitem>
<para>
Returns TRUE if the indexed value satisfies the query operator with
- strategy number <literal>n</> (or may satisfy, if the operator is
+ strategy number <literal>n</> (or would satisfy, if the operator is
marked RECHECK in the operator class). The <literal>check</> array has
the same length as the number of keys previously returned by
<function>extractQuery</> for this query. Each element of the
</para>
<para>
- <acronym>GIN</acronym> searches keys only by equality matching. This may
+ <acronym>GIN</acronym> searches keys only by equality matching. This might
be improved in future.
</para>
</sect1>
-<!-- $PostgreSQL: pgsql/doc/src/sgml/gist.sgml,v 1.27 2006/10/23 18:10:31 petere Exp $ -->
+<!-- $PostgreSQL: pgsql/doc/src/sgml/gist.sgml,v 1.28 2007/01/31 20:56:17 momjian Exp $ -->
<chapter id="GiST">
<title>GiST Indexes</title>
Usually, replay of the WAL log is sufficient to restore the integrity
of a GiST index following a database crash. However, there are some
corner cases in which the index state is not fully rebuilt. The index
- will still be functionally correct, but there may be some performance
+ will still be functionally correct, but there might be some performance
degradation. When this occurs, the index can be repaired by
<command>VACUUM</>ing its table, or by rebuilding the index using
<command>REINDEX</>. In some cases a plain <command>VACUUM</> is
-<!-- $PostgreSQL: pgsql/doc/src/sgml/indexam.sgml,v 2.20 2007/01/20 23:13:01 tgl Exp $ -->
+<!-- $PostgreSQL: pgsql/doc/src/sgml/indexam.sgml,v 2.21 2007/01/31 20:56:17 momjian Exp $ -->
<chapter id="indexam">
<title>Index Access Method Interface Definition</title>
block number and an item number within that block (see <xref
linkend="storage-page-layout">). This is sufficient
information to fetch a particular row version from the table.
- Indexes are not directly aware that under MVCC, there may be multiple
+ Indexes are not directly aware that under MVCC, there might be multiple
extant versions of the same logical row; to an index, each tuple is
an independent object that needs its own index entry. Thus, an
update of a row always creates all-new index entries for the row, even if
<literal>heap_tid</> is the TID to be indexed.
If the access method supports unique indexes (its
<structname>pg_am</>.<structfield>amcanunique</> flag is true) then
- <literal>check_uniqueness</> may be true, in which case the access method
+ <literal>check_uniqueness</> might be true, in which case the access method
must verify that there is no conflicting row; this is the only situation in
which the access method normally needs the <literal>heapRelation</>
parameter. See <xref linkend="index-unique-checks"> for details.
<para>
Because of limited <varname>maintenance_work_mem</>,
- <function>ambulkdelete</> may need to be called more than once when many
+ <function>ambulkdelete</> might need to be called more than once when many
tuples are to be deleted. The <literal>stats</> argument is the result
of the previous call for this index (it is NULL for the first call within a
<command>VACUUM</> operation). This allows the AM to accumulate statistics
</programlisting>
Clean up after a <command>VACUUM</command> operation (zero or more
<function>ambulkdelete</> calls). This does not have to do anything
- beyond returning index statistics, but it may perform bulk cleanup
+ beyond returning index statistics, but it might perform bulk cleanup
such as reclaiming empty index pages. <literal>stats</> is whatever the
last <function>ambulkdelete</> call returned, or NULL if
<function>ambulkdelete</> was not called because no tuples needed to be
</para>
<para>
- The operator family may indicate that the index is <firstterm>lossy</> for a
+ The operator family can indicate that the index is <firstterm>lossy</> for a
particular operator; this implies that the index scan will return all the
entries that pass the scan key, plus possibly additional entries that do
not. The core system's index-scan machinery will then apply that operator
<para>
The access method must support <quote>marking</> a position in a scan
- and later returning to the marked position. The same position may be
+ and later returning to the marked position. The same position might be
restored multiple times. However, only one position need be remembered
per scan; a new <function>ammarkpos</> call overrides the previously
marked position.
would have found the entry if it had existed when the scan started, or for
the scan to return such an entry upon rescanning or backing
up even though it had not been returned the first time through. Similarly,
- a concurrent delete may or may not be reflected in the results of a scan.
+ a concurrent delete might or might not be reflected in the results of a scan.
What is important is that insertions or deletions not cause the scan to
miss or multiply return entries that were not themselves being inserted or
deleted.
<literal>RowExclusiveLock</> when updating the index (including plain
<command>VACUUM</>). Since these lock
types do not conflict, the access method is responsible for handling any
- fine-grained locking it may need. An exclusive lock on the index as a whole
+ fine-grained locking it might need. An exclusive lock on the index as a whole
will be taken only during index creation, destruction,
<command>REINDEX</>, or <command>VACUUM FULL</>.
</para>
<firstterm>heap</>) and the index. Because
<productname>PostgreSQL</productname> separates accesses
and updates of the heap from those of the index, there are windows in
- which the index may be inconsistent with the heap. We handle this problem
+ which the index might be inconsistent with the heap. We handle this problem
with the following rules:
<itemizedlist>
against this scenario by requiring the scan keys to be rechecked
against the heap row in all cases, but that is too expensive. Instead,
we use a pin on an index page as a proxy to indicate that the reader
- may still be <quote>in flight</> from the index entry to the matching
+ might still be <quote>in flight</> from the index entry to the matching
heap entry. Making <function>ambulkdelete</> block on such a pin ensures
that <command>VACUUM</> cannot delete the heap entry before the reader
is done with it. This solution costs little in run time, and adds blocking
entry. This is expensive for a number of reasons. An
<quote>asynchronous</> scan in which we collect many TIDs from the index,
and only visit the heap tuples sometime later, requires much less index
- locking overhead and may allow a more efficient heap access pattern.
+ locking overhead and can allow a more efficient heap access pattern.
Per the above analysis, we must use the synchronous approach for
non-MVCC-compliant snapshots, but an asynchronous scan is workable
for a query using an MVCC snapshot.
-<!-- $PostgreSQL: pgsql/doc/src/sgml/indices.sgml,v 1.67 2006/12/23 00:43:08 tgl Exp $ -->
+<!-- $PostgreSQL: pgsql/doc/src/sgml/indices.sgml,v 1.68 2007/01/31 20:56:17 momjian Exp $ -->
<chapter id="indexes">
<title id="indexes-title">Indexes</title>
Once an index is created, no further intervention is required: the
system will update the index when the table is modified, and it will
use the index in queries when it thinks this would be more efficient
- than a sequential table scan. But you may have to run the
+ than a sequential table scan. But you might have to run the
<command>ANALYZE</command> command regularly to update
statistics to allow the query planner to make educated decisions.
See <xref linkend="performance-tips"> for information about
how to find out whether an index is used and when and why the
- planner may choose <emphasis>not</emphasis> to use an index.
+ planner might choose <emphasis>not</emphasis> to use an index.
</para>
<para>
indexes to perform no better than B-tree indexes, and the
index size and build time for hash indexes is much worse.
Furthermore, hash index operations are not presently WAL-logged,
- so hash indexes may need to be rebuilt with <command>REINDEX</>
+ so hash indexes might need to be rebuilt with <command>REINDEX</>
after a database crash.
For these reasons, hash index use is presently discouraged.
</para>
<programlisting>
SELECT name FROM test2 WHERE major = <replaceable>constant</replaceable> AND minor = <replaceable>constant</replaceable>;
</programlisting>
- then it may be appropriate to define an index on the columns
+ then it might be appropriate to define an index on the columns
<structfield>major</structfield> and
<structfield>minor</structfield> together, e.g.,
<programlisting>
<para>
Currently, only the B-tree and GiST index types support multicolumn
- indexes. Up to 32 columns may be specified. (This limit can be
+ indexes. Up to 32 columns can be specified. (This limit can be
altered when building <productname>PostgreSQL</productname>; see the
file <filename>pg_config_manual.h</filename>.)
</para>
<para>
In all but the simplest applications, there are various combinations of
- indexes that may be useful, and the database developer must make
+ indexes that might be useful, and the database developer must make
trade-offs to decide which indexes to provide. Sometimes multicolumn
indexes are best, but sometimes it's better to create separate indexes
and rely on the index-combination feature. For example, if your
</indexterm>
<para>
- Indexes may also be used to enforce uniqueness of a column's value,
+ Indexes can also be used to enforce uniqueness of a column's value,
or the uniqueness of the combined values of more than one column.
<synopsis>
CREATE UNIQUE INDEX <replaceable>name</replaceable> ON <replaceable>table</replaceable> (<replaceable>column</replaceable> <optional>, ...</optional>);
<para>
The syntax of the <command>CREATE INDEX</> command normally requires
writing parentheses around index expressions, as shown in the second
- example. The parentheses may be omitted when the expression is just
+ example. The parentheses can be omitted when the expression is just
a function call, as in the first example.
</para>
<programlisting>
SELECT * FROM orders WHERE order_nr = 3501;
</programlisting>
- The order 3501 may be among the billed or among the unbilled
+ The order 3501 might be among the billed or among the unbilled
orders.
</para>
</example>
<para>
Finally, a partial index can also be used to override the system's
- query plan choices. It may occur that data sets with peculiar
- distributions will cause the system to use an index when it really
+ query plan choices. Also, data sets with peculiar
+ distributions might cause the system to use an index when it really
should not. In that case the index can be set up so that it is not
available for the offending query. Normally,
<productname>PostgreSQL</> makes reasonable choices about index
</indexterm>
<para>
- An index definition may specify an <firstterm>operator
+ An index definition can specify an <firstterm>operator
class</firstterm> for each column of an index.
<synopsis>
CREATE INDEX <replaceable>name</replaceable> ON <replaceable>table</replaceable> (<replaceable>column</replaceable> <replaceable>opclass</replaceable> <optional>, ...</optional>);
via run-time parameters (described in <xref
linkend="runtime-config-query-constants">).
An inaccurate selectivity estimate is due to
- insufficient statistics. It may be possible to improve this by
+ insufficient statistics. It might be possible to improve this by
tuning the statistics-gathering parameters (see
<xref linkend="sql-altertable" endterm="sql-altertable-title">).
</para>
<para>
If you do not succeed in adjusting the costs to be more
- appropriate, then you may have to resort to forcing index usage
- explicitly. You may also want to contact the
+ appropriate, then you might have to resort to forcing index usage
+ explicitly. You might also want to contact the
<productname>PostgreSQL</> developers to examine the issue.
</para>
</listitem>
-<!-- $PostgreSQL: pgsql/doc/src/sgml/information_schema.sgml,v 1.29 2006/10/23 18:10:31 petere Exp $ -->
+<!-- $PostgreSQL: pgsql/doc/src/sgml/information_schema.sgml,v 1.30 2007/01/31 20:56:17 momjian Exp $ -->
<chapter id="information-schema">
<title>The Information Schema</title>
<entry><literal>grantee</literal></entry>
<entry><type>sql_identifier</type></entry>
<entry>
- Name of the role to which this role membership was granted (may
+ Name of the role to which this role membership was granted (can
be the current user, or a different role in case of nested role
memberships)
</entry>
<entry><literal>grantee</literal></entry>
<entry><type>sql_identifier</type></entry>
<entry>
- Name of the role to which this role membership was granted (may
+ Name of the role to which this role membership was granted (can
be the current user, or a different role in case of nested role
memberships)
</entry>
If <literal>data_type</literal> identifies a numeric type, this
column contains the (declared or implicit) precision of the
type for this attribute. The precision indicates the number of
- significant digits. It may be expressed in decimal (base 10)
+ significant digits. It can be expressed in decimal (base 10)
or binary (base 2) terms, as specified in the column
<literal>numeric_precision_radix</literal>. For all other data
types, this column is null.
If <literal>data_type</literal> identifies an exact numeric
type, this column contains the (declared or implicit) scale of
the type for this attribute. The scale indicates the number of
- significant digits to the right of the decimal point. It may
+ significant digits to the right of the decimal point. It can
be expressed in decimal (base 10) or binary (base 2) terms, as
specified in the column
<literal>numeric_precision_radix</literal>. For all other data
<literal>YES</literal> if the column is possibly nullable,
<literal>NO</literal> if it is known not nullable. A not-null
constraint is one way a column can be known not nullable, but
- there may be others.
+ there can be others.
</entry>
</row>
If <literal>data_type</literal> identifies a numeric type, this
column contains the (declared or implicit) precision of the
type for this column. The precision indicates the number of
- significant digits. It may be expressed in decimal (base 10)
+ significant digits. It can be expressed in decimal (base 10)
or binary (base 2) terms, as specified in the column
<literal>numeric_precision_radix</literal>. For all other data
types, this column is null.
If <literal>data_type</literal> identifies an exact numeric
type, this column contains the (declared or implicit) scale of
the type for this column. The scale indicates the number of
- significant digits to the right of the decimal point. It may
+ significant digits to the right of the decimal point. It can
be expressed in decimal (base 10) or binary (base 2) terms, as
specified in the column
<literal>numeric_precision_radix</literal>. For all other data
is supposed to identify the underlying built-in type of the column.
In <productname>PostgreSQL</productname>, this means that the type
is defined in the system catalog schema
- <literal>pg_catalog</literal>. This column may be useful if the
+ <literal>pg_catalog</literal>. This column might be useful if the
application can handle the well-known built-in types specially (for
example, format the numeric types differently or use the data in
the precision columns). The columns <literal>udt_name</literal>,
If the domain has a numeric type, this column contains the
(declared or implicit) precision of the type for this column.
The precision indicates the number of significant digits. It
- may be expressed in decimal (base 10) or binary (base 2) terms,
+ can be expressed in decimal (base 10) or binary (base 2) terms,
as specified in the column
<literal>numeric_precision_radix</literal>. For all other data
types, this column is null.
If the domain has an exact numeric type, this column contains
the (declared or implicit) scale of the type for this column.
The scale indicates the number of significant digits to the
- right of the decimal point. It may be expressed in decimal
+ right of the decimal point. It can be expressed in decimal
(base 10) or binary (base 2) terms, as specified in the column
<literal>numeric_precision_radix</literal>. For all other data
types, this column is null.
<para>
For permission checking, the set of <quote>applicable roles</quote>
- is applied, which may be broader than the set of enabled roles. So
+ is applied, which can be broader than the set of enabled roles. So
generally, it is better to use the view
<literal>applicable_roles</literal> instead of this one; see also
there.
<row>
<entry><literal>routine_name</literal></entry>
<entry><type>sql_identifier</type></entry>
- <entry>Name of the function (may be duplicated in case of overloading)</entry>
+ <entry>Name of the function (might be duplicated in case of overloading)</entry>
</row>
<row>
applies to domains, and since domains do not have real privileges
in <productname>PostgreSQL</productname>, this view is empty.
Further information can be found under
- <literal>usage_privileges</literal>. In the future, this view may
+ <literal>usage_privileges</literal>. In the future, this view might
contain more useful information.
</para>
<row>
<entry><literal>routine_name</literal></entry>
<entry><type>sql_identifier</type></entry>
- <entry>Name of the function (may be duplicated in case of overloading)</entry>
+ <entry>Name of the function (might be duplicated in case of overloading)</entry>
</row>
<row>
<row>
<entry><literal>routine_name</literal></entry>
<entry><type>sql_identifier</type></entry>
- <entry>Name of the function (may be duplicated in case of overloading)</entry>
+ <entry>Name of the function (might be duplicated in case of overloading)</entry>
</row>
<row>
<entry>
This column contains the (declared or implicit) precision of
the sequence data type (see above). The precision indicates
- the number of significant digits. It may be expressed in
+ the number of significant digits. It can be expressed in
decimal (base 10) or binary (base 2) terms, as specified in the
column <literal>numeric_precision_radix</literal>.
</entry>
This column contains the (declared or implicit) scale of the
sequence data type (see above). The scale indicates the number
of significant digits to the right of the decimal point. It
- may be expressed in decimal (base 10) or binary (base 2) terms,
+ can be expressed in decimal (base 10) or binary (base 2) terms,
as specified in the column
<literal>numeric_precision_radix</literal>.
</entry>
incompatibilities with the SQL standard that affect the
representation in the information schema. First, trigger names are
local to the table in <productname>PostgreSQL</productname>, rather
- than being independent schema objects. Therefore there may be duplicate
+ than being independent schema objects. Therefore there can be duplicate
trigger names defined in one schema, as long as they belong to
different tables. (<literal>trigger_catalog</literal> and
<literal>trigger_schema</literal> are really the values pertaining
in <productname>PostgreSQL</productname>, this view shows implicit
<literal>USAGE</literal> privileges granted to
<literal>PUBLIC</literal> for all domains. In the future, this
- view may contain more useful information.
+ view might contain more useful information.
</para>
<table>
-<!-- $PostgreSQL: pgsql/doc/src/sgml/installation.sgml,v 1.278 2007/01/29 21:49:17 momjian Exp $ -->
+<!-- $PostgreSQL: pgsql/doc/src/sgml/installation.sgml,v 1.279 2007/01/31 20:56:17 momjian Exp $ -->
<chapter id="installation">
<title><![%standalone-include[<productname>PostgreSQL</>]]>
executables considerably, and on non-GCC compilers it usually
also disables compiler optimization, causing slowdowns. However,
having the symbols available is extremely helpful for dealing
- with any problems that may arise. Currently, this option is
+ with any problems that might arise. Currently, this option is
recommended for production installations only if you use GCC.
But you should always have it on if you are doing development work
or running a beta version.
investigates (for example, software upgrades), then it's a good
idea to do <command>gmake distclean</> before reconfiguring and
rebuilding. Without this, your changes in configuration choices
- may not propagate everywhere they need to.
+ might not propagate everywhere they need to.
</para>
</sect1>
</para>
<para>
- To stop a server running in the background you can type
+ To stop a server running in the background you can type:
<programlisting>
kill `cat /usr/local/pgsql/data/postmaster.pid`
</programlisting>
-<!-- $PostgreSQL: pgsql/doc/src/sgml/intro.sgml,v 1.31 2006/03/10 19:10:48 momjian Exp $ -->
+<!-- $PostgreSQL: pgsql/doc/src/sgml/intro.sgml,v 1.32 2007/01/31 20:56:17 momjian Exp $ -->
<preface id="preface">
<title>Preface</title>
<listitem>
<para>
- <xref linkend="internals"> contains assorted information that may be of
+ <xref linkend="internals"> contains assorted information that might be of
use to <productname>PostgreSQL</> developers.
</para>
</listitem>
-<!-- $PostgreSQL: pgsql/doc/src/sgml/libpq.sgml,v 1.222 2007/01/30 22:29:22 momjian Exp $ -->
+<!-- $PostgreSQL: pgsql/doc/src/sgml/libpq.sgml,v 1.223 2007/01/31 20:56:17 momjian Exp $ -->
<chapter id="libpq">
<title><application>libpq</application> - C Library</title>
</para>
<para>
Using <literal>hostaddr</> instead of <literal>host</> allows the
- application to avoid a host name look-up, which may be important in
+ application to avoid a host name look-up, which might be important in
applications with time constraints. However, Kerberos authentication
requires the host name. The following therefore applies: If
<literal>host</> is specified without <literal>hostaddr</>, a host name
</para>
<para>
If <function>PQconnectStart</> succeeds, the next stage is to poll
- <application>libpq</> so that it may proceed with the connection sequence.
+ <application>libpq</> so that it can proceed with the connection sequence.
Use <function>PQsocket(conn)</function> to obtain the descriptor of the
socket underlying the database connection.
Loop thus: If <function>PQconnectPoll(conn)</function> last returned
</para>
<para>
- At any time during connection, the status of the connection may be
+ At any time during connection, the status of the connection can be
checked by calling <function>PQstatus</>. If this gives <symbol>CONNECTION_BAD</>, then the
connection procedure has failed; if it gives <function>CONNECTION_OK</>, then the
connection is ready. Both of these states are equally detectable
- from the return value of <function>PQconnectPoll</>, described above. Other states may also occur
+ from the return value of <function>PQconnectPoll</>, described above. Other states might also occur
during (and only during) an asynchronous connection procedure. These
- indicate the current stage of the connection procedure and may be useful
+ indicate the current stage of the connection procedure and might be useful
to provide feedback to the user for example. These statuses are:
<variablelist>
</para>
<para>
- Returns a connection options array. This may be used to determine
+ Returns a connection options array. This can be used to determine
all possible <function>PQconnectdb</function> options and their
current default values. The return value points to an array of
<structname>PQconninfoOption</structname> structures, which ends
This function will close the connection
to the server and attempt to reestablish a new
connection to the same server, using all the same
- parameters previously used. This may be useful for
+ parameters previously used. This might be useful for
error recovery if a working connection is lost.
</para>
</listitem>
<para>
These functions will close the connection to the server and attempt to
reestablish a new connection to the same server, using all the same
- parameters previously used. This may be useful for error recovery if a
+ parameters previously used. This can be useful for error recovery if a
working connection is lost. They differ from <function>PQreset</function> (above) in that they
act in a nonblocking manner. These functions suffer from the same
restrictions as <function>PQconnectStart</> and <function>PQconnectPoll</>.
<title>Connection Status Functions</title>
<para>
- These functions may be used to interrogate the status
+ These functions can be used to interrogate the status
of an existing database connection object.
</para>
<para>
If no value for <literal>standard_conforming_strings</> is reported,
-applications may assume it is <literal>off</>, that is, backslashes
+applications can assume it is <literal>off</>, that is, backslashes
are treated as escapes in string literals. Also, the presence of this
-parameter may be taken as an indication that the escape string syntax
+parameter can be taken as an indication that the escape string syntax
(<literal>E'...'</>) is accepted.
</para>
<synopsis>
int PQprotocolVersion(const PGconn *conn);
</synopsis>
-Applications may wish to use this to determine whether certain features
+Applications might wish to use this to determine whether certain features
are supported.
Currently, the possible values are 2 (2.0 protocol), 3 (3.0 protocol),
or zero (connection bad). This will not change after connection
<synopsis>
int PQserverVersion(const PGconn *conn);
</synopsis>
-Applications may use this to determine the version of the database server they
+Applications might use this to determine the version of the database server they
are connected to. The number is formed by converting the major, minor, and
revision numbers into two-decimal-digit numbers and appending them
together. For example, version 8.1.5 will be returned as 80105, and version
The number of parameters supplied; it is the length of the arrays
<parameter>paramTypes[]</>, <parameter>paramValues[]</>,
<parameter>paramLengths[]</>, and <parameter>paramFormats[]</>. (The
- array pointers
may be <symbol>NULL</symbol> when <parameter>nParams</>
+ array pointers
can be <symbol>NULL</symbol> when <parameter>nParams</>
is zero.)
</para>
</listitem>
<para>
Specifies the actual data lengths of binary-format parameters.
It is ignored for null parameters and text-format parameters.
- The array pointer may be null when there are no binary parameters.
+ The array pointer can be null when there are no binary parameters.
</para>
</listitem>
</varlistentry>
<para>
The primary advantage of <function>PQexecParams</> over <function>PQexec</>
-is that parameter values may be separated from the command string, thus
+is that parameter values can be separated from the command string, thus
avoiding the need for tedious and error-prone quoting and escaping.
</para>
<para>
The function creates a prepared statement named <parameter>stmtName</>
from the <parameter>query</> string, which must contain a single SQL command.
-<parameter>stmtName</> may be <literal>""</> to create an unnamed statement,
+<parameter>stmtName</> can be <literal>""</> to create an unnamed statement,
in which case any pre-existing unnamed statement is automatically replaced;
otherwise it is an error if the statement name is already defined in the
current session.
to in the query as <literal>$1</>, <literal>$2</>, etc.
<parameter>nParams</> is the number of parameters for which types are
pre-specified in the array <parameter>paramTypes[]</>. (The array pointer
-
may be <symbol>NULL</symbol> when <parameter>nParams</> is zero.)
+
can be <symbol>NULL</symbol> when <parameter>nParams</> is zero.)
<parameter>paramTypes[]</> specifies, by OID, the data types to be assigned to
the parameter symbols. If <parameter>paramTypes</> is <symbol>NULL</symbol>,
or any particular element in the array is zero, the server assigns a data type
to the parameter symbol in the same way it would do for an untyped literal
-string. Also, the query may use parameter symbols with numbers higher than
+string. Also, the query can use parameter symbols with numbers higher than
<parameter>nParams</>; data types will be inferred for these symbols as
well. (See <function>PQdescribePrepared</function> for a means to find out
what data types were inferred.)
</para>
<para>
-<parameter>stmtName</> may be <literal>""</> or NULL to reference the unnamed
+<parameter>stmtName</> can be <literal>""</> or NULL to reference the unnamed
statement, otherwise it must be the name of an existing prepared statement.
On success, a <structname>PGresult</> with status
<literal>PGRES_COMMAND_OK</literal> is returned. The functions
<function>PQnparams</function> and <function>PQparamtype</function>
-may be applied to this <structname>PGresult</> to obtain information
+can be applied to this <structname>PGresult</> to obtain information
about the parameters of the prepared statement, and the functions
<function>PQnfields</function>, <function>PQfname</function>,
<function>PQftype</function>, etc provide information about the result
</para>
<para>
-<parameter>portalName</> may be <literal>""</> or NULL to reference the unnamed
+<parameter>portalName</> can be <literal>""</> or NULL to reference the unnamed
portal, otherwise it must be the name of an existing portal.
On success, a <structname>PGresult</> with status
<literal>PGRES_COMMAND_OK</literal> is returned. The functions
<function>PQnfields</function>, <function>PQfname</function>,
-<function>PQftype</function>, etc may be applied to the
+<function>PQftype</function>, etc can be applied to the
<structname>PGresult</> to obtain information about the result
columns (if any) of the portal.
</para>
to retrieve zero rows still shows <literal>PGRES_TUPLES_OK</literal>.
<literal>PGRES_COMMAND_OK</literal> is for commands that can never
return rows (<command>INSERT</command>, <command>UPDATE</command>,
-etc.). A response of <literal>PGRES_EMPTY_QUERY</literal> may indicate
+etc.). A response of <literal>PGRES_EMPTY_QUERY</literal> might indicate
a bug in the client software.
</para>
<listitem>
<para>
Detail: an optional secondary error message carrying more detail about
-the problem. May run to multiple lines.
+the problem. Might run to multiple lines.
</para>
</listitem>
</varlistentry>
<para>
Hint: an optional suggestion what to do about the problem. This is
intended to differ from detail in that it offers advice (potentially
-inappropriate) rather than hard facts. May run to multiple lines.
+inappropriate) rather than hard facts. Might run to multiple lines.
</para>
</listitem>
</varlistentry>
</synopsis>
</para>
<para>
-Commonly this is just the name of the command, but it may include additional
+Commonly this is just the name of the command, but it might include additional
data such as the number of rows processed. The caller should
not free the result directly. It will be freed when the
associated <structname>PGresult</> handle is passed to
<function>PQescapeStringConn</>; the difference is that it does not
take <parameter>conn</> or <parameter>error</> parameters. Because of this,
it cannot adjust its behavior depending on the connection properties (such as
-character encoding) and therefore <emphasis>it may give the wrong results</>.
+character encoding) and therefore <emphasis>it might give the wrong results</>.
Also, it has no way to report error conditions.
</para>
<para>
take a <structname>PGconn</> parameter. Because of this, it cannot adjust
its behavior depending on the connection properties (in particular,
whether standard-conforming strings are enabled)
- and therefore <emphasis>it may give the wrong results</>. Also, it
+ and therefore <emphasis>it might give the wrong results</>. Also, it
has no way to return an error message on failure.
</para>
<itemizedlist>
<listitem>
<para>
-<function>PQexec</function> waits for the command to be completed. The application may have other
+<function>PQexec</function> waits for the command to be completed. The application might have other
work to do (such as maintaining a user interface), in which case it won't
want to block waiting for the response.
</para>
After successfully calling <function>PQsendQuery</function>, call
<function>PQgetResult</function> one or more
- times to obtain the results. <function>PQsendQuery</function> may not be called
+ times to obtain the results. <function>PQsendQuery</function> cannot be called
again (on the same connection) until <function>PQgetResult</function> has returned a null pointer,
indicating that the command is done.
</para>
<function>PQerrorMessage</function> can be consulted). Note that the result
does not say
whether any input data was actually collected. After calling
-<function>PQconsumeInput</function>, the application may check
+<function>PQconsumeInput</function>, the application can check
<function>PQisBusy</function> and/or <function>PQnotifies</function> to see if
their state has changed.
</para>
<para>
-<function>PQconsumeInput</function> may be called even if the application is not
+<function>PQconsumeInput</function> can be called even if the application is not
prepared to deal with a result or notification just yet. The
function will read available data and save it in a buffer, thereby
causing a <function>select()</function> read-ready indication to go away. The
or data values are sent. (It is much more probable if the application
sends data via <command>COPY IN</command>, however.) To prevent this possibility and achieve
completely nonblocking database operation, the following additional
-functions may be used.
+functions can be used.
<variablelist>
<varlistentry>
<tip>
<para>
-This interface is somewhat obsolete, as one may achieve similar performance
+This interface is somewhat obsolete, as one can achieve similar performance
and greater functionality by setting up a prepared statement to define the
function call. Then, executing the statement with binary transmission of
parameters and results substitutes for a fast-path function call.
is returned to indicate success or failure of the transfer. Its status
will be <literal>PGRES_COMMAND_OK</literal> for success or
<literal>PGRES_FATAL_ERROR</literal> if some problem was encountered.
- At this point further SQL commands may be issued via
+ At this point further SQL commands can be issued via
<function>PQexec</function>. (It is not possible to execute other SQL
commands using the same connection while the <command>COPY</command>
operation is in progress.)
</para>
<para>
-The application may divide the <command>COPY</command> data stream into buffer loads of any
+The application can divide the <command>COPY</command> data stream into buffer loads of any
convenient size. Buffer-load boundaries have no semantic significance when
sending. The contents of the data stream must match the data format expected
by the <command>COPY</> command; see
<para>
After successfully calling <function>PQputCopyEnd</>, call
<function>PQgetResult</> to obtain the final result status of the
-<command>COPY</> command. One may wait for
+<command>COPY</> command. One can wait for
this result to be available in the usual way. Then return to normal
operation.
</para>
<para>
After <function>PQgetCopyData</> returns -1, call
<function>PQgetResult</> to obtain the final result status of the
-<command>COPY</> command. One may wait for
+<command>COPY</> command. One can wait for
this result to be available in the usual way. Then return to normal
operation.
</para>
returned messages include severity, primary text, and position only;
this will normally fit on a single line. The default mode produces
messages that include the above plus any detail, hint, or context
-fields (these may span multiple lines). The <firstterm>VERBOSE</>
+fields (these might span multiple lines). The <firstterm>VERBOSE</>
mode includes all available fields. Changing the verbosity does not
affect the messages available from already-existing
<structname>PGresult</> objects, only subsequently-created ones.
form before it is sent. The arguments are the cleartext password, and the SQL
name of the user it is for. The return value is a string allocated by
<function>malloc</function>, or <symbol>NULL</symbol> if out of memory.
-The caller may assume the string doesn't contain any special
+The caller can assume the string doesn't contain any special
characters that would require escaping. Use <function>PQfreemem</> to free
the result when done with it.
</para>
<synopsis>
<replaceable>hostname</replaceable>:<replaceable>port</replaceable>:<replaceable>database</replaceable>:<replaceable>username</replaceable>:<replaceable>password</replaceable>
</synopsis>
-Each of the first four fields may be a literal value, or <literal>*</literal>,
+Each of the first four fields can be a literal value, or <literal>*</literal>,
which matches anything. The password field from the first line that matches the
current connection parameters will be used. (Therefore, put more-specific
entries first when you are using wildcards.)
-<!-- $PostgreSQL: pgsql/doc/src/sgml/lobj.sgml,v 1.42 2006/10/23 18:10:31 petere Exp $ -->
+<!-- $PostgreSQL: pgsql/doc/src/sgml/lobj.sgml,v 1.43 2007/01/31 20:56:17 momjian Exp $ -->
<chapter id="largeObjects">
<title id="largeObjects-title">Large Objects</title>
large object data. We use the <application>libpq</application> C
library for the examples in this chapter, but most programming
interfaces native to <productname>PostgreSQL</productname> support
- equivalent functionality. Other interfaces may use the large
+ equivalent functionality. Other interfaces might use the large
object interface internally to provide generic support for large
values. This is not described here.
</para>
<title>Closing a Large Object Descriptor</title>
<para>
- A large object descriptor may be closed by calling
+ A large object descriptor can be closed by calling
<synopsis>
int lo_close(PGconn *conn, int fd);
</synopsis>
<para>
<xref linkend="lo-example"> is a sample program which shows how the large object
interface
- in <application>libpq</> can be used. Parts of the program are
+ in <application>libpq</> can be used. Parts of the program are
commented out but are left in the source for the reader's
benefit. This program can also be found in
<filename>src/test/examples/testlo.c</filename> in the source distribution.
-<!-- $PostgreSQL: pgsql/doc/src/sgml/maintenance.sgml,v 1.67 2007/01/31 04:13:22 momjian Exp $ -->
+<!-- $PostgreSQL: pgsql/doc/src/sgml/maintenance.sgml,v 1.68 2007/01/31 20:56:17 momjian Exp $ -->
<chapter id="maintenance">
<title>Routine Database Maintenance Tasks</title>
<para>
Clearly, a table that receives frequent updates or deletes will need
to be vacuumed more often than tables that are seldom updated. It
- m
ay be useful to set up periodic <application>cron</> tasks that
+ m
ight be useful to set up periodic <application>cron</> tasks that
<command>VACUUM</command> only selected tables, skipping tables that are known not to
change often. This is only likely to be helpful if you have both
large heavily-updated tables and large seldom-updated tables — the
If you have multiple databases
in a cluster, don't forget to <command>VACUUM</command> each one;
the program <xref linkend="app-vacuumdb" endterm="app-vacuumdb-title">
- may be helpful.
+ might be helpful.
</para>
<para>
generate good plans for queries. These statistics are gathered by
the <command>ANALYZE</> command, which can be invoked by itself or
as an optional step in <command>VACUUM</>. It is important to have
- reasonably accurate statistics, otherwise poor choices of plans may
+ reasonably accurate statistics, otherwise poor choices of plans might
degrade database performance.
</para>
<para>
As with vacuuming for space recovery, frequent updates of statistics
are more useful for heavily-updated tables than for seldom-updated
- ones. But even for a heavily-updated table, there may be no need for
+ ones. But even for a heavily-updated table, there might be no need for
statistics updates if the statistical distribution of the data is
not changing much. A simple rule of thumb is to think about how much
the minimum and maximum values of the columns in the table change.
of row update will have a constantly-increasing maximum value as
rows are added and updated; such a column will probably need more
frequent statistics updates than, say, a column containing URLs for
- pages accessed on a website. The URL column may receive changes just
+ pages accessed on a website. The URL column might receive changes just
as often, but the statistical distribution of its values probably
changes relatively slowly.
</para>
<tip>
<para>
- Although per-column tweaking of <command>ANALYZE</> frequency may not be
- very productive, you may well find it worthwhile to do per-column
+ Although per-column tweaking of <command>ANALYZE</> frequency might not be
+ very productive, you might well find it worthwhile to do per-column
adjustment of the level of detail of the statistics collected by
<command>ANALYZE</>. Columns that are heavily used in <literal>WHERE</> clauses
- and have highly irregular data distributions may require a finer-grain
+ and have highly irregular data distributions might require a finer-grain
data histogram than other columns. See <command>ALTER TABLE SET
STATISTICS</>.
</para>
Recommended practice for most sites is to schedule a database-wide
<command>ANALYZE</> once a day at a low-usage time of day; this can
usefully be combined with a nightly <command>VACUUM</>. However,
- sites with relatively slowly changing table statistics may find that
+ sites with relatively slowly changing table statistics might find that
this is overkill, and that less-frequent <command>ANALYZE</> runs
are sufficient.
</para>
<para>
One disadvantage of decreasing <varname>vacuum_freeze_min_age</> is that
- it may cause <command>VACUUM</> to do useless work: changing a table row's
+ it might cause <command>VACUUM</> to do useless work: changing a table row's
XID to <literal>FrozenXID</> is a waste of time if the row is modified
soon thereafter (causing it to acquire a new XID). So the setting should
be large enough that rows are not frozen until they are unlikely to change
The number of obsolete tuples is obtained from the statistics
collector; it is a semi-accurate count updated by each
<command>UPDATE</command> and <command>DELETE</command> operation. (It
- is only semi-accurate because some information may be lost under heavy
+ is only semi-accurate because some information might be lost under heavy
load.) For analyze, a similar condition is used: the threshold, defined as
<programlisting>
analyze threshold = analyze base threshold + analyze scale factor * number of tuples
<command>postgres</command> into a
file, you will have log output, but
the only way to truncate the log file is to stop and restart
- the server. This may be OK if you are using
+ the server. This might be OK if you are using
<productname>PostgreSQL</productname> in a development environment,
but few production servers would find this behavior acceptable.
</para>
<para>
On many systems, however, <application>syslog</> is not very reliable,
- particularly with large log messages; it may truncate or drop messages
+ particularly with large log messages; it might truncate or drop messages
just when you need them the most. Also, on <productname>Linux</>,
<application>syslog</> will sync each message to disk, yielding poor
performance. (You can use a <literal>-</> at the start of the file name
-<!-- $PostgreSQL: pgsql/doc/src/sgml/manage-ag.sgml,v 2.51 2007/01/22 02:47:56 momjian Exp $ -->
+<!-- $PostgreSQL: pgsql/doc/src/sgml/manage-ag.sgml,v 2.52 2007/01/31 20:56:17 momjian Exp $ -->
<chapter id="managing-databases">
<title>Managing Databases</title>
exactly as described above.
The <xref linkend="app-createdb"> reference page contains the invocation
details. Note that <command>createdb</> without any arguments will create
- a database with the current user name, which may or may not be what
+ a database with the current user name, which might or might not be what
you want.
</para>
<literal>pg_dump</> dump: the dump script should be restored in a
virgin database to ensure that one recreates the correct contents
of the dumped database, without any conflicts with additions that
- may now be present in <literal>template1</>.
+ can now be present in <literal>template1</>.
</para>
<para>
<para>
It is possible to create additional template databases, and indeed
- one may copy any database in a cluster by specifying its name
+ one can copy any database in a cluster by specifying its name
as the template for <command>CREATE DATABASE</>. It is important to
understand, however, that this is not (yet) intended as
a general-purpose <quote><command>COPY DATABASE</command></quote> facility.
Two useful flags exist in <literal>pg_database</literal><indexterm><primary>pg_database</></> for each
database: the columns <literal>datistemplate</literal> and
<literal>datallowconn</literal>. <literal>datistemplate</literal>
- may be set to indicate that a database is intended as a template for
- <command>CREATE DATABASE</>. If this flag is set, the database may be
+ can be set to indicate that a database is intended as a template for
+ <command>CREATE DATABASE</>. If this flag is set, the database can be
cloned by
any user with <literal>CREATEDB</> privileges; if it is not set, only superusers
- and the owner of the database may clone it.
+ and the owner of the database can clone it.
If <literal>datallowconn</literal> is false, then no new connections
to that database will be allowed (but existing sessions are not killed
simply by setting the flag false). The <literal>template0</literal>
The <literal>postgres</> database is also created when a database
cluster is initialized. This database is meant as a default database for
users and applications to connect to. It is simply a copy of
- <literal>template1</> and may be dropped and recreated if required.
+ <literal>template1</> and can be dropped and recreated if required.
</para>
</note>
</sect1>
-<!-- $PostgreSQL: pgsql/doc/src/sgml/monitoring.sgml,v 1.43 2006/12/08 19:16:17 tgl Exp $ -->
+<!-- $PostgreSQL: pgsql/doc/src/sgml/monitoring.sgml,v 1.44 2007/01/31 20:56:18 momjian Exp $ -->
<chapter id="monitoring">
<title>Monitoring Database Activity</title>
but one should not neglect regular Unix monitoring programs such as
<command>ps</>, <command>top</>, <command>iostat</>, and <command>vmstat</>.
Also, once one has identified a
- poorly-performing query, further investigation may be needed using
+ poorly-performing query, further investigation might be needed using
<productname>PostgreSQL</productname>'s <xref linkend="sql-explain"
endterm="sql-explain-title"> command.
<xref linkend="using-explain"> discusses <command>EXPLAIN</>
The user, database, and connection source host items remain the same for
the life of the client connection, but the activity indicator changes.
- The activity may be <literal>idle</> (i.e., waiting for a client command),
+ The activity can be <literal>idle</> (i.e., waiting for a client command),
<literal>idle in transaction</> (waiting for client inside a <command>BEGIN</> block),
or a command type name such as <literal>SELECT</>. Also,
<literal>waiting</> is attached if the server process is presently waiting
<para>
The parameter <xref linkend="guc-stats-start-collector"> must be
set to <literal>true</> for the statistics collector to be launched
- at all. This is the default and recommended setting, but it may be
+ at all. This is the default and recommended setting, but it can be
turned off if you have no interest in statistics and want to
squeeze out every last drop of overhead. (The savings is likely to
be small, however.) Note that this option cannot be changed while
invoking a kernel call. However, these statistics do not give the
entire story: due to the way in which <productname>PostgreSQL</>
handles disk I/O, data that is not in the
- <productname>PostgreSQL</> buffer cache may still reside in the
- kernel's I/O cache, and may therefore still be fetched without
+ <productname>PostgreSQL</> buffer cache might still reside in the
+ kernel's I/O cache, and might therefore still be fetched without
requiring a physical read. Users interested in obtaining more
detailed information on <productname>PostgreSQL</> I/O behavior are
advised to use the <productname>PostgreSQL</> statistics collector
</para>
<para>
You should remember that trace programs need to be carefully written and
- debugged prior to their use, otherwise the trace information collected may
+ debugged prior to their use, otherwise the trace information collected might
be meaningless. In most cases where problems are found it is the
instrumentation that is at fault, not the underlying system. When
discussing information found using dynamic tracing, be sure to enclose
</para>
<para>
- The dynamic tracing utility may require you to further define these trace
+ The dynamic tracing utility might require you to further define these trace
points. For example, DTrace requires you to add new probes to the file
<filename>src/backend/utils/probes.d</> as shown here:
<programlisting>
-<!-- $PostgreSQL: pgsql/doc/src/sgml/mvcc.sgml,v 2.65 2006/12/01 01:04:36 tgl Exp $ -->
+<!-- $PostgreSQL: pgsql/doc/src/sgml/mvcc.sgml,v 2.66 2007/01/31 20:56:18 momjian Exp $ -->
<chapter id="mvcc">
<title>Concurrency Control</title>
Committed and Serializable. When you select the level Read
Uncommitted you really get Read Committed, and when you select
Repeatable Read you really get Serializable, so the actual
- isolation level may be stricter than what you select. This is
+ isolation level might be stricter than what you select. This is
permitted by the SQL standard: the four isolation levels only
define which phenomena must not happen, they do not define which
phenomena must happen. The reason that <productname>PostgreSQL</>
behave the same as <command>SELECT</command>
in terms of searching for target rows: they will only find target rows
that were committed as of the command start time. However, such a target
- row may have already been updated (or deleted or locked) by
+ row might have already been updated (or deleted or locked) by
another concurrent transaction by the time it is found. In this case, the
would-be updater will wait for the first updating transaction to commit or
roll back (if it is still in progress). If the first updater rolls back,
<para>
The partial transaction isolation provided by Read Committed mode is
adequate for many applications, and this mode is fast and simple to use.
- However, for applications that do complex queries and updates, it may
+ However, for applications that do complex queries and updates, it might
be necessary to guarantee a more rigorously consistent view of the
database than the Read Committed mode provides.
</para>
in terms of searching for target rows: they will only find target rows
that were committed as of the transaction start time. However, such a
target
- row may have already been updated (or deleted or locked) by
+ row might have already been updated (or deleted or locked) by
another concurrent transaction by the time it is found. In this case, the
serializable transaction will wait for the first updating transaction to commit or
roll back (if it is still in progress). If the first updater rolls back,
</para>
<para>
- Note that only updating transactions may need to be retried; read-only
+ Note that only updating transactions might need to be retried; read-only
transactions will never have serialization conflicts.
</para>
transaction sees a wholly consistent view of the database. However,
the application has to be prepared to retry transactions when concurrent
updates make it impossible to sustain the illusion of serial execution.
- Since the cost of redoing complex transactions may be significant,
+ Since the cost of redoing complex transactions might be significant,
this mode is recommended only when updating transactions contain logic
- sufficiently complex that they may give wrong answers in Read
+ sufficiently complex that they might give wrong answers in Read
Committed mode. Most commonly, Serializable mode is necessary when
a transaction executes several successive commands that must see
identical views of the database.
The intuitive meaning (and mathematical definition) of
<quote>serializable</> execution is that any two successfully committed
concurrent transactions will appear to have executed strictly serially,
- one after the other — although which one appeared to occur first may
+ one after the other — although which one appeared to occur first might
not be predictable in advance. It is important to realize that forbidding
the undesirable behaviors listed in <xref linkend="mvcc-isolevel-table">
is not sufficient to guarantee true serializability, and in fact
between one lock mode and another is the set of lock modes with
which each conflicts. Two transactions cannot hold locks of conflicting
modes on the same table at the same time. (However, a transaction
- never conflicts with itself. For example, it may acquire
+ never conflicts with itself. For example, it might acquire
<literal>ACCESS EXCLUSIVE</literal> lock and later acquire
<literal>ACCESS SHARE</literal> lock on the same table.) Non-conflicting
- lock modes may be held concurrently by many transactions. Notice in
+ lock modes can be held concurrently by many transactions. Notice in
particular that some lock modes are self-conflicting (for example,
an <literal>ACCESS EXCLUSIVE</literal> lock cannot be held by more than one
transaction at a time) while others are not self-conflicting (for example,
To acquire an exclusive row-level lock on a row without actually
modifying the row, select the row with <command>SELECT FOR
UPDATE</command>. Note that once the row-level lock is acquired,
- the transaction may update the row multiple times without
+ the transaction can update the row multiple times without
fear of conflicts.
</para>
<productname>PostgreSQL</productname> doesn't remember any
information about modified rows in memory, so it has no limit to
the number of rows locked at one time. However, locking a row
- may cause a disk write; thus, for example, <command>SELECT FOR
+ might cause a disk write; thus, for example, <command>SELECT FOR
UPDATE</command> will modify selected rows to mark them locked, and so
will result in disk writes.
</para>
occurred. One should also ensure that the first lock acquired on
an object in a transaction is the highest mode that will be
needed for that object. If it is not feasible to verify this in
- advance, then deadlocks may be handled on-the-fly by retrying
+ advance, then deadlocks can be handled on-the-fly by retrying
transactions that are aborted due to deadlock.
</para>
<para>
Another way to think about it is that each
transaction sees a snapshot of the database contents, and concurrently
- executing transactions may very well see different snapshots. So the
+ executing transactions might very well see different snapshots. So the
whole concept of <quote>now</quote> is somewhat ill-defined anyway.
This is not normally
a big problem if the client applications are isolated from each other,
but if the clients can communicate via channels outside the database
- then serious confusion may ensue.
+ then serious confusion might ensue.
</para>
<para>
lock(s) before performing queries. A lock obtained by a
serializable transaction guarantees that no other transactions modifying
the table are still running, but if the snapshot seen by the
- transaction predates obtaining the lock, it may predate some now-committed
+ transaction predates obtaining the lock, it might predate some now-committed
changes in the table. A serializable transaction's snapshot is actually
frozen at the start of its first query or data-modification command
(<literal>SELECT</>, <literal>INSERT</>,
read/write access. Locks are released immediately after each
index row is fetched or inserted. But note that a GIN-indexed
value insertion usually produces several index key insertions
- per row, so GIN may do substantial work for a single value's
+ per row, so GIN might do substantial work for a single value's
insertion.
</para>
</listitem>
-<!-- $PostgreSQL: pgsql/doc/src/sgml/nls.sgml,v 1.14 2006/03/10 19:10:48 momjian Exp $ -->
+<!-- $PostgreSQL: pgsql/doc/src/sgml/nls.sgml,v 1.15 2007/01/31 20:56:18 momjian Exp $ -->
<chapter id="nls">
<chapterinfo>
<para>
The # character introduces a comment. If whitespace immediately
follows the # character, then this is a comment maintained by the
- translator. There may also be automatic comments, which have a
+ translator. There can also be automatic comments, which have a
non-whitespace character immediately following the #. These are
maintained by the various tools that operate on the PO files and
are intended to aid the translator.
ISO 639-1 two-letter language code (in lower case)</ulink>, e.g.,
<filename>fr.po</filename> for French. If there is really a need
for more than one translation effort per language then the files
- may also be named
+ can also be named
<filename><replaceable>language</replaceable>_<replaceable>region</replaceable>.po</filename>
where <replaceable>region</replaceable> is the
<ulink url="http://www.din.de/gremien/nas/nabd/iso3166ma/codlstp1/en_listp1.html">
<programlisting>
AVAIL_LANGUAGES := de fr
</programlisting>
- (Other languages may appear, of course.)
+ (Other languages can appear, of course.)
</para>
<para>
- As the underlying program or library changes, messages may be
+ As the underlying program or library changes, messages might be
changed or added by the programmers. In this case you do not need
to start from scratch. Instead, run the command
<programlisting>
<para>
The PO files can be edited with a regular text editor. The
translator should only change the area between the quotes after
- the msgstr directive, may add comments and alter the fuzzy flag.
+ the msgstr directive, add comments, and alter the fuzzy flag.
There is (unsurprisingly) a PO mode for Emacs, which I find quite
useful.
</para>
<literal><replaceable>digits</replaceable>$</literal> needs to
follow the % immediately, before any other format manipulators.
(This feature really exists in the <function>printf</function>
- family of functions. You may not have heard of it before because
+ family of functions. You might not have heard of it before because
there is little use for it outside of message
internationalization.)
</para>
normally. The corrected string can be merged in when the
program sources have been updated. If the original string
contains a factual mistake, report that (or fix it yourself)
- and do not translate it. Instead, you may mark the string with
+ and do not translate it. Instead, you can mark the string with
a comment in the PO file.
</para>
</listitem>
open file %s</literal>) should probably not start with a
capital letter (if your language distinguishes letter case) or
end with a period (if your language uses punctuation marks).
- It may help to read <xref linkend="error-style-guide">.
+ It might help to read <xref linkend="error-style-guide">.
</para>
</listitem>
</para>
<para>
- This may tend to add a lot of clutter. One common shortcut is to use
+ This tends to add a lot of clutter. One common shortcut is to use
<programlisting>
#define _(x) gettext(x)
</programlisting>
<programlisting>
printf("Files were %s.\n", flag ? "copied" : "removed");
</programlisting>
- The word order within the sentence may be different in other
+ The word order within the sentence might be different in other
languages. Also, even if you remember to call gettext() on each
- fragment, the fragments may not translate well separately. It's
+ fragment, the fragments might not translate well separately. It's
better to duplicate a little code so that each message to be
translated is a coherent whole. Only numbers, file names, and
such-like run-time variables should be inserted at run time into
printf("copied %d files", n):
</programlisting>
then be disappointed. Some languages have more than two forms,
- with some peculiar rules. We may have a solution for this in
+ with some peculiar rules. We might have a solution for this in
the future, but for now the matter is best avoided altogether.
You could write:
<programlisting>
-<!-- $PostgreSQL: pgsql/doc/src/sgml/perform.sgml,v 1.60 2007/01/25 02:17:25 momjian Exp $ -->
+<!-- $PostgreSQL: pgsql/doc/src/sgml/perform.sgml,v 1.61 2007/01/31 20:56:18 momjian Exp $ -->
<chapter id="performance-tips">
<title>Performance Tips</title>
<listitem>
<para>
- Estimated total cost (If all rows were to be retrieved, which they may
+ Estimated total cost (If all rows were to be retrieved, though they might
not be: for example, a query with a <literal>LIMIT</> clause will stop
short of paying the total cost of the <literal>Limit</> plan node's
input node.)
</para>
<para>
- If the <literal>WHERE</> condition is selective enough, the planner may
+ If the <literal>WHERE</> condition is selective enough, the planner might
switch to a <quote>simple</> index scan plan:
<programlisting>
run time will normally be just a little larger than the total time
reported for the top-level plan node. For <command>INSERT</>,
<command>UPDATE</>, and <command>DELETE</> commands, the total run time
- may be considerably larger, because it includes the time spent processing
+ might be considerably larger, because it includes the time spent processing
the result rows. In these commands, the time for the top plan node
essentially is the time spent computing the new rows and/or locating the
old ones, but it doesn't include the time spent applying the changes.
It is worth noting that <command>EXPLAIN</> results should not be extrapolated
to situations other than the one you are actually testing; for example,
results on a toy-sized table can't be assumed to apply to large tables.
- The planner's cost estimates are not linear and so it may well choose
+ The planner's cost estimates are not linear and so it might choose
a different plan for a larger or smaller table. An extreme example
is that on a table that only occupies one disk page, you'll nearly
always get a sequential scan plan whether indexes are available or not.
command, or globally by setting the
<xref linkend="guc-default-statistics-target"> configuration variable.
The default limit is presently 10 entries. Raising the limit
- may allow more accurate planner estimates to be made, particularly for
+ might allow more accurate planner estimates to be made, particularly for
columns with irregular data distributions, at the price of consuming
more space in <structname>pg_statistic</structname> and slightly more
- time to compute the estimates. Conversely, a lower limit may be
+ time to compute the estimates. Conversely, a lower limit might be
appropriate for columns with simple data distributions.
</para>
between two input tables, so it's necessary to build up the result
in one or another of these fashions.) The important point is that
these different join possibilities give semantically equivalent
- results but may have hugely different execution costs. Therefore,
+ results but might have hugely different execution costs. Therefore,
the planner will explore all of them to try to find the most
efficient query plan.
</para>
orders to worry about. But the number of possible join orders grows
exponentially as the number of tables expands. Beyond ten or so input
tables it's no longer practical to do an exhaustive search of all the
- possibilities, and even for six or seven tables planning may take an
+ possibilities, and even for six or seven tables planning might take an
annoyingly long time. When there are too many input tables, the
<productname>PostgreSQL</productname> planner will switch from exhaustive
search to a <firstterm>genetic</firstterm> probabilistic search
Therefore the planner has no choice of join order here: it must join
B to C and then join A to that result. Accordingly, this query takes
less time to plan than the previous query. In other cases, the planner
- may be able to determine that more than one join order is safe.
+ might be able to determine that more than one join order is safe.
For example, given
<programlisting>
SELECT * FROM a LEFT JOIN b ON (a.bid = b.id) LEFT JOIN c ON (a.cid = c.id);
<title>Populating a Database</title>
<para>
- One may need to insert a large amount of data when first populating
+ One might need to insert a large amount of data when first populating
a database. This section contains some suggestions on how to make
this process as efficient as possible.
</para>
<para>
Turn off autocommit and just do one commit at the end. (In plain
SQL, this means issuing <command>BEGIN</command> at the start and
- <command>COMMIT</command> at the end. Some client libraries may
+ <command>COMMIT</command> at the end. Some client libraries might
do this behind your back, in which case you need to make sure the
library does it when you want it done.) If you allow each
insertion to be committed separately,
</para>
<para>
- If you cannot use <command>COPY</command>, it may help to use <xref
+ If you cannot use <command>COPY</command>, it might help to use <xref
linkend="sql-prepare" endterm="sql-prepare-title"> to create a
prepared <command>INSERT</command> statement, and then use
<command>EXECUTE</command> as many times as required. This avoids
<para>
If you are adding large amounts of data to an existing table,
- it may be a win to drop the index,
+ it might be a win to drop the index,
load the table, and then recreate the index. Of course, the
- database performance for other users may be adversely affected
+ database performance for other users might be adversely affected
during the time that the index is missing. One should also think
twice before dropping unique indexes, since the error checking
afforded by the unique constraint will be lost while the index is
<para>
Just as with indexes, a foreign key constraint can be checked
- <quote>in bulk</> more efficiently than row-by-row. So it may be
+ <quote>in bulk</> more efficiently than row-by-row. So it might be
useful to drop foreign key constraints, load data, and re-create
the constraints. Again, there is a trade-off between data load
speed and loss of error checking while the constraint is missing.
<title>Turn off <varname>archive_command</varname></title>
<para>
- When loading large amounts of data you may want to unset the
- <xref linkend="guc-archive-command"> before loading. It may be
+ When loading large amounts of data you might want to unset the
+ <xref linkend="guc-archive-command"> before loading. It might be
faster to take a new base backup once the load has completed
than to allow a large archive to accumulate.
</para>
includes bulk loading large amounts of data into the table. Running
<command>ANALYZE</command> (or <command>VACUUM ANALYZE</command>)
ensures that the planner has up-to-date statistics about the
- table. With no statistics or obsolete statistics, the planner may
+ table. With no statistics or obsolete statistics, the planner might
make poor decisions during query planning, leading to poor
performance on any tables with inaccurate or nonexistent
statistics.
-<!-- $PostgreSQL: pgsql/doc/src/sgml/planstats.sgml,v 1.7 2006/09/16 00:30:14 momjian Exp $ -->
+<!-- $PostgreSQL: pgsql/doc/src/sgml/planstats.sgml,v 1.8 2007/01/31 20:56:18 momjian Exp $ -->
<chapter id="planner-stats-details">
<title>How the Planner Uses Statistics</title>
<para>
The outputs and algorithms shown below are taken from version 8.0.
- The behavior of earlier (or later) versions may vary.
+ The behavior of earlier (or later) versions might vary.
</para>
<sect1 id="row-estimation-examples">
345 | 10000
</programlisting>
The planner will check the <structfield>relpages</structfield>
- estimate (this is a cheap operation) and if incorrect may scale
+ estimate (this is a cheap operation) and if incorrect might scale
<structfield>reltuples</structfield> to obtain a row estimate. In this
case it does not, thus:
-<!-- $PostgreSQL: pgsql/doc/src/sgml/plperl.sgml,v 2.60 2007/01/30 22:29:23 momjian Exp $ -->
+<!-- $PostgreSQL: pgsql/doc/src/sgml/plperl.sgml,v 2.61 2007/01/31 20:56:18 momjian Exp $ -->
<chapter id="plperl">
<title>PL/Perl - Perl Procedural Language</title>
<para> The usual advantage to using PL/Perl is that this allows use,
within stored functions, of the manyfold <quote>string
munging</quote> operators and functions available for Perl. Parsing
- complex strings may be be easier using Perl than it is with the
+ complex strings might be be easier using Perl than it is with the
string functions and control structures provided in PL/pgSQL.</para>
<para>
</para>
<para>
<literal>spi_query</literal> and <literal>spi_fetchrow</literal>
- work together as a pair for row sets which may be large, or for cases
+ work together as a pair for row sets which might be large, or for cases
where you wish to return rows as they arrive.
<literal>spi_fetchrow</literal> works <emphasis>only</emphasis> with
<literal>spi_query</literal>. The following example illustrates how
<para>
The advantage of prepared queries is that is it possible to use one prepared plan for more
- than one query execution. After the plan is not needed anymore, it may be freed with
+ than one query execution. After the plan is not needed anymore, it can be freed with
<literal>spi_freeplan</literal>:
</para>
external modules). There is no way to access internals of the
database server process or to gain OS-level access with the
permissions of the server process,
- as a C function can do. Thus, any unprivileged database user may
+ as a C function can do. Thus, any unprivileged database user can
be permitted to use this language.
</para>
-<!-- $PostgreSQL: pgsql/doc/src/sgml/plpgsql.sgml,v 1.103 2007/01/30 22:29:23 momjian Exp $ -->
+<!-- $PostgreSQL: pgsql/doc/src/sgml/plpgsql.sgml,v 1.104 2007/01/31 20:56:18 momjian Exp $ -->
<chapter id="plpgsql">
<title><application>PL/pgSQL</application> - <acronym>SQL</acronym> Procedural Language</title>
substantially reduce the total amount of time required to parse
and generate execution plans for the statements in a
<application>PL/pgSQL</> function. A disadvantage is that errors
- in a specific expression or command may not be detected until that
+ in a specific expression or command cannot be detected until that
part of the function is reached in execution.
</para>
</para>
<para>
- <application>PL/pgSQL</> functions may also be declared to accept
+ <application>PL/pgSQL</> functions can also be declared to accept
and return the polymorphic types
<type>anyelement</type> and <type>anyarray</type>. The actual
data types handled by a polymorphic function can vary from call to
</para>
<para>
- Finally, a <application>PL/pgSQL</> function may be declared to return
+ Finally, a <application>PL/pgSQL</> function can be declared to return
<type>void</> if it has no useful return value.
</para>
<para>
The following chart shows what you have to do when writing quote
- marks without dollar quoting. It may be useful when translating
+ marks without dollar quoting. It might be useful when translating
pre-dollar quoting code into something more comprehensible.
</para>
type of the structure you are referencing, and most importantly,
if the data type of the referenced item changes in the future (for
instance: you change the type of <literal>user_id</>
- from <type>integer</type> to <type>real</type>), you may not need
+ from <type>integer</type> to <type>real</type>), you might not need
to change your function definition.
</para>
<para>
<literal>%TYPE</literal> is particularly valuable in polymorphic
- functions, since the data types needed for internal variables may
+ functions, since the data types needed for internal variables can
change from one call to the next. Appropriate variables can be
created by applying <literal>%TYPE</literal> to the function's
arguments or result placeholders.
Note that <literal>RECORD</> is not a true data type, only a placeholder.
One should also realize that when a <application>PL/pgSQL</application>
function is declared to return type <type>record</>, this is not quite the
- same concept as a record variable, even though such a function may well
+ same concept as a record variable, even though such a function might
use a record variable to hold its result. In both cases the actual row
structure is unknown when the function is written, but for a function
returning <type>record</> the actual structure is determined when the
loops). <literal>FOUND</literal> is set this way when the
<command>FOR</> loop exits; inside the execution of the loop,
<literal>FOUND</literal> is not modified by the
- <command>FOR</> statement, although it may be changed by the
+ <command>FOR</> statement, although it might be changed by the
execution of other statements within the loop body.
</para>
</listitem>
for <application>PL/pgSQL</> stores the entire result set
before returning from the function, as discussed above. That
means that if a <application>PL/pgSQL</> function produces a
- very large result set, performance may be poor: data will be
+ very large result set, performance might be poor: data will be
written to disk to avoid memory exhaustion, but the function
itself will not return until the entire result set has been
- generated. A future version of <application>PL/pgSQL</> may
+ generated. A future version of <application>PL/pgSQL</> might
allow users to define set-returning functions
that do not have this limitation. Currently, the point at
which data begins being written to disk is controlled by the
<synopsis>
<replaceable>name</replaceable> CURSOR <optional> ( <replaceable>arguments</replaceable> ) </optional> FOR <replaceable>query</replaceable>;
</synopsis>
- (<literal>FOR</> may be replaced by <literal>IS</> for
+ (<literal>FOR</> can be replaced by <literal>IS</> for
<productname>Oracle</productname> compatibility.)
<replaceable>arguments</replaceable>, if specified, is a
comma-separated list of pairs <literal><replaceable>name</replaceable>
curs3 CURSOR (key integer) IS SELECT * FROM tenk1 WHERE unique1 = key;
</programlisting>
All three of these variables have the data type <type>refcursor</>,
- but the first may be used with any query, while the second has
+ but the first can be used with any query, while the second has
a fully specified query already <firstterm>bound</> to it, and the last
has a parameterized query bound to it. (<literal>key</> will be
replaced by an integer parameter value when the cursor is opened.)
<para>
<command>FETCH</command> retrieves the next row from the
- cursor into a target, which may be a row variable, a record
+ cursor into a target, which might be a row variable, a record
variable, or a comma-separated list of simple variables, just like
<command>SELECT INTO</command>. As with <command>SELECT
- INTO</command>, the special variable <literal>FOUND</literal> may
+ INTO</command>, the special variable <literal>FOUND</literal> can
be checked to see whether a row was obtained or not.
</para>
</para>
<para>
- Row-level triggers fired <literal>BEFORE</> may return null to signal the
+ Row-level triggers fired <literal>BEFORE</> can return null to signal the
trigger manager to skip the rest of the operation for this row
(i.e., subsequent triggers are not fired, and the
<command>INSERT</>/<command>UPDATE</>/<command>DELETE</> does not occur
<para>
The return value of a <literal>BEFORE</> or <literal>AFTER</>
statement-level trigger or an <literal>AFTER</> row-level trigger is
- always ignored; it may as well be null. However, any of these types of
- triggers can still abort the entire operation by raising an error.
+ always ignored; it might as well be null. However, any of these types of
+ triggers might still abort the entire operation by raising an error.
</para>
<para>
original table for certain queries — often with vastly reduced run
times.
This technique is commonly used in Data Warehousing, where the tables
- of measured or observed data (called fact tables) can be extremely large.
+ of measured or observed data (called fact tables) might be extremely large.
<xref linkend="plpgsql-trigger-summary-example"> shows an example of a
trigger procedure in <application>PL/pgSQL</application> that maintains
a summary table for a fact table in a data warehouse.
-<!-- $PostgreSQL: pgsql/doc/src/sgml/plpython.sgml,v 1.36 2006/10/23 18:10:31 petere Exp $ -->
+<!-- $PostgreSQL: pgsql/doc/src/sgml/plpython.sgml,v 1.37 2007/01/31 20:56:18 momjian Exp $ -->
<chapter id="plpython">
<title>PL/Python - Python Procedural Language</title>
available as an <quote>untrusted</> language (meaning it does not
offer any way of restricting what users can do in it). It has
therefore been renamed to <literal>plpythonu</>. The trusted
- variant <literal>plpython</> may become available again in future,
+ variant <literal>plpython</> might become available again in future,
if a new secure execution mechanism is developed in Python.
</para>
</para>
<para>
- If <literal>TD["when"]</literal> is <literal>BEFORE</>, you may
+ If <literal>TD["when"]</literal> is <literal>BEFORE</>, you can
return <literal>None</literal> or <literal>"OK"</literal> from the
Python function to indicate the row is unmodified,
<literal>"SKIP"</> to abort the event, or <literal>"MODIFY"</> to
-<!-- $PostgreSQL: pgsql/doc/src/sgml/pltcl.sgml,v 2.43 2007/01/30 22:29:23 momjian Exp $ -->
+<!-- $PostgreSQL: pgsql/doc/src/sgml/pltcl.sgml,v 2.44 2007/01/31 20:56:18 momjian Exp $ -->
<chapter id="pltcl">
<title>PL/Tcl - Tcl Procedural Language</title>
provides no way to access internals of the database server or to
gain OS-level access under the permissions of the
<productname>PostgreSQL</productname> server process, as a C
- function can do. Thus, unprivileged database users may be trusted
+ function can do. Thus, unprivileged database users can be trusted
to use this language; it does not give them unlimited authority.
</para>
<para>
The other notable implementation restriction is that Tcl functions
- may not be used to create input/output functions for new data
+ cannot be used to create input/output functions for new data
types.
</para>
<para>
PL/Tcl</></>
</para>
<para>
- The query may use parameters, that is, placeholders for
+ The query can use parameters, that is, placeholders for
values to be supplied whenever the plan is actually executed.
In the query string, refer to parameters
by the symbols <literal>$1</literal> ... <literal>$<replaceable>n</replaceable></literal>.
<listitem>
<para>
Doubles all occurrences of single quote and backslash characters
- in the given string. This may be used to safely quote strings
+ in the given string. This can be used to safely quote strings
that are to be inserted into SQL commands given
to <function>spi_exec</function> or
<function>spi_prepare</function>.
<title>Tcl Procedure Names</title>
<para>
- In <productname>PostgreSQL</productname>, one and the
- same function name can be used for
- different functions as long as the number of arguments or their types
+ In <productname>PostgreSQL</productname>, the same function name can be used for
+ different function definitions as long as the number of arguments or their types
differ. Tcl, however, requires all procedure names to be distinct.
PL/Tcl deals with this by making the internal Tcl procedure names contain
the object
-<!-- $PostgreSQL: pgsql/doc/src/sgml/postgres.sgml,v 1.80 2006/11/17 16:38:44 momjian Exp $ -->
+<!-- $PostgreSQL: pgsql/doc/src/sgml/postgres.sgml,v 1.81 2007/01/31 20:56:18 momjian Exp $ -->
<!DOCTYPE book PUBLIC "-//OASIS//DTD DocBook V4.2//EN" [
<partintro>
<para>
- This part contains assorted information that can be of use to
+ This part contains assorted information that might be of use to
<productname>PostgreSQL</> developers.
</para>
</partintro>
-<!-- $PostgreSQL: pgsql/doc/src/sgml/problems.sgml,v 2.27 2006/09/16 00:30:15 momjian Exp $ -->
+<!-- $PostgreSQL: pgsql/doc/src/sgml/problems.sgml,v 2.28 2007/01/31 20:56:18 momjian Exp $ -->
<sect1 id="bug-reporting">
<title>Bug Reporting Guidelines</title>
If the function or the options do not exist then your version is
more than old enough to warrant an upgrade.
If you run a prepackaged version, such as RPMs, say so, including any
- subversion the package may have. If you are talking about a CVS
+ subversion the package might have. If you are talking about a CVS
snapshot, mention that, including its date and time.
</para>
-<!-- $PostgreSQL: pgsql/doc/src/sgml/protocol.sgml,v 1.66 2006/09/06 20:40:47 tgl Exp $ -->
+<!-- $PostgreSQL: pgsql/doc/src/sgml/protocol.sgml,v 1.67 2007/01/31 20:56:18 momjian Exp $ -->
<chapter id="protocol">
<title>Frontend/Backend Protocol</title>
The protocol is supported over <acronym>TCP/IP</acronym> and also over
Unix-domain sockets. Port number 5432 has been registered with IANA as
the customary TCP port number for servers supporting this protocol, but
- in practice any non-privileged port number may be used.
+ in practice any non-privileged port number can be used.
</para>
<para>
count) before attempting to process its contents. This allows easy
recovery if an error is detected while processing the contents. In
extreme situations (such as not having enough memory to buffer the
- message), the receiver may use the byte count to determine how much
+ message), the receiver can use the byte count to determine how much
input to skip before it resumes reading messages.
</para>
<firstterm>portals</>. A prepared statement represents the result of
parsing, semantic analysis, and (optionally) planning of a textual query
string.
- A prepared statement is not necessarily ready to execute, because it may
+ A prepared statement is not necessarily ready to execute, because it might
lack specific values for <firstterm>parameters</>. A portal represents
a ready-to-execute or already-partially-executed statement, with any
missing parameter values filled in. (For <command>SELECT</> statements,
<firstterm>execute</> step that runs a portal's query. In the case of
a query that returns rows (<command>SELECT</>, <command>SHOW</>, etc),
the execute step can be told to fetch only
- a limited number of rows, so that multiple execute steps may be needed
+ a limited number of rows, so that multiple execute steps might be needed
to complete the operation.
</para>
the only supported formats are <quote>text</> and <quote>binary</>,
but the protocol makes provision for future extensions. The desired
format for any value is specified by a <firstterm>format code</>.
- Clients may specify a format code for each transmitted parameter value
+ Clients can specify a format code for each transmitted parameter value
and for each column of a query result. Text has format code zero,
binary has format code one, and all other format codes are reserved
for future definition.
Binary representations for integers use network byte order (most
significant byte first). For other data types consult the documentation
or source code to learn about the binary representation. Keep in mind
- that binary representations for complex data types may change across
+ that binary representations for complex data types might change across
server versions; the text format is usually the more portable choice.
</para>
</sect2>
<para>
This message informs the frontend about the current (initial)
setting of backend parameters, such as <xref
- linkend="guc-client-encoding"> or <xref linkend="guc-datestyle">.
- The frontend may ignore this message, or record the settings
+ linkend="guc-client-encoding"> or <xref linkend="guc-datestyle">.
+ The frontend can ignore this message, or record the settings
for its future use; see <xref linkend="protocol-async"> for
more details. The frontend should not respond to this
message, but should continue listening for a ReadyForQuery
<term>ReadyForQuery</term>
<listitem>
<para>
- Start-up is completed. The frontend may now issue commands.
+ Start-up is completed. The frontend can now issue commands.
</para>
</listitem>
</varlistentry>
The backend then sends one or more response
messages depending on the contents of the query command string,
and finally a ReadyForQuery response message. ReadyForQuery
- informs the frontend that it may safely send a new command.
+ informs the frontend that it can safely send a new command.
(It is not actually necessary for the frontend to wait for
ReadyForQuery before issuing another command, but the frontend must
then take responsibility for figuring out what happens if the earlier
<listitem>
<para>
Processing of the query string is complete. A separate
- message is sent to indicate this because the query string may
+ message is sent to indicate this because the query string might
contain multiple SQL commands. (CommandComplete marks the
end of processing one SQL command, not the whole string.)
ReadyForQuery will always be sent, whether processing
In the event of an error, ErrorResponse is issued followed by
ReadyForQuery. All further processing of the query string is aborted by
ErrorResponse (even if more queries remained in it). Note that this
- may occur partway through the sequence of messages generated by an
+ might occur partway through the sequence of messages generated by an
individual query.
</para>
A frontend must be prepared to accept ErrorResponse and
NoticeResponse messages whenever it is expecting any other type of
message. See also <xref linkend="protocol-async"> concerning messages
- that the backend may generate due to outside events.
+ that the backend might generate due to outside events.
</para>
<para>
about data types of parameter placeholders, and the
name of a destination prepared-statement object (an empty string
selects the unnamed prepared statement). The response is
- either ParseComplete or ErrorResponse. Parameter data types may be
+ either ParseComplete or ErrorResponse. Parameter data types can be
specified by OID; if not given, the parser attempts to infer the
data types in the same way as it would do for untyped literal string
constants.
<para>
Query planning for named prepared-statement objects occurs when the Parse
message is processed. If a query will be repeatedly executed with
- different parameters, it may be beneficial to send a single Parse message
+ different parameters, it might be beneficial to send a single Parse message
containing a parameterized query, followed by multiple Bind
and Execute messages. This will avoid replanning the query on each
execution.
<note>
<para>
- Query plans generated from a parameterized query may be less
+ Query plans generated from a parameterized query might be less
efficient than query plans generated from an equivalent query with actual
parameter values substituted. The query planner cannot make decisions
based on actual parameter values (for example, index selectivity) when
FunctionCall message to the backend. The backend then sends one
or more response messages depending on the results of the function
call, and finally a ReadyForQuery response message. ReadyForQuery
- informs the frontend that it may safely send a new query or
+ informs the frontend that it can safely send a new query or
function call.
</para>
<para>
At present, NotificationResponse can only be sent outside a
transaction, and thus it will not occur in the middle of a
- command-response series, though it may occur just before ReadyForQuery.
+ command-response series, though it might occur just before ReadyForQuery.
It is unwise to design frontend logic that assumes that, however.
Good practice is to be able to accept NotificationResponse at any
point in the protocol.
<title>Cancelling Requests in Progress</title>
<para>
- During the processing of a query, the frontend may request
+ During the processing of a query, the frontend might request
cancellation of the query. The cancel request is not sent
directly on the open connection to the backend for reasons of
implementation efficiency: we don't want to have the backend
</para>
<para>
- The cancellation signal may or may not have any effect — for
+ The cancellation signal might or might not have any effect — for
example, if it arrives after the backend has finished processing
the query, then it will have no effect. If the cancellation is
effective, it results in the current command being terminated
server and not across the regular frontend/backend communication
link, it is possible for the cancel request to be issued by any
process, not just the frontend whose query is to be canceled.
- This may have some benefits of flexibility in building
+ This might provide additional flexibility when building
multiple-process applications. It also introduces a security
risk, in that unauthorized persons might try to cancel queries.
The security risk is addressed by requiring a dynamically
<para>
In rare cases (such as an administrator-commanded database shutdown)
- the backend may disconnect without any frontend request to do so.
+ the backend might disconnect without any frontend request to do so.
In such cases the backend will attempt to send an error or notice message
giving the reason for the disconnection before it closes the connection.
</para>
is being processed, the backend will probably finish the query
before noticing the disconnection. If the query is outside any
transaction block (<command>BEGIN</> ... <command>COMMIT</>
- sequence) then its results may be committed before the
+ sequence) then its results might be committed before the
disconnection is recognized.
</para>
</sect2>
StartupMessage. The server then responds with a single byte
containing <literal>S</> or <literal>N</>, indicating that it is
willing or unwilling to perform <acronym>SSL</acronym>,
- respectively. The frontend may close the connection at this point
+ respectively. The frontend might close the connection at this point
if it is dissatisfied with the response. To continue after
<literal>S</>, perform an <acronym>SSL</acronym> startup handshake
(not described here, part of the <acronym>SSL</acronym>
response to SSLRequest from the server. This would only occur if
the server predates the addition of <acronym>SSL</acronym> support
to <productname>PostgreSQL</>. In this case the connection must
- be closed, but the frontend may choose to open a fresh connection
+ be closed, but the frontend might choose to open a fresh connection
and proceed without requesting <acronym>SSL</acronym>.
</para>
<para>
- An initial SSLRequest may also be used in a connection that is being
+ An initial SSLRequest can also be used in a connection that is being
opened to send a CancelRequest message.
</para>
<para>
While the protocol itself does not provide a way for the server to
- force <acronym>SSL</acronym> encryption, the administrator may
+ force <acronym>SSL</acronym> encryption, the administrator can
configure the server to reject unencrypted sessions as a byproduct
of authentication checking.
</para>
<para>
This section describes the detailed format of each message. Each is marked to
-indicate that it may be sent by a frontend (F), a backend (B), or both
+indicate that it can be sent by a frontend (F), a backend (B), or both
(F & B).
Notice that although each message includes a byte count at the beginning,
the message format is defined so that the message end can be found without
reference to the byte count. This aids validity checking. (The CopyData
message is an exception, because it forms part of a data stream; the contents
-of any individual CopyData message may not be interpretable on their own.)
+of any individual CopyData message cannot be interpretable on their own.)
</para>
<variablelist>
<para>
Data that forms part of a <command>COPY</command> data stream. Messages sent
from the backend will always correspond to single data rows,
- but messages sent by frontends may divide the data stream
+ but messages sent by frontends might divide the data stream
arbitrarily.
</para>
</listitem>
</varlistentry>
</variablelist>
The message body consists of one or more identified fields,
- followed by a zero byte as a terminator. Fields may appear in
+ followed by a zero byte as a terminator. Fields can appear in
any order. For each field there is the following:
<variablelist>
<varlistentry>
the message terminator and no string follows.
The presently defined field types are listed in
<xref linkend="protocol-error-fields">.
- Since more field types may be added in future,
+ Since more field types might be added in future,
frontends should silently ignore fields of unrecognized
type.
</para>
</varlistentry>
</variablelist>
The message body consists of one or more identified fields,
- followed by a zero byte as a terminator. Fields may appear in
+ followed by a zero byte as a terminator. Fields can appear in
any order. For each field there is the following:
<variablelist>
<varlistentry>
the message terminator and no string follows.
The presently defined field types are listed in
<xref linkend="protocol-error-fields">.
- Since more field types may be added in future,
+ Since more field types might be added in future,
frontends should silently ignore fields of unrecognized
type.
</para>
<listitem>
<para>
The number of parameters used by the statement
- (may be zero).
+ (can be zero).
</para>
</listitem>
</varlistentry>
<listitem>
<para>
The number of parameter data types specified
- (may be zero). Note that this is not an indication of
+ (can be zero). Note that this is not an indication of
the number of parameters that might appear in the
query string, only the number that the frontend wants to
prespecify types for.
</term>
<listitem>
<para>
- Specifies the number of fields in a row (may be zero).
+ Specifies the number of fields in a row (can be zero).
</para>
</listitem>
</varlistentry>
</variablelist>
In addition to the above, any run-time parameter that can be
- set at backend start time may be listed. Such settings
+ set at backend start time might be listed. Such settings
will be applied during backend start (after parsing the
command-line options if any). The values will act as
session defaults.
<title>Error and Notice Message Fields</title>
<para>
-This section describes the fields that may appear in ErrorResponse and
+This section describes the fields that can appear in ErrorResponse and
NoticeResponse messages. Each field type has a single-byte identification
token. Note that any given field type should appear at most once per
message.
<listitem>
<para>
Detail: an optional secondary error message carrying more
- detail about the problem. May run to multiple lines.
+ detail about the problem. Might run to multiple lines.
</para>
</listitem>
</varlistentry>
Hint: an optional suggestion what to do about the problem.
This is intended to differ from Detail in that it offers advice
(potentially inappropriate) rather than hard facts.
- May run to multiple lines.
+ Might run to multiple lines.
</para>
</listitem>
</varlistentry>
<para>
ErrorResponse and NoticeResponse ('<literal>E</>' and '<literal>N</>')
-messages now contain multiple fields, from which the client code may
+messages now contain multiple fields, from which the client code can
assemble an error message of the desired level of verbosity. Note that
individual fields will typically not end with a newline, whereas the single
string sent in the older protocol always did.
backend message types ParseComplete, BindComplete, PortalSuspended,
ParameterDescription, NoData, and CloseComplete. Existing clients do not
have to concern themselves with this sub-protocol, but making use of it
-may allow improvements in performance or functionality.
+might allow improvements in performance or functionality.
</para>
<para>
<para>
The NotificationResponse ('<literal>A</>') message has an additional string
-field, which is presently empty but may someday carry additional data passed
+field, which is presently empty but might someday carry additional data passed
from the <command>NOTIFY</command> event sender.
</para>
-<!-- $PostgreSQL: pgsql/doc/src/sgml/queries.sgml,v 1.41 2007/01/09 16:59:20 tgl Exp $ -->
+<!-- $PostgreSQL: pgsql/doc/src/sgml/queries.sgml,v 1.42 2007/01/31 20:56:18 momjian Exp $ -->
<chapter id="queries">
<title>Queries</title>
FROM <replaceable>table_reference</replaceable> <optional>, <replaceable>table_reference</replaceable> <optional>, ...</optional></optional>
</synopsis>
- A table reference may be a table name (possibly schema-qualified),
+ A table reference can be a table name (possibly schema-qualified),
or a derived table such as a subquery, a table join, or complex
combinations of these. If more than one table reference is listed
in the <literal>FROM</> clause they are cross-joined (see below)
- to form the intermediate virtual table that may then be subject to
+ to form the intermediate virtual table that can then be subject to
transformations by the <literal>WHERE</>, <literal>GROUP BY</>,
and <literal>HAVING</> clauses and is finally the result of the
overall table expression.
<para>
Joins of all types can be chained together or nested: either or
both of <replaceable>T1</replaceable> and
- <replaceable>T2</replaceable> may be joined tables. Parentheses
- may be used around <literal>JOIN</> clauses to control the join
+ <replaceable>T2</replaceable> might be joined tables. Parentheses
+ can be used around <literal>JOIN</> clauses to control the join
order. In the absence of parentheses, <literal>JOIN</> clauses
nest left-to-right.
</para>
of either base data types (scalar types) or composite data types
(table rows). They are used like a table, view, or subquery in
the <literal>FROM</> clause of a query. Columns returned by table
- functions may be included in <literal>SELECT</>,
+ functions can be included in <literal>SELECT</>,
<literal>JOIN</>, or <literal>WHERE</> clauses in the same manner
as a table, view, or subquery column.
</para>
</para>
<para>
- A table function may be aliased in the <literal>FROM</> clause,
- but it also may be left unaliased. If a function is used in the
+ A table function can be aliased in the <literal>FROM</> clause,
+ but it also can be left unaliased. If a function is used in the
<literal>FROM</> clause with no alias, the function name is used
as the resulting table name.
</para>
<para>
After passing the <literal>WHERE</> filter, the derived input
- table may be subject to grouping, using the <literal>GROUP BY</>
+ table might be subject to grouping, using the <literal>GROUP BY</>
clause, and elimination of group rows using the <literal>HAVING</>
clause.
</para>
<literal>p.name</literal>, and <literal>p.price</literal> must be
in the <literal>GROUP BY</> clause since they are referenced in
the query select list. (Depending on how exactly the products
- table is set up, name and price may be fully dependent on the
+ table is set up, name and price might be fully dependent on the
product ID, so the additional groupings could theoretically be
unnecessary, but this is not implemented yet.) The column
<literal>s.units</> does not have to be in the <literal>GROUP
</indexterm>
<para>
- After the select list has been processed, the result table may
+ After the select list has been processed, the result table can
optionally be subject to the elimination of duplicate rows. The
<literal>DISTINCT</literal> key word is written directly after
<literal>SELECT</literal> to specify this:
</programlisting>
When more than one expression is specified,
the later values are used to sort rows that are equal according to the
- earlier values. Each expression may be followed by an optional
+ earlier values. Each expression can be followed by an optional
<literal>ASC</> or <literal>DESC</> keyword to set the sort direction to
ascending or descending. <literal>ASC</> order is the default.
Ascending order puts smaller values first, where
When using <literal>LIMIT</>, it is important to use an
<literal>ORDER BY</> clause that constrains the result rows into a
unique order. Otherwise you will get an unpredictable subset of
- the query's rows. You may be asking for the tenth through
+ the query's rows. You might be asking for the tenth through
twentieth rows, but tenth through twentieth in what ordering? The
ordering is unknown, unless you specified <literal>ORDER BY</>.
</para>
<para>
The rows skipped by an <literal>OFFSET</> clause still have to be
computed inside the server; therefore a large <literal>OFFSET</>
- can be inefficient.
+ might be inefficient.
</para>
</sect1>
-<!-- $PostgreSQL: pgsql/doc/src/sgml/query.sgml,v 1.48 2006/10/21 23:12:57 tgl Exp $ -->
+<!-- $PostgreSQL: pgsql/doc/src/sgml/query.sgml,v 1.49 2007/01/31 20:56:18 momjian Exp $ -->
<chapter id="tutorial-sql">
<title>The <acronym>SQL</acronym> Language</title>
</para>
<para>
- White space (i.e., spaces, tabs, and newlines) may be used freely
+ White space (i.e., spaces, tabs, and newlines) can be used freely
in SQL commands. That means you can type the command aligned
differently than above, or even all on one line. Two dashes
(<quote><literal>--</literal></quote>) introduce comments.
a type for storing single precision floating-point numbers.
<type>date</type> should be self-explanatory. (Yes, the column of
type <type>date</type> is also named <literal>date</literal>.
- This may be convenient or confusing — you choose.)
+ This might be convenient or confusing — you choose.)
</para>
<para>
You can update existing rows using the
<command>UPDATE</command> command.
Suppose you discover the temperature readings are
- all off by 2 degrees after November 28. You may correct the
+ all off by 2 degrees after November 28. You can correct the
data as follows:
<programlisting>
-<!-- $PostgreSQL: pgsql/doc/src/sgml/reference.sgml,v 1.61 2007/01/23 05:07:17 tgl Exp $ -->
+<!-- $PostgreSQL: pgsql/doc/src/sgml/reference.sgml,v 1.62 2007/01/31 20:56:18 momjian Exp $ -->
<part id="reference">
<title>Reference</title>
length an authoritative, complete, and formal summary about their
respective subjects. More information about the use of
<productname>PostgreSQL</productname>, in narrative, tutorial, or
- example form, may be found in other parts of this book. See the
+ example form, can be found in other parts of this book. See the
cross-references listed on each reference page.
</para>
This part contains reference information for
<productname>PostgreSQL</productname> client applications and
utilities. Not all of these commands are of general utility, some
- may require special privileges. The common feature of these
+ might require special privileges. The common feature of these
applications is that they can be run on any host, independent of
where the database server resides.
</para>
-<!-- $PostgreSQL: pgsql/doc/src/sgml/regress.sgml,v 1.55 2006/09/16 00:30:15 momjian Exp $ -->
+<!-- $PostgreSQL: pgsql/doc/src/sgml/regress.sgml,v 1.56 2007/01/31 20:56:18 momjian Exp $ -->
<chapter id="regress">
<title id="regress-title">Regression Tests</title>
If you have configured <productname>PostgreSQL</productname> to install
into a location where an older <productname>PostgreSQL</productname>
installation already exists, and you perform <literal>gmake check</>
- before installing the new version, you may find that the tests fail
+ before installing the new version, you might find that the tests fail
because the new programs try to use the already-installed shared
libraries. (Typical symptoms are complaints about undefined symbols.)
If you wish to run the tests before overwriting the old installation,
scripts, which means forty processes: there's a server process and a
<application>psql</> process for each test script.
So if your system enforces a per-user limit on the number of processes,
- make sure this limit is at least fifty or so, else you may get
+ make sure this limit is at least fifty or so, else you might get
random-seeming failures in the parallel test. If you are not in
a position to raise the limit, you can cut down the degree of parallelism
by setting the <literal>MAX_CONNECTIONS</> parameter. For example,
generated on a reference system, so the results are sensitive to
small system differences. When a test is reported as
<quote>failed</quote>, always examine the differences between
- expected and actual results; you may well find that the
+ expected and actual results; you might find that the
differences are not significant. Nonetheless, we still strive to
maintain accurate reference files across all supported platforms,
so it can be expected that all tests pass.
Some of the regression tests involve intentional invalid input
values. Error messages can come from either the
<productname>PostgreSQL</productname> code or from the host
- platform system routines. In the latter case, the messages may
+ platform system routines. In the latter case, the messages can
vary between platforms, but should reflect similar
information. These differences in messages will result in a
<quote>failed</quote> regression test that can be validated by
<para>
If you run the tests against an already-installed server that was
initialized with a collation-order locale other than C, then
- there may be differences due to sort order and follow-up
+ there might be differences due to sort order and follow-up
failures. The regression test suite is set up to handle this
problem by providing alternative result files that together are
known to handle a large number of locales.
-<!-- $PostgreSQL: pgsql/doc/src/sgml/release.sgml,v 1.491 2007/01/06 15:19:45 momjian Exp $ -->
+<!-- $PostgreSQL: pgsql/doc/src/sgml/release.sgml,v 1.492 2007/01/31 20:56:18 momjian Exp $ -->
<!--
Typical markup:
</para>
<para>
- This may eliminate the need to set unrealistically small
+ This might eliminate the need to set unrealistically small
values of <link
linkend="guc-random-page-cost"><varname>random_page_cost</></link>.
If you have been using a very small <varname>random_page_cost</>,
<para>
On platforms where it is expensive to update the <application>ps</>
- display, it may be worthwhile to turn this off and rely solely on
+ display, it might be worthwhile to turn this off and rely solely on
<structname>pg_stat_activity</> for status information.
</para>
</listitem>
<para>
This prevents surprising behavior due to multiple evaluation
of a <literal>volatile</> function (such as <function>random()</>
- or <function>nextval()</>). It may cause performance
+ or <function>nextval()</>). It might cause performance
degradation in the presence of functions that are unnecessarily
marked as <literal>volatile</>.
</para>
<para>
Full security against the SQL-injection attacks described in
- CVE-2006-2313 and CVE-2006-2314 may require changes in application
+ CVE-2006-2313 and CVE-2006-2314 might require changes in application
code. If you have applications that embed untrustworthy strings
into SQL commands, you should examine them as soon as possible to
ensure that they are using recommended escaping techniques. In
GB18030, or UHC), which is the scenario in which SQL injection is possible.
A new configuration parameter <varname>backslash_quote</> is available to
adjust this behavior when needed. Note that full security against
-CVE-2006-2314 may require client-side changes; the purpose of
+CVE-2006-2314 might require client-side changes; the purpose of
<varname>backslash_quote</> is in part to make it obvious that insecure
clients are insecure.
</para></listitem>
<para>This fixes a problem that occurred if the <application>postmaster</> was
started with environment variables specifying a different locale than what
<application>initdb</> had been told. Under these conditions, any use of
-<application>plperl</> was likely to lead to corrupt indexes. You may need
+<application>plperl</> was likely to lead to corrupt indexes. You might need
<command>REINDEX</> to fix existing indexes on
textual columns if this has happened to you.</para></listitem>
</para>
<para>
- It may be necessary to set <varname>add_missing_from</> to true
+ It might be necessary to set <varname>add_missing_from</> to true
in order to load an existing dump file, if the dump contains any
views or rules created using the implicit-<literal>FROM</> syntax.
This should be a one-time annoyance, because
<listitem>
<para>
- <command>CREATE LANGUAGE</> may ignore the provided arguments
+ <command>CREATE LANGUAGE</> can ignore the provided arguments
in favor of information from <structname>pg_pltemplate</>
(Tom)
</para>
<para>
Full security against the SQL-injection attacks described in
- CVE-2006-2313 and CVE-2006-2314 may require changes in application
+ CVE-2006-2313 and CVE-2006-2314 might require changes in application
code. If you have applications that embed untrustworthy strings
into SQL commands, you should examine them as soon as possible to
ensure that they are using recommended escaping techniques. In
GB18030, or UHC), which is the scenario in which SQL injection is possible.
A new configuration parameter <varname>backslash_quote</> is available to
adjust this behavior when needed. Note that full security against
-CVE-2006-2314 may require client-side changes; the purpose of
+CVE-2006-2314 might require client-side changes; the purpose of
<varname>backslash_quote</> is in part to make it obvious that insecure
clients are insecure.
</para></listitem>
<para>This fixes a problem that occurred if the <application>postmaster</> was
started with environment variables specifying a different locale than what
<application>initdb</> had been told. Under these conditions, any use of
-<application>plperl</> was likely to lead to corrupt indexes. You may need
+<application>plperl</> was likely to lead to corrupt indexes. You might need
<command>REINDEX</> to fix existing indexes on
textual columns if this has happened to you.</para></listitem>
It is strongly recommended that all installations repair these errors,
either by initdb or by following the manual repair procedure given
below. The errors at least allow unprivileged database users to crash
- their server process, and may allow unprivileged users to gain the
+ their server process, and might allow unprivileged users to gain the
privileges of a database superuser.
</para>
behavior across all platforms. In most cases, there should be
little noticeable difference in time zone behavior, except that
the time zone names used by <command>SET</>/<command>SHOW</>
- <varname>TimeZone</> may
- be different from what your platform provides.
+ <varname>TimeZone</> might be different from what your platform provides.
</para>
</listitem>
Some logging-related configuration parameters could formerly be adjusted
by ordinary users, but only in the <quote>more verbose</> direction.
They are now treated more strictly: only superusers can set them.
- However, a superuser may use <command>ALTER USER</> to provide per-user
+ However, a superuser can use <command>ALTER USER</> to provide per-user
settings of these values for non-superusers. Also, it is now possible
for superusers to set values of superuser-only configuration parameters
via <literal>PGOPTIONS</>.
<para>
Full security against the SQL-injection attacks described in
- CVE-2006-2313 and CVE-2006-2314 may require changes in application
+ CVE-2006-2313 and CVE-2006-2314 might require changes in application
code. If you have applications that embed untrustworthy strings
into SQL commands, you should examine them as soon as possible to
ensure that they are using recommended escaping techniques. In
GB18030, or UHC), which is the scenario in which SQL injection is possible.
A new configuration parameter <varname>backslash_quote</> is available to
adjust this behavior when needed. Note that full security against
-CVE-2006-2314 may require client-side changes; the purpose of
+CVE-2006-2314 might require client-side changes; the purpose of
<varname>backslash_quote</> is in part to make it obvious that insecure
clients are insecure.
</para></listitem>
<para>This fixes a problem that occurred if the <application>postmaster</> was
started with environment variables specifying a different locale than what
<application>initdb</> had been told. Under these conditions, any use of
-<application>plperl</> was likely to lead to corrupt indexes. You may need
+<application>plperl</> was likely to lead to corrupt indexes. You might need
<command>REINDEX</> to fix existing indexes on
textual columns if this has happened to you.</para></listitem>
It is strongly recommended that all installations repair these errors,
either by initdb or by following the manual repair procedures given
below. The errors at least allow unprivileged database users to crash
- their server process, and may allow unprivileged users to gain the
+ their server process, and might allow unprivileged users to gain the
privileges of a database superuser.
</para>
<para>
A dump/restore is not required for those running 7.4.X. However,
- it may be advisable as the easiest method of incorporating fixes for
+ it might be advisable as the easiest method of incorporating fixes for
two errors that have been found in the initial contents of 7.4.X system
catalogs. A dump/initdb/reload sequence using 7.4.2's initdb will
automatically correct these problems.
<listitem>
<para>
Inner joins using the explicit <literal>JOIN</literal> syntax
- may behave differently because they are now better
+ might behave differently because they are now better
optimized.
</para>
</listitem>
<listitem>
<para>
- Arrays may now be specified as <literal>ARRAY[1,2,3]</literal>,
+ Arrays can now be specified as <literal>ARRAY[1,2,3]</literal>,
<literal>ARRAY[['a','b'],['c','d']]</literal>, or
<literal>ARRAY[ARRAY[ARRAY[2]]]</literal> (Joe)
</para>
<para>
Full security against the SQL-injection attacks described in
- CVE-2006-2313 and CVE-2006-2314 may require changes in application
+ CVE-2006-2313 and CVE-2006-2314 might require changes in application
code. If you have applications that embed untrustworthy strings
into SQL commands, you should examine them as soon as possible to
ensure that they are using recommended escaping techniques. In
GB18030, or UHC), which is the scenario in which SQL injection is possible.
A new configuration parameter <varname>backslash_quote</> is available to
adjust this behavior when needed. Note that full security against
-CVE-2006-2314 may require client-side changes; the purpose of
+CVE-2006-2314 might require client-side changes; the purpose of
<varname>backslash_quote</> is in part to make it obvious that insecure
clients are insecure.
</para></listitem>
<para>This fixes a problem that occurred if the <application>postmaster</> was
started with environment variables specifying a different locale than what
<application>initdb</> had been told. Under these conditions, any use of
-<application>plperl</> was likely to lead to corrupt indexes. You may need
+<application>plperl</> was likely to lead to corrupt indexes. You might need
<command>REINDEX</> to fix existing indexes on
textual columns if this has happened to you.</para></listitem>
It is strongly recommended that all installations repair this error,
either by initdb or by following the manual repair procedure given
below. The error at least allows unprivileged database users to crash
- their server process, and may allow unprivileged users to gain the
+ their server process, and might allow unprivileged users to gain the
privileges of a database superuser.
</para>
A dump/restore is <emphasis>not</emphasis> required for those
running version 7.3. However, it should be noted that the main
<productname>PostgreSQL</productname> interface library, libpq,
- has a new major version number for this release, which may require
+ has a new major version number for this release, which might require
recompilation of client code in certain cases.
</para>
</sect2>
<para>
<command>COPY</command> no longer considers missing trailing
columns to be null. All columns need to be specified.
- (However, one may achieve a similar effect by specifying a
+ (However, one can achieve a similar effect by specifying a
column list in the <command>COPY</command> command.)
</para>
</listitem>
<listitem>
<para>
The semantics of the <command>VACUUM</command> command have
- changed in this release. You may wish to update your
+ changed in this release. You might wish to update your
maintenance procedures accordingly.
</para>
</listitem>
all data modifications to disk before each transaction commit. With
WAL, only one log file must be flushed to disk, greatly improving
performance. If you have been using -F in previous releases to
-disable disk flushes, you may want to consider discontinuing its use.
+disable disk flushes, you might want to consider discontinuing its use.
</para>
</listitem>
</varlistentry>
The previous C function manager did not
handle null values properly, nor did it support 64-bit <acronym>CPU</acronym>'s (Alpha). The new
function manager does. You can continue using your old custom
-functions, but you may want to rewrite them in the future to use the new
+functions, but you might want to rewrite them in the future to use the new
function manager call interface.
</para>
</listitem>
A dump/restore using <application>pg_dump</application>
is required for those wishing to migrate data from any
previous release of <productname>PostgreSQL</productname>.
- For those upgrading from 6.5.*, you may instead use
+ For those upgrading from 6.5.*, you can instead use
<application>pg_upgrade</application> to upgrade to this
release; however, a full dump/reload installation is always the
most robust method for upgrades.
ease the transition by allowing
<productname>PostgreSQL</productname> to recognize
the deprecated type names and translate them to the new type
- names, this mechanism may not be completely transparent to
+ names, this mechanism cannot be completely transparent to
your existing application.
</para>
</listitem>
decreased query times as the optimizer makes a better choice
for the preferred plan. However, in a small number of cases,
usually involving pathological distributions of data, your
- query times may go up. If you are dealing with large amounts
- of data, you may want to check your queries to verify
+ query times might go up. If you are dealing with large amounts
+ of data, you might want to check your queries to verify
performance.
</para>
</listitem>
<listitem>
<para>
Socket interface for client/server connection. This is the default now
- so you m
ay need to start <application>postmaster</application> with the
+ so you m
ight need to start <application>postmaster</application> with the
<option>-i</option> flag.
</para>
</listitem>
mention that without subselects, SQL is a very limited language.
Subselects are a major feature, and you should review your code for
places where subselects provide a better solution for your queries. I
- think you will find that there are more uses for subselects than you may
+ think you will find that there are more uses for subselects than you might
think. Vadim has put us on the big SQL map with subselects, and fully
functional ones too. The only thing you can't do with subselects is to
use them in the target list.
Third, <type>char()</type> fields will now allow faster access than <type>varchar()</type> or
<type>text</type>. Specifically, the <type>text</> and <type>varchar()</type> have a penalty for access to
any columns after the first column of this type. <type>char()</type> used to also
- have this access penalty, but it no longer does. This may suggest that
+ have this access penalty, but it no longer does. This might suggest that
you redesign some of your tables, especially if you have short character
columns that you have defined as <type>varchar()</type> or <type>text</type>. This and other
changes make 6.3 even faster than earlier releases.
The interpretation of array specifiers (the curly braces around atomic
values) appears to have changed sometime after the original regression
tests were generated. The current <filename>./expected/*.out</filename> files reflect this
- new interpretation, which may not be correct!
+ new interpretation, which might not be correct!
</para>
<para>
* float literals (eg. 3.14) are now of type float4 (instead of float8 in
previous releases); you might have to do typecasting if you depend on it
being of type float8. If you neglect to do the typecasting and you assign
- a float literal to a field of type float8, you may get incorrect values
+ a float literal to a field of type float8, you might get incorrect values
stored!
* LIBPQ has been totally revamped so that frontend applications
can connect to multiple backends
-<!-- $PostgreSQL: pgsql/doc/src/sgml/rowtypes.sgml,v 2.7 2007/01/30 22:29:23 momjian Exp $ -->
+<!-- $PostgreSQL: pgsql/doc/src/sgml/rowtypes.sgml,v 2.8 2007/01/31 20:56:18 momjian Exp $ -->
<sect1 id="rowtypes">
<title>Composite Types</title>
<para>
To write a composite value as a literal constant, enclose the field
- values within parentheses and separate them by commas. You may put double
+ values within parentheses and separate them by commas. You can put double
quotes around any field value, and must do so if it contains commas or
parentheses. (More details appear below.) Thus, the general format of a
composite constant is the following:
</para>
<para>
- The <literal>ROW</literal> expression syntax may also be used to
+ The <literal>ROW</literal> expression syntax can also be used to
construct composite values. In most cases this is considerably
simpler to use than the string-literal syntax, since you don't have
to worry about multiple layers of quoting. We already used this
The decoration consists of parentheses (<literal>(</> and <literal>)</>)
around the whole value, plus commas (<literal>,</>) between adjacent
items. Whitespace outside the parentheses is ignored, but within the
- parentheses it is considered part of the field value, and may or may not be
+ parentheses it is considered part of the field value, and might or might not be
significant depending on the input conversion rules for the field data type.
For example, in
<programlisting>
</para>
<para>
- As shown previously, when writing a composite value you may write double
+ As shown previously, when writing a composite value you can write double
quotes around any individual field value.
You <emphasis>must</> do so if the field value would otherwise
confuse the composite-value parser. In particular, fields containing
with a data type whose input routine also treated backslashes specially,
<type>bytea</> for example, we might need as many as eight backslashes
in the command to get one backslash into the stored composite field.)
- Dollar quoting (see <xref linkend="sql-syntax-dollar-quoting">) may be
+ Dollar quoting (see <xref linkend="sql-syntax-dollar-quoting">) can be
used to avoid the need to double backslashes.
</para>
</note>
-<!-- $PostgreSQL: pgsql/doc/src/sgml/rules.sgml,v 1.48 2006/12/27 16:07:36 tgl Exp $ -->
+<!-- $PostgreSQL: pgsql/doc/src/sgml/rules.sgml,v 1.49 2007/01/31 20:56:18 momjian Exp $ -->
<chapter id="rules">
<title>The Rule System</title>
the originally given query will be executed, and its command
status will be returned as usual. (But note that if there were
any conditional <literal>INSTEAD</> rules, the negation of their qualifications
- will have been added to the original query. This may reduce the
+ will have been added to the original query. This might reduce the
number of rows it processes, and if so the reported status will
be affected.)
</para>
-<!-- $PostgreSQL: pgsql/doc/src/sgml/runtime.sgml,v 1.374 2006/11/25 22:44:48 petere Exp $ -->
+<!-- $PostgreSQL: pgsql/doc/src/sgml/runtime.sgml,v 1.375 2007/01/31 20:56:19 momjian Exp $ -->
<chapter Id="runtime">
<title>Operating System Environment</title>
This usually means just what it suggests: you tried to start
another server on the same port where one is already running.
However, if the kernel error message is not <computeroutput>Address
- already in use</computeroutput> or some variant of that, there may
+ already in use</computeroutput> or some variant of that, there might
be a different problem. For example, trying to start a server
- on a reserved port number may draw something like:
+ on a reserved port number might draw something like:
<screen>
$ <userinput>postgres -p 666</userinput>
LOG: could not bind IPv4 socket: Permission denied
can try starting the server with a smaller-than-normal number of
buffers (<xref linkend="guc-shared-buffers">). You will eventually want
to reconfigure your kernel to increase the allowed shared memory
- size. You may also see this message when trying to start multiple
+ size. You might also see this message when trying to start multiple
servers on the same machine, if their total space requested
exceeds the kernel limit.
</para>
space. It means your kernel's limit on the number of <systemitem
class="osname">System V</> semaphores is smaller than the number
<productname>PostgreSQL</productname> wants to create. As above,
- you may be able to work around the problem by starting the
+ you might be able to work around the problem by starting the
server with a reduced number of allowed connections
(<xref linkend="guc-max-connections">), but you'll eventually want to
increase the kernel limit.
connection request and rejected it. That case will produce a
different message, as shown in <xref
linkend="client-authentication-problems">.) Other error messages
- such as <computeroutput>Connection timed out</computeroutput> may
+ such as <computeroutput>Connection timed out</computeroutput> might
indicate more fundamental problems, like lack of network
connectivity.
</para>
</para>
<para>
- Older distributions may not have the <command>sysctl</command> program,
+ Older distributions might not have the <command>sysctl</command> program,
but equivalent changes can be made by manipulating the
<filename>/proc</filename> file system:
<screen>
<para>
In OS X 10.3.9 and later, instead of editing <filename>/etc/rc</>
- you may create a file named <filename>/etc/sysctl.conf</>,
+ you can create a file named <filename>/etc/sysctl.conf</>,
containing variable assignments such as
<programlisting>
kern.sysv.shmmax=4194304
sort of configuration commonly used for other databases such
as <application>DB/2</application>.</para>
- <para> It may, however, be necessary to modify the global
+ <para> It might , however, be necessary to modify the global
<command>ulimit</command> information in
<filename>/etc/security/limits</filename>, as the default hard
limits for file sizes (<varname>fsize</varname>) and numbers of
- files (<varname>nofiles</varname>) may be too low.
+ files (<varname>nofiles</varname>) might be too low.
</para>
</listitem>
</varlistentry>
<quote>socially friendly</quote> values that allow many users to
coexist on a machine without using an inappropriate fraction of
the system resources. If you run many servers on a machine this
- is perhaps what you want, but on dedicated servers you may want to
+ is perhaps what you want, but on dedicated servers you might want to
raise this limit.
</para>
<para>
In Linux 2.4 and later, the default virtual memory behavior is not
optimal for <productname>PostgreSQL</productname>. Because of the
- way that the kernel implements memory overcommit, the kernel may
+ way that the kernel implements memory overcommit, the kernel might
terminate the <productname>PostgreSQL</productname> server (the
master server process) if the memory demands of
another process cause the system to run out of virtual memory.
sysctl -w vm.overcommit_memory=2
</programlisting>
or placing an equivalent entry in <filename>/etc/sysctl.conf</>.
- You may also wish to modify the related setting
+ You might also wish to modify the related setting
<literal>vm.overcommit_ratio</>. For details see the kernel documentation
file <filename>Documentation/vm/overcommit-accounting</>.
</para>
<para>
It is best not to use <systemitem>SIGKILL</systemitem> to shut down
the server. Doing so will prevent the server from releasing
- shared memory and semaphores, which may then have to be done
+ shared memory and semaphores, which might then have to be done
manually before a new server can be started. Furthermore,
<systemitem>SIGKILL</systemitem> kills the <command>postgres</command>
process without letting it relay the signal to its subprocesses,
-<!-- $PostgreSQL: pgsql/doc/src/sgml/sources.sgml,v 2.20 2006/10/23 18:10:32 petere Exp $ -->
+<!-- $PostgreSQL: pgsql/doc/src/sgml/sources.sgml,v 2.21 2007/01/31 20:56:19 momjian Exp $ -->
<chapter id="source">
<title>PostgreSQL Coding Conventions</title>
func_signature_string(funcname, nargs,
actual_arg_types)),
errhint("Unable to choose a best candidate function. "
- "You may need to add explicit typecasts.")));
+ "You might need to add explicit typecasts.")));
</programlisting>
This illustrates the use of format codes to embed run-time values into
a message text. Also, an optional <quote>hint</> message is provided.
<para>
Rationale: keeping the primary message short helps keep it to the point,
and lets clients lay out screen space on the assumption that one line is
- enough for error messages. Detail and hint messages may be relegated to a
+ enough for error messages. Detail and hint messages can be relegated to a
verbose mode, or perhaps a pop-up error-details window. Also, details and
hints would normally be suppressed from the server log to save
space. Reference to implementation details is best avoided since users
<para>
Don't put any specific assumptions about formatting into the message
texts. Expect clients and the server log to wrap lines to fit their own
- needs. In long messages, newline characters (\n) may be used to indicate
+ needs. In long messages, newline characters (\n) can be used to indicate
suggested paragraph breaks. Don't end a message with a newline. Don't
use tabs or other formatting characters. (In error context displays,
newlines are automatically added to separate levels of context such as
messages are not grammatically complete sentences anyway. (And if they're
long enough to be more than one sentence, they should be split into
primary and detail parts.) However, detail and hint messages are longer
- and may need to include multiple sentences. For consistency, they should
+ and might need to include multiple sentences. For consistency, they should
follow complete-sentence style even when there's only one sentence.
</para>
The first one means that the attempt to open the file failed. The
message should give a reason, such as <quote>disk full</quote> or
<quote>file doesn't exist</quote>. The past tense is appropriate because
- next time the disk might not be full anymore or the file in question may
+ next time the disk might not be full anymore or the file in question might
exist.
</para>
-<!-- $PostgreSQL: pgsql/doc/src/sgml/spi.sgml,v 1.49 2006/10/23 18:10:32 petere Exp $ -->
+<!-- $PostgreSQL: pgsql/doc/src/sgml/spi.sgml,v 1.50 2007/01/31 20:56:19 momjian Exp $ -->
<chapter id="spi">
<title>Server Programming Interface</title>
Note that if a command invoked via SPI fails, then control will not be
returned to your procedure. Rather, the
transaction or subtransaction in which your procedure executes will be
- rolled back. (This may seem surprising given that the SPI functions mostly
+ rolled back. (This might seem surprising given that the SPI functions mostly
have documented error-return conventions. Those conventions only apply
for errors detected within the SPI functions themselves, however.)
It is possible to recover control after an error by establishing your own
<function>SPI_connect</function> opens a connection from a
procedure invocation to the SPI manager. You must call this
function if you want to execute commands through SPI. Some utility
- SPI functions may be called from unconnected procedures.
+ SPI functions can be called from unconnected procedures.
</para>
<para>
</para>
<para>
- This function may only be called from a connected procedure.
+ This function can only be called from a connected procedure.
</para>
<para>
</para>
<para>
- You may pass multiple commands in one string.
+ You can pass multiple commands in one string.
<function>SPI_execute</function> returns the
result for the command executed last. The <parameter>count</parameter>
limit applies to each command separately, but it is not applied to
<symbol>SPI_OK_INSERT_RETURNING</symbol>,
<symbol>SPI_OK_DELETE_RETURNING</symbol>, or
<symbol>SPI_OK_UPDATE_RETURNING</symbol>,
- then you may use the
+ then you can use the
global pointer <literal>SPITupleTable *SPI_tuptable</literal> to
access the result rows. Some utility commands (such as
<command>EXPLAIN</>) also return row sets, and <literal>SPI_tuptable</>
</programlisting>
<structfield>vals</> is an array of pointers to rows. (The number
of valid entries is given by <varname>SPI_processed</varname>.)
- <structfield>tupdesc</> is a row descriptor which you may pass to
+ <structfield>tupdesc</> is a row descriptor which you can pass to
SPI functions dealing with rows. <structfield>tuptabcxt</>,
<structfield>alloced</>, and <structfield>free</> are internal
fields not intended for use by SPI callers.
<para>
When the same or a similar command is to be executed repeatedly, it
- may be advantageous to perform the planning only once.
+ might be advantageous to perform the planning only once.
<function>SPI_prepare</function> converts a command string into an
execution plan that can be executed repeatedly using
<function>SPI_execute_plan</function>.
<para>
There is a disadvantage to using parameters: since the planner does
not know the values that will be supplied for the parameters, it
- may make worse planning choices than it would make for a normal
+ might make worse planning choices than it would make for a normal
command with all constants visible.
</para>
</refsect1>
</para>
<para>
- All functions described in this section may be used by both
+ All functions described in this section can be used by both
connected and unconnected procedures.
</para>
</para>
<para>
- All functions described in this section may be used by both
+ All functions described in this section can be used by both
connected and unconnected procedures. In an unconnected procedure,
they act the same as the underlying ordinary server functions
(<function>palloc</>, etc.).
-<!-- $PostgreSQL: pgsql/doc/src/sgml/sql.sgml,v 1.43 2007/01/09 02:14:10 tgl Exp $ -->
+<!-- $PostgreSQL: pgsql/doc/src/sgml/sql.sgml,v 1.44 2007/01/31 20:56:19 momjian Exp $ -->
<chapter id="sql-intro">
<title>SQL</title>
</para>
<para>
- The tables PART and SUPPLIER may be regarded as
+ The tables PART and SUPPLIER can be regarded as
<firstterm>entities</firstterm> and
- SELLS may be regarded as a <firstterm>relationship</firstterm>
+ SELLS can be regarded as a <firstterm>relationship</firstterm>
between a particular
part and a particular supplier.
</para>
</para>
<para>
- Arithmetic operations may be used in the target list and in the WHERE
+ Arithmetic operations can be used in the target list and in the WHERE
clause. For example if we want to know how much it would cost if we
take two pieces of a part we could use the following query:
A joined table, created using JOIN syntax, is a table reference list
item that occurs in a FROM clause and before any WHERE, GROUP BY,
or HAVING clause. Other table references, including table names or
- other JOIN clauses, may be included in the FROM clause if separated
+ other JOIN clauses, can be included in the FROM clause if separated
by commas. JOINed tables are logically like any other
table listed in the FROM clause.
</para>
<para>
JOINs of all types can be chained together or nested where either or both of
<replaceable class="parameter">T1</replaceable> and
- <replaceable class="parameter">T2</replaceable> may be JOINed tables.
+ <replaceable class="parameter">T2</replaceable> can be JOINed tables.
Parenthesis can be used around JOIN clauses to control the order
of JOINs which are otherwise processed left to right.
</para>
<para>
Note that for a query using GROUP BY and aggregate
functions to make sense, the target list can only refer directly to
- the attributes being grouped by. Other attributes may only be used
+ the attributes being grouped by. Other attributes can only be used
inside the arguments of aggregate functions. Otherwise there would
not be a unique value to associate with the other attributes.
</para>
<title>Create View</title>
<para>
- A view may be regarded as a <firstterm>virtual table</firstterm>,
+ A view can be regarded as a <firstterm>virtual table</firstterm>,
i.e. a table that
does not <emphasis>physically</emphasis> exist in the database
but looks to the user
-<!-- $PostgreSQL: pgsql/doc/src/sgml/start.sgml,v 1.43 2006/10/21 23:12:57 tgl Exp $ -->
+<!-- $PostgreSQL: pgsql/doc/src/sgml/start.sgml,v 1.44 2007/01/31 20:56:19 momjian Exp $ -->
<chapter id="tutorial-start">
<title>Getting Started</title>
<para>
If your site administrator has not set things up in the default
- way, you may have some more work to do. For example, if the
+ way, you might have some more work to do. For example, if the
database server machine is a remote machine, you will need to set
the <envar>PGHOST</envar> environment variable to the name of the
database server machine. The environment variable
- <envar>PGPORT</envar> may also have to be set. The bottom line is
+ <envar>PGPORT</envar> might also have to be set. The bottom line is
this: if you try to start an application program and it complains
that it cannot connect to the database, you should consult your
site administrator or, if that is you, the documentation to make
</para>
<para>
- More about <command>createdb</command> and <command>dropdb</command> may
+ More about <command>createdb</command> and <command>dropdb</command> can
be found in <xref linkend="APP-CREATEDB"> and <xref linkend="APP-DROPDB">
respectively.
</para>
-<!-- $PostgreSQL: pgsql/doc/src/sgml/storage.sgml,v 1.13 2006/11/25 22:55:59 petere Exp $ -->
+<!-- $PostgreSQL: pgsql/doc/src/sgml/storage.sgml,v 1.14 2007/01/31 20:56:19 momjian Exp $ -->
<chapter id="storage">
when compiling the server). In a table, all the pages are logically
equivalent, so a particular item (row) can be stored in any page. In
indexes, the first page is generally reserved as a <firstterm>metapage</>
-holding control information, and there may be different types of pages
+holding control information, and there can be different types of pages
within the index, depending on the index access method.
</para>
</table>
<para>
- All the details may be found in
+ All the details can be found in
<filename>src/include/storage/bufpage.h</filename>.
</para>
The number of item identifiers present can be determined by looking at
<structfield>pd_lower</>, which is increased to allocate a new identifier.
Because an item
- identifier is never moved until it is freed, its index may be used on a
+ identifier is never moved until it is freed, its index can be used on a
long-term basis to reference an item, even when the item itself is moved
around on the page to compact free space. In fact, every pointer to an
item (<type>ItemPointer</type>, also known as
<para>
- The final section is the <quote>special section</quote> which may
- contain anything the access method wishes to store. For example,
+ The final section is the <quote>special section</quote> which can
+ contain anything the access method wishes to store. For example,
b-tree indexes store links to the page's left and right siblings,
as well as some other data relevant to the index structure.
Ordinary tables do not use a special section at all (indicated by setting
</table>
<para>
- All the details may be found in
+ All the details can be found in
<filename>src/include/access/htup.h</filename>.
</para>
variable length field (attlen = -1) then it's a bit more complicated.
All variable-length datatypes share the common header structure
<type>varattrib</type>, which includes the total length of the stored
- value and some flag bits. Depending on the flags, the data may be either
+ value and some flag bits. Depending on the flags, the data can be either
inline or in a <acronym>TOAST</> table;
it might be compressed, too (see <xref linkend="storage-toast">).
-<!-- $PostgreSQL: pgsql/doc/src/sgml/syntax.sgml,v 1.111 2006/10/22 03:03:41 tgl Exp $ -->
+<!-- $PostgreSQL: pgsql/doc/src/sgml/syntax.sgml,v 1.112 2007/01/31 20:56:19 momjian Exp $ -->
<chapter id="sql-syntax">
<title>SQL Syntax</title>
key word can be letters, underscores, digits
(<literal>0</literal>-<literal>9</literal>), or dollar signs
(<literal>$</>). Note that dollar signs are not allowed in identifiers
- according to the letter of the SQL standard, so their use may render
+ according to the letter of the SQL standard, so their use might render
applications less portable.
The SQL standard will not define a key word that contains
digits or starts or ends with an underscore, so identifiers of this
digits (0 through 9). At least one digit must be before or after the
decimal point, if one is used. At least one digit must follow the
exponent marker (<literal>e</literal>), if one is present.
- There may not be any spaces or other characters embedded in the
+ There cannot be any spaces or other characters embedded in the
constant. Note that any leading plus or minus sign is not actually
considered part of the constant; it is an operator applied to the
constant.
The string constant's text is passed to the input conversion
routine for the type called <replaceable>type</replaceable>. The
result is a constant of the indicated type. The explicit type
- cast may be omitted if there is no ambiguity as to the type the
+ cast can be omitted if there is no ambiguity as to the type the
constant must be (for example, when it is assigned directly to a
table column), in which case it is automatically coerced.
</para>
<synopsis>
<replaceable>typename</replaceable> ( '<replaceable>string</replaceable>' )
</synopsis>
- but not all type names may be used in this way; see <xref
+ but not all type names can be used in this way; see <xref
linkend="sql-syntax-type-casts"> for details.
</para>
A dollar sign (<literal>$</literal>) followed by digits is used
to represent a positional parameter in the body of a function
definition or a prepared statement. In other contexts the
- dollar sign may be part of an identifier or a dollar-quoted string
+ dollar sign can be part of an identifier or a dollar-quoted string
constant.
</para>
</listitem>
where the comment begins with <literal>/*</literal> and extends to
the matching occurrence of <literal>*/</literal>. These block
comments nest, as specified in the SQL standard but unlike C, so that one can
- comment out larger blocks of code that may contain existing block
+ comment out larger blocks of code that might contain existing block
comments.
</para>
associativity of the operators in <productname>PostgreSQL</>.
Most operators have the same precedence and are left-associative.
The precedence and associativity of the operators is hard-wired
- into the parser. This may lead to non-intuitive behavior; for
+ into the parser. This can lead to non-intuitive behavior; for
example the Boolean operators <literal><</> and
<literal>></> have a different precedence than the Boolean
operators <literal><=</> and <literal>>=</>. Also, you will
the key words <literal>NEW</literal> or <literal>OLD</literal>.
(<literal>NEW</literal> and <literal>OLD</literal> can only appear in rewrite rules,
while other correlation names can be used in any SQL statement.)
- The correlation name and separating dot may be omitted if the column name
+ The correlation name and separating dot can be omitted if the column name
is unique across all the tables being used in the current query. (See also <xref linkend="queries">.)
</para>
</sect2>
<para>
In general the array <replaceable>expression</replaceable> must be
- parenthesized, but the parentheses may be omitted when the expression
+ parenthesized, but the parentheses can be omitted when the expression
to be subscripted is just a column reference or positional parameter.
Also, multiple subscripts can be concatenated when the original array
is multidimensional.
<para>
In general the row <replaceable>expression</replaceable> must be
- parenthesized, but the parentheses may be omitted when the expression
+ parenthesized, but the parentheses can be omitted when the expression
to be selected from is just a table reference or positional parameter.
For example,
<para>
The list of built-in functions is in <xref linkend="functions">.
- Other functions may be added by the user.
+ Other functions can be added by the user.
</para>
</sect2>
<para>
The predefined aggregate functions are described in <xref
- linkend="functions-aggregate">. Other aggregate functions may be added
+ linkend="functions-aggregate">. Other aggregate functions can be added
by the user.
</para>
<para>
- An aggregate expression may only appear in the result list or
+ An aggregate expression can only appear in the result list or
<literal>HAVING</> clause of a <command>SELECT</> command.
It is forbidden in other clauses, such as <literal>WHERE</>,
because those clauses are logically evaluated before the results
</para>
<para>
- An explicit type cast may usually be omitted if there is no ambiguity as
+ An explicit type cast can usually be omitted if there is no ambiguity as
to the type that a value expression must produce (for example, when it is
assigned to a table column); the system will automatically apply a
type cast in such cases. However, automatic casting is only done for
<para>
Multidimensional array values can be built by nesting array
constructors.
- In the inner constructors, the key word <literal>ARRAY</literal> may
+ In the inner constructors, the key word <literal>ARRAY</literal> can
be omitted. For example, these produce the same result:
<programlisting>
By default, the value created by a <literal>ROW</> expression is of
an anonymous record type. If necessary, it can be cast to a named
composite type — either the row type of a table, or a composite type
- created with <command>CREATE TYPE AS</>. An explicit cast may be needed
+ created with <command>CREATE TYPE AS</>. An explicit cast might be needed
to avoid ambiguity. For example:
<programlisting>
CREATE TABLE mytable(f1 int, f2 float, f3 text);
rely on side effects or evaluation order in <literal>WHERE</> and <literal>HAVING</> clauses,
since those clauses are extensively reprocessed as part of
developing an execution plan. Boolean
- expressions (<literal>AND</>/<literal>OR</>/<literal>NOT</> combinations) in those clauses may be reorganized
+ expressions (<literal>AND</>/<literal>OR</>/<literal>NOT</> combinations) in those clauses can be reorganized
in any manner allowed by the laws of Boolean algebra.
</para>
<para>
When it is essential to force evaluation order, a <literal>CASE</>
- construct (see <xref linkend="functions-conditional">) may be
+ construct (see <xref linkend="functions-conditional">) can be
used. For example, this is an untrustworthy way of trying to
avoid division by zero in a <literal>WHERE</> clause:
<programlisting>
-<!-- $PostgreSQL: pgsql/doc/src/sgml/trigger.sgml,v 1.47 2006/09/16 00:30:16 momjian Exp $ -->
+<!-- $PostgreSQL: pgsql/doc/src/sgml/trigger.sgml,v 1.48 2007/01/31 20:56:19 momjian Exp $ -->
<chapter id="triggers">
<title>Triggers</title>
<para>
The return value is ignored for row-level triggers fired after an
- operation, and so they may as well return <symbol>NULL</>.
+ operation, and so they can return <symbol>NULL</>.
</para>
<para>
<para>
If a trigger function executes SQL commands then these
- commands may fire triggers again. This is known as cascading
+ commands might fire triggers again. This is known as cascading
triggers. There is no direct limitation on the number of cascade
levels. It is possible for cascades to cause a recursive invocation
of the same trigger; for example, an <command>INSERT</command>
changes for rows previously processed in the same outer
command. This requires caution, since the ordering of these
change events is not in general predictable; a SQL command that
- affects multiple rows may visit the rows in any order.
+ affects multiple rows can visit the rows in any order.
</para>
</listitem>
<term><structfield>tg_event</></term>
<listitem>
<para>
- Describes the event for which the function is called. You may use the
+ Describes the event for which the function is called. You can use the
following macros to examine <literal>tg_event</literal>:
<variablelist>
<para>
Here is a very simple example of a trigger function written in C.
- (Examples of triggers written in procedural languages may be found
+ (Examples of triggers written in procedural languages can be found
in the documentation of the procedural languages.)
</para>
-<!-- $PostgreSQL: pgsql/doc/src/sgml/typeconv.sgml,v 1.48 2006/09/18 19:54:01 tgl Exp $ -->
+<!-- $PostgreSQL: pgsql/doc/src/sgml/typeconv.sgml,v 1.49 2007/01/31 20:56:19 momjian Exp $ -->
<chapter Id="typeconv">
<title>Type Conversion</title>
SELECT ~ '20' AS "negation";
ERROR: operator is not unique: ~ "unknown"
-HINT: Could not choose a best candidate operator. You may need to add explicit
+HINT: Could not choose a best candidate operator. You might need to add explicit
type casts.
</screen>
This happens because the system can't decide which of the several
<para>
Since numeric constants with decimal points are initially assigned the
type <type>numeric</type>, the following query will require no type
-conversion and may therefore be slightly more efficient:
+conversion and might therefore be slightly more efficient:
<screen>
SELECT round(4.0, 4);
</screen>
-<!-- $PostgreSQL: pgsql/doc/src/sgml/user-manag.sgml,v 1.37 2006/09/05 21:08:34 tgl Exp $ -->
+<!-- $PostgreSQL: pgsql/doc/src/sgml/user-manag.sgml,v 1.38 2007/01/31 20:56:19 momjian Exp $ -->
<chapter id="user-manag">
<title>Database Roles and Privileges</title>
</para>
<para>
- The set of database roles a given client connection may connect as
+ The set of database roles a given client connection can connect as
is determined by the client authentication setup, as explained in
<xref linkend="client-authentication">. (Thus, a client is not
necessarily limited to connect as the role with the same name as
<title>Role Attributes</title>
<para>
- A database role may have a number of attributes that define its
+ A database role can have a number of attributes that define its
privileges and interact with the client authentication system.
<variablelist>
<para>
Functions and triggers allow users to insert code into the backend
- server that other users may execute unintentionally. Hence, both
+ server that other users might execute unintentionally. Hence, both
mechanisms permit users to <quote>Trojan horse</quote>
others with relative ease. The only real protection is tight
control over who can define functions.
-<!-- $PostgreSQL: pgsql/doc/src/sgml/wal.sgml,v 1.42 2006/11/25 22:44:48 petere Exp $ -->
+<!-- $PostgreSQL: pgsql/doc/src/sgml/wal.sgml,v 1.43 2007/01/31 20:56:19 momjian Exp $ -->
<chapter id="wal">
<title>Reliability and the Write-Ahead Log</title>
</para>
<para>
- Next, there may be a cache in the disk drive controller; this is
+ Next, there might be a cache in the disk drive controller; this is
particularly common on <acronym>RAID</> controller cards. Some of
these caches are <firstterm>write-through</>, meaning writes are passed
along to the drive as soon as they arrive. Others are
<para>
<firstterm>Write-Ahead Logging</firstterm> (<acronym>WAL</acronym>)
is a standard approach to transaction logging. Its detailed
- description may be found in most (if not all) books about
+ description can be found in most (if not all) books about
transaction processing. Briefly, <acronym>WAL</acronym>'s central
concept is that changes to data files (where tables and indexes
reside) must be written only after those changes have been logged,
file needs to be flushed to disk at the time of transaction
commit, rather than every data file changed by the transaction.
In multiuser environments, commits of many transactions
- may be accomplished with a single <function>fsync</function> of
+ can be accomplished with a single <function>fsync</function> of
the log file. Furthermore, the log file is written sequentially,
and so the cost of syncing the log is much less than the cost of
flushing the data pages. This is especially true for servers
<varname>checkpoint_segments</varname>. Occasional appearance of such
a message is not cause for alarm, but if it appears often then the
checkpoint control parameters should be increased. Bulk operations such
- as large <command>COPY</> transfers may cause a number of such warnings
+ as large <command>COPY</> transfers might cause a number of such warnings
to appear if you have not set <varname>checkpoint_segments</> high
enough.
</para>
is used on every database low level modification (for example, row
insertion) at a time when an exclusive lock is held on affected
data pages, so the operation needs to be as fast as possible. What
- is worse, writing <acronym>WAL</acronym> buffers may also force the
+ is worse, writing <acronym>WAL</acronym> buffers might also force the
creation of a new log segment, which takes even more
time. Normally, <acronym>WAL</acronym> buffers should be written
and flushed by a <function>LogFlush</function> request, which is
made, for the most part, at transaction commit time to ensure that
transaction records are flushed to permanent storage. On systems
- with high log output, <function>LogFlush</function> requests may
+ with high log output, <function>LogFlush</function> requests might
not occur often enough to prevent <function>LogInsert</function>
from having to do writes. On such systems
one should increase the number of <acronym>WAL</acronym> buffers by
compiled with support for it) will result in each
<function>LogInsert</function> and <function>LogFlush</function>
<acronym>WAL</acronym> call being logged to the server log. This
- option may be replaced by a more general mechanism in the future.
+ option might be replaced by a more general mechanism in the future.
</para>
</sect1>
<para>
It is of advantage if the log is located on another disk than the
- main database files. This may be achieved by moving the directory
+ main database files. This can be achieved by moving the directory
<filename>pg_xlog</filename> to another location (while the server
is shut down, of course) and creating a symbolic link from the
original location in the main data directory to the new location.
<para>
The aim of <acronym>WAL</acronym>, to ensure that the log is
- written before database records are altered, may be subverted by
+ written before database records are altered, can be subverted by
disk drives<indexterm><primary>disk drive</></> that falsely report a
successful write to the kernel,
when in fact they have only cached the data and not yet stored it
- on the disk. A power failure in such a situation may still lead to
+ on the disk. A power failure in such a situation might still lead to
irrecoverable data corruption. Administrators should try to ensure
that disks holding <productname>PostgreSQL</productname>'s
<acronym>WAL</acronym> log files do not make such false reports.
-<!-- $PostgreSQL: pgsql/doc/src/sgml/xaggr.sgml,v 1.33 2006/09/16 00:30:16 momjian Exp $ -->
+<!-- $PostgreSQL: pgsql/doc/src/sgml/xaggr.sgml,v 1.34 2007/01/31 20:56:20 momjian Exp $ -->
<sect1 id="xaggr">
<title>User-Defined Aggregates</title>
<para>
Thus, in addition to the argument and result data types seen by a user
of the aggregate, there is an internal state-value data type that
- may be different from both the argument and result types.
+ might be different from both the argument and result types.
</para>
<para>
</para>
<para>
- Aggregate functions may use polymorphic
+ Aggregate functions can use polymorphic
state transition functions or final functions, so that the same functions
can be used to implement multiple aggregates.
See <xref linkend="extend-types-polymorphic">
for an explanation of polymorphic functions.
- Going a step further, the aggregate function itself may be specified
+ Going a step further, the aggregate function itself can be specified
with polymorphic input type(s) and state type, allowing a single
aggregate definition to serve for multiple input data types.
Here is an example of a polymorphic aggregate:
-<!-- $PostgreSQL: pgsql/doc/src/sgml/xfunc.sgml,v 1.122 2007/01/30 22:29:23 momjian Exp $ -->
+<!-- $PostgreSQL: pgsql/doc/src/sgml/xfunc.sgml,v 1.123 2007/01/31 20:56:20 momjian Exp $ -->
<sect1 id="xfunc">
<title>User-Defined Functions</title>
of function can take base types, composite types, or
combinations of these as arguments (parameters). In addition,
every kind of function can return a base type or
- a composite type. Functions may also be defined to return
+ a composite type. Functions can also be defined to return
sets of base or composite values.
</para>
<para>
<indexterm><primary>SETOF</><seealso>function</></> Alternatively,
- an SQL function may be declared to return a set, by specifying the
+ an SQL function can be declared to return a set, by specifying the
function's return type as <literal>SETOF
<replaceable>sometype</></literal>. In this case all rows of the
last query's result are returned. Further details appear below.
body using the syntax <literal>$<replaceable>n</></>: <literal>$1</>
refers to the first argument, <literal>$2</> to the second, and so on.
If an argument is of a composite type, then the dot notation,
- e.g., <literal>$1.name</literal>, may be used to access attributes
+ e.g., <literal>$1.name</literal>, can be used to access attributes
of the argument. The arguments can only be used as data values,
not as identifiers. Thus for example this is reasonable:
<programlisting>
<title><acronym>SQL</acronym> Functions as Table Sources</title>
<para>
- All SQL functions may be used in the <literal>FROM</> clause of a query,
+ All SQL functions can be used in the <literal>FROM</> clause of a query,
but it is particularly useful for functions returning composite types.
If the function is defined to return a base type, the table function
produces a one-column table. If the function is defined to return
</para>
<para>
- Currently, functions returning sets may also be called in the select list
+ Currently, functions returning sets can also be called in the select list
of a query. For each row that the query
generates by itself, the function returning set is invoked, and an output
row is generated for each element of the function's result set. Note,
- however, that this capability is deprecated and may be removed in future
+ however, that this capability is deprecated and might be removed in future
releases. The following is an example function returning a set from the
select list:
<title>Polymorphic <acronym>SQL</acronym> Functions</title>
<para>
- <acronym>SQL</acronym> functions may be declared to accept and
+ <acronym>SQL</acronym> functions can be declared to accept and
return the polymorphic types <type>anyelement</type> and
<type>anyarray</type>. See <xref
linkend="extend-types-polymorphic"> for a more detailed
</indexterm>
<para>
- More than one function may be defined with the same SQL name, so long
+ More than one function can be defined with the same SQL name, so long
as the arguments they take are different. In other words,
function names can be <firstterm>overloaded</firstterm>. When a
query is executed, the server will determine which function to
a lot whether a function is executed once during planning or once during
query execution startup. But there is a big difference if the plan is
saved and reused later. Labeling a function <literal>IMMUTABLE</> when
- it really isn't may allow it to be prematurely folded to a constant during
+ it really isn't might allow it to be prematurely folded to a constant during
planning, resulting in a stale value being re-used during subsequent uses
of the plan. This is a hazard when using prepared statements or when
using function languages that cache plans (such as
</para>
<para>
- On the other hand, fixed-length types of any size may
+ On the other hand, fixed-length types of any size can
be passed by-reference. For example, here is a sample
implementation of a <productname>PostgreSQL</productname> type:
<para>
<emphasis>Never</> modify the contents of a pass-by-reference input
value. If you do so you are likely to corrupt on-disk data, since
- the pointer you are given may well point directly into a disk buffer.
+ the pointer you are given might point directly into a disk buffer.
The sole exception to this rule is explained in
<xref linkend="xaggr">.
</para>
that uses a built-in type of <productname>PostgreSQL</>.
The <quote>Defined In</quote> column gives the header file that
needs to be included to get the type definition. (The actual
- definition may be in a different file that is included by the
+ definition might be in a different file that is included by the
listed file. It is recommended that users stick to the defined
interface.) Note that you should always include
<filename>postgres.h</filename> first in any source file, because
(Better style would be to use just <literal>'funcs'</> in the
<literal>AS</> clause, after having added
<replaceable>DIRECTORY</replaceable> to the search path. In any
- case, we may omit the system-specific extension for a shared
+ case, we can omit the system-specific extension for a shared
library, commonly <literal>.so</literal> or
<literal>.sl</literal>.)
</para>
</para>
<para>
- At first glance, the version-1 coding conventions may appear to
+ At first glance, the version-1 coding conventions might appear to
be just pointless obscurantism. They do, however, offer a number
of improvements, because the macros can hide unnecessary detail.
An example is that in coding <function>add_one_float8</>, we no longer need to
<para>
Before we turn to the more advanced topics, we should discuss
some coding rules for <productname>PostgreSQL</productname>
- C-language functions. While it may be possible to load functions
+ C-language functions. While it might be possible to load functions
written in languages other than C into
<productname>PostgreSQL</productname>, this is usually difficult
(when it is possible at all) because other languages, such as
<function>memset</function>. Without this, it's difficult to
support hash indexes or hash joins, as you must pick out only
the significant bits of your data structure to compute a hash.
- Even if you initialize all fields of your structure, there may be
- alignment padding (holes in the structure) that may contain
+ Even if you initialize all fields of your structure, there might be
+ alignment padding (holes in the structure) that contain
garbage values.
</para>
</listitem>
<para>
Composite types do not have a fixed layout like C structures.
- Instances of a composite type may contain null fields. In
+ Instances of a composite type can contain null fields. In
addition, composite types that are part of an inheritance
- hierarchy may have different fields than other members of the
+ hierarchy can have different fields than other members of the
same inheritance hierarchy. Therefore,
<productname>PostgreSQL</productname> provides a function
interface for accessing fields of composite types from C.
<title>Polymorphic Arguments and Return Types</title>
<para>
- C-language functions may be declared to accept and
+ C-language functions can be declared to accept and
return the polymorphic types
<type>anyelement</type> and <type>anyarray</type>.
See <xref linkend="extend-types-polymorphic"> for a more detailed explanation
<title>Shared Memory and LWLocks</title>
<para>
- Add-ins may reserve LWLocks and an allocation of shared memory on server
+ Add-ins can reserve LWLocks and an allocation of shared memory on server
startup. The add-in's shared library must be preloaded by specifying
it in
<xref linkend="guc-shared-preload-libraries"><indexterm><primary>shared-preload-libraries</></>.
-<!-- $PostgreSQL: pgsql/doc/src/sgml/xindex.sgml,v 1.56 2007/01/23 20:45:28 tgl Exp $ -->
+<!-- $PostgreSQL: pgsql/doc/src/sgml/xindex.sgml,v 1.57 2007/01/31 20:56:20 momjian Exp $ -->
<sect1 id="xindex">
<title>Interfacing Extensions To Indexes</title>
called because one thing they specify is the set of
<literal>WHERE</>-clause operators that can be used with an index
(i.e., can be converted into an index-scan qualification). An
- operator class may also specify some <firstterm>support
+ operator class can also specify some <firstterm>support
procedures</> that are needed by the internal operations of the
index method, but do not directly correspond to any
<literal>WHERE</>-clause operator that can be used with the index.
To handle these needs, <productname>PostgreSQL</productname>
uses the concept of an <firstterm>operator
family</><indexterm><primary>operator family</></indexterm>.
- An operator family contains one or more operator classes, and may also
+ An operator family contains one or more operator classes, and can also
contain indexable operators and corresponding support functions that
belong to the family as a whole but not to any single class within the
family. We say that such operators and functions are <quote>loose</>
Consider again the situation where we are storing in the index only
the bounding box of a complex object such as a polygon. In this
case there's not much value in storing the whole polygon in the index
- entry — we may as well store just a simpler object of type
+ entry — we might as well store just a simpler object of type
<type>box</>. This situation is expressed by the <literal>STORAGE</>
option in <command>CREATE OPERATOR CLASS</>: we'd write something like
-<!-- $PostgreSQL: pgsql/doc/src/sgml/xoper.sgml,v 1.38 2007/01/20 20:45:38 tgl Exp $ -->
+<!-- $PostgreSQL: pgsql/doc/src/sgml/xoper.sgml,v 1.39 2007/01/31 20:56:20 momjian Exp $ -->
<sect1 id="xoper">
<title>User-Defined Operators</title>
This assists the optimizer by
giving it some idea of how many rows will be eliminated by <literal>WHERE</>
clauses that have this form. (What happens if the constant is on
- the left, you may be wondering? Well, that's one of the things that
+ the left, you might be wondering? Well, that's one of the things that
<literal>COMMUTATOR</> is for...)
</para>
<para>
There are additional selectivity estimation functions designed for geometric
operators in <filename>src/backend/utils/adt/geo_selfuncs.c</filename>: <function>areasel</function>, <function>positionsel</function>,
- and <function>contsel</function>. At this writing these are just stubs, but you may want
+ and <function>contsel</function>. At this writing these are just stubs, but you might want
to use them (or even better, improve them) anyway.
</para>
</sect2>
<para>
Care should be exercised when preparing a hash function, because there
are machine-dependent ways in which it might fail to do the right thing.
- For example, if your data type is a structure in which there may be
+ For example, if your data type is a structure in which there might be
uninteresting pad bits, you can't simply pass the whole structure to
<function>hash_any</>. (Unless you write your other operators and
functions to ensure that the unused bits are always zero, which is the
strict, the
function must also be complete: that is, it should return true or
false, never null, for any two nonnull inputs. If this rule is
- not followed, hash-optimization of <literal>IN</> operations may
+ not followed, hash-optimization of <literal>IN</> operations might
generate wrong results. (Specifically, <literal>IN</> might return
false where the correct answer according to the standard would be null;
or it might yield an error complaining that it wasn't prepared for a
A merge-joinable operator must have a commutator (itself if the two
operand data types are the same, or a related equality operator
if they are different) that appears in the same operator family.
- If this is not the case, planner errors may occur when the operator
+ If this is not the case, planner errors might occur when the operator
is used. Also, it is a good idea (but not strictly required) for
a btree operator family that supports multiple datatypes to provide
equality operators for every combination of the datatypes; this
-<!-- $PostgreSQL: pgsql/doc/src/sgml/xplang.sgml,v 1.32 2006/11/20 17:42:16 neilc Exp $ -->
+<!-- $PostgreSQL: pgsql/doc/src/sgml/xplang.sgml,v 1.33 2007/01/31 20:56:20 momjian Exp $ -->
<chapter id="xplang">
<title id="xplang-title">Procedural Languages</title>
only necessary to execute <command>CREATE LANGUAGE</>
<replaceable>language_name</> to install the language into the
current database. Alternatively, the program <xref
- linkend="app-createlang"> may be used to do this from the shell
+ linkend="app-createlang"> can be used to do this from the shell
command line. For example, to install the language
<application>PL/pgSQL</application> into the database
<literal>template1</>, use
<step performance="optional" id="xplang-install-cr3">
<para>
- Optionally, the language handler may provide a <quote>validator</>
+ Optionally, the language handler can provide a <quote>validator</>
function that checks a function definition for correctness without
actually executing it. The validator function is called by
<command>CREATE FUNCTION</> if it exists. If a validator function
*
*
* IDENTIFICATION
- * $PostgreSQL: pgsql/src/backend/parser/parse_func.c,v 1.191 2007/01/05 22:19:34 momjian Exp $
+ * $PostgreSQL: pgsql/src/backend/parser/parse_func.c,v 1.192 2007/01/31 20:56:20 momjian Exp $
*
*-------------------------------------------------------------------------
*/
func_signature_string(funcname, nargs,
actual_arg_types)),
errhint("Could not choose a best candidate function. "
- "You may need to add explicit type casts."),
+ "You might need to add explicit type casts."),
parser_errposition(pstate, location)));
else
ereport(ERROR,
func_signature_string(funcname, nargs,
actual_arg_types)),
errhint("No function matches the given name and argument types. "
- "You may need to add explicit type casts."),
+ "You might need to add explicit type casts."),
parser_errposition(pstate, location)));
}
*
*
* IDENTIFICATION
- * $PostgreSQL: pgsql/src/backend/parser/parse_oper.c,v 1.91 2007/01/05 22:19:34 momjian Exp $
+ * $PostgreSQL: pgsql/src/backend/parser/parse_oper.c,v 1.92 2007/01/31 20:56:20 momjian Exp $
*
*-------------------------------------------------------------------------
*/
errmsg("operator is not unique: %s",
op_signature_string(op, oprkind, arg1, arg2)),
errhint("Could not choose a best candidate operator. "
- "You may need to add explicit type casts."),
+ "You might need to add explicit type casts."),
parser_errposition(pstate, location)));
else
ereport(ERROR,
errmsg("operator does not exist: %s",
op_signature_string(op, oprkind, arg1, arg2)),
errhint("No operator matches the given name and argument type(s). "
- "You may need to add explicit type casts."),
+ "You might need to add explicit type casts."),
parser_errposition(pstate, location)));
}
projects / postgresql / commitdiff