Frequently Asked Questions (FAQ) for PostgreSQL
- Last updated: Fri Oct 11 13:54:56 EDT 2002
+ Last updated: Sun Oct 13 22:49:56 EDT 2002
Current maintainer: Bruce Momjian (pgman@candle.pha.pa.us)
General Questions
- 1.1) What is PostgreSQL?
+ 1.1) What is PostgreSQL? How is it pronounced?
PostgreSQL is pronounced Post-Gres-Q-L.
replaces the PostQuel query language with an extended subset of SQL.
PostgreSQL is free and the complete source is available.
- PostgreSQL development is performed by a team of Internet developers
- who all subscribe to the PostgreSQL development mailing list. The
- current coordinator is Marc G. Fournier (scrappy@PostgreSQL.org). (See
- below on how to join). This team is now responsible for all
- development of PostgreSQL.
+ PostgreSQL development is performed by a team of developers who all
+ subscribe to the PostgreSQL development mailing list. The current
+ coordinator is Marc G. Fournier (scrappy@PostgreSQL.org). (See section
+ 1.6 on how to join). This team is now responsible for all development
+ of PostgreSQL.
The authors of PostgreSQL 1.01 were Andrew Yu and Jolly Chen. Many
others have contributed to the porting, testing, debugging, and
Client
It is possible to compile the libpq C library, psql, and other
- interfaces and binaries to run on MS Windows platforms. In this case,
- the client is running on MS Windows, and communicates via TCP/IP to a
- server running on one of our supported Unix platforms. A file
- win31.mak is included in the distribution for making a Win32 libpq
- library and psql. PostgreSQL also communicates with ODBC clients.
+ interfaces and client applications to run on MS Windows platforms. In
+ this case, the client is running on MS Windows, and communicates via
+ TCP/IP to a server running on one of our supported Unix platforms. A
+ file win32.mak is included in the distribution for making a Win32
+ libpq library and psql. PostgreSQL also communicates with ODBC
+ clients.
Server
The database server can run on Windows NT and Win2k using Cygwin, the
Cygnus Unix/NT porting library. See pgsql/doc/FAQ_MSWIN in the
- distribution or the MS Windows FAQ on our web site. We have no plan to
- do a native port to any Microsoft platform.
+ distribution or the MS Windows FAQ at
+ http://www.PostgreSQL.org/docs/faq-mswin.html.
+
+ A native port to some Microsoft platforms is currently being worked
+ upon.
1.5) Where can I get PostgreSQL?
Unix command irc -c '#PostgreSQL' "$USER" irc.phoenix.net.
A list of commercial support companies is available at
- http://www.postgresql.org/users-lounge/commercial-support.html.
+ http://www.PostgreSQL.org/users-lounge/commercial-support.html.
1.7) What is the latest release?
There are two PostgreSQL books available online at
http://www.PostgreSQL.org/docs/awbook.html and
http://www.commandprompt.com/ppbook/. There is a list of PostgreSQL
- books available for purchase at http://www.postgresql.org/books/.
+ books available for purchase at http://www.ca.PostgreSQL.org/books/.
There is also a collection of PostgreSQL technical articles at
- http://techdocs.postgresql.org/.
+ http://techdocs.PostgreSQL.org/.
psql has some nice \d commands to show information about types,
operators, functions, aggregates, etc.
1.13) How do I submit a bug report?
- Please visit the PostgreSQL BugTool page, which gives guidelines and
- directions on how to submit a bug.
+ Please visit the PostgreSQL BugTool page at
+ http://www.PostgreSQL.org/bugs/bugs.php, which gives guidelines and
+ directions on how to submit a bug report.
Also check out our ftp site ftp://ftp.PostgreSQL.org/pub to see if
there is a more recent PostgreSQL version or patches.
compare favorably to other database software in this area.
Support
- Our mailing list provides a large group of developers and users
- to help resolve any problems encountered. While we can not
- guarantee a fix, commercial DBMSs do not always supply a fix
- either. Direct access to developers, the user community,
- manuals, and the source code often make PostgreSQL support
- superior to other DBMSs. There is commercial per-incident
- support available for those who need it. (See support FAQ
- item.)
+ Our mailing lists provide contact with a large group of
+ developers and users to help resolve any problems encountered.
+ While we cannot guarantee a fix, commercial DBMSs do not always
+ supply a fix either. Direct access to developers, the user
+ community, manuals, and the source code often make PostgreSQL
+ support superior to other DBMSs. There is commercial
+ per-incident support available for those who need it. (See FAQ
+ section 1.6.)
Price
We are free for all use, both commercial and non-commercial.
1.15) How can I financially assist PostgreSQL?
- PostgreSQL has had a first-class infrastructure since we started six
- years ago. This is all thanks to Marc Fournier, who has created and
- managed this infrastructure over the years.
+ PostgreSQL has had a first-class infrastructure since we started in
+ 1994. This is all thanks to Marc Fournier, who has created and managed
+ this infrastructure over the years.
Quality infrastructure is very important to an open-source project. It
prevents disruptions that can greatly delay forward movement of the
Of course, this infrastructure is not cheap. There are a variety of
monthly and one-time expenses that are required to keep it going. If
you or your company has money it can donate to help fund this effort,
- please go to https://store.pgsql.com/shopping/index.php?id=1 and make
- a donation.
+ please go to http://store.pgsql.com/shopping/ and make a donation.
Although the web page mentions PostgreSQL, Inc, the "contributions"
item is solely to support the PostgreSQL project and does not fund any
A nice introduction to Database-backed Web pages can be seen at:
http://www.webreview.com
- There is also one at http://www.phone.net/home/mwm/hotlist/.
-
For Web integration, PHP is an excellent interface. It is at
http://www.php.net.
- For complex cases, many use the Perl interface and CGI.pm.
+ For complex cases, many use the Perl interface and CGI.pm or mod_perl.
2.3) Does PostgreSQL have a graphical user interface? A report generator?
An embedded query language interface?
- We have a nice graphical user interface called pgaccess, which is
- shipped as part of the distribution. pgaccess also has a report
- generator. The Web page is http://www.flex.ro/pgaccess
+ We have a nice graphical user interface called PgAccess, which is
+ shipped as part of the distribution. PgAccess also has a report
+ generator. The Web page is http://www.pgaccess.org/.
We also include ecpg, which is an embedded SQL query language
interface for C.
- 2.4) What languages are available to communicate with PostgreSQL?
+ 2.4) What languages are able to communicate with PostgreSQL?
We have:
* C (libpq)
* C++ (libpq++)
* Embedded C (ecpg)
* Java (jdbc)
- * Perl (perl5)
+ * Perl (DBD::Pg)
* ODBC (odbc)
* Python (PyGreSQL)
* TCL (libpgtcl)
* C Easy API (libpgeasy)
- * Embedded HTML (PHP from http://www.php.net)
+ * PHP ('pg_' functions, Pear::DB)
Additional interfaces are available at
- http://www.postgresql.org/interfaces.html.
+ http://www.PostgreSQL.org/interfaces.html.
_________________________________________________________________
Administrative Questions
was 64, and changing it required a rebuild after altering the
MaxBackendId constant in include/storage/sinvaladt.h.
- 3.9) What are the pg_sorttempNNN.NN files in my database directory?
+ 3.9) What are the pg_tempNNN.NN files in my database directory?
They are temporary files generated by the query executor. For example,
if a sort needs to be done to satisfy an ORDER BY, and the sort
The PostgreSQL team makes only small changes between minor releases,
so upgrading from 7.2 to 7.2.1 does not require a dump and restore.
- However, major releases often change the internal format of system
- tables and data files. These changes are often complex, so we don't
- maintain backward compatability for data files. A dump outputs data in
- a generic format that can then be loaded in using the new internal
- format.
+ However, major releases (e.g. from 7.2 to 7.3) often change the
+ internal format of system tables and data files. These changes are
+ often complex, so we don't maintain backward compatability for data
+ files. A dump outputs data in a generic format that can then be loaded
+ in using the new internal format.
In releases where the on-disk format does not change, the pg_upgrade
script can be used to upgrade without a dump/restore. The release
See the FETCH manual page, or use SELECT ... LIMIT....
The entire query may have to be evaluated, even if you only want the
- first few rows. Consider a query that has an ORDER BY. If there is an
- index that matches the ORDER BY, PostgreSQL may be able to evaluate
- only the first few records requested, or the entire query may have to
- be evaluated until the desired rows have been generated.
+ first few rows. Consider using a query that has an ORDER BY. If there
+ is an index that matches the ORDER BY, PostgreSQL may be able to
+ evaluate only the first few records requested, or the entire query may
+ have to be evaluated until the desired rows have been generated.
4.3) How do I get a list of tables or other things I can see in psql?
4.4) How do you remove a column from a table?
- We do not support ALTER TABLE DROP COLUMN, but do this:
+ Prior to version 7.3, ALTER TABLE DROP COLUMN is not supported. You
+ can do this instead:
BEGIN;
LOCK TABLE old_table;
SELECT ... -- select all columns but the one you want to remove
PostgreSQL database file containing this data can be estimated as 6.4
MB:
36 bytes: each row header (approximate)
- 24 bytes: one int field and one text filed
+ 24 bytes: one int field and one text field
+ 4 bytes: pointer on page to tuple
----------------------------------------
64 bytes per row
SELECT col
FROM tab
ORDER BY col [ DESC ]
- LIMIT 1
+ LIMIT 1;
When using wild-card operators such as LIKE or ~, indexes can only be
used in certain circumstances:
The search string can not start with a character class, e.g. [a-e].
- Case-insensitive searches like ILIKE and ~* can not be used.
- Instead, use functional indexes, which are described later in this
- FAQ.
+ Case-insensitive searches such as ILIKE and ~* do not utilise
+ indexes. Instead, use functional indexes, which are described in
+ section 4.12.
- The default C local must be used during initdb.
+ The default C locale must be used during initdb.
4.9) How do I see how the query optimizer is evaluating my query?
Case-insensitive equality comparisons are normally expressed as:
SELECT *
FROM tab
- WHERE lower(col) = 'abc'
+ WHERE LOWER(col) = 'abc';
This will not use an standard index. However, if you create a
functional index, it will be used:
- CREATE INDEX tabindex on tab (lower(col));
+ CREATE INDEX tabindex ON tab (LOWER(col));
4.13) In a query, how do I detect if a field is NULL?
Type Internal Name Notes
--------------------------------------------------
"char" char 1 character
-CHAR(#) bpchar blank padded to the specified fixed length
-VARCHAR(#) varchar size specifies maximum length, no padding
+CHAR(n) bpchar blank padded to the specified fixed length
+VARCHAR(n) varchar size specifies maximum length, no padding
TEXT text no specific upper limit on length
BYTEA bytea variable-length byte array (null-byte safe)
data types are also subject to compression or being stored out-of-line
by TOAST, so the space on disk might also be less than expected.
- CHAR() is best when storing strings that are usually the same length.
- VARCHAR() is best when storing variable-length strings but it limits
+ CHAR(n) is best when storing strings that are usually the same length.
+ VARCHAR(n) is best when storing variable-length strings but it limits
how long a string can be. TEXT is for strings of unlimited length,
maximum 1 gigabyte. BYTEA is for storing binary data, particularly
values that include NULL bytes.
One approach is to retrieve the next SERIAL value from the sequence
object with the nextval() function before inserting and then insert it
- explicitly. Using the example table in 4.15.1, that might look like
- this in Perl:
- new_id = output of "SELECT nextval('person_id_seq')"
- INSERT INTO person (id, name) VALUES (new_id, 'Blaise Pascal');
+ explicitly. Using the example table in 4.15.1, an example in a
+ pseudo-language would look like this:
+ new_id = execute("SELECT nextval('person_id_seq')");
+ execute("INSERT INTO person (id, name) VALUES (new_id, 'Blaise Pascal')");
You would then also have the new value stored in new_id for use in
other queries (e.g., as a foreign key to the person table). Note that
Alternatively, you could retrieve the assigned SERIAL value with the
currval() function after it was inserted by default, e.g.,
- INSERT INTO person (name) VALUES ('Blaise Pascal');
- new_id = output of "SELECT currval('person_id_seq')";
+ execute("INSERT INTO person (name) VALUES ('Blaise Pascal')");
+ new_id = execute("SELECT currval('person_id_seq')");
Finally, you could use the OID returned from the INSERT statement to
look up the default value, though this is probably the least portable
4.15.3) Don't currval() and nextval() lead to a race condition with other
users?
- No. Currval() returns the current value assigned by your backend, not
+ No. currval() returns the current value assigned by your backend, not
by all users.
4.15.4) Why aren't my sequence numbers reused on transaction abort? Why are
OIDs are PostgreSQL's answer to unique row ids. Every row that is
created in PostgreSQL gets a unique OID. All OIDs generated during
- initdb are less than 16384 (from backend/access/transam.h). All
+ initdb are less than 16384 (from include/access/transam.h). All
user-created OIDs are equal to or greater than this. By default, all
these OIDs are unique not only within a table or database, but unique
within the entire PostgreSQL installation.
* range variable, table name, table alias
A list of general database terms can be found at:
- http://www.comptechnews.com/~reaster/dbdesign.html
+ http://hea-www.harvard.edu/MST/simul/software/docs/pkgs/pgsql/glossary
+ /glossary.html
4.18) Why do I get the error "ERROR: Memory exhausted in AllocSetAlloc()"?
4.19) How do I tell what PostgreSQL version I am running?
- From psql, type select version();
+ From psql, type SELECT version();
4.20) Why does my large-object operations get "invalid large obj
descriptor"?
If the subquery returns only a few rows and the outer query returns
many rows, IN is fastest. To speed up other queries, replace IN with
EXISTS:
-SELECT *
+ SELECT *
FROM tab
- WHERE col IN (SELECT subcol FROM subtab)
+ WHERE col IN (SELECT subcol FROM subtab);
to:
-SELECT *
+ SELECT *
FROM tab
- WHERE EXISTS (SELECT subcol FROM subtab WHERE subcol = col)
+ WHERE EXISTS (SELECT subcol FROM subtab WHERE subcol = col);
For this to be fast, subcol should be an indexed column. We hope to
fix this limitation in a future release.
4.25) How do I return multiple rows or columns from a function?
You can return result sets from PL/pgSQL functions using refcursors.
- See
- http://developer.postgresql.org/docs/postgres/plpgsql-cursors.html,
+ See http://www.PostgreSQL.org/idocs/index.php?plpgsql-cursors.html,
section 23.7.3.3.
4.26) Why can't I reliably create/drop temporary tables in PL/PgSQL
There are several master/slave replication options available. These
allow only the master to make database changes and the slave can only
do database reads. The bottom of
- http://gborg.postgresql.org/genpage?replication_research lists them. A
+ http://gborg.PostgreSQL.org/genpage?replication_research lists them. A
multi-master replication solution is being worked on at
- http://gborg.postgresql.org/project/pgreplication/projdisplay.php.
+ http://gborg.PostgreSQL.org/project/pgreplication/projdisplay.php.
- 4.27) What encryption options are available?
+ 4.28) What encryption options are available?
* /contrib/pgcrypto contains many encryption functions for use in
SQL queries.
* The only way to encrypt transmission from the client to the server
is by using hostssl in pg_hba.conf.
* Database user passwords are automatically encrypted when stored in
- version 7.3. In previous versions, you must enable
- password_encryption in postgresql.conf.
+ version 7.3. In previous versions, you must enable the option
+ PASSWORD_ENCRYPTION in postgresql.conf.
* The server can run using an encrypted file system.
_________________________________________________________________
alink="#0000ff">
<H1>Frequently Asked Questions (FAQ) for PostgreSQL</H1>
- <P>Last updated: Fri Oct 11 13:54:56 EDT 2002</P>
+ <P>Last updated: Sun Oct 13 22:49:56 EDT 2002</P>
<P>Current maintainer: Bruce Momjian (<A href=
"mailto:pgman@candle.pha.pa.us">pgman@candle.pha.pa.us</A>)<BR>
<P>The most recent version of this document can be viewed at <A
href=
- "http://www.Postgresql.org/docs/faq-english.html">http://www.PostgreSQL.org/docs/faq-english.html</A>.</P>
+ "http://www.PostgreSQL.org/docs/faq-english.html">http://www.PostgreSQL.org/docs/faq-english.html</A>.</P>
<P>Platform-specific questions are answered at <A href=
"http://www.PostgreSQL.org/users-lounge/docs/faq.html">http://www.PostgreSQL.org/users-lounge/docs/faq.html</A>.</P>
<HR>
<H2 align="center">General Questions</H2>
- <A href="#1.1">1.1</A>) What is PostgreSQL? How is it
- pronounced?<BR>
+ <A href="#1.1">1.1</A>) What is PostgreSQL? How is it pronounced?<BR>
<A href="#1.2">1.2</A>) What is the copyright on PostgreSQL?<BR>
<A href="#1.3">1.3</A>) What Unix platforms does PostgreSQL run
on?<BR>
<H2 align="center">General Questions</H2>
- <H4><A name="1.1">1.1</A>) What is PostgreSQL?</H4>
+ <H4><A name="1.1">1.1</A>) What is PostgreSQL? How is it pronounced?</H4>
<P>PostgreSQL is pronounced <I>Post-Gres-Q-L</I>.</P>
extended subset of <SMALL>SQL</SMALL>. PostgreSQL is free and the
complete source is available.</P>
- <P>PostgreSQL development is performed by a team of Internet
+ <P>PostgreSQL development is performed by a team of
developers who all subscribe to the PostgreSQL development mailing
list. The current coordinator is Marc G. Fournier (<A href=
"mailto:scrappy@PostgreSQL.org">scrappy@PostgreSQL.org</A>). (See
- below on how to join). This team is now responsible for all
- development of PostgreSQL.</P>
+ section <a href="#1.6">1.6</a> on how to join). This team is now
+ responsible for all development of PostgreSQL.</P>
<P>The authors of PostgreSQL 1.01 were Andrew Yu and Jolly Chen.
Many others have contributed to the porting, testing, debugging,
<P><STRONG>Client</STRONG></P>
<P>It is possible to compile the <I>libpq</I> C library, psql, and
- other interfaces and binaries to run on MS Windows platforms. In
- this case, the client is running on MS Windows, and communicates
+ other interfaces and client applications to run on MS Windows platforms.
+ In this case, the client is running on MS Windows, and communicates
via TCP/IP to a server running on one of our supported Unix
- platforms. A file <I>win31.mak</I> is included in the distribution
+ platforms. A file <I>win32.mak</I> is included in the distribution
for making a Win32 <I>libpq</I> library and <I>psql</I>. PostgreSQL
also communicates with <SMALL>ODBC</SMALL> clients.</P>
<P>The database server can run on Windows NT and Win2k using
Cygwin, the Cygnus Unix/NT porting library. See
- <I>pgsql/doc/FAQ_MSWIN</I> in the distribution or the <A href=
- "http://www.postgresql.org/docs/faq-mswin.html">MS Windows FAQ</A>
- on our web site. We have no plan to do a native port to any
- Microsoft platform.</P>
+ <I>pgsql/doc/FAQ_MSWIN</I> in the distribution or the MS Windows FAQ
+ at <A href="http://www.PostgreSQL.org/docs/faq-mswin.html">http://www.PostgreSQL.org/docs/faq-mswin.html</A>.</P>
+
+ <p>A native port to some Microsoft platforms is currently being worked
+ upon.</p>
<H4><A name="1.5">1.5</A>) Where can I get PostgreSQL?</H4>
'#PostgreSQL' "$USER" irc.phoenix.net.</CODE></P>
<P>A list of commercial support companies is available at <A href=
- "http://www.postgresql.org/users-lounge/commercial-support.html">http://www.postgresql.org/users-lounge/commercial-support.html</A>.</P>
+ "http://www.PostgreSQL.org/users-lounge/commercial-support.html">http://www.PostgreSQL.org/users-lounge/commercial-support.html</A>.</P>
<H4><A name="1.7">1.7</A>) What is the latest release?</H4>
"http://www.commandprompt.com/ppbook/">http://www.commandprompt.com/ppbook/</A>.
There is a list of PostgreSQL books available for purchase at <A
href=
- "http://www.postgresql.org/books/">http://www.postgresql.org/books/</A>.
+ "http://www.ca.PostgreSQL.org/books/">http://www.ca.PostgreSQL.org/books/</A>.
There is also a collection of PostgreSQL technical articles at <A
href=
- "http://techdocs.postgresql.org/">http://techdocs.postgresql.org/</A>.</P>
+ "http://techdocs.PostgreSQL.org/">http://techdocs.PostgreSQL.org/</A>.</P>
<P><I>psql</I> has some nice \d commands to show information about
types, operators, functions, aggregates, etc.</P>
<H4><A name="1.13">1.13</A>) How do I submit a bug report?</H4>
- <P>Please visit the <A href=
- "http://www.postgresql.org/bugs/bugs.php">PostgreSQL BugTool</A>
- page, which gives guidelines and directions on how to submit a
- bug.</P>
+ <P>Please visit the
PostgreSQL BugTool page at <A href=
+ "http://www.PostgreSQL.org/bugs/bugs.php">http://www.PostgreSQL.org/bugs/bugs.php</A>,
+ which gives guidelines and directions on how to submit a
+ bug report.</P>
<P>Also check out our ftp site <A href=
"ftp://ftp.PostgreSQL.org/pub">ftp://ftp.PostgreSQL.org/pub</A> to
<DT><B>Support</B></DT>
- <DD>Our mailing list provides a large group of developers and
- users to help resolve any problems encountered. While we can not
+ <DD>Our mailing lists provide contact with a large group of developers
+ and users to help resolve any problems encountered. While we cannot
guarantee a fix, commercial <SMALL>DBMS</SMALL>s do not always
supply a fix either. Direct access to developers, the user
community, manuals, and the source code often make PostgreSQL
support superior to other <SMALL>DBMS</SMALL>s. There is
commercial per-incident support available for those who need it.
- (See <A href="#1.6">support FAQ item</A>.)<BR>
+ (See <A href="#1.6">FAQ section 1.6</A>.)<BR>
<BR>
</DD>
PostgreSQL?</H4>
<P>PostgreSQL has had a first-class infrastructure since we started
- six years ago. This is all thanks to Marc Fournier, who has created
+ in 1994. This is all thanks to Marc Fournier, who has created
and managed this infrastructure over the years.</P>
<P>Quality infrastructure is very important to an open-source
<P>Of course, this infrastructure is not cheap. There are a variety
of monthly and one-time expenses that are required to keep it
going. If you or your company has money it can donate to help fund
- this effort, please go to <A href=
- "https://store.pgsql.com/shopping/index.php?id=1">
- https://store.pgsql.com/shopping/index.php?id=1</A>
+ this effort, please go to <A href="http://store.pgsql.com/shopping/">http://store.pgsql.com/shopping/</A>
and make a donation.</P>
<P>Although the web page mentions PostgreSQL, Inc, the
<P>A nice introduction to Database-backed Web pages can be seen at:
<A href="http://www.webreview.com">http://www.webreview.com</A></P>
- <P>There is also one at <A href=
- "http://www.phone.net/home/mwm/hotlist/">http://www.phone.net/home/mwm/hotlist/.</A></P>
-
<P>For Web integration, PHP is an excellent interface. It is at <A
href="http://www.php.net">http://www.php.net</A>.</P>
- <P>For complex cases, many use the Perl interface and CGI.pm.</P>
+ <P>For complex cases, many use the Perl interface and CGI.pm or mod_perl.</P>
<H4><A name="2.3">2.3</A>) Does PostgreSQL have a graphical user
interface? A report generator? An embedded query language
interface?</H4>
- <P>We have a nice graphical user interface called <I>pgaccess</I>,
- which is shipped as part of the distribution. <I>pgaccess</I> also
- has a report generator. The Web page is <A href=
- "http://www.flex.ro/pgaccess">http://www.flex.ro/pgaccess</A></P>
+ <P>We have a nice graphical user interface called PgAccess, which is
+ shipped as part of the distribution. PgAccess also has a report
+ generator. The Web page is <A href="http://www.pgaccess.org/">http://www.pgaccess.org/</A>.</P>
<P>We also include <I>ecpg</I>, which is an embedded SQL query
language interface for C.</P>
- <H4><A name="2.4">2.4</A>) What languages are available to
- communicate with PostgreSQL?</H4>
+ <H4><A name="2.4">2.4</A>) What languages are able to communicate with PostgreSQL?</H4>
<P>We have:</P>
<LI>Java (jdbc)</LI>
- <LI>Perl (perl5)</LI>
+ <LI>Perl (DBD::Pg)</LI>
<LI>ODBC (odbc)</LI>
<LI>C Easy API (libpgeasy)</LI>
- <LI>Embedded <SMALL>HTML</SMALL> (<A href=
- "http://www.php.net">PHP from http://www.php.net</A>)</LI>
+ <LI>PHP ('pg_' functions, Pear::DB)</LI>
</UL>
- <P>Additional interfaces are available at <a
- href="http://www.postgresql.org/interfaces.html">
- http://www.postgresql.org/interfaces.html.</A>
+ <P>Additional interfaces are available at
+ <a href="http://www.PostgreSQL.org/interfaces.html">http://www.PostgreSQL.org/interfaces.html</A>.
</P>
<HR>
the MaxBackendId constant in
<I>include/storage/sinvaladt.h</I>.</P>
- <H4><A name="3.9">3.9</A>) What are the <I>pg_sorttempNNN.NN</I>
+ <H4><A name="3.9">3.9</A>) What are the <I>pg_tempNNN.NN</I>
files in my database directory?</H4>
<P>They are temporary files generated by the query executor. For
<P>The PostgreSQL team makes only small changes between minor releases,
so upgrading from 7.2 to 7.2.1 does not require a dump and restore.
- However, major releases often change the internal format of system
- tables and data files. These changes are often complex, so we don't
- maintain backward compatability for data files. A dump outputs data
- in a generic format that can then be loaded in using the new internal
- format.
+ However, major releases (e.g. from 7.2 to 7.3) often change the internal
+ format of system tables and data files. These changes are often complex,
+ so we don't maintain backward compatability for data files. A dump outputs
+ data in a generic format that can then be loaded in using the new internal
+ format.</P>
- <P>In releases where the on-disk format does not change, the
+ <p>In releases where the on-disk format does not change, the
<i>pg_upgrade</i> script can be used to upgrade without a dump/restore.
The release notes mention whether <i>pg_upgrade</i> is available for the
- release.
+ release.</p>
<HR>
<SMALL>SELECT</SMALL> ... <SMALL>LIMIT</SMALL>....</P>
<P>The entire query may have to be evaluated, even if you only want
- the first few rows. Consider a query that has an <SMALL>ORDER
+ the first few rows. Consider using a query that has an <SMALL>ORDER
BY.</SMALL> If there is an index that matches the <SMALL>ORDER
BY</SMALL>, PostgreSQL may be able to evaluate only the first few
records requested, or the entire query may have to be evaluated
<H4><A name="4.4">4.4</A>) How do you remove a column from a
table?</H4>
- <P>We do not support <SMALL>ALTER TABLE DROP COLUMN,</SMALL> but do
- this:</P>
+ <P>Prior to version 7.3, <SMALL>ALTER TABLE DROP COLUMN</SMALL> is not supported.
+ You can do this instead:</P>
<PRE>
BEGIN;
LOCK TABLE old_table;
be estimated as 6.4 MB:</P>
<PRE>
36 bytes: each row header (approximate)
- 24 bytes: one int field and one text filed
+ 24 bytes: one int field and one text field
+ 4 bytes: pointer on page to tuple
----------------------------------------
64 bytes per row
SELECT col
FROM tab
ORDER BY col [ DESC ]
- LIMIT 1
+ LIMIT 1;
</PRE>
<P>When using wild-card operators such as <SMALL>LIKE</SMALL> or
</UL>
<LI>The search string can not start with a character class,
e.g. [a-e].</LI>
- <LI>Case-insensitive searches like <SMALL>ILIKE</SMALL> and
- <I>~*</I> can not be used. Instead, use functional
- indexes, which are described later in this FAQ.</LI>
- <LI>The default <I>C</I> local must be used during
+ <LI>Case-insensitive searches such as <SMALL>ILIKE</SMALL> and
+ <I>~*</I> do not utilise indexes. Instead, use functional
+ indexes, which are described in section <a href="#4.12">4.12</a>.</LI>
+ <LI>The default <I>C</I> locale must be used during
<i>initdb.</i></LI>
</UL>
<P>
<PRE>
SELECT *
FROM tab
- WHERE lower(col) = 'abc'
+ WHERE LOWER(col) = 'abc';
</PRE>
This will not use an standard index. However, if you create a
functional index, it will be used:
<PRE>
- CREATE INDEX tabindex on tab (lower(col));
+ CREATE INDEX tabindex ON tab (LOWER(col));
</PRE>
<H4><A name="4.13">4.13</A>) In a query, how do I detect if a field
Type Internal Name Notes
--------------------------------------------------
"char" char 1 character
-CHAR(#) bpchar blank padded to the specified fixed length
-VARCHAR(#) varchar size specifies maximum length, no padding
+CHAR(n) bpchar blank padded to the specified fixed length
+VARCHAR(n) varchar size specifies maximum length, no padding
TEXT text no specific upper limit on length
BYTEA bytea variable-length byte array (null-byte safe)
</PRE>
stored out-of-line by <SMALL>TOAST</SMALL>, so the space on disk
might also be less than expected.</P>
- <P><SMALL>CHAR()</SMALL> is best when storing strings that are
- usually the same length. <SMALL>VARCHAR()</SMALL> is best when
+ <P><SMALL>CHAR(n)</SMALL> is best when storing strings that are
+ usually the same length. <SMALL>VARCHAR(n)</SMALL> is best when
storing variable-length strings but it limits how long a string can
be. <SMALL>TEXT</SMALL> is for strings of unlimited length, maximum
1 gigabyte. <SMALL>BYTEA</SMALL> is for storing binary data,
<P>One approach is to retrieve the next <SMALL>SERIAL</SMALL> value
from the sequence object with the <I>nextval()</I> function
<I>before</I> inserting and then insert it explicitly. Using the
- example table in <A href="#4.15.1">4.15.1</A>, that might look like
- this in Perl:</P>
+ example table in <A href="#4.15.1">4.15.1</A>, an example in a
+ pseudo-language would look like this:</P>
<PRE>
- new_id = output of "SELECT nextval('person_id_seq')"
- INSERT INTO person (id, name) VALUES (new_id, 'Blaise Pascal');
+ new_id = execute("SELECT nextval('person_id_seq')");
+ execute("INSERT INTO person (id, name) VALUES (new_id, 'Blaise Pascal')");
</PRE>
You would then also have the new value stored in
automatically created <SMALL>SEQUENCE</SMALL> object will be named
<<I>table</I>>_<<I>serialcolumn</I>>_<I>seq</I>, where
<I>table</I> and <I>serialcolumn</I> are the names of your table
- and your <SMALL>SERIAL</SMALL> column, respectively.
+ and your <SMALL>SERIAL</SMALL> column, respectively.
<P>Alternatively, you could retrieve the assigned
<SMALL>SERIAL</SMALL> value with the <I>currval</I>() function
<I>after</I> it was inserted by default, e.g.,</P>
<PRE>
- INSERT INTO person (name) VALUES ('Blaise Pascal');
- new_id = output of "SELECT currval('person_id_seq')";
+ execute("INSERT INTO person (name) VALUES ('Blaise Pascal')");
+ new_id = execute("SELECT currval('person_id_seq')");
</PRE>
Finally, you could use the <A href="#4.16"><SMALL>OID</SMALL></A>
default value, though this is probably the least portable approach.
In Perl, using DBI with Edmund Mergl's DBD::Pg module, the oid
value is made available via <I>$sth->{pg_oid_status}</I> after
- <I>$sth->execute()</I>.
+ <I>$sth->execute()</I>.
<H4><A name="4.15.3">4.15.3</A>) Don't <I>currval()</I> and
<I>nextval()</I> lead to a race condition with other users?</H4>
- <P>No. Currval() returns the current value assigned by your
+ <P>No. <i>currval</i>() returns the current value assigned by your
backend, not by all users.</P>
<H4><A name="4.15.4">4.15.4</A>) Why aren't my sequence numbers
Every row that is created in PostgreSQL gets a unique
<SMALL>OID</SMALL>. All <SMALL>OID</SMALL>s generated during
<I>initdb</I> are less than 16384 (from
- <I>backend/access/transam.h</I>). All user-created
+ <I>include/access/transam.h</I>). All user-created
<SMALL>OID</SMALL>s are equal to or greater than this. By default,
all these <SMALL>OID</SMALL>s are unique not only within a table or
database, but unique within the entire PostgreSQL installation.</P>
COPY new TO '/tmp/pgtable';
DELETE FROM new;
COPY new WITH OIDS FROM '/tmp/pgtable';
+</PRE>
<!--
CREATE TABLE new_table (mycol int);
INSERT INTO new_table (oid, mycol) SELECT oid, mycol FROM old_table;
-->
-</PRE>
-
<P>O<SMALL>ID</SMALL>s are stored as 4-byte integers, and will
overflow at 4 billion. No one has reported this ever happening, and
we plan to have the limit removed before anyone does.</P>
</UL>
<P>A list of general database terms can be found at: <A href=
- "http://www.comptechnews.com/~reaster/dbdesign.html">http://www.comptechnews.com/~reaster/dbdesign.html</A></P>
+ "http://hea-www.harvard.edu/MST/simul/software/docs/pkgs/pgsql/glossary/glossary.html">http://hea-www.harvard.edu/MST/simul/software/docs/pkgs/pgsql/glossary/glossary.html</A></P>
<H4><A name="4.18">4.18</A>) Why do I get the error <I>"ERROR:
Memory exhausted in AllocSetAlloc()"</I>?</H4>
and all subprocesses created after the command is run. If you are
having a problem with the <SMALL>SQL</SMALL> client because the
backend is returning too much data, try it before starting the
- client.
+ client.
<H4><A name="4.19">4.19</A>) How do I tell what PostgreSQL version
I am running?</H4>
- <P>From <I>psql</I>, type <CODE>select version();</CODE></P>
+ <P>From <I>psql</I>, type <CODE>SELECT version();</CODE></P>
<H4><A name="4.20">4.20</A>) Why does my large-object operations
get <I>"invalid large obj descriptor"</I>?</H4>
returns many rows, <CODE><SMALL>IN</SMALL></CODE> is fastest. To
speed up other queries, replace <CODE>IN</CODE> with
<CODE>EXISTS</CODE>:</P>
-<PRE>
-<CODE>SELECT *
+<PRE> SELECT *
FROM tab
- WHERE col IN (SELECT subcol FROM subtab)
-</CODE>
+ WHERE col IN (SELECT subcol FROM subtab);
</PRE>
- to:
-<PRE>
-<CODE>SELECT *
+ to:
+<PRE> SELECT *
FROM tab
- WHERE EXISTS (SELECT subcol FROM subtab WHERE subcol = col)
-</CODE>
+ WHERE EXISTS (SELECT subcol FROM subtab WHERE subcol = col);
</PRE>
For this to be fast, <CODE>subcol</CODE> should be an indexed column.
- We hope to fix this limitation in a future release.
+ We hope to fix this limitation in a future release.
<H4><A name="4.23">4.23</A>) How do I perform an outer join?</H4>
<P>You can return result sets from PL/pgSQL functions using
<I>refcursors</I>. See <A href=
- "http://developer.postgresql.org/docs/postgres/plpgsql-cursors.html">
- http://developer.postgresql.org/docs/postgres/plpgsql-cursors.html,</A>
+ "http://www.PostgreSQL.org/idocs/index.php?plpgsql-cursors.html">
+ http://www.PostgreSQL.org/idocs/index.php?plpgsql-cursors.html</A>,
section 23.7.3.3.</P>
<H4><A name="4.26">4.26</A>) Why can't I reliably create/drop
<P>There are several master/slave replication options available.
These allow only the master to make database changes and the slave
can only do database reads. The bottom of <a
- href="http://gborg.postgresql.org/genpage?replication_research">
- http://gborg.postgresql.org/genpage?replication_research</a> lists
+ href="http://gborg.PostgreSQL.org/genpage?replication_research">
+ http://gborg.PostgreSQL.org/genpage?replication_research</a> lists
them. A multi-master replication solution is being worked on at <a
- href="http://gborg.postgresql.org/project/pgreplication/projdisplay.
- php">http://gborg.postgresql.org/project/pgreplication/projdisplay.php</a>.</P>
+ href="http://gborg.PostgreSQL.org/project/pgreplication/projdisplay.php">http://gborg.PostgreSQL.org/project/pgreplication/projdisplay.php</a>.</P>
- <H4><A name="4.27">4.27</A>) What encryption options are available?
+ <H4><A name="4.28">4.28</A>) What encryption options are available?
</H4>
<UL>
<LI><I>/contrib/pgcrypto</I> contains many encryption functions for
<LI>The only way to encrypt transmission from the client to the
server is by using <I>hostssl</I> in <I>pg_hba.conf</I>.</LI>
<LI>Database user passwords are automatically encrypted when stored
- in version 7.3. In previous versions, you must enable
- <I>password_encryption</I> in <I>postgresql.conf</I>.</LI>
+ in version 7.3. In previous versions, you must enable the option
+ <i>PASSWORD_ENCRYPTION</i> in <i>postgresql.conf</i>.</LI>
<LI>The server can run using an encrypted file system.</LI>
</UL>
projects / postgresql / commitdiff