Frequently Asked Questions (FAQ) for PostgreSQL
- Last updated: Mon Jan 31 20:41:21 EST 2005
+ Last updated: Mon Jan 31 21:31:39 EST 2005
Current maintainer: Bruce Momjian (pgman@candle.pha.pa.us)
4.11.3) Why aren't my sequence numbers reused on transaction abort?
Why are there gaps in the numbering of my sequence/SERIAL column?
4.12) What is an OID? What is a TID?
- 4.13) What is the meaning of some of the terms used in PostgreSQL?
- 4.14) Why do I get the error "ERROR: Memory exhausted in
+ 4.13) Why do I get the error "ERROR: Memory exhausted in
AllocSetAlloc()"?
- 4.15) How do I tell what PostgreSQL version I am running?
- 4.16) Why does my large-object operations get "invalid large obj
+ 4.14) How do I tell what PostgreSQL version I am running?
+ 4.15) Why does my large-object operations get "invalid large obj
descriptor"?
- 4.17) How do I create a column that will default to the current time?
- 4.18) How do I perform an outer join?
- 4.19) How do I perform queries using multiple databases?
- 4.20) How do I return multiple rows or columns from a function?
- 4.21) Why can't I reliably create/drop temporary tables in PL/PgSQL
+ 4.16) How do I create a column that will default to the current time?
+ 4.17) How do I perform an outer join?
+ 4.18) How do I perform queries using multiple databases?
+ 4.19) How do I return multiple rows or columns from a function?
+ 4.20) Why can't I reliably create/drop temporary tables in PL/PgSQL
functions?
- 4.22) What encryption options are available?
+ 4.21) What encryption options are available?
Extending PostgreSQL
properly. First, by running configure with the --enable-cassert
option, many assert()s monitor the progress of the backend and halt
the program when something unexpected occurs.
+
The postmaster has a -d option that allows even more detailed
information to be reported. The -d option takes a number that
specifies the debug level. Be warned that high debug level values
4.4) What is the maximum size for a row, a table, and a database?
These are the limits:
- Maximum size for a database? unlimited (32 TB databases exist)
- Maximum size for a table? 32 TB
- Maximum size for a row? 1.6TB
- Maximum size for a field? 1 GB
- Maximum number of rows in a table? unlimited
- Maximum number of columns in a table? 250-1600 depending on column types
- Maximum number of indexes on a table? unlimited
-
+
+ Maximum size for a database? unlimited (32 TB databases exist)
+ Maximum size for a table? 32 TB
+ Maximum size for a row? 1.6TB
+ Maximum size for a field? 1 GB
+ Maximum number of rows in a table? unlimited
+ Maximum number of columns in a table? 250-1600 depending on column
+ types
+ Maximum number of indexes on a table? unlimited
+
Of course, these are not actually unlimited, but limited to available
disk space and memory/swap space. Performance may suffer when these
values get unusually large.
* The search string can not start with a character class, e.g.
[a-e].
* Case-insensitive searches such as ILIKE and ~* do not utilize
- indexes. Instead, use functional indexes, which are described in
- section 4.10.
+ indexes. Instead, use expression indexes, which are described in
+ section 4.8.
* The default C locale must be used during initdb because it is not
- possible to know the next-greater character in a non-C locale. You
- can create a special text_pattern_ops index for such cases that
- work only for LIKE indexing.
+ possible to know the next-greatest character in a non-C locale.
+ You can create a special text_pattern_ops index for such cases
+ that work only for LIKE indexing.
In pre-8.0 releases, indexes often can not be used unless the data
- types exactly match the index's column types. This is particularly
+ types exactly match the index's column types. This was particularly
true of int2, int8, and numeric column indexes.
4.7) How do I see how the query optimizer is evaluating my query?
WHERE lower(col) = 'abc';
This will not use an standard index. However, if you create a
- functional index, it will be used:
+ expresssion index, it will be used:
CREATE INDEX tabindex ON tab (lower(col));
4.9) In a query, how do I detect if a field is NULL?
4.10) What is the difference between the various character types?
-Type Internal Name Notes
---------------------------------------------------
-VARCHAR(n) varchar size specifies maximum length, no padding
-CHAR(n) bpchar blank padded to the specified fixed length
-TEXT text no specific upper limit on length
-BYTEA bytea variable-length byte array (null-byte safe)
-"char" char one character
-
+ Type Internal Name Notes
+ VARCHAR(n) varchar size specifies maximum length, no padding
+ CHAR(n) bpchar blank padded to the specified fixed length
+ TEXT text no specific upper limit on length
+ BYTEA bytea variable-length byte array (null-byte safe)
+ "char" char one character
+
You will see the internal name when examining system catalogs and in
some error messages.
);
See the create_sequence manual page for more information about
- sequences. You can also use each row's OID field as a unique value.
- However, if you need to dump and reload the database, you need to use
- pg_dump's -o option or COPY WITH OIDS option to preserve the OIDs.
+ sequences.
4.11.2) How do I get the value of a SERIAL insert?
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
- approach, and the oid value will wrap around when it reaches 4
- billion. In Perl, using DBI with the DBD::Pg module, the oid value is
- made available via $sth->{pg_oid_status} after $sth->execute().
-
4.11.3) Doesn't currval() lead to a race condition with other users?
- No. currval() returns the current value assigned by your backend, not
- by all users.
+ No. currval() returns the current value assigned by your session, not
+ by all sessions.
4.11.4) Why aren't my sequence numbers reused on transaction abort? Why are
there gaps in the numbering of my sequence/SERIAL column?
values. TIDs change after rows are modified or reloaded. They are used
by index entries to point to physical rows.
- 4.13) What is the meaning of some of the terms used in PostgreSQL?
-
- Some of the source code and older documentation use terms that have
- more common usage. Here are some:
- * table, relation, class
- * row, record, tuple
- * column, field, attribute
- * retrieve, select
- * replace, update
- * append, insert
- * OID, serial value
- * portal, cursor
- * range variable, table name, table alias
-
- A list of general database terms can be found at:
- http://hea-www.harvard.edu/MST/simul/software/docs/pkgs/pgsql/glossary
- /glossary.html
-
- 4.14) Why do I get the error "ERROR: Memory exhausted in AllocSetAlloc()"?
+ 4.13) Why do I get the error "ERROR: Memory exhausted in AllocSetAlloc()"?
You probably have run out of virtual memory on your system, or your
kernel has a low limit for certain resources. Try this before starting
problem with the SQL client because the backend is returning too much
data, try it before starting the client.
- 4.15) How do I tell what PostgreSQL version I am running?
+ 4.14) How do I tell what PostgreSQL version I am running?
From psql, type SELECT version();
- 4.16) Why does my large-object operations get "invalid large obj
+ 4.15) Why does my large-object operations get "invalid large obj
descriptor"?
You need to put BEGIN WORK and COMMIT around any use of a large object
If you are using a client interface like ODBC you may need to set
auto-commit off.
- 4.17) How do I create a column that will default to the current time?
+ 4.16) How do I create a column that will default to the current time?
Use CURRENT_TIMESTAMP:
CREATE TABLE test (x int, modtime timestamp DEFAULT CURRENT_TIMESTAMP );
- 4.18) How do I perform an outer join?
+ 4.17) How do I perform an outer join?
PostgreSQL supports outer joins using the SQL standard syntax. Here
are two examples:
WHERE tab1.col1 NOT IN (SELECT tab2.col1 FROM tab2)
ORDER BY col1
- 4.19) How do I perform queries using multiple databases?
+ 4.18) How do I perform queries using multiple databases?
There is no way to query a database other than the current one.
Because PostgreSQL loads database-specific system catalogs, it is
course, a client can make simultaneous connections to different
databases and merge the results on the client side.
- 4.20) How do I return multiple rows or columns from a function?
+ 4.19) How do I return multiple rows or columns from a function?
In 7.3, you can easily return multiple rows or columns from a
function, http://techdocs.postgresql.org/guides/SetReturningFunctions.
- 4.21) Why can't I reliably create/drop temporary tables in PL/PgSQL
+ 4.20) Why can't I reliably create/drop temporary tables in PL/PgSQL
functions?
PL/PgSQL caches function contents, and an unfortunate side effect is
table access in PL/PgSQL. This will cause the query to be reparsed
every time.
- 4.22) What encryption options are available?
+ 4.21) What encryption options are available?
* contrib/pgcrypto contains many encryption functions for use in SQL
queries.
alink="#0000ff">
<H1>Frequently Asked Questions (FAQ) for PostgreSQL</H1>
- <P>Last updated: Mon Jan 31 20:41:21 EST 2005</P>
+ <P>Last updated: Mon Jan 31 21:31:39 EST 2005</P>
<P>Current maintainer: Bruce Momjian (<A href=
"mailto:pgman@candle.pha.pa.us">pgman@candle.pha.pa.us</A>)
my sequence/SERIAL column?<BR>
<A href="#4.12">4.12</A>) What is an <SMALL>OID</SMALL>? What is a
<SMALL>TID</SMALL>?<BR>
- <A href="#4.13">4.13</A>) What is the meaning of some of the terms
- used in PostgreSQL?<BR>
- <A href="#4.14">4.14</A>) Why do I get the error <I>"ERROR: Memory
+ <A href="#4.12">4.13</A>) Why do I get the error <I>"ERROR: Memory
exhausted in AllocSetAlloc()"</I>?<BR>
- <A href="#4.15">4.15</A>) How do I tell what PostgreSQL version I
+ <A href="#4.14">4.14</A>) How do I tell what PostgreSQL version I
am running?<BR>
- <A href="#4.16">4.16</A>) Why does my large-object operations get
+ <A href="#4.15">4.15</A>) Why does my large-object operations get
<I>"invalid large obj descriptor"</I>?<BR>
- <A href="#4.17">4.17</A>) How do I create a column that will
+ <A href="#4.16">4.16</A>) How do I create a column that will
default to the current time?<BR>
- <A href="#4.18">4.18</A>) How do I perform an outer join?<BR>
- <A href="#4.19">4.19</A>) How do I perform queries using multiple
+ <A href="#4.17">4.17</A>) How do I perform an outer join?<BR>
+ <A href="#4.18">4.18</A>) How do I perform queries using multiple
databases?<BR>
- <A href="#4.20">4.20</A>) How do I return multiple rows or columns
+ <A href="#4.19">4.19</A>) How do I return multiple rows or columns
from a function?<BR>
- <A href="#4.21">4.21</A>) Why can't I reliably create/drop
+ <A href="#4.20">4.20</A>) Why can't I reliably create/drop
temporary tables in PL/PgSQL functions?<BR>
- <A href="#4.22">4.22</A>) What encryption options are available?<BR>
+ <A href="#4.21">4.21</A>) What encryption options are available?<BR>
<H2 align="center">Extending PostgreSQL</H2>
option, many <I>assert()</I>s monitor the progress of the backend
and halt the program when something unexpected occurs.</P>
- The <I>postmaster</I> has a <I>-d</I> option that allows even more
+ <P>The <I>postmaster</I> has a <I>-d</I> option that allows even more
detailed information to be reported. The <I>-d</I> option takes a
number that specifies the debug level. Be warned that high debug
level values generate large log files.</P>
<H4><A name="3.5">3.5</A>) Why do I get <I>"Sorry, too many
clients"</I> when trying to connect?</H4>
- You have reached the default limit is 100 database sessions. You
+ <P>You have reached the default limit is 100 database sessions. You
need to increase the <I>postmaster</I>'s limit on how many
concurrent backend processes it can start by changing the
<I>max_connections</I> value in <I>postgresql.conf</I> and
table, and a database?</H4>
<P>These are the limits:</P>
-<PRE>
- Maximum size for a database? unlimited (32 TB databases exist)
- Maximum size for a table? 32 TB
- Maximum size for a row? 1.6TB
- Maximum size for a field? 1 GB
- Maximum number of rows in a table? unlimited
- Maximum number of columns in a table? 250-1600 depending on column types
- Maximum number of indexes on a table? unlimited
-</PRE>
-
- Of course, these are not actually unlimited, but limited to
+<CENTER>
+<TABLE BORDER=1>
+<TR><TD>Maximum size for a database?</TD><TD>unlimited (32 TB databases
+exist)</TD></TR>
+<TR><TD>Maximum size for a table?</TD><TD>32 TB</TD></TR>
+<TR><TD>Maximum size for a row?</TD><TD>1.6TB</TD></TR>
+<TR><TD>Maximum size for a field?</TD><TD>1 GB</TD></TR>
+<TR><TD>Maximum number of rows in a table?</TD><TD>unlimited</TD></TR>
+<TR><TD>Maximum number of columns in a table?</TD><TD>250-1600 depending
+on column types</TD></TR>
+<TR><TD>Maximum number of indexes on a
+table?</TD><TD>unlimited</TD></TR>
+</TABLE>
+</CENTER>
+<BR>
+
+ <P>Of course, these are not actually unlimited, but limited to
available disk space and memory/swap space. Performance may suffer
- when these values get unusually large.
+ when these values get unusually large.</P>
<P>The maximum table size of 32 TB does not require large file
support from the operating system. Large tables are stored as
<LI>The search string can not start with a character class,
e.g. [a-e].</LI>
<LI>Case-insensitive searches such as <SMALL>ILIKE</SMALL> and
- <I>~*</I> do not utilize indexes. Instead, use functional
- indexes, which are described in section <a href="#4.10">4.10</a>.</LI>
+ <I>~*</I> do not utilize indexes. Instead, use expression
+ indexes, which are described in section <a href="#4.8">4.8</a>.</LI>
<LI>The default <I>C</I> locale must be used during
- <i>initdb</i> because it is not possible to know the next-greater
+ <i>initdb</i> because it is not possible to know the next-greatest
character in a non-C locale. You can create a special
<CODE>text_pattern_ops</CODE> index for such cases that work only
for <SMALL>LIKE</SMALL> indexing.
</UL>
<P>In pre-8.0 releases, indexes often can not be used unless the data
- types exactly match the index's column types. This is particularly
+ types exactly match the index's column types. This was particularly
true of int2, int8, and numeric column indexes.</P>
<H4><A name="4.7">4.7</A>) How do I see how the query optimizer is
</PRE>
This will not use an standard index. However, if you create a
- functional index, it will be used:
+ expresssion index, it will be used:
<PRE>
CREATE INDEX tabindex ON tab (lower(col));
</PRE>
<H4><A name="4.10">4.10</A>) What is the difference between the
various character types?</H4>
-<PRE>
-Type Internal Name Notes
---------------------------------------------------
-VARCHAR(n) varchar size specifies maximum length, no padding
-CHAR(n) bpchar blank padded to the specified fixed length
-TEXT text no specific upper limit on length
-BYTEA bytea variable-length byte array (null-byte safe)
-"char" char one character
-</PRE>
-
+<CENTER>
+<TABLE BORDER=1>
+<TR><TH>Type</TH><TH>Internal Name</TH><TH>Notes</TH></TR>
+<TR><TD>VARCHAR(n)</TD><TD>varchar</TD><TD>size specifies maximum
+length, no padding</TD></TR>
+<TR><TD>CHAR(n)</TD><TD>bpchar</TD><TD>blank padded to the specified
+fixed length</TD></TR>
+<TR><TD>TEXT</TD><TD>text</TD><TD>no specific upper limit on
+length</TD></TR>
+<TR><TD>BYTEA</TD><TD>bytea</TD><TD>variable-length byte array
+(null-byte safe)</TD></TR>
+<TR><TD>"char"</TD><TD>char</TD><TD>one character</TD></TR>
+</TABLE>
+</CENTER>
<P>You will see the internal name when examining system catalogs
and in some error messages.</P>
</PRE>
See the <I>create_sequence</I> manual page for more information
- about sequences. You can also use each row's <I>OID</I> field as a
- unique value. However, if you need to dump and reload the database,
- you need to use <I>pg_dump</I>'s <I>-o</I> option or <SMALL>COPY
- WITH OIDS</SMALL> option to preserve the <SMALL>OID</SMALL>s.
+ about sequences.
<H4><A name="4.11.2">4.11.2</A>) How do I get the value of a
<SMALL>SERIAL</SMALL> insert?</H4>
new_id = execute("SELECT currval('person_id_seq')");
</PRE>
- <P>Finally, you could use the <A href="#4.12"><SMALL>OID</SMALL></A>
- returned from the <SMALL>INSERT</SMALL> statement to look up the
- default value, though this is probably the least portable approach,
- and the oid value will wrap around when it reaches 4 billion.
- In Perl, using DBI with the DBD::Pg module, the oid value is made
- available via <I>$sth->{pg_oid_status}</I> after
- <I>$sth->execute()</I>.</P>
-
<H4><A name="4.11.3">4.11.3</A>) Doesn't <I>currval()</I>
lead to a race condition with other users?</H4>
<P>No. <I>currval()</I> returns the current value assigned by your
- backend, not by all users.</P>
+ session, not by all sessions.</P>
<H4><A name="4.11.4">4.11.4</A>) Why aren't my sequence numbers
reused on transaction abort? Why are there gaps in the numbering of
are modified or reloaded. They are used by index entries to point
to physical rows.</P>
- <H4><A name="4.13">4.13</A>) What is the meaning of some of the
- terms used in PostgreSQL?</H4>
-
- <P>Some of the source code and older documentation use terms that
- have more common usage. Here are some:</P>
-
- <UL>
- <LI>table, relation, class</LI>
-
- <LI>row, record, tuple</LI>
-
- <LI>column, field, attribute</LI>
-
- <LI>retrieve, select</LI>
-
- <LI>replace, update</LI>
-
- <LI>append, insert</LI>
-
- <LI><SMALL>OID</SMALL>, serial value</LI>
-
- <LI>portal, cursor</LI>
-
- <LI>range variable, table name, table alias</LI>
- </UL>
-
- <P>A list of general database terms can be found at: <A href=
- "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="4.14">4.14</A>) Why do I get the error <I>"ERROR:
+ <H4><A name="4.13">4.13</A>) Why do I get the error <I>"ERROR:
Memory exhausted in AllocSetAlloc()"</I>?</H4>
<P>You probably have run out of virtual memory on your system,
backend is returning too much data, try it before starting the
client.
- <H4><A name="4.15">4.15</A>) How do I tell what PostgreSQL version
+ <H4><A name="4.14">4.14</A>) How do I tell what PostgreSQL version
I am running?</H4>
<P>From <I>psql</I>, type <CODE>SELECT version();</CODE></P>
- <H4><A name="4.16">4.16</A>) Why does my large-object operations
+ <H4><A name="4.15">4.15</A>) Why does my large-object operations
get <I>"invalid large obj descriptor"</I>?</H4>
<P>You need to put <CODE>BEGIN WORK</CODE> and <CODE>COMMIT</CODE>
<P>If you are using a client interface like <SMALL>ODBC</SMALL> you
may need to set <CODE>auto-commit off.</CODE></P>
- <H4><A name="4.17">4.17</A>) How do I create a column that will
+ <H4><A name="4.16">4.16</A>) How do I create a column that will
default to the current time?</H4>
<P>Use <I>CURRENT_TIMESTAMP</I>:</P>
CREATE TABLE test (x int, modtime timestamp DEFAULT CURRENT_TIMESTAMP );
</PRE>
- <H4><A name="4.18">4.18</A>) How do I perform an outer join?</H4>
+ <H4><A name="4.17">4.17</A>) How do I perform an outer join?</H4>
<P>PostgreSQL supports outer joins using the SQL standard syntax.
Here are two examples:</P>
ORDER BY col1
</PRE>
- <H4><A name="4.19">4.19</A>) How do I perform queries using
+ <H4><A name="4.18">4.18</A>) How do I perform queries using
multiple databases?</H4>
<P>There is no way to query a database other than the current one.
connections to different databases and merge the results on the
client side.</P>
- <H4><A name="4.20">4.20</A>) How do I return multiple rows or
+ <H4><A name="4.19">4.19</A>) How do I return multiple rows or
columns from a function?</H4>
<P>In 7.3, you can easily return multiple rows or columns from a
<a href="http://techdocs.postgresql.org/guides/SetReturningFunctions">
http://techdocs.postgresql.org/guides/SetReturningFunctions</a>.
- <H4><A name="4.21">4.21</A>) Why can't I reliably create/drop
+ <H4><A name="4.20">4.20</A>) Why can't I reliably create/drop
temporary tables in PL/PgSQL functions?</H4>
<P>PL/PgSQL caches function contents, and an unfortunate side effect
is that if a PL/PgSQL function accesses a temporary table, and that
<SMALL>EXECUTE</SMALL> for temporary table access in PL/PgSQL. This
will cause the query to be reparsed every time.</P>
- <H4><A name="4.22">4.22</A>) What encryption options are available?
+ <H4><A name="4.21">4.21</A>) What encryption options are available?
</H4>
<UL>
<LI><I>contrib/pgcrypto</I> contains many encryption functions for
projects / postgresql / commitdiff