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<TITLE>PostgreSQL FAQ</TITLE>
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alink="#0000ff">
<H1>Frequently Asked Questions (FAQ) for PostgreSQL</H1>
<P>Last updated: Tue Feb 1 16:10:47 EST 2005</P>
<P>Current maintainer: Bruce Momjian (<A href=
"mailto:pgman@candle.pha.pa.us">pgman@candle.pha.pa.us</A>)
</P>
<P>The most recent version of this document can be viewed at <A href=
"http://www.postgresql.org/files/documentation/faqs/FAQ.html">
http://www.postgresql.org/files/documentation/faqs/FAQ.html</A>.</P>
<P>Platform-specific questions are answered at <A href=
"http://www.postgresql.org/docs/faq/">
http://www.postgresql.org/docs/faq/</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="#1.2">1.2</A>) What is the copyright of PostgreSQL?<BR>
<A href="#1.3">1.3</A>) What platforms does PostgreSQL support?<BR>
<A href="#1.4">1.4</A>) Where can I get PostgreSQL?<BR>
<A href="#1.5">1.5</A>) Where can I get support?<BR>
<A href="#1.6">1.6</A>) How do I submit a bug report?<BR>
<A href="#1.7">1.7</A>) What is the latest release?<BR>
<A href="#1.8">1.8</A>) What documentation is available?<BR>
<A href="#1.9">1.9</A>) How do I find out about known bugs or
missing features?<BR>
<A href="#1.10">1.10</A>) How can I learn <SMALL>SQL</SMALL>?<BR>
<A href="#1.11">1.11</A>) How do I join the development team?<BR>
<A href="#1.12">1.12</A>) How does PostgreSQL compare to other
<SMALL>DBMS</SMALL>s?<BR>
<H2 align="center">User Client Questions</H2>
<A href="#2.1">2.1</A>) What interfaces are available for
PostgreSQL?<BR>
<A href="#2.2">2.2</A>) What tools are available for using
PostgreSQL with Web pages?<BR>
<A href="#2.3">2.3</A>) Does PostgreSQL have a graphical user
interface?<BR>
<H2 align="center">Administrative Questions</H2>
<A href="#3.1">3.1</A>) How do I install PostgreSQL somewhere other
than <I>/usr/local/pgsql</I>?<BR>
<A href="#3.2">3.2</A>) How do I control connections from other
hosts?<BR>
<A href="#3.3">3.3</A>) How do I tune the database engine for
better performance?<BR>
<A href="#3.4">3.4</A>) What debugging features are available?<BR>
<A href="#3.5">3.5</A>) Why do I get <I>"Sorry, too many
clients"</I> when trying to connect?<BR>
<A href="#3.6">3.6</A>) What is in the <I>pgsql_tmp</I>
directory?<BR>
<A href="#3.7">3.7</A>) Why do I need to do a dump and restore
to upgrade PostgreSQL releases?<BR>
<A href="#3.8">3.8</A>) What computer hardware should I use?<BR>
<H2 align="center">Operational Questions</H2>
<A href="#4.1">4.1</A>) How do I <SMALL>SELECT</SMALL> only the
first few rows of a query? A random row?<BR>
<A href="#4.2">4.2</A>) How do I find out what tables, indexes,
databases, and users are defined? How do I see the queries used
by <I>psql</I> to display them?<BR>
<A href="#4.3">4.3</A>) How do you change a column's data type?<BR>
<A href="#4.4">4.4</A>) What is the maximum size for a row, a
table, and a database?<BR>
<A href="#4.5">4.5</A>) How much database disk space is required
to store data from a typical text file?<BR>
<A href="#4.6">4.6</A>) Why are my queries slow? Why don't they
use my indexes?<BR>
<A href="#4.7">4.7</A>) How do I see how the query optimizer is
evaluating my query?<BR>
<A href="#4.8">4.8</A>) How do I perform regular expression
searches and case-insensitive regular expression searches? How do I
use an index for case-insensitive searches?<BR>
<A href="#4.9">4.9</A>) In a query, how do I detect if a field
is <SMALL>NULL</SMALL>?<BR>
<A href="#4.10">4.10</A>) What is the difference between the
various character types?<BR>
<A href="#4.11.0">4.11.0</A>) How do I create a
serial/auto-incrementing field?<BR>
<A href="#4.11.1">4.11.1</A>) How do I get the value of a
<SMALL>SERIAL</SMALL> insert?<BR>
<A href="#4.11.2">4.11.2</A>) Doesn't <I>currval()</I>
lead to a race condition with other users?<BR>
<A href="#4.11.3">4.11.3</A>) Why aren't my sequence numbers
reused on transaction abort? Why are there gaps in the numbering of
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="#4.12">4.13</A>) Why do I get the error <I>"ERROR: Memory
exhausted in AllocSetAlloc()"</I>?<BR>
<A href="#4.14">4.14</A>) How do I tell what PostgreSQL version I
am running?<BR>
<A href="#4.15">4.15</A>) Why does my large-object operations get
<I>"invalid large obj descriptor"</I>?<BR>
<A href="#4.16">4.16</A>) How do I create a column that will
default to the current time?<BR>
<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="#4.19">4.19</A>) How do I return multiple rows or columns
from a function?<BR>
<A href="#4.20">4.20</A>) Why can't I reliably create/drop
temporary tables in PL/PgSQL functions?<BR>
<A href="#4.21">4.21</A>) What encryption options are available?<BR>
<H2 align="center">Extending PostgreSQL</H2>
<A href="#5.1">5.1</A>) I wrote a user-defined function. When I run
it in <I>psql</I>, why does it dump core?<BR>
<A href="#5.2">5.2</A>) How can I contribute some nifty new types
and functions to PostgreSQL?<BR>
<A href="#5.3">5.3</A>) How do I write a C function to return a
tuple?<BR>
<A href="#5.4">5.4</A>) I have changed a source file. Why does the
recompile not see the change?<BR>
<HR>
<H2 align="center">General Questions</H2>
<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>, also called just
<I>Postgres</I>.</P>
<P>PostgreSQL is an object-relational database system that has the
features of traditional commercial database systems with
enhancements to be found in next-generation <SMALL>DBMS</SMALL>
systems. PostgreSQL is free and the complete source code is
available.</P>
<P>PostgreSQL development is performed by a team of mostly volunteer
developers spread throughout the world and communicating via the
Internet. It is a community project and is not controlled by any
company. To get involved, see the developer's FAQ at <A href=
"http://www.postgresql.org/files/documentation/faqs/FAQ_DEV.html">
http://www.postgresql.org/files/documentation/faqs/FAQ_DEV.html</A>
</P>
<H4><A name="1.2">1.2</A>) What is the copyright of
PostgreSQL?</H4>
<P>PostgreSQL is distributed under the classic BSD license. It has
no restrictions on how the source code can be used. We like it and
have no intention of changing it.</P>
<P>This is the BSD license we use:</P>
<P>PostgreSQL Data Base Management System</P>
<P>Portions Copyright (c) 1996-2005, PostgreSQL Global Development Group
Portions Copyright (c) 1994-6 Regents of the University of California</P>
<P>Permission to use, copy, modify, and distribute this software
and its documentation for any purpose, without fee, and without a
written agreement is hereby granted, provided that the above
copyright notice and this paragraph and the following two
paragraphs appear in all copies.</P>
<P>IN NO EVENT SHALL THE UNIVERSITY OF CALIFORNIA BE LIABLE TO ANY
PARTY FOR DIRECT, INDIRECT, SPECIAL, INCIDENTAL, OR CONSEQUENTIAL
DAMAGES, INCLUDING LOST PROFITS, ARISING OUT OF THE USE OF THIS
SOFTWARE AND ITS DOCUMENTATION, EVEN IF THE UNIVERSITY OF
CALIFORNIA HAS BEEN ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.</P>
<P>THE UNIVERSITY OF CALIFORNIA SPECIFICALLY DISCLAIMS ANY
WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE. THE
SOFTWARE PROVIDED HEREUNDER IS ON AN "AS IS" BASIS, AND THE
UNIVERSITY OF CALIFORNIA HAS NO OBLIGATIONS TO PROVIDE MAINTENANCE,
SUPPORT, UPDATES, ENHANCEMENTS, OR MODIFICATIONS.</P>
<H4><A name="1.3">1.3</A>) What platforms does PostgreSQL support?</H4>
<P>In general, any modern Unix-compatible platform should be able to
run PostgreSQL. The platforms that had received explicit testing at
the time of release are listed in the installation
instructions.</P>
<P>Starting with version 8.0, PostgreSQL now runs natively on
Microsoft Windows NT-based operating systems like Win2000, WinXP,
and Win2003. A prepackaged installer is available at <a href=
"http://pgfoundry.org/projects/pginstaller">
http://pgfoundry.org/projects/pginstaller</a>. MSDOS-based versions
of Windows (Win95, Win98, WinMe) can run PostgreSQL using Cygwin.
<p>There is also a Novell Netware 6 port at
<a href="http://forge.novell.com">http://forge.novell.com</a>,
and an OS/2 (eComStation) version at <a href=
"http://hobbes.nmsu.edu/cgi-bin/h-search?sh=1&button=Search&key=postgreSQL&stype=all&sort=type&dir=%2F">
http://hobbes.nmsu.edu/cgi-bin/h-search?sh=1&button=Search&key=postgreSQL&stype=all&sort=type&dir=%2F</a>.</p>
<H4><A name="1.4">1.4</A>) Where can I get PostgreSQL?</H4>
<P>The primary anonymous ftp site for PostgreSQL is <A href=
"ftp://ftp.PostgreSQL.org/pub">ftp://ftp.PostgreSQL.org/pub</A>.
For mirror sites, see our main web site.</P>
<H4><A name="1.5">1.5</A>) Where can I get support?</H4>
<P>The PostgreSQL community provides assistance to many of its users
via email. The main web site to subscribe to the email lists is
<a href="http://www.postgresql.org/community/lists/">
http://www.postgresql.org/community/lists/</a>. The <I>general</I>
or <I>bugs</I> lists are a good place to start.
<P>The major IRC channel is <I>#postgresql</I> on Freenode
(<I>irc.freenode.net</I>). To connect you can use the Unix
program <CODE>irc -c '#postgresql' "$USER" irc.freenode.net</CODE>
or use any of the other popular IRC clients. A Spanish one also exists
on the same network, (<I>#postgresql-es</I>), and a French one,
(<I>#postgresqlfr</I>). There is also a PostgreSQL channel on EFNet.
<P>A list of commercial support companies is available at <A href=
"http://techdocs.postgresql.org/companies.php">http://techdocs.postg
resql.org/companies.php</A>.</P>
<H4><A name="1.6">1.6</A>) How do I submit a bug report?</H4>
<P>Visit the PostgreSQL bug form at <A href=
"http://www.postgresql.org/support/submitbug">
http://www.postgresql.org/support/submitbug</A>.</P>
<P>Also check out our ftp site <A href=
"ftp://ftp.PostgreSQL.org/pub">ftp://ftp.PostgreSQL.org/pub</A> to
see if there is a more recent PostgreSQL version.</P>
<H4><A name="1.7">1.7</A>) What is the latest release?</H4>
<P>The latest release of PostgreSQL is version 8.0.0.</P>
<P>We plan to have major releases every ten to twelve months.</P>
<H4><A name="1.8">1.8</A>) What documentation is available?</H4>
<P>PostgreSQL includes extensive documentation, including a large
manual, manual pages, and some test examples. See the <I>/doc</I>
directory. You can also browse the manuals online at <A href=
"http://www.PostgreSQL.org/docs">http://www.PostgreSQL.org/docs</A>.
</P>
<P>There are two PostgreSQL books available online at <A href=
"http://www.PostgreSQL.org/docs/awbook.html">http://www.PostgreSQL.org/docs/awbook.html</A>
and <A href=
"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://techdocs.postgresql.org/techdocs/bookreviews.php">http://techdocs.PostgreSQL.org/techdocs/bookreviews.php</A>.
There is also a collection of PostgreSQL technical articles at <A
href=
"http://techdocs.PostgreSQL.org/">http://techdocs.PostgreSQL.org/</A>.</P>
<P>The command line client program <I>psql</I> has some \d commands to show
information about types, operators, functions, aggregates, etc. - use \? to
display the available commands.</P>
<P>Our web site contains even more documentation.</P>
<H4><A name="1.9">1.9</A>) How do I find out about known bugs or
missing features?</H4>
<P>PostgreSQL supports an extended subset of <SMALL>SQL</SMALL>-92.
See our <A href="http://developer.PostgreSQL.org/todo.php">TODO</A>
list for known bugs, missing features, and future plans.</P>
<H4><A name="1.10">1.10</A>) How can I learn
<SMALL>SQL</SMALL>?</H4>
<P>The PostgreSQL book at <A href=
"http://www.PostgreSQL.org/docs/awbook.html">http://www.PostgreSQL.org/docs/awbook.html</A>
teaches <SMALL>SQL</SMALL>. There is another PostgreSQL book at <A
href=
"http://www.commandprompt.com/ppbook/">http://www.commandprompt.com/ppbook.</A>
There is a nice tutorial at <A href=
"http://www.intermedia.net/support/sql/sqltut.shtm">http://www.intermedia.net/support/sql/sqltut.shtm,</A>
at <A href=
"http://ourworld.compuserve.com/homepages/graeme_birchall/HTM_COOK.HTM">
http://ourworld.compuserve.com/homepages/graeme_birchall/HTM_COOK.HTM,</A>
and at <A href=
"http://sqlcourse.com/">http://sqlcourse.com.</A></P>
<P>Another one is "Teach Yourself SQL in 21 Days, Second Edition"
at <A href=
"http://members.tripod.com/er4ebus/sql/index.htm">http://members.tripod.com/er4ebus/sql/index.htm</A></P>
<P>Many of our users like <I>The Practical SQL Handbook</I>,
Bowman, Judith S., et al., Addison-Wesley. Others like <I>The
Complete Reference SQL</I>, Groff et al., McGraw-Hill.</P>
<H4><A name="1.11">1.11</A>) How do I join the development
team?</H4>
<P>First, download the latest source and read the PostgreSQL
Developers FAQ and documentation on our web site, or in the
distribution. Second, subscribe to the <I>pgsql-hackers</I> and
<I>pgsql-patches</I> mailing lists. Third, submit high quality
patches to pgsql-patches.</P>
<P>There are about a dozen people who have commit privileges to the
PostgreSQL <SMALL>CVS</SMALL> archive. They each have submitted so
many high-quality patches that it was impossible for the existing
committers to keep up, and we had confidence that patches they
committed were of high quality.</P>
<H4><A name="1.12">1.12</A>) How does PostgreSQL compare to other
<SMALL>DBMS</SMALL>s?</H4>
<P>There are several ways of measuring software: features,
performance, reliability, support, and price.</P>
<DL>
<DT><B>Features</B></DT>
<DD>PostgreSQL has most features present in large commercial
<SMALL>DBMS</SMALL>s, like transactions, subselects, triggers,
views, foreign key referential integrity, and sophisticated
locking. We have some features they do not have, like
user-defined types, inheritance, rules, and multi-version
concurrency control to reduce lock contention.<BR>
<BR>
</DD>
<DT><B>Performance</B></DT>
<DD>PostgreSQL's performance is comparable to other commercial and
open source databases. It is faster for some things, slower for
others. In comparison to MySQL or leaner database systems, we are
faster for multiple users, complex queries, and a read/write query
load. MySQL is faster for simple SELECT queries done by a few users.
Of course, MySQL does not have most of the features mentioned in the
<I>Features</I> section above. We are built for reliability and
features, and we continue to improve performance in every
release. <BR>
<BR>
</DD>
<DT><B>Reliability</B></DT>
<DD>We realize that a <SMALL>DBMS</SMALL> must be reliable, or it
is worthless. We strive to release well-tested, stable code that
has a minimum of bugs. Each release has at least one month of
beta testing, and our release history shows that we can provide
stable, solid releases that are ready for production use. We
believe we compare favorably to other database software in this
area.<BR>
<BR>
</DD>
<DT><B>Support</B></DT>
<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="#1.5">FAQ section 1.5</A>.)<BR>
<BR>
</DD>
<DT><B>Price</B></DT>
<DD>We are free for all use, both commercial and non-commercial.
You can add our code to your product with no limitations, except
those outlined in our BSD-style license stated above.<BR>
<BR>
</DD>
</DL>
<HR>
<H2 align="center">User Client Questions</H2>
<H4><A name="2.1">2.1</A>) What interfaces are available for
PostgreSQL?</H4>
<P>The PostgreSQL install includes only the <SMALL>C</SMALL> and embedded
<SMALL>C</SMALL> interfaces. All other interfaces are independent projects
that are downloaded separately; being separate allows them to have their
own release schedule and development teams.</P>
<P>Some programming languages like <SMALL>PHP</SMALL> include an
interface to PostgreSQL. Interfaces for languages like Perl,
<SMALL>TCL</SMALL>, Python, and many others are available at
<a href="http://gborg.postgresql.org">http://gborg.postgresql.org</A>
in the <I>Drivers/Interfaces</I> section and via Internet search.
</P>
<H4><A name="2.2">2.2</A>) What tools are available for using
PostgreSQL with Web pages?</H4>
<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>For Web integration, PHP (<A
href="http://www.php.net">http://www.php.net</A>) is an excellent
interface.</P>
<P>For complex cases, many use the Perl and CGI.pm or mod_perl.</P>
<H4><A name="2.3">2.3</A>) Does PostgreSQL have a graphical user
interface?</H4>
<P>Yes, there are several graphical interfaces to PostgreSQL
available. These include pgAdmin III (<a
href="http://www.pgadmin.org">http://www.pgadmin.org</a>, PgAccess
<a href="http://www.pgaccess.org"> http://www.pgaccess.org</a>),
RHDB Admin (<a
href="http://sources.redhat.com/rhdb/">http://sources.redhat.com/rhd
b/ </a>), TORA (<a
href="http://www.globecom.net/tora/">http://www.globecom.net/tora/</a>,
partly commercial), and Rekall (<a
href="http://www.rekallrevealed.org/">
http://www.rekallrevealed.org/</a>). There is also PhpPgAdmin (<a
href="http://phppgadmin.sourceforge.net/">
http://phppgadmin.sourceforge.net/ </a>), a web-based interface to
PostgreSQL.</P>
<P>See <a href="http://techdocs.postgresql.org/guides/GUITools">
http://techdocs.postgresql.org/guides/GUITools</a> for a more
detailed list.</P>
<HR>
<H2 align="center">Administrative Questions</H2>
<H4><A name="3.1">3.1</A>) How do I install PostgreSQL somewhere
other than <I>/usr/local/pgsql</I>?</H4>
<P>Specify the <I>--prefix</I> option when running
<I>configure</I>.</P>
<H4><A name="3.2">3.2</A>) How do I control connections from other
hosts?</H4>
<P>By default, PostgreSQL only allows connections from the local
machine using Unix domain sockets or TCP/IP connections. Other
machines will not be able to connect unless you modify
<I>listen_addresses</I> in the <I>postgresql.conf</I> file, enable
host-based authentication by modifying the
<I>$PGDATA/pg_hba.conf</I> file, and restart the server.</P>
<H4><A name="3.3">3.3</A>) How do I tune the database engine for
better performance?</H4>
<P>There are three major areas for potential performance
improvement:</P>
<DL>
<DT><B>Query Changes</B></DT>
<DD>This involves modifying queries to obtain better
performance:
<ul>
<li>Creation of indexes, including expression and partial
indexes</li>
<li>Use of COPY instead of multiple <SMALL>INSERT</SMALL>s</li>
<li>Grouping of multiple statements into a single transaction to
reduce commit overhead</li>
<li>Use of <SMALL>CLUSTER</SMALL> when retrieving many rows from an
index</li>
<li>Use of <SMALL>LIMIT</SMALL> for returning a subset of a query's
output</li>
<li>Use of Prepared queries</li>
<li>Use of <SMALL>ANALYZE</SMALL> to maintain accurate optimizer
statistics</li>
<li>Regular use of <SMALL>VACUUM</SMALL> or <I>pg_autovacuum</I>
<li>Dropping of indexes during large data changes</li>
</ul><BR>
<BR>
</DD>
<DT><B>Server Configuration</B></DT>
<DD>A number of <I>postgresql.conf</I> settings affect performance.
For more details, see <a href=
"http://www.postgresql.org/docs/current/static/runtime.html">
Administration Guide/Server Run-time Environment/Run-time
Configuration</a> for a full listing, and for commentary see <a
href="http://www.varlena.com/varlena/GeneralBits/Tidbits/annotated_conf_e.html">
http://www.varlena.com/varlena/GeneralBits/Tidbits/annotated_conf_e.html</a>
and <a href="http://www.varlena.com/varlena/GeneralBits/Tidbits/perf.html">
http://www.varlena.com/varlena/GeneralBits/Tidbits/perf.html</a>.
<BR>
<BR>
</DD>
<DT><B>Hardware Selection</B></DT>
<DD>The effect of hardware on performance is detailed in <a
href="http://candle.pha.pa.us/main/writings/pgsql/hw_performance/index.html">
http://candle.pha.pa.us/main/writings/pgsql/hw_performance/index.html</a> and
<a href="http://www.powerpostgresql.com/PerfList/">
http://www.powerpostgresql.com/PerfList/</a>.
<BR>
<BR>
</DD>
</DL>
<H4><A name="3.4">3.4</A>) What debugging features are
available?</H4>
<P>There are many <CODE>log_*</CODE> server configuration variables
that enable printing of query and process statistics which can be
very useful for debugging and performance measurements.</P>
<P><B>The following detailed debug instructions are to be used to
provide more detailed information for server developers debugging a
problem.</B></P>
<P>It is also possible to debug the server if it isn't operating
properly. First, by running <I>configure</I> with the --enable-cassert
option, many <I>assert()</I>s monitor the progress of the backend
and halt the program when something unexpected occurs.</P>
<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>
<P>If <I>postmaster</I> is not running, you can actually run the
<I>postgres</I> backend from the command line, and type your
<SMALL>SQL</SMALL> statement directly. This is recommended
<B>only</B> for debugging purposes. Note that a newline terminates
the query, not a semicolon. If you have compiled with debugging
symbols, you can use a debugger to see what is happening. Because
the backend was not started from <I>postmaster</I>, it is not
running in an identical environment and locking/backend interaction
problems may not be duplicated.</P>
<P>If <I>postmaster</I> is running, start <I>psql</I> in one
window, then find the <SMALL>PID</SMALL> of the <I>postgres</I>
process used by <I>psql</I> using <CODE>SELECT pg_backend_pid()</CODE>.
Use a debugger to attach to the <I>postgres</I> <SMALL>PID</SMALL>.
You can set breakpoints in the debugger and issue queries from
<I>psql</I>. If you are debugging <I>postgres</I> startup, you can
set PGOPTIONS="-W n", then start <I>psql</I>. This will cause startup
to delay for <I>n</I> seconds so you can attach to the process with
the debugger, set any breakpoints, and continue through the startup
sequence.</P>
<P>You can also compile with profiling to see what functions are
taking execution time. The backend profile files will be deposited
in the <I>pgsql/data/base/dbname</I> directory. The client profile
file will be put in the client's current directory. Linux requires
a compile with <I>-DLINUX_PROFILE</I> for proper profiling.</P>
<H4><A name="3.5">3.5</A>) Why do I get <I>"Sorry, too many
clients"</I> when trying to connect?</H4>
<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
restarting the <I>postmaster</I>.</P>
<H4><A name="3.6">3.6</A>) What is in the <I>pgsql_tmp</I> directory?</H4>
<P>This directory contains temporary files generated by the query
executor. For example, if a sort needs to be done to satisfy an
<SMALL>ORDER BY</SMALL> and the sort requires more space than the
backend's <I>-S</I> parameter allows, then temporary files are created
here to hold the extra data.</P>
<P>The temporary files are usually deleted automatically, but might
remain if a backend crashes during a sort. A stop and restart of the
<I>postmaster</I> will remove files from those directories.</P>
<H4><A name="3.7">3.7</A>) Why do I need to do a dump and restore
to upgrade between major PostgreSQL releases?</H4>
<P>The PostgreSQL team makes only small changes between minor releases,
so upgrading from 7.4 to 7.4.1 does not require a dump and restore.
However, major releases (e.g. from 7.3 to 7.4) often change the internal
format of system tables and data files. These changes are often complex,
so we don't maintain backward compatibility for data files. A dump outputs
data in a generic format that can then be loaded in using the new internal
format.</P>
<H4><A name="3.8">3.8</A>) What computer hardware should I use?</H4>
<P>Because PC hardware is mostly compatible, people tend to believe that
all PC hardware is of equal quality. It is not. ECC RAM, SCSI, and
quality motherboards are more reliable and have better performance than
less expensive hardware. PostgreSQL will run on almost any hardware,
but if reliability and performance are important it is wise to
research your hardware options thoroughly. Our email lists can be used
to discuss hardware options and tradeoffs.</P>
<HR>
<H2 align="center">Operational Questions</H2>
<H4><A name="4.1">4.1</A>) How do I <SMALL>SELECT</SMALL> only the
first few rows of a query? A random row?</H4>
<P>To retrieve only a few rows, if you know at the number of rows
needed at the time of the <SMALL>SELECT</SMALL> use
<SMALL>LIMIT</SMALL> . If an index matches the <SMALL>ORDER
BY</SMALL> it is possible the entire query does not have to be
executed. If you don't know the number of rows at
<SMALL>SELECT</SMALL> time, use a cursor and
<SMALL>FETCH</SMALL>.</P>
<P>To <SMALL>SELECT</SMALL> a random row, use:
<PRE>
SELECT col
FROM tab
ORDER BY random()
LIMIT 1;
</PRE>
<H4><A name="4.2">4.2</A>) How do I find out what tables, indexes,
databases, and users are defined? How do I see the queries used
by <I>psql</I> to display them?</H4>
<P>Use the \dt command to see tables in <I>psql</I>. For a complete list of
commands inside psql you can use \?. Alternatively you can read the source
code for <I>psql</I> in file <I>pgsql/src/bin/psql/describe.c</I>, it
contains <SMALL>SQL</SMALL> commands that generate the output for
<I>psql</I>'s backslash commands. You can also start <I>psql</I> with the
<I>-E</I> option so it will print out the queries it uses to execute the
commands you give. PostgreSQL also provides an <SMALL>SQL</SMALL> compliant
INFORMATION SCHEMA interface you can query to get information about the
database.</P>
<P>There are also system tables beginning with <I>pg_</I> that describe
these too.</P>
<P>Use <I>psql -l</I> will list all databases.</P>
<P>Also try the file <I>pgsql/src/tutorial/syscat.source</I>. It
illustrates many of the <SMALL>SELECT</SMALL>s needed to get
information from the database system tables.</P>
<H4><A name="4.3">4.3</A>) How do you change a column's data type?</H4>
<P>Changing the data type of a column can be done easily in 8.0
and later with <SMALL>ALTER TABLE ALTER COLUMN TYPE</SMALL>.
<P>In earlier releases, do this:</P>
<PRE>
BEGIN;
ALTER TABLE tab ADD COLUMN new_col <i>new_data_type</i>;
UPDATE tab SET new_col = CAST(old_col AS <i>new_data_type</i>);
ALTER TABLE tab DROP COLUMN old_col;
COMMIT;
</PRE>
<P>You might then want to do <I>VACUUM FULL tab</I> to reclaim the
disk space used by the expired rows.</P>
<H4><A name="4.4">4.4</A>) What is the maximum size for a row, a
table, and a database?</H4>
<P>These are the limits:</P>
<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.</P>
<P>The maximum table size of 32 TB does not require large file
support from the operating system. Large tables are stored as
multiple 1 GB files so file system size limits are not
important.</P>
<P>The maximum table size and maximum number of columns can be
quadrupled by increasing the default block size to 32k.</P>
<H4><A name="4.5">4.5</A>) How much database disk space is required
to store data from a typical text file?</H4>
<P>A PostgreSQL database may require up to five times the disk
space to store data from a text file.</P>
<P>As an example, consider a file of 100,000 lines with an integer
and text description on each line. Suppose the text string
avergages twenty bytes in length. The flat file would be 2.8 MB.
The size of the PostgreSQL database file containing this data can
be estimated as 6.4 MB:</P>
<PRE>
32 bytes: each row header (approximate)
24 bytes: one int field and one text field
+ 4 bytes: pointer on page to tuple
----------------------------------------
60 bytes per row
The data page size in PostgreSQL is 8192 bytes (8 KB), so:
8192 bytes per page
------------------- = 136 rows per database page (rounded down)
60 bytes per row
100000 data rows
-------------------- = 735 database pages (rounded up)
128 rows per page
735 database pages * 8192 bytes per page = 6,021,120 bytes (6 MB)
</PRE>
<P>Indexes do not require as much overhead, but do contain the data
that is being indexed, so they can be large also.</P>
<P><SMALL>NULL</SMALL>s are stored as bitmaps, so they
use very little space.</P>
<H4><A name="4.6">4.6</A>) Why are my queries slow? Why don't they
use my indexes?</H4>
<P>Indexes are not automatically used by every query. Indexes are only
used if the table is larger than a minimum size, and the query
selects only a small percentage of the rows in the table. This is
because the random disk access caused by an index scan can be
slower than a straight read through the table, or sequential scan.
<P>To determine if an index should be used, PostgreSQL must have
statistics about the table. These statistics are collected using
<SMALL>VACUUM ANALYZE</SMALL>, or simply <SMALL>ANALYZE</SMALL>.
Using statistics, the optimizer knows how many rows are in the
table, and can better determine if indexes should be used.
Statistics are also valuable in determining optimal join order and
join methods. Statistics collection should be performed
periodically as the contents of the table change.</P>
<P>Indexes are normally not used for <SMALL>ORDER BY</SMALL> or to
perform joins. A sequential scan followed by an explicit sort is
usually faster than an index scan of a large table.</P>
However, <SMALL>LIMIT</SMALL> combined with <SMALL>ORDER BY</SMALL>
often will use an index because only a small portion of the table
is returned. In fact, though MAX() and MIN() don't use indexes,
it is possible to retrieve such values using an index with ORDER BY
and LIMIT:
<PRE>
SELECT col
FROM tab
ORDER BY col [ DESC ]
LIMIT 1;
</PRE>
<P>If you believe the optimizer is incorrect in choosing a
sequential scan, use <CODE>SET enable_seqscan TO 'off'</CODE> and
run tests to see if an index scan is indeed faster.</P>
<P>When using wild-card operators such as <SMALL>LIKE</SMALL> or
<I>~</I>, indexes can only be used in certain circumstances:</P>
<UL>
<LI>The beginning of the search string must be anchored to the start
of the string, i.e.
<UL>
<LI><SMALL>LIKE</SMALL> patterns must not start with <I>%</I>.</LI>
<LI><I>~</I> (regular expression) patterns must start with
<I>^</I>.</LI>
</UL></LI>
<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 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-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.
</LI>
</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 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
evaluating my query?</H4>
<P>See the <SMALL>EXPLAIN</SMALL> manual page.</P>
<H4><A name="4.8">4.8</A>) How do I perform regular expression
searches and case-insensitive regular expression searches? How do I
use an index for case-insensitive searches?</H4>
<P>The <I>~</I> operator does regular expression matching, and
<I>~*</I> does case-insensitive regular expression matching. The
case-insensitive variant of <SMALL>LIKE</SMALL> is called
<SMALL>ILIKE</SMALL>.</P>
<P>Case-insensitive equality comparisons are normally expressed
as:</P>
<PRE>
SELECT *
FROM tab
WHERE lower(col) = 'abc';
</PRE>
This will not use an standard index. However, if you create a
expresssion index, it will be used:
<PRE>
CREATE INDEX tabindex ON tab (lower(col));
</PRE>
<H4><A name="4.9">4.9</A>) In a query, how do I detect if a field
is <SMALL>NULL</SMALL>?</H4>
<P>You test the column with <SMALL>IS NULL</SMALL> and <SMALL>IS
NOT NULL</SMALL>.</P>
<H4><A name="4.10">4.10</A>) What is the difference between the
various character types?</H4>
<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>
<P>The first four types above are "varlena" types (i.e., the first
four bytes on disk are the length, followed by the data). Thus the
actual space used is slightly greater than the declared size.
However, these data types are also subject to compression or being
stored out-of-line by <SMALL>TOAST</SMALL>, so the space on disk
might also be less than expected.</P>
<SMALL>VARCHAR(n)</SMALL> is best when storing variable-length
strings and it limits how long a string can be. <SMALL>TEXT</SMALL>
is for strings of unlimited length, with a maximum of one gigabyte.
<P><SMALL>CHAR(n)</SMALL> is for storing strings that are all the
same length. <SMALL>CHAR(n)</SMALL> pads with blanks to the specified
length, while <SMALL>VARCHAR(n)</SMALL> only stores the characters
supplied. <SMALL>BYTEA</SMALL> is for storing binary data,
particularly values that include <SMALL>NULL</SMALL> bytes. All the
types described here have similar performance characteristics.</P>
<H4><A name="4.11.1">4.11.1</A>) How do I create a
serial/auto-incrementing field?</H4>
<P>PostgreSQL supports a <SMALL>SERIAL</SMALL> data type. It
auto-creates a sequence. For example,
this:</P>
<PRE>
CREATE TABLE person (
id SERIAL,
name TEXT
);
</PRE>
is automatically translated into this:
<PRE>
CREATE SEQUENCE person_id_seq;
CREATE TABLE person (
id INT4 NOT NULL DEFAULT nextval('person_id_seq'),
name TEXT
);
</PRE>
See the <I>create_sequence</I> manual page for more information
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>
<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="#4.11.1">4.11.1</A>, an example in a
pseudo-language would look like this:</P>
<PRE>
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
<CODE>new_id</CODE> for use in other queries (e.g., as a foreign
key to the <CODE>person</CODE> table). Note that the name of the
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.
<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>
execute("INSERT INTO person (name) VALUES ('Blaise Pascal')");
new_id = execute("SELECT currval('person_id_seq')");
</PRE>
<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
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
my sequence/SERIAL column?</H4>
<P>To improve concurrency, sequence values are given out to running
transactions as needed and are not locked until the transaction
completes. This causes gaps in numbering from aborted
transactions.</P>
<H4><A name="4.12">4.12</A>) What is an <SMALL>OID</SMALL>? What is
a <SMALL>TID</SMALL>?</H4>
<P>Every row that is created in PostgreSQL gets a unique
<SMALL>OID</SMALL> unless created <SMALL>WITHOUT OIDS</SMALL>.
O<SMALL>ID</SMALL>s are autotomatically assigned unique 4-byte
integers that are unique across the entire installation. However,
they overflow at 4 billion, and then the O<SMALL>ID</SMALL>s start
being duplicated. PostgreSQL uses <SMALL>OID</SMALL>s to link its
internal system tables together.</P>
<P>To uniquely number columns in user tables, it is best to use
<SMALL>SERIAL</SMALL> rather than O<SMALL>ID</SMALL>s because
<SMALL>SERIAL</SMALL> sequences are unique only within a single
table. and are therefore less likely to overflow.
<SMALL>SERIAL8</SMALL> is available for storing eight-byte sequence
values.</P>
<P>T<SMALL>ID</SMALL>s are used to identify specific physical rows
with block and offset values. T<SMALL>ID</SMALL>s change after rows
are modified or reloaded. They are used by index entries to point
to physical rows.</P>
<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,
or your kernel has a low limit for certain resources. Try this
before starting <I>postmaster</I>:</P>
<PRE>
ulimit -d 262144
limit datasize 256m
</PRE>
Depending on your shell, only one of these may succeed, but it will
set your process data segment limit much higher and perhaps allow
the query to complete. This command applies to the current process,
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.
<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="4.15">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>
around any use of a large object handle, that is, surrounding
<CODE>lo_open</CODE> ... <CODE>lo_close.</CODE></P>
<P>Currently PostgreSQL enforces the rule by closing large object
handles at transaction commit. So the first attempt to do anything
with the handle will draw <I>invalid large obj descriptor</I>. So
code that used to work (at least most of the time) will now
generate that error message if you fail to use a transaction.</P>
<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="4.16">4.16</A>) How do I create a column that will
default to the current time?</H4>
<P>Use <I>CURRENT_TIMESTAMP</I>:</P>
<PRE>
CREATE TABLE test (x int, modtime TIMESTAMP DEFAULT CURRENT_TIMESTAMP );
</PRE>
<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>
<PRE>
SELECT *
FROM t1 LEFT OUTER JOIN t2 ON (t1.col = t2.col);
</PRE>
or
<PRE>
SELECT *
FROM t1 LEFT OUTER JOIN t2 USING (col);
</PRE>
<P>These identical queries join t1.col to t2.col, and also return
any unjoined rows in t1 (those with no match in t2). A
<SMALL>RIGHT</SMALL> join would add unjoined rows of t2. A
<SMALL>FULL</SMALL> join would return the matched rows plus all
unjoined rows from t1 and t2. The word <SMALL>OUTER</SMALL> is
optional and is assumed in <SMALL>LEFT</SMALL>,
<SMALL>RIGHT</SMALL>, and <SMALL>FULL</SMALL> joins. Ordinary joins
are called <SMALL>INNER</SMALL> joins.</P>
<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.
Because PostgreSQL loads database-specific system catalogs, it is
uncertain how a cross-database query should even behave.</P>
<P><I>contrib/dblink</I> allows cross-database queries using
function calls. Of course, a client can also make simultaneous
connections to different databases and merge the results on the
client side.</P>
<H4><A name="4.19">4.19</A>) How do I return multiple rows or
columns from a function?</H4>
<P>It is easy using set-returning functions,
<a href="http://techdocs.postgresql.org/guides/SetReturningFunctions">
http://techdocs.postgresql.org/guides/SetReturningFunctions</a>.
<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 scripts, and an unfortunate side effect
is that if a PL/PgSQL function accesses a temporary table, and that
table is later dropped and recreated, and the function called again,
the function will fail because the cached function contents still
point to the old temporary table. The solution is to use
<SMALL>EXECUTE</SMALL> for temporary table access in PL/PgSQL. This
will cause the query to be reparsed every time.</P>
<H4><A name="4.21">4.21</A>) What encryption options are available?
</H4>
<UL>
<LI><I>contrib/pgcrypto</I> contains many encryption functions for
use in <SMALL>SQL</SMALL> queries.</LI>
<LI>To encrypt transmission from the client to the server, the server
must have the <I>ssl</I> option set to <I>true</I> in <I>postgresql.conf,
</I> and an applicable <I>host</I> or <I>hostssl</I> record must exist in
<I>pg_hba.conf</I>, and the client <I>sslmode</I> must not be
<I>disable.</I> (Note that it is also possible to use a third-party
encrypted transport, such as stunnel or ssh, rather than PostgreSQL's
native SSL connections.)
<LI>Database user passwords are automatically encrypted when stored in
the system tables.</LI>
<LI>The server can run using an encrypted file system.</LI>
</UL>
<HR>
<H2 align="center">Extending PostgreSQL</H2>
<H4><A name="5.1">5.1</A>) I wrote a user-defined function. When I
run it in <I>psql</I>, why does it dump core?</H4>
<P>The problem could be a number of things. Try testing your
user-defined function in a stand-alone test program first.</P>
<H4><A name="5.2">5.2</A>) How can I contribute some nifty new
types and functions to PostgreSQL?</H4>
<P>Send your extensions to the <I>pgsql-hackers</I> mailing list,
and they will eventually end up in the <I>contrib/</I>
subdirectory.</P>
<H4><A name="5.3">5.3</A>) How do I write a C function to return a
tuple?</H4>
<P>In versions of PostgreSQL beginning with 7.3, table-returning
functions are fully supported in C, PL/PgSQL, and SQL. See the
Programmer's Guide for more information. An example of a
table-returning function defined in C can be found in
<I>contrib/tablefunc</I>.</P>
<H4><A name="5.4">5.4</A>) I have changed a source file. Why does
the recompile not see the change?</H4>
<P>The <I>Makefiles</I> do not have the proper dependencies for
include files. You have to do a <I>make clean</I> and then another
<I>make</I>. If you are using <SMALL>GCC</SMALL> you can use the
<I>--enable-depend</I> option of <I>configure</I> to have the
compiler compute the dependencies automatically.</P>
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