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<H1>Frequently Asked Questions (FAQ) for PostgreSQL</H1>
<P>Last updated: Fri Feb 24 23:28:40 EST 2006</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="#item1.1">1.1</A>) What is PostgreSQL? How is it pronounced?<BR>
<A href="#item1.2">1.2</A>) Who controls PostgreSQL?<BR>
<A href="#item1.3">1.3</A>) What is the copyright of PostgreSQL?<BR>
<A href="#item1.4">1.4</A>) What platforms does PostgreSQL support?<BR>
<A href="#item1.5">1.5</A>) Where can I get PostgreSQL?<BR>
<A href="#item1.6">1.6</A>) What is the latest release?<BR>
<A href="#item1.7">1.7</A>) Where can I get support?<BR>
<A href="#item1.8">1.8</A>) How do I submit a bug report?<BR>
<A href="#item1.9">1.9</A>) How do I find out about known bugs or
missing features?<BR>
<A href="#item1.10">1.10</A>) What documentation is available?<BR>
<A href="#item1.11">1.11</A>) How can I learn <SMALL>SQL</SMALL>?<BR>
<A href="#item1.12">1.12</A>) How do I join the development team?<BR>
<A href="#item1.13">1.13</A>) How does PostgreSQL compare to other
<SMALL>DBMS</SMALL>s?<BR>
<H2 align="center">User Client Questions</H2>
<A href="#item2.1">2.1</A>) What interfaces are available for
PostgreSQL?<BR>
<A href="#item2.2">2.2</A>) What tools are available for using
PostgreSQL with Web pages?<BR>
<A href="#item2.3">2.3</A>) Does PostgreSQL have a graphical user
interface?<BR>
<H2 align="center">Administrative Questions</H2>
<A href="#item3.1">3.1</A>) How do I install PostgreSQL somewhere other
than <I>/usr/local/pgsql</I>?<BR>
<A href="#item3.2">3.2</A>) How do I control connections from other
hosts?<BR>
<A href="#item3.3">3.3</A>) How do I tune the database engine for
better performance?<BR>
<A href="#item3.4">3.4</A>) What debugging features are available?<BR>
<A href="#item3.5">3.5</A>) Why do I get <I>"Sorry, too many
clients"</I> when trying to connect?<BR>
<A href="#item3.6">3.6</A>) Why do I need to do a dump and restore
to upgrade PostgreSQL releases?<BR>
<A href="#item3.7">3.7</A>) What computer hardware should I use?<BR>
<H2 align="center">Operational Questions</H2>
<A href="#item4.1">4.1</A>) How do I <SMALL>SELECT</SMALL> only the
first few rows of a query? A random row?<BR>
<A href="#item4.2">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="#item4.3">4.3</A>) How do you change a column's data type?<BR>
<A href="#item4.4">4.4</A>) What is the maximum size for a row, a
table, and a database?<BR>
<A href="#item4.5">4.5</A>) How much database disk space is required
to store data from a typical text file?<BR>
<A href="#item4.6">4.6</A>) Why are my queries slow? Why don't they
use my indexes?<BR>
<A href="#item4.7">4.7</A>) How do I see how the query optimizer is
evaluating my query?<BR>
<A href="#item4.8">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="#item4.9">4.9</A>) In a query, how do I detect if a field
is <SMALL>NULL</SMALL>? How can I sort on whether a field is <SMALL>
NULL</SMALL> or not?<BR>
<A href="#item4.10">4.10</A>) What is the difference between the
various character types?<BR>
<A href="#item4.11.1">4.11.1</A>) How do I create a
serial/auto-incrementing field?<BR>
<A href="#item4.11.2">4.11.2</A>) How do I get the value of a
<SMALL>SERIAL</SMALL> insert?<BR>
<A href="#item4.11.3">4.11.3</A>) Doesn't <I>currval()</I>
lead to a race condition with other users?<BR>
<A href="#item4.11.4">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?<BR>
<A href="#item4.12">4.12</A>) What is an <SMALL>OID</SMALL>? What is a
<SMALL>CTID</SMALL>?<BR>
<A href="#item4.13">4.13</A>) Why do I get the error <I>"ERROR: Memory
exhausted in AllocSetAlloc()"</I>?<BR>
<A href="#item4.14">4.14</A>) How do I tell what PostgreSQL version I
am running?<BR>
<A href="#item4.15">4.15</A>) How do I create a column that will
default to the current time?<BR>
<A href="#item4.16">4.16</A>) How do I perform an outer join?<BR>
<A href="#item4.17">4.17</A>) How do I perform queries using multiple
databases?<BR>
<A href="#item4.18">4.18</A>) How do I return multiple rows or columns
from a function?<BR>
<A href="#item4.19">4.19</A>) Why do I get "relation with OID #####
does not exist" errors when accessing temporary tables in PL/PgSQL
functions?<BR>
<A href="#item4.20">4.20</A>) What replication solutions are available?<BR>
<A href="#item4.21">4.21</A>) Why are my table and column names not
recognized in my query?<BR>
<HR>
<H2 align="center">General Questions</H2>
<H3 id="item1.1">1.1) What is PostgreSQL? How is it pronounced?</H3>
<P>PostgreSQL is pronounced <I>Post-Gres-Q-L</I>, and is also sometimes
referred to as just <I>Postgres</I>. An audio file is available in
<a href="http://www.postgresql.org/files/postgresql.mp3">MP3 format</a> for
those would like to hear the pronunciation.</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>
<H3 id="item1.2">1.2) Who controls PostgreSQL?<BR></H3>
<P>If you are looking for a PostgreSQL gatekeeper, central committee,
or controlling company, give up --- there isn't one. We do have a
core committee and CVS committers, but these groups are more for
administrative purposes than control. The project is directed by
the community of developers and users, which anyone can join. All
you need to do is subscribe to the mailing lists and participate in the
discussions. (See the <a href="http://www.postgresql.org/docs/faqs.FAQ_DEV.html">
Developer's FAQ</A> for information on how to get involved in PostgreSQL
development.)</P>
<H3 id="item1.3">1.3) What is the copyright of
PostgreSQL?</H3>
<P>PostgreSQL is distributed under the classic BSD license. Basically,
it allows users to do anything they want with the code, including
reselling binaries without the source code. The only restriction is
that you not hold us legally liable for problems with the software.
There is also the requirement that this copyright appear in all copies
of the software. Here is the actual 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) 1994-1996 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>
<H3 id="item1.4">1.4) What platforms does PostgreSQL support?</H3>
<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>PostgreSQL also 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>
<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&amp;button=Search&amp;key=postgreSQL&amp;stype=all&amp;sort=type&amp;dir=%2F">
http://hobbes.nmsu.edu/cgi-bin/h-search?sh=1&amp;button=Search&amp;key=postgreSQL&amp;stype=all&amp;sort=type&amp;dir=%2F</a>.</p>
<H3 id="item1.5">1.5) Where can I get PostgreSQL?</H3>
<P>Via web browser, use <a href="http://www.postgresql.org/ftp/">
http://www.postgresql.org/ftp/</a>, and via ftp, use
<A href="ftp://ftp.PostgreSQL.org/pub/">
ftp://ftp.PostgreSQL.org/pub/</A>.</P>
<H3 id="item1.6">1.6) What is the latest release?</H3>
<P>The latest release of PostgreSQL is version 8.1.3.</P>
<P>We plan to have a major release every year, with minor releases
every few months.</P>
<H3 id="item1.7">1.7) Where can I get support?</H3>
<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>
<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 other 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>
<P>A list of commercial support companies is available at <A href=
"http://techdocs.postgresql.org/companies.php">
http://techdocs.postgresql.org/companies.php</A>.</P>
<H3 id="item1.8">1.8) How do I submit a bug report?</H3>
<P>Visit the PostgreSQL bug form at <A href=
"http://www.postgresql.org/support/submitbug">
http://www.postgresql.org/support/submitbug</A>.
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>
<P>Bugs submitted using the bug form or posted to any PostgreSQL mailing
list typically generates one of the following replies:</P>
<ul>
<li>It is not a bug, and why</li>
<li>It is a known bug and is already on the
<A href="http://www.postgresql.org/docs/faqs.TODO.html">TODO</A> list</li>
<li>The bug has been fixed in the current release</li>
<li>The bug has been fixed but is not packaged yet in an official
release</li>
<li>A request is made for more detailed information:
<ul>
<li>Operating system</li>
<li>PostgreSQL version</li>
<li>Reproducible test case</li>
<li>Debugging information</li>
<li>Debugger backtrace output</li>
</ul>
</li>
<li>The bug is new. The following might happen:
<ul>
<li>A patch is created and will be included in the next major
or minor release</li>
<li>The bug cannot be fixed immediately and is added
to the <A href="http://www.postgresql.org/docs/faqs.TODO.html">TODO</A>
list</li>
</ul>
</li>
</ul>
<H3 id="item1.9">1.9) How do I find out about known bugs or
missing features?</H3>
<P>PostgreSQL supports an extended subset of <SMALL>SQL:2003</SMALL>.
See our <A href="http://www.postgresql.org/docs/faqs.TODO.html">TODO</A>
list for known bugs, missing features, and future plans.</P>
<P>A feature request usually results in one of the following
replies:</P>
<ul>
<li>The feature is already on the
<A href="http://www.postgresql.org/docs/faqs.TODO.html">TODO</A>
list</li>
<li>The feature is not desired because:
<ul>
<li>It duplicates existing functionality that already
follows the SQL standard</li>
<li>The feature would increase code complexity but add little
benefit</li>
<li>The feature would be insecure or unreliable</li>
</ul>
</li>
<li>The new feature is added to the
<A href="http://www.postgresql.org/docs/faqs.TODO.html">TODO</A> list</li>
</ul>
<P>PostgreSQL does not use a bug tracking system because we find
it more efficient to respond directly to email and keep the
<A href="http://www.postgresql.org/docs/faqs.TODO.html">TODO</A>
list up-to-date. In practice, bugs don't last very long in the
software, and bugs that affect a large number of users are fixed
rapidly. The only place to find all changes, improvements, and
fixes in a PostgreSQL release is to read the
<a href="http://www.postgresql.org/developer/sourcecode/">CVS</a>
log messages. Even the release notes do not list every change
made to the software.</P>
<H3 id="item1.10">1.10) What documentation is available?</H3>
<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/books/awbook.html">http://www.postgresql.org/docs/books/awbook.html</A>
and <A href=
"http://www.commandprompt.com/ppbook/">http://www.commandprompt.com/ppbook/</A>.
There are a number of PostgreSQL books available for purchase. One
of the most popular ones is by Korry Douglas. A list of book reviews
can be found 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>
<H3 id="item1.11">1.11) How can I learn
<SMALL>SQL</SMALL>?</H3>
<P>First, consider the PostgreSQL-specific books mentioned above.
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>.
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>
<P>There is also 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>
<H3 id="item1.12">1.12) How do I join the development
team?</H3>
<P>See the <a href="http://www.postgresql.org/docs/faqs.FAQ_DEV.html">
Developer's FAQ</A>.</P>
<H3 id="item1.13">1.13) How does PostgreSQL compare to other
<SMALL>DBMS</SMALL>s?</H3>
<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. Our performance is usually +/-10% compared to other databases.
<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="#item1.7">FAQ section 1.7</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>
<H3 id="item2.1">2.1) What interfaces are available for
PostgreSQL?</H3>
<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>
<H3 id="item2.2">2.2) What tools are available for using
PostgreSQL with Web pages?</H3>
<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 DBD::Pg with CGI.pm or
mod_perl.</P>
<H3 id="item2.3">2.3) Does PostgreSQL have a graphical user
interface?</H3>
<P>Yes, see <a href="http://techdocs.postgresql.org/guides/GUITools">
http://techdocs.postgresql.org/guides/GUITools</a> for a
detailed list.</P>
<HR>
<H2 align="center">Administrative Questions</H2>
<H3 id="item3.1">3.1) How do I install PostgreSQL somewhere
other than <I>/usr/local/pgsql</I>?</H3>
<P>Specify the <I>--prefix</I> option when running
<I>configure</I>.</P>
<H3 id="item3.2">3.2) How do I control connections from other
hosts?</H3>
<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>
<H3 id="item3.3">3.3) How do I tune the database engine for
better performance?</H3>
<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>
<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>
<H3 id="item3.4">3.4) What debugging features are
available?</H3>
<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>
<H3 id="item3.5">3.5) Why do I get <I>"Sorry, too many
clients"</I> when trying to connect?</H3>
<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>
<H3 id="item3.6">3.6) Why do I need to do a dump and restore
to upgrade between major PostgreSQL releases?</H3>
<P>The PostgreSQL team makes only small changes between minor releases,
so upgrading from 7.4.0 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>
<H3 id="item3.7">3.7) What computer hardware should I use?</H3>
<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>
<H3 id="item4.1">4.1) How do I <SMALL>SELECT</SMALL> only the
first few rows of a query? A random row?</H3>
<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:</P>
<PRE>
SELECT col
FROM tab
ORDER BY random()
LIMIT 1;
</PRE>
<H3 id="item4.2">4.2) 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?</H3>
<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>
<H3 id="item4.3">4.3) How do you change a column's data type?</H3>
<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>
<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>
<H3 id="item4.4">4.4) What is the maximum size for a row, a
table, and a database?</H3>
<P>These are the limits:</P>
<BLOCKQUOTE>
<TABLE>
<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>
</BLOCKQUOTE>
<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>
<P>One limitation is that indexes can not be created on columns
longer than about 2,000 characters. Fortunately, such indexes are
rarely needed. Uniqueness is best guaranteed by a funtion index
of an MD5 hash of the long column, and full text indexing
allows for searching of words within the column.</P>
<H3 id="item4.5">4.5) How much database disk space is required
to store data from a typical text file?</H3>
<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 5.6 MB:</P>
<PRE>
28 bytes: each row header (approximate)
24 bytes: one int field and one text field
+ 4 bytes: pointer on page to tuple
----------------------------------------
56 bytes per row
The data page size in PostgreSQL is 8192 bytes (8 KB), so:
8192 bytes per page
------------------- = 146 rows per database page (rounded down)
56 bytes per row
100000 data rows
-------------------- = 685 database pages (rounded up)
146 rows per page
685 database pages * 8192 bytes per page = 5,611,520 bytes (5.6 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>
<H3 id="item4.6">4.6) Why are my queries slow? Why don't they
use my indexes?</H3>
<P>Indexes are not used by every query. Indexes are used only 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>
<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.</P>
<P>If you believe the optimizer is incorrect in choosing a
sequential scan, use <CODE>SET enable_seqscan TO 'off'</CODE> and
run query again 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="#item4.8">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>
<H3 id="item4.7">4.7) How do I see how the query optimizer is
evaluating my query?</H3>
<P>See the <SMALL>EXPLAIN</SMALL> manual page.</P>
<H3 id="item4.8">4.8) How do I perform regular expression
searches and case-insensitive regular expression searches? How do I
use an index for case-insensitive searches?</H3>
<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>
<P>If the above index is created as <SMALL>UNIQUE</SMALL>, though
the column can store upper and lowercase characters, it can not have
identical values that differ only in case. To force a particular
case to be stored in the column, use a <SMALL>CHECK</SMALL>
constraint or a trigger.</P>
<H3 id="item4.9">4.9) In a query, how do I detect if a field
is <SMALL>NULL</SMALL>? How can I sort on whether a field is <SMALL>
NULL</SMALL> or not?</H3>
<P>You test the column with <SMALL>IS NULL</SMALL> and <SMALL>IS
NOT NULL</SMALL>, like this:</P>
<PRE>
SELECT *
FROM tab
WHERE col IS NULL;
</PRE>
<P>To sort by the <SMALL>NULL</SMALL> status, use the <SMALL>IS NULL</SMALL>
and <SMALL>IS NOT NULL</SMALL> modifiers in your <SMALL>ORDER BY</SMALL> clause.
Things that are <I>true</I> will sort higher than things that are <I>false</I>,
so the following will put NULL entries at the top of the resulting list:</P>
<PRE>
SELECT *
FROM tab
ORDER BY (col IS NOT NULL)
</PRE>
<H3 id="item4.10">4.10) What is the difference between the
various character types?</H3>
<BLOCKQUOTE>
<TABLE>
<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>
</BLOCKQUOTE>
<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, long values are also subject to compression, 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>
<H3 id="item4.11.1">4.11.1) How do I create a
serial/auto-incrementing field?</H3>
<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.
<H3 id="item4.11.2">4.11.2) How do I get the value of a
<SMALL>SERIAL</SMALL> insert?</H3>
<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="#item4.11.1">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 &lt;<I>table</I>&gt;_&lt;<I>
serialcolumn</I>&gt;_<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>
<H3 id="item4.11.3">4.11.3) Doesn't <I>currval()</I>
lead to a race condition with other users?</H3>
<P>No. <I>currval()</I> returns the current value assigned by your
session, not by all sessions.</P>
<H3 id="item4.11.4">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?</H3>
<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>
<H3 id="item4.12">4.12) What is an <SMALL>OID</SMALL>? What is
a <SMALL>CTID</SMALL>?</H3>
<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>C<SMALL>TID</SMALL>s are used to identify specific physical rows
with block and offset values. C<SMALL>TID</SMALL>s change after rows
are modified or reloaded. They are used by index entries to point
to physical rows.</P>
<H3 id="item4.13">4.13) Why do I get the error <I>"ERROR:
Memory exhausted in AllocSetAlloc()"</I>?</H3>
<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.
<H3 id="item4.14">4.14) How do I tell what PostgreSQL version
I am running?</H3>
<P>From <I>psql</I>, type <CODE>SELECT version();</CODE></P>
<H3 id="item4.15">4.15) How do I create a column that will
default to the current time?</H3>
<P>Use <I>CURRENT_TIMESTAMP</I>:</P>
<PRE>
CREATE TABLE test (x int, modtime TIMESTAMP DEFAULT CURRENT_TIMESTAMP );
</PRE>
<H3 id="item4.16">4.16) How do I perform an outer join?</H3>
<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>
<H3 id="item4.17">4.17) How do I perform queries using
multiple databases?</H3>
<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>
<H3 id="item4.18">4.18) How do I return multiple rows or
columns from a function?</H3>
<P>It is easy using set-returning functions,
<a href="http://techdocs.postgresql.org/guides/SetReturningFunctions">
http://techdocs.postgresql.org/guides/SetReturningFunctions</a></P>.
<H3 id="item4.19">4.19) Why do I get "relation with OID #####
does not exist" errors when accessing temporary tables in PL/PgSQL
functions?</H3>
<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>
<H3 id="item4.20">4.20) What replication solutions are available?
</H3>
<P>Though "replication" is a single term, there are several technologies
for doing replication, with advantages and disadvantages for each.</P>
<P>Master/slave replication allows a single master to receive read/write
queries, while slaves can only accept read/<SMALL>SELECT</SMALL>
queries. The most popular freely available master-slave PostgreSQL
replication solution is <A
href="http://gborg.postgresql.org/project/slony1/projdisplay.php">
Slony-I</A>.</P>
<P>Multi-master replication allows read/write queries to be sent to
multiple replicated computers. This capability also has a severe impact
on performance due to the need to synchronize changes between servers.
<A href="http://pgfoundry.org/projects/pgcluster/">Pgcluster</a> is the
most popular such solution freely available for PostgreSQL.</P>
<P>There are also commercial and hardware-based replication solutions
available supporting a variety of replication models.</P>
<H3 id="item4.21">4.21) Why are my table and column names not
recognized in my query?</H3>
<P>The most common cause is the use of double-quotes around table or
column names during table creation. When double-quotes are used,
table and column names (called identifiers) are stored <a
href="http://www.postgresql.org/docs/8.0/static/sql-syntax.html#SQL-SYNTAX-IDENTIFIERS">
case-sensitive</a>, meaning you must use double-quotes when
referencing the names in a query. Some interfaces, like pgAdmin,
automatically double-quote identifiers during table creation. So,
for identifiers to be recognized, you must either:
<UL>
<LI>Avoid double-quoting identifiers when creating tables</LI>
<LI>Use only lowercase characters in identifiers</LI>
<LI>Double-quote identifiers when referencing them in queries</LI>
</UL>
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