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        <H1>Frequently Asked Questions (FAQ) for PostgreSQL</H1>
    
        <P>Last updated: Tue Oct  9 15:52:10 EDT 2007</P>
    
        <P>Current maintainer: Bruce Momjian (<A href=
        "mailto:bruce@momjian.us">bruce@momjian.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 most recent 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 submit a patch or join the
         development team?<BR>
         <A href="#item1.13">1.13</A>) How does PostgreSQL compare to other
         <SMALL>DBMS</SMALL>s?<BR>
         <A href="#item1.14">1.14</A>) Will PostgreSQL handle recent daylight
         saving time changes in various countries?<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>  What is the upgrade process for PostgreSQL?<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 do I concatenate possible <SMALL>NULL</SMALL>s?
        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?  Why is capitalization not preserved?<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>, but can also be
        referred to as simply <I>Postgres</I>, particularly in conversation.
        (For those curious about how to say "PostgreSQL", an <a
        href="http://www.postgresql.org/files/postgresql.mp3">audio file</a>
        is available.)</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 Postgres
        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) 1996-2007, 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 SP4, 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 Postgres 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 most recent release?</H3>
    
        <P>The latest release of PostgreSQL is version 8.2.5.</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>), a French one,
        (<I>#postgresqlfr</I>), and a Brazilian one, (<I>#postgresql-br</I>).
        There is also a PostgreSQL channel on EFNet.</P>
    
        <P>A list of commercial support companies is available at <A href=
        "http://www.postgresql.org/support/professional_support">
        http://www.postgresql.org/support/professional_support</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 Postgres 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 Postgres 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 are also many nice tutorials available online:
        <UL>
        <LI> <A href="http://www.intermedia.net/support/sql/sqltut.shtm">http://www.intermedia.net/support/sql/sqltut.shtm</A>
        </LI>
        <LI>
        <A href="http://sqlcourse.com/">http://sqlcourse.com</A>
        </LI>
        <LI>
        <A href="http://www.w3schools.com/sql/default.asp">http://www.w3schools.com/sql/default.asp</A>
        </LI>
        <LI><A href=
        "http://mysite.verizon.net/Graeme_Birchall/id1.html">http://mysite.verizon.net/Graeme_Birchall/id1.html</A>
        </LI>
        </UL>
    
        <H3 id="item1.12">1.12) How do I submit a patch or 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>
    
        <H3 id="item1.14">1.14) Will PostgreSQL handle recent daylight
        saving time changes in various countries?</H3>
    
        <P>USA daylight saving time changes are included in PostgreSQL release
        8.0.[4+], and all later major releases, e.g. 8.1.  Canada and Western
        Australia changes are included in 8.0.[10+], 8.1.[6+], and all later
        major releases.  PostgreSQL releases prior to 8.0 use the operating
        system's timezone database for daylight saving information.</P>
    
        <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>There are a large number of GUI Tools that are available for PostgreSQL
        from both commercial and open source developers. A detailed list can be
        found in the <A href="http://www.postgresql.org/docs/techdocs.54">
        PostgreSQL Community Documentation</A></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://www.powerpostgresql.com/PerfList/">
        http://www.powerpostgresql.com/PerfList/</a> and <a
        href="http://momjian.us/main/writings/pgsql/hw_performance/index.html">
        http://momjian.us/main/writings/pgsql/hw_performance/index.html</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 of 100 database sessions. You
        need to increase the server'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 server.</P>
    
        <H3 id="item3.6">3.6) What is the upgrade process for PostgreSQL?</H3>
    
        <P>See <a
        href="http://www.postgresql.org/support/versioning">http://www.postgresql.org/support/versioning</a>
        for a general discussion about upgrading, and <a href=
        "http://www.postgresql.org/docs/current/static/install-upgrading.html">
        http://www.postgresql.org/docs/current/static/install-upgrading.html</a>
        for specific instructions.</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>400 GB</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, row size, and maximum number of columns
        can be quadrupled by increasing the default block size to 32k.  The
        maximum table size can also be increased using table partitioning.</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 function 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 Postgres database file containing this data can
        be estimated as 5.2 MB:</P>
    <PRE>
        24 bytes: each row header (approximate)
        24 bytes: one int field and one text field
       + 4 bytes: pointer on page to tuple
       ----------------------------------------
        52 bytes per row
    
       The data page size in Postgres is 8192 bytes (8 KB), so:
    
       8192 bytes per page
       -------------------   =  158 rows per database page (rounded down)
         52 bytes per row
    
       100000 data rows
       --------------------  =  633 database pages (rounded up)
          158 rows per page
    
    633 database pages * 8192 bytes per page  =  5,185,536 bytes (5.2 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.
        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 an
        expression 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 do I concatenate possible <SMALL>NULL</SMALL>s?
        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 concatentate with possible <SMALL>NULL</SMALL>s, use <I>COALESCE()</I>,
       like this:</P>
    <PRE>
       SELECT COALESCE(col1, '') || COALESCE(col2, '')
       FROM tab
    </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>
    
        <P>Automatically created sequence are 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 the table and
        <SMALL>SERIAL</SMALL> column, respectively.  See the
        <I>create_sequence</I> manual page for more information about
        sequences.</P>
    
        <H3 id="item4.11.2">4.11.2) How do I get the value of a
        <SMALL>SERIAL</SMALL> insert?</H3>
    
        <P>The simplest way is to retrieve the assigned <SMALL>SERIAL</SMALL>
        value with <SMALL>RETURNING</SMALL>.  Using the example table in <A
        href="#item4.11.1">4.11.1</A>, it would look like this:</P>
    
    <PRE>
        INSERT INTO person (name) VALUES ('Blaise Pascal') RETURNING id;
    </PRE>
    
        You can also call <I>nextval()</I> and use that value in the
        <SMALL>INSERT</SMALL>, or call <I>currval()</I> <I>after</I> the
        <SMALL>INSERT</SMALL>.
        
        <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>If a table is created <SMALL>WITH OIDS</SMALL>, each row
        gets a unique a <SMALL>OID</SMALL>.  O<SMALL>ID</SMALL>s are
        automatically 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 rows 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 the server:</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://www.postgresql.org/docs/techdocs.17">
        http://www.postgresql.org/docs/techdocs.17</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?  Why is capitalization not preserved?</H3>
    
        <P>The most common cause of unrecognized names 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/current/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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