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    <H1>Frequently Asked Questions (FAQ) for PostgreSQL</H1>

    <P>Last updated: Sun Oct 13 22:49:56 EDT 2002</P>

    <P>Current maintainer: Bruce Momjian (<A href=
    "mailto:pgman@candle.pha.pa.us">pgman@candle.pha.pa.us</A>)<BR>
    </P>

    <P>The most recent version of this document can be viewed at <A
    href=
    "http://www.PostgreSQL.org/docs/faq-english.html">http://www.PostgreSQL.org/docs/faq-english.html</A>.</P>

    <P>Platform-specific questions are answered at <A href=
    "http://www.PostgreSQL.org/users-lounge/docs/faq.html">http://www.PostgreSQL.org/users-lounge/docs/faq.html</A>.</P>
    <HR>

    <H2 align="center">General Questions</H2>
    <A href="#1.1">1.1</A>) What is PostgreSQL? How is it pronounced?<BR>
     <A href="#1.2">1.2</A>) What is the copyright on PostgreSQL?<BR>
     <A href="#1.3">1.3</A>) What Unix platforms does PostgreSQL run
    on?<BR>
     <A href="#1.4">1.4</A>) What non-Unix ports are available?<BR>
     <A href="#1.5">1.5</A>) Where can I get PostgreSQL?<BR>
     <A href="#1.6">1.6</A>) Where can I get support?<BR>
     <A href="#1.7">1.7</A>) What is the latest release?<BR>
     <A href="#1.8">1.8</A>) What documentation is available?<BR>
     <A href="#1.9">1.9</A>) How do I find out about known bugs or
    missing features?<BR>
     <A href="#1.10">1.10</A>) How can I learn <SMALL>SQL</SMALL>?<BR>
     <A href="#1.11">1.11</A>) Is PostgreSQL Y2K compliant?<BR>
     <A href="#1.12">1.12</A>) How do I join the development team?<BR>
     <A href="#1.13">1.13</A>) How do I submit a bug report?<BR>
     <A href="#1.14">1.14</A>) How does PostgreSQL compare to other
    <SMALL>DBMS</SMALL>s?<BR>
     <A href="#1.15">1.15</A>) How can I financially assist
    PostgreSQL?<BR>
     

    <H2 align="center">User Client Questions</H2>
    <A href="#2.1">2.1</A>) Are there <SMALL>ODBC</SMALL> drivers for
    PostgreSQL?<BR>
     <A href="#2.2">2.2</A>) What tools are available for using
    PostgreSQL with Web pages?<BR>
     <A href="#2.3">2.3</A>) Does PostgreSQL have a graphical user
    interface? A report generator? An embedded query language
    interface?<BR>
     <A href="#2.4">2.4</A>) What languages are available to
    communicate with PostgreSQL?<BR>
     

    <H2 align="center">Administrative Questions</H2>
    <A href="#3.1">3.1</A>) How do I install PostgreSQL somewhere other
    than <I>/usr/local/pgsql</I>?<BR>
     <A href="#3.2">3.2</A>) When I start <I>postmaster</I>, I get a
    <I>Bad System Call</I> or core dumped message. Why?<BR>
     <A href="#3.3">3.3</A>) When I try to start <I>postmaster</I>, I
    get <I>IpcMemoryCreate</I> errors. Why?<BR>
     <A href="#3.4">3.4</A>) When I try to start <I>postmaster</I>, I
    get <I>IpcSemaphoreCreate</I> errors. Why?<BR>
     <A href="#3.5">3.5</A>) How do I control connections from other
    hosts?<BR>
     <A href="#3.6">3.6</A>) How do I tune the database engine for
    better performance?<BR>
     <A href="#3.7">3.7</A>) What debugging features are available?<BR>
     <A href="#3.8">3.8</A>) Why do I get <I>"Sorry, too many
    clients"</I> when trying to connect?<BR>
     <A href="#3.9">3.9</A>) What are the <I>pg_sorttempNNN.NN</I>
    files in my database directory?<BR>
     <A href="#3.10">3.10</A>) Why do I need to do a dump and restore 
    to upgrade PostgreSQL releases?<BR>
     

    <H2 align="center">Operational Questions</H2>
    <A href="#4.1">4.1</A>) What is the difference between binary
    cursors and normal cursors?<BR>
     <A href="#4.2">4.2</A>) How do I <SMALL>SELECT</SMALL> only the
    first few rows of a query?<BR>
     <A href="#4.3">4.3</A>) How do I get a list of tables or other
    things I can see in <I>psql</I>?<BR>
     <A href="#4.4">4.4</A>) How do you remove a column from a
    table?<BR>
     <A href="#4.5">4.5</A>) What is the maximum size for a row, a
    table, and a database?<BR>
     <A href="#4.6">4.6</A>) How much database disk space is required
    to store data from a typical text file?<BR>
     <A href="#4.7">4.7</A>) How do I find out what tables, indexes,
    databases, and users are defined?<BR>
     <A href="#4.8">4.8</A>) My queries are slow or don't make use of
    the indexes. Why?<BR>
     <A href="#4.9">4.9</A>) How do I see how the query optimizer is
    evaluating my query?<BR>
     <A href="#4.10">4.10</A>) What is an R-tree index?<BR>
     <A href="#4.11">4.11</A>) What is the Genetic Query Optimizer?<BR>
     <A href="#4.12">4.12</A>) How do I perform regular expression
    searches and case-insensitive regular expression searches? How do I
    use an index for case-insensitive searches?<BR>
     <A href="#4.13">4.13</A>) In a query, how do I detect if a field
    is <SMALL>NULL</SMALL>?<BR>
     <A href="#4.14">4.14</A>) What is the difference between the
    various character types?<BR>
     <A href="#4.15.1">4.15.1</A>) How do I create a
    serial/auto-incrementing field?<BR>
     <A href="#4.15.2">4.15.2</A>) How do I get the value of a
    <SMALL>SERIAL</SMALL> insert?<BR>
     <A href="#4.15.3">4.15.3</A>) Don't <I>currval()</I> and
    <I>nextval()</I> lead to a race condition with other users?<BR>
     <A href="#4.15.4">4.15.4</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="#4.16">4.16</A>) What is an <SMALL>OID</SMALL>? What is a
    <SMALL>TID</SMALL>?<BR>
     <A href="#4.17">4.17</A>) What is the meaning of some of the terms
    used in PostgreSQL?<BR>
     <A href="#4.18">4.18</A>) Why do I get the error <I>"ERROR: Memory
    exhausted in AllocSetAlloc()"</I>?<BR>
     <A href="#4.19">4.19</A>) How do I tell what PostgreSQL version I
    am running?<BR>
     <A href="#4.20">4.20</A>) Why does my large-object operations get
    <I>"invalid large obj descriptor"</I>?<BR>
     <A href="#4.21">4.21</A>) How do I create a column that will
    default to the current time?<BR>
     <A href="#4.22">4.22</A>) Why are my subqueries using
    <CODE><SMALL>IN</SMALL></CODE> so slow?<BR>
     <A href="#4.23">4.23</A>) How do I perform an outer join?<BR>
     <A href="#4.24">4.24</A>) How do I perform queries using multiple
    databases?<BR>
     <A href="#4.25">4.25</A>) How do I return multiple rows or columns
    from a function?<BR>
     <A href="#4.26">4.26</A>) Why can't I reliably create/drop
    temporary tables in PL/PgSQL functions?<BR>
     <A href="#4.27">4.27</A>) What replication options are available?<BR>
     <A href="#4.28">4.28</A>) What encryption options are available?<BR>
     

    <H2 align="center">Extending PostgreSQL</H2>
    <A href="#5.1">5.1</A>) I wrote a user-defined function. When I run
    it in <I>psql</I>, why does it dump core?<BR>
     <A href="#5.2">5.2</A>) How can I contribute some nifty new types
    and functions to PostgreSQL?<BR>
     <A href="#5.3">5.3</A>) How do I write a C function to return a
    tuple?<BR>
     <A href="#5.4">5.4</A>) I have changed a source file. Why does the
    recompile not see the change?<BR>
     
    <HR>

    <H2 align="center">General Questions</H2>

    <H4><A name="1.1">1.1</A>) What is PostgreSQL? How is it pronounced?</H4>

    <P>PostgreSQL is pronounced <I>Post-Gres-Q-L</I>.</P>

    <P>PostgreSQL is an enhancement of the POSTGRES database management
    system, a next-generation <SMALL>DBMS</SMALL> research prototype.
    While PostgreSQL retains the powerful data model and rich data
    types of POSTGRES, it replaces the PostQuel query language with an
    extended subset of <SMALL>SQL</SMALL>. PostgreSQL is free and the
    complete source is available.</P>

    <P>PostgreSQL development is performed by a team of
    developers who all subscribe to the PostgreSQL development mailing
    list. The current coordinator is Marc G. Fournier (<A href=
    "mailto:scrappy@PostgreSQL.org">scrappy@PostgreSQL.org</A>). (See
    section <a href="#1.6">1.6</a> on how to join). This team is now 
    responsible for all development of PostgreSQL.</P>

    <P>The authors of PostgreSQL 1.01 were Andrew Yu and Jolly Chen.
    Many others have contributed to the porting, testing, debugging,
    and enhancement of the code. The original Postgres code, from which
    PostgreSQL is derived, was the effort of many graduate students,
    undergraduate students, and staff programmers working under the
    direction of Professor Michael Stonebraker at the University of
    California, Berkeley.</P>

    <P>The original name of the software at Berkeley was Postgres. When
    <SMALL>SQL</SMALL> functionality was added in 1995, its name was
    changed to Postgres95. The name was changed at the end of 1996 to
    PostgreSQL.</P>

    <H4><A name="1.2">1.2</A>) What is the copyright on
    PostgreSQL?</H4>

    <P>PostgreSQL is subject to the following COPYRIGHT:</P>

    <P>PostgreSQL Data Base Management System</P>

    <P>Portions copyright (c) 1996-2002, PostgreSQL Global Development
    Group Portions Copyright (c) 1994-6 Regents of the University of
    California</P>

    <P>Permission to use, copy, modify, and distribute this software
    and its documentation for any purpose, without fee, and without a
    written agreement is hereby granted, provided that the above
    copyright notice and this paragraph and the following two
    paragraphs appear in all copies.</P>

    <P>IN NO EVENT SHALL THE UNIVERSITY OF CALIFORNIA BE LIABLE TO ANY
    PARTY FOR DIRECT, INDIRECT, SPECIAL, INCIDENTAL, OR CONSEQUENTIAL
    DAMAGES, INCLUDING LOST PROFITS, ARISING OUT OF THE USE OF THIS
    SOFTWARE AND ITS DOCUMENTATION, EVEN IF THE UNIVERSITY OF
    CALIFORNIA HAS BEEN ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.</P>

    <P>THE UNIVERSITY OF CALIFORNIA SPECIFICALLY DISCLAIMS ANY
    WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
    OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE. THE
    SOFTWARE PROVIDED HEREUNDER IS ON AN "AS IS" BASIS, AND THE
    UNIVERSITY OF CALIFORNIA HAS NO OBLIGATIONS TO PROVIDE MAINTENANCE,
    SUPPORT, UPDATES, ENHANCEMENTS, OR MODIFICATIONS.</P>

    <P>The above is the BSD license, the classic open-source license.
    It has no restrictions on how the source code may be used. We like
    it and have no intention of changing it.</P>

    <H4><A name="1.3">1.3</A>) What Unix platforms does PostgreSQL run
    on?</H4>

    <P>In general, a 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>

    <H4><A name="1.4">1.4</A>) What non-Unix ports are available?</H4>

    <P><STRONG>Client</STRONG></P>

    <P>It is possible to compile the <I>libpq</I> C library, psql, and
    other interfaces and client applications to run on MS Windows platforms.
    In this case, the client is running on MS Windows, and communicates
    via TCP/IP to a server running on one of our supported Unix
    platforms. A file <I>win32.mak</I> is included in the distribution
    for making a Win32 <I>libpq</I> library and <I>psql</I>. PostgreSQL
    also communicates with <SMALL>ODBC</SMALL> clients.</P>

    <P><STRONG>Server</STRONG></P>

    <P>The database server can run on Windows NT and Win2k using
    Cygwin, the Cygnus Unix/NT porting library. See
    <I>pgsql/doc/FAQ_MSWIN</I> in the distribution or the MS Windows FAQ
    at <A href="http://www.PostgreSQL.org/docs/faq-mswin.html">http://www.PostgreSQL.org/docs/faq-mswin.html</A>.</P>

    <p>A native port to some Microsoft platforms is currently being worked
    upon.</p>

    <H4><A name="1.5">1.5</A>) Where can I get PostgreSQL?</H4>

    <P>The primary anonymous ftp site for PostgreSQL is <A href=
    "ftp://ftp.PostgreSQL.org/pub">ftp://ftp.PostgreSQL.org/pub</A>.
    For mirror sites, see our main web site.</P>

    <H4><A name="1.6">1.6</A>) Where can I get support?</H4>

    <P>The main mailing list is: <A href=
    "mailto:pgsql-general@PostgreSQL.org">pgsql-general@PostgreSQL.org</A>.
    It is available for discussion of matters pertaining to PostgreSQL.
    To subscribe, send mail with the following lines in the body (not
    the subject line):</P>
<PRE>
    subscribe
    end
</PRE>

    <P>to <A href=
    "mailto:pgsql-general-request@PostgreSQL.org">pgsql-general-request@PostgreSQL.org</A>.</P>

    <P>There is also a digest list available. To subscribe to this
    list, send email to: <A href=
    "mailto:pgsql-general-digest-request@PostgreSQL.org">pgsql-general-digest-request@PostgreSQL.org</A>
    with a body of:</P>
<PRE>
    subscribe
    end
</PRE>

    Digests are sent out to members of this list whenever the main list
    has received around 30k of messages. 

    <P>The bugs mailing list is available. To subscribe to this list,
    send email to <A href=
    "mailto:pgsql-bugs-request@PostgreSQL.org">pgsql-bugs-request@PostgreSQL.org</A>
    with a body of:</P>
<PRE>
    subscribe
    end
</PRE>

    There is also a developers discussion mailing list available. To
    subscribe to this list, send email to <A href=
    "mailto:pgsql-hackers-request@PostgreSQL.org">pgsql-hackers-request@PostgreSQL.org</A>
    with a body of: 
<PRE>
    subscribe
    end
</PRE>

    <P>Additional mailing lists and information about PostgreSQL can be
    found via the PostgreSQL WWW home page at:</P>

    <BLOCKQUOTE>
      <A href="http://www.PostgreSQL.org">http://www.PostgreSQL.org</A>
    </BLOCKQUOTE>

    <P>There is also an IRC channel on EFNet, channel
    <I>#PostgreSQL</I>. I use the Unix command <CODE>irc -c
    '#PostgreSQL' "$USER" irc.phoenix.net.</CODE></P>

    <P>A list of commercial support companies is available at <A href=
    "http://www.PostgreSQL.org/users-lounge/commercial-support.html">http://www.PostgreSQL.org/users-lounge/commercial-support.html</A>.</P>

    <H4><A name="1.7">1.7</A>) What is the latest release?</H4>

    <P>The latest release of PostgreSQL is version 7.2.3.</P>

    <P>We plan to have major releases every four months.</P>

    <H4><A name="1.8">1.8</A>) What documentation is available?</H4>

    <P>Several manuals, manual pages, and some small test examples are
    included in the distribution. See the <I>/doc</I> directory. You
    can also browse the manual online at <A href=
    "http://www.PostgreSQL.org/users-lounge/docs/">http://www.PostgreSQL.org/users-lounge/docs/</A>.</P>

    <P>There are two PostgreSQL books available online at <A href=
    "http://www.PostgreSQL.org/docs/awbook.html">http://www.PostgreSQL.org/docs/awbook.html</A>
    and <A href=
    "http://www.commandprompt.com/ppbook/">http://www.commandprompt.com/ppbook/</A>.
    There is a list of PostgreSQL books available for purchase at <A
    href=
    "http://www.ca.PostgreSQL.org/books/">http://www.ca.PostgreSQL.org/books/</A>.
    There is also a collection of PostgreSQL technical articles at <A
    href=
    "http://techdocs.PostgreSQL.org/">http://techdocs.PostgreSQL.org/</A>.</P>

    <P><I>psql</I> has some nice \d commands to show information about
    types, operators, functions, aggregates, etc.</P>

    <P>Our web site contains even more documentation.</P>

    <H4><A name="1.9">1.9</A>) How do I find out about known bugs or
    missing features?</H4>

    <P>PostgreSQL supports an extended subset of <SMALL>SQL</SMALL>-92.
    See our <A href="http://developer.PostgreSQL.org/todo.php">TODO</A>
    list for known bugs, missing features, and future plans.</P>

    <H4><A name="1.10">1.10</A>) How can I learn
    <SMALL>SQL</SMALL>?</H4>

    <P>The PostgreSQL book at <A href=
    "http://www.PostgreSQL.org/docs/awbook.html">http://www.PostgreSQL.org/docs/awbook.html</A>
    teaches <SMALL>SQL</SMALL>. There is another PostgreSQL book at <A
    href=
    "http://www.commandprompt.com/ppbook/">http://www.commandprompt.com/ppbook.</A>
    There is a nice tutorial at <A href=
    "http://www.intermedia.net/support/sql/sqltut.shtm">http://www.intermedia.net/support/sql/sqltut.shtm,</A>
    at <A href=
    "http://ourworld.compuserve.com/homepages/graeme_birchall/HTM_COOK.HTM">
    http://ourworld.compuserve.com/homepages/graeme_birchall/HTM_COOK.HTM,</A>
    and at <A href=
    "http://sqlcourse.com/">http://sqlcourse.com.</A></P>

    <P>Another one is "Teach Yourself SQL in 21 Days, Second Edition"
    at <A href=
    "http://members.tripod.com/er4ebus/sql/index.htm">http://members.tripod.com/er4ebus/sql/index.htm</A></P>

    <P>Many of our users like <I>The Practical SQL Handbook</I>,
    Bowman, Judith S., et al., Addison-Wesley. Others like <I>The
    Complete Reference SQL</I>, Groff et al., McGraw-Hill.</P>

    <H4><A name="1.11">1.11</A>) Is PostgreSQL Y2K compliant?</H4>

    <P>Yes, we easily handle dates past the year 2000 AD, and before
    2000 BC.</P>

    <H4><A name="1.12">1.12</A>) How do I join the development
    team?</H4>

    <P>First, download the latest source and read the PostgreSQL
    Developers documentation on our web site, or in the distribution.
    Second, subscribe to the <I>pgsql-hackers</I> and
    <I>pgsql-patches</I> mailing lists. Third, submit high quality
    patches to pgsql-patches.</P>

    <P>There are about a dozen people who have commit privileges to the
    PostgreSQL <SMALL>CVS</SMALL> archive. They each have submitted so
    many high-quality patches that it was impossible for the existing
    committers to keep up, and we had confidence that patches they
    committed were of high quality.</P>

    <H4><A name="1.13">1.13</A>) How do I submit a bug report?</H4>

    <P>Please visit the PostgreSQL BugTool page at <A href=
    "http://www.PostgreSQL.org/bugs/bugs.php">http://www.PostgreSQL.org/bugs/bugs.php</A>,
    which gives guidelines and directions on how to submit a
    bug report.</P>

    <P>Also check out our ftp site <A href=
    "ftp://ftp.PostgreSQL.org/pub">ftp://ftp.PostgreSQL.org/pub</A> to
    see if there is a more recent PostgreSQL version or patches.</P>

    <H4><A name="1.14">1.14</A>) How does PostgreSQL compare to other
    <SMALL>DBMS</SMALL>s?</H4>

    <P>There are several ways of measuring software: features,
    performance, reliability, support, and price.</P>

    <DL>
      <DT><B>Features</B></DT>

      <DD>PostgreSQL has most features present in large commercial
      <SMALL>DBMS</SMALL>s, like transactions, subselects, triggers,
      views, foreign key referential integrity, and sophisticated
      locking. We have some features they do not have, like
      user-defined types, inheritance, rules, and multi-version
      concurrency control to reduce lock contention.<BR>
      <BR>
      </DD>

      <DT><B>Performance</B></DT>

      <DD>PostgreSQL has performance similar to other commercial and
      open source databases. it is faster for some things, slower for
      others. In comparison to MySQL or leaner database systems, we are
      slower on inserts/updates because of transaction overhead. Of
      course, MySQL does not have any of the features mentioned in the
      <I>Features</I> section above. We are built for reliability and
      features, though we continue to improve performance in every
      release. There is an interesting Web page comparing PostgreSQL to
      MySQL at <A href= "http://openacs.org/why-not-mysql.html">

      http://openacs.org/why-not-mysql.html</A><BR>

      <BR>
      </DD>

      <DT><B>Reliability</B></DT>

      <DD>We realize that a <SMALL>DBMS</SMALL> must be reliable, or it
      is worthless. We strive to release well-tested, stable code that
      has a minimum of bugs. Each release has at least one month of
      beta testing, and our release history shows that we can provide
      stable, solid releases that are ready for production use. We
      believe we compare favorably to other database software in this
      area.<BR>
      <BR>
      </DD>

      <DT><B>Support</B></DT>

      <DD>Our mailing lists provide contact with a large group of developers
      and users to help resolve any problems encountered. While we cannot
      guarantee a fix, commercial <SMALL>DBMS</SMALL>s do not always
      supply a fix either. Direct access to developers, the user
      community, manuals, and the source code often make PostgreSQL
      support superior to other <SMALL>DBMS</SMALL>s. There is
      commercial per-incident support available for those who need it.
      (See <A href="#1.6">FAQ section 1.6</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>

    <H4><A name="1.15">1.15</A>) How can I financially assist
    PostgreSQL?</H4>

    <P>PostgreSQL has had a first-class infrastructure since we started
    in 1994. This is all thanks to Marc Fournier, who has created
    and managed this infrastructure over the years.</P>

    <P>Quality infrastructure is very important to an open-source
    project. It prevents disruptions that can greatly delay forward
    movement of the project.</P>

    <P>Of course, this infrastructure is not cheap. There are a variety
    of monthly and one-time expenses that are required to keep it
    going. If you or your company has money it can donate to help fund
    this effort, please go to <A href="http://store.pgsql.com/shopping/">http://store.pgsql.com/shopping/</A>
    and make a donation.</P>

    <P>Although the web page mentions PostgreSQL, Inc, the
    "contributions" item is solely to support the PostgreSQL project
    and does not fund any specific company. If you prefer, you can also
    send a check to the contact address.</P>
    <HR>

    <H2 align="center">User Client Questions</H2>

    <H4><A name="2.1">2.1</A>) Are there <SMALL>ODBC</SMALL> drivers
    for PostgreSQL?</H4>

    <P>There are two <SMALL>ODBC</SMALL> drivers available, PsqlODBC
    and OpenLink <SMALL>ODBC</SMALL>.</P>

    <P>PsqlODBC is included in the distribution. More information about
    it can be gotten from <A href=
    "ftp://ftp.PostgreSQL.org/pub/odbc/">ftp://ftp.PostgreSQL.org/pub/odbc/</A>.</P>

    <P>OpenLink <SMALL>ODBC</SMALL> can be gotten from <A href=
    "http://www.openlinksw.com/">http://www.openlinksw.com</A>. It
    works with their standard <SMALL>ODBC</SMALL> client software so
    you'll have PostgreSQL <SMALL>ODBC</SMALL> available on every
    client platform they support (Win, Mac, Unix, VMS).</P>

    <P>They will probably be selling this product to people who need
    commercial-quality support, but a freeware version will always be
    available. Please send questions to <A href=
    "mailto:postgres95@openlink.co.uk">postgres95@openlink.co.uk</A>.</P>

    <P>See also the <A href=
    "http://www.postgresql.org/devel-corner/docs/programmer/odbc.html">ODBC
    chapter of the Programmer's Guide</A>.</P>

    <H4><A name="2.2">2.2</A>) What tools are available for using
    PostgreSQL with Web pages?</H4>

    <P>A nice introduction to Database-backed Web pages can be seen at:
    <A href="http://www.webreview.com">http://www.webreview.com</A></P>

    <P>For Web integration, PHP is an excellent interface. It is at <A
    href="http://www.php.net">http://www.php.net</A>.</P>

    <P>For complex cases, many use the Perl interface and CGI.pm or mod_perl.</P>

    <H4><A name="2.3">2.3</A>) Does PostgreSQL have a graphical user
    interface? A report generator? An embedded query language
    interface?</H4>

    <P>We have a nice graphical user interface called PgAccess, which is
    shipped as part of the distribution. PgAccess also has a report
    generator. The Web page is <A href="http://www.pgaccess.org/">http://www.pgaccess.org/</A>.</P>

    <P>We also include <I>ecpg</I>, which is an embedded SQL query
    language interface for C.</P>

    <H4><A name="2.4">2.4</A>) What languages are able to communicate with PostgreSQL?</H4>

    <P>We have:</P>

    <UL>
      <LI>C (libpq)</LI>

      <LI>C++ (libpq++)</LI>

      <LI>Embedded C (ecpg)</LI>

      <LI>Java (jdbc)</LI>

      <LI>Perl (DBD::Pg)</LI>

      <LI>ODBC (odbc)</LI>

      <LI>Python (PyGreSQL)</LI>

      <LI>TCL (libpgtcl)</LI>

      <LI>C Easy API (libpgeasy)</LI>

      <LI>PHP ('pg_' functions, Pear::DB)</LI>
    </UL>
    <P>Additional interfaces are available at 
    <a href="http://www.PostgreSQL.org/interfaces.html">http://www.PostgreSQL.org/interfaces.html</A>.
    </P>
    <HR>

    <H2 align="center">Administrative Questions</H2>

    <H4><A name="3.1">3.1</A>) How do I install PostgreSQL somewhere
    other than <I>/usr/local/pgsql</I>?</H4>

    <P>Specify the <I>--prefix</I> option when running
    <I>configure</I>.</P>

    <H4><A name="3.2">3.2</A>) When I start <I>postmaster</I>, I get a
    <I>Bad System Call</I> or core dumped message. Why?</H4>

    <P>It could be a variety of problems, but first check to see that
    you have System V extensions installed in your kernel. PostgreSQL
    requires kernel support for shared memory and semaphores.</P>

    <H4><A name="3.3">3.3</A>) When I try to start <I>postmaster</I>, I
    get <I>IpcMemoryCreate</I> errors. Why?</H4>

    <P>You either do not have shared memory configured properly in your
    kernel or you need to enlarge the shared memory available in the
    kernel. The exact amount you need depends on your architecture and
    how many buffers and backend processes you configure for
    <I>postmaster</I>. For most systems, with default numbers of
    buffers and processes, you need a minimum of ~1 MB. See the <A
    href=
    "http://www.postgresql.org/idocs/index.php?kernel-resources.html">PostgreSQL
    Administrator's Guide</A> for more detailed information about
    shared memory and semaphores.</P>

    <H4><A name="3.4">3.4</A>) When I try to start <I>postmaster</I>, I
    get <I>IpcSemaphoreCreate</I> errors. Why?</H4>

    <P>If the error message is <I>IpcSemaphoreCreate: semget failed (No
    space left on device)</I> then your kernel is not configured with
    enough semaphores. Postgres needs one semaphore per potential
    backend process. A temporary solution is to start <I>postmaster</I>
    with a smaller limit on the number of backend processes. Use
    <I>-N</I> with a parameter less than the default of 32. A more
    permanent solution is to increase your kernel's
    <SMALL>SEMMNS</SMALL> and <SMALL>SEMMNI</SMALL> parameters.</P>

    <P>Inoperative semaphores can also cause crashes during heavy
    database access.</P>

    <P>If the error message is something else, you might not have
    semaphore support configured in your kernel at all. See the
    PostgreSQL Administrator's Guide for more detailed information
    about shared memory and semaphores.</P>

    <H4><A name="3.5">3.5</A>) How do I control connections from other
    hosts?</H4>

    <P>By default, PostgreSQL only allows connections from the local
    machine using Unix domain sockets. Other machines will not be able
    to connect unless you add the <I>-i</I> flag to <I>postmaster</I>,
    <B>and</B> enable host-based authentication by modifying the file
    <I>$PGDATA/pg_hba.conf</I> accordingly. This will allow TCP/IP
    connections.</P>

    <H4><A name="3.6">3.6</A>) How do I tune the database engine for
    better performance?</H4>

    <P>Certainly, indexes can speed up queries. The
    <SMALL>EXPLAIN</SMALL> command allows you to see how PostgreSQL is
    interpreting your query, and which indexes are being used.</P>

    <P>If you are doing many <SMALL>INSERTs</SMALL>, consider doing
    them in a large batch using the <SMALL>COPY</SMALL> command. This
    is much faster than individual <SMALL>INSERTS.</SMALL> Second,
    statements not in a <SMALL>BEGIN WORK/COMMIT</SMALL> transaction
    block are considered to be in their own transaction. Consider
    performing several statements in a single transaction block. This
    reduces the transaction overhead. Also, consider dropping and
    recreating indexes when making large data changes.</P>

    <P>There are several tuning options. You can disable <I>fsync()</I>
    by starting <I>postmaster</I> with a <I>-o -F</I> option. This will
    prevent <I>fsync()</I>s from flushing to disk after every
    transaction.</P>

    <P>You can also use the <I>postmaster</I> <I>-B</I> option to
    increase the number of shared memory buffers used by the backend
    processes. If you make this parameter too high, the
    <I>postmaster</I> may not start because you have exceeded your
    kernel's limit on shared memory space. Each buffer is 8K and the
    default is 64 buffers.</P>

    <P>You can also use the backend <I>-S</I> option to increase the
    maximum amount of memory used by the backend process for temporary
    sorts. The <I>-S</I> value is measured in kilobytes, and the
    default is 512 (i.e. 512K).</P>

    <P>You can also use the <SMALL>CLUSTER</SMALL> command to group
    data in tables to match an index. See the <SMALL>CLUSTER</SMALL>
    manual page for more details.</P>

    <H4><A name="3.7">3.7</A>) What debugging features are
    available?</H4>

    <P>PostgreSQL has several features that report status information
    that can be valuable for debugging purposes.</P>

    <P>First, by running <I>configure</I> with the --enable-cassert
    option, many <I>assert()</I>s monitor the progress of the backend
    and halt the program when something unexpected occurs.</P>

    <P>Both <I>postmaster</I> and <I>postgres</I> have several debug
    options available. First, whenever you start <I>postmaster</I>,
    make sure you send the standard output and error to a log file,
    like:</P>
<PRE>
    cd /usr/local/pgsql
    ./bin/postmaster &gt;server.log 2&gt;&amp;1 &amp;
</PRE>

    <P>This will put a server.log file in the top-level PostgreSQL
    directory. This file contains useful information about problems or
    errors encountered by the server. <I>Postmaster</I> has a <I>-d</I>
    option that allows even more detailed information to be reported.
    The <I>-d</I> option takes a number that specifies the debug level.
    Be warned that high debug level values generate large log
    files.</P>

    <P>If <I>postmaster</I> is not running, you can actually run the
    <I>postgres</I> backend from the command line, and type your
    <SMALL>SQL</SMALL> statement directly. This is recommended
    <B>only</B> for debugging purposes. Note that a newline terminates
    the query, not a semicolon. If you have compiled with debugging
    symbols, you can use a debugger to see what is happening. Because
    the backend was not started from <I>postmaster</I>, it is not
    running in an identical environment and locking/backend interaction
    problems may not be duplicated.</P>

    <P>If <I>postmaster</I> is running, start <I>psql</I> in one
    window, then find the <SMALL>PID</SMALL> of the <I>postgres</I>
    process used by <I>psql</I>. Use a debugger to attach to the
    <I>postgres</I> <SMALL>PID.</SMALL> You can set breakpoints in the
    debugger and issue queries from <I>psql</I>. If you are debugging
    <I>postgres</I> startup, you can set PGOPTIONS="-W n", then start
    <I>psql</I>. This will cause startup to delay for <I>n</I> seconds
    so you can attach to the process with the debugger, set any
    breakpoints, and continue through the startup sequence.</P>

    <P>The <I>postgres</I> program has <I>-s, -A</I>, and <I>-t</I>
    options that can be very useful for debugging and performance
    measurements.</P>

    <P>You can also compile with profiling to see what functions are
    taking execution time. The backend profile files will be deposited
    in the <I>pgsql/data/base/dbname</I> directory. The client profile
    file will be put in the client's current directory. Linux requires
    a compile with <I>-DLINUX_PROFILE</I> for proper profiling.</P>

    <H4><A name="3.8">3.8</A>) Why do I get <I>"Sorry, too many
    clients"</I> when trying to connect?</H4>

    <P>You need to increase <I>postmaster</I>'s limit on how many
    concurrent backend processes it can start.</P>

    <P>The default limit is 32 processes. You can increase it by
    restarting <I>postmaster</I> with a suitable <I>-N</I> value or
    modifying <I>postgresql.conf</I>.</P>

    <P>Note that if you make <I>-N</I> larger than 32, you must also
    increase <I>-B</I> beyond its default of 64; <I>-B</I> must be at
    least twice <I>-N</I>, and probably should be more than that for
    best performance. For large numbers of backend processes, you are
    also likely to find that you need to increase various Unix kernel
    configuration parameters. Things to check include the maximum size
    of shared memory blocks, <SMALL>SHMMAX;</SMALL> the maximum number
    of semaphores, <SMALL>SEMMNS</SMALL> and <SMALL>SEMMNI;</SMALL> the
    maximum number of processes, <SMALL>NPROC;</SMALL> the maximum
    number of processes per user, <SMALL>MAXUPRC;</SMALL> and the
    maximum number of open files, <SMALL>NFILE</SMALL> and
    <SMALL>NINODE.</SMALL> The reason that PostgreSQL has a limit on
    the number of allowed backend processes is so your system won't run
    out of resources.</P>

    <P>In PostgreSQL versions prior to 6.5, the maximum number of
    backends was 64, and changing it required a rebuild after altering
    the MaxBackendId constant in
    <I>include/storage/sinvaladt.h</I>.</P>

    <H4><A name="3.9">3.9</A>) What are the <I>pg_tempNNN.NN</I>
    files in my database directory?</H4>

    <P>They are temporary files generated by the query executor. For
    example, if a sort needs to be done to satisfy an <SMALL>ORDER
    BY,</SMALL> and the sort requires more space than the backend's
    <I>-S</I> parameter allows, then temporary files are created to
    hold the extra data.</P>

    <P>The temporary files should be deleted automatically, but might
    not if a backend crashes during a sort. If you have no backends
    running at the time, it is safe to delete the pg_tempNNN.NN
    files.</P>

    <H4><A name="3.10">3.10</A>) Why do I need to do a dump and restore
    to upgrade between major PostgreSQL releases?</H4>

    <P>The PostgreSQL team makes only small changes between minor releases,
    so upgrading from 7.2 to 7.2.1 does not require a dump and restore.
    However, major releases (e.g. from 7.2 to 7.3) often change the internal
    format of system tables and data files. These changes are often complex,
    so we don't maintain backward compatability for data files. A dump outputs
    data in a generic format that can then be loaded in using the new internal
    format.</P>

    <p>In releases where the on-disk format does not change, the
    <i>pg_upgrade</i> script can be used to upgrade without a dump/restore.
    The release notes mention whether <i>pg_upgrade</i> is available for the
    release.</p>

    <HR>

    <H2 align="center">Operational Questions</H2>

    <H4><A name="4.1">4.1</A>) What is the difference between binary
    cursors and normal cursors?</H4>

    <P>See the <SMALL>DECLARE</SMALL> manual page for a
    description.</P>

    <H4><A name="4.2">4.2</A>) How do I <SMALL>SELECT</SMALL> only the
    first few rows of a query?</H4>

    <P>See the <SMALL>FETCH</SMALL> manual page, or use
    <SMALL>SELECT</SMALL> ... <SMALL>LIMIT</SMALL>....</P>

    <P>The entire query may have to be evaluated, even if you only want
    the first few rows. Consider using a query that has an <SMALL>ORDER
    BY.</SMALL> If there is an index that matches the <SMALL>ORDER
    BY</SMALL>, PostgreSQL may be able to evaluate only the first few
    records requested, or the entire query may have to be evaluated
    until the desired rows have been generated.</P>

    <H4><A name="4.3">4.3</A>) How do I get a list of tables or other
    things I can see in <I>psql</I>?</H4>

    <P>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 psql'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.</P>

    <H4><A name="4.4">4.4</A>) How do you remove a column from a
    table?</H4>

    <P>Prior to version 7.3, <SMALL>ALTER TABLE DROP COLUMN</SMALL> is not supported.
    You can do this instead:</P>
<PRE>
    BEGIN;
    LOCK TABLE old_table;
    SELECT ...  -- select all columns but the one you want to remove
    INTO TABLE new_table
    FROM old_table;
    DROP TABLE old_table;
    ALTER TABLE new_table RENAME TO old_table;
    COMMIT;
</PRE>

    <H4><A name="4.5">4.5</A>) What is the maximum size for a row, a
    table, and a database?</H4>

    <P>These are the limits:</P>
<PRE>
    Maximum size for a database?             unlimited (1 TB databases exist)
    Maximum size for a table?                16 TB
    Maximum size for a row?                  1.6TB
    Maximum size for a field?                1 GB
    Maximum number of rows in a table?       unlimited
    Maximum number of columns in a table?    250-1600 depending on column types
    Maximum number of indexes on a table?    unlimited
</PRE>

    Of course, these are not actually unlimited, but limited to
    available disk space and memory/swap space. Performance may suffer
    when these values get unusually large. 

    <P>The maximum table size of 16 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
    increased if the default block size is increased to 32k.</P>

    <H4><A name="4.6">4.6</A>) How much database disk space is required
    to store data from a typical text file?</H4>

    <P>A PostgreSQL database may require up to five times the disk
    space to store data from a text file.</P>

    <P>As an example, consider a file of 100,000 lines with an integer
    and text description on each line. Suppose the text string
    avergages twenty bytes in length. The flat file would be 2.8 MB.
    The size of the PostgreSQL database file containing this data can
    be estimated as 6.4 MB:</P>
<PRE>
    36 bytes: each row header (approximate)
    24 bytes: one int field and one text field
   + 4 bytes: pointer on page to tuple
   ----------------------------------------
    64 bytes per row

   The data page size in PostgreSQL is 8192 bytes (8 KB), so:

   8192 bytes per page
   -------------------   =  128 rows per database page (rounded down)
     64 bytes per row

   100000 data rows
   --------------------  =  782 database pages (rounded up)
      128 rows per page

782 database pages * 8192 bytes per page  =  6,406,144 bytes (6.4 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 in bitmaps, so they
    use very little space.</P>
    
    <H4><A name="4.7">4.7</A>) How do I find out what tables, indexes,
    databases, and users are defined?</H4>

    <P><I>psql</I> has a variety of backslash commands to show such
    information. Use \? to see them. There are also system tables
    beginning with <I>pg_</I> that describe these too. Also, <I>psql
    -l</I> will list all databases.</P>

    <P>Also try the file <I>pgsql/src/tutorial/syscat.source</I>. It
    illustrates many of the <SMALL>SELECT</SMALL>s needed to get
    information from the database system tables.</P>

    <H4><A name="4.8">4.8</A>) My queries are slow or don't make use of
    the indexes. Why?</H4>
    Indexes are not automatically used by every query. Indexes are only
    used if the table is larger than a minimum size, and the query
    selects only a small percentage of the rows in the table. This is
    because the random disk access caused by an index scan can be
    slower than a straight read through the table, or sequential scan. 

    <P>To determine if an index should be used, PostgreSQL must have
    statistics about the table. These statistics are collected using
    <SMALL>VACUUM ANALYZE</SMALL>, or simply <SMALL>ANALYZE</SMALL>.
    Using statistics, the optimizer knows how many rows are in the
    table, and can better determine if indexes should be used.
    Statistics are also valuable in determining optimal join order and
    join methods. Statistics collection should be performed
    periodically as the contents of the table change.</P>

    <P>Indexes are normally not used for <SMALL>ORDER BY</SMALL> or to
    perform joins. A sequential scan followed by an explicit sort is
    usually faster than an index scan of a large table.</P>
    However, <SMALL>LIMIT</SMALL> combined with <SMALL>ORDER BY</SMALL>
    often will use an index because only a small portion of the table
    is returned.  In fact, though MAX() and MIN() don't use indexes,
    it is possible to retrieve such values using an index with ORDER BY
    and LIMIT:
<PRE>
    SELECT col
    FROM tab
    ORDER BY col [ DESC ]
    LIMIT 1;
</PRE>

    <P>When using wild-card operators such as <SMALL>LIKE</SMALL> or
    <I>~</I>, indexes can only be used in certain circumstances:
    <UL>
    <LI>The beginning of the search string must be anchored to the start
    of the string, i.e.:</LI>
    <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>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 utilise indexes. Instead, use functional
    indexes, which are described in section <a href="#4.12">4.12</a>.</LI>
    <LI>The default <I>C</I> locale must be used during
    <i>initdb.</i></LI>
    </UL>
    <P>

    <H4><A name="4.9">4.9</A>) How do I see how the query optimizer is
    evaluating my query?</H4>

    <P>See the <SMALL>EXPLAIN</SMALL> manual page.</P>

    <H4><A name="4.10">4.10</A>) What is an R-tree index?</H4>

    <P>An R-tree index is used for indexing spatial data. A hash index
    can't handle range searches. A B-tree index only handles range
    searches in a single dimension. R-trees can handle
    multi-dimensional data. For example, if an R-tree index can be
    built on an attribute of type <I>point</I>, the system can more
    efficiently answer queries such as "select all points within a
    bounding rectangle."</P>

    <P>The canonical paper that describes the original R-tree design
    is:</P>

    <P>Guttman, A. "R-trees: A Dynamic Index Structure for Spatial
    Searching." Proceedings of the 1984 ACM SIGMOD Int'l Conf on Mgmt
    of Data, 45-57.</P>

    <P>You can also find this paper in Stonebraker's "Readings in
    Database Systems".</P>

    <P>Built-in R-trees can handle polygons and boxes. In theory,
    R-trees can be extended to handle higher number of dimensions. In
    practice, extending R-trees requires a bit of work and we don't
    currently have any documentation on how to do it.</P>

    <H4><A name="4.11">4.11</A>) What is the Genetic Query
    Optimizer?</H4>

    <P>The <SMALL>GEQO</SMALL> module speeds query optimization when
    joining many tables by means of a Genetic Algorithm (GA). It allows
    the handling of large join queries through nonexhaustive
    search.</P>

    <H4><A name="4.12">4.12</A>) How do I perform regular expression
    searches and case-insensitive regular expression searches? How do I
    use an index for case-insensitive searches?</H4>

    <P>The <I>~</I> operator does regular expression matching, and
    <I>~*</I> does case-insensitive regular expression matching. The
    case-insensitive variant of <SMALL>LIKE</SMALL> is called
    <SMALL>ILIKE</SMALL>.</P>

    <P>Case-insensitive equality comparisons are normally expressed
    as:</P>
<PRE>
    SELECT *
    FROM tab
    WHERE LOWER(col) = 'abc';
</PRE>

    This will not use an standard index. However, if you create a
    functional index, it will be used: 
<PRE>
    CREATE INDEX tabindex ON tab (LOWER(col));
</PRE>

    <H4><A name="4.13">4.13</A>) In a query, how do I detect if a field
    is <SMALL>NULL</SMALL>?</H4>

    <P>You test the column with <SMALL>IS NULL</SMALL> and <SMALL>IS
    NOT NULL</SMALL>.</P>

    <H4><A name="4.14">4.14</A>) What is the difference between the
    various character types?</H4>
<PRE>
Type            Internal Name   Notes
--------------------------------------------------
"char"          char            1 character
CHAR(n)         bpchar          blank padded to the specified fixed length
VARCHAR(n)      varchar         size specifies maximum length, no padding
TEXT            text            no specific upper limit on length
BYTEA           bytea           variable-length byte array (null-byte safe)
</PRE>

    <P>You will see the internal name when examining system catalogs
    and in some error messages.</P>

    <P>The last four types above are "varlena" types (i.e., the first
    four bytes on disk are the length, followed by the data). Thus the
    actual space used is slightly greater than the declared size.
    However, these data types are also subject to compression or being
    stored out-of-line by <SMALL>TOAST</SMALL>, so the space on disk
    might also be less than expected.</P>

    <P><SMALL>CHAR(n)</SMALL> is best when storing strings that are
    usually the same length. <SMALL>VARCHAR(n)</SMALL> is best when
    storing variable-length strings but it limits how long a string can
    be. <SMALL>TEXT</SMALL> is for strings of unlimited length, maximum
    1 gigabyte. <SMALL>BYTEA</SMALL> is for storing binary data,
    particularly values that include <SMALL>NULL</SMALL> bytes.</P>

    <H4><A name="4.15.1">4.15.1</A>) How do I create a
    serial/auto-incrementing field?</H4>

    <P>PostgreSQL supports a <SMALL>SERIAL</SMALL> data type. It
    auto-creates a sequence and index on the column. 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 
    );
    CREATE UNIQUE INDEX person_id_key ON person ( id );
</PRE>

    See the <I>create_sequence</I> manual page for more information
    about sequences. You can also use each row's <I>OID</I> field as a
    unique value. However, if you need to dump and reload the database,
    you need to use <I>pg_dump</I>'s <I>-o</I> option or <SMALL>COPY
    WITH OIDS</SMALL> option to preserve the <SMALL>OID</SMALL>s. 

    <H4><A name="4.15.2">4.15.2</A>) How do I get the value of a
    <SMALL>SERIAL</SMALL> insert?</H4>

    <P>One approach is to retrieve the next <SMALL>SERIAL</SMALL> value
    from the sequence object with the <I>nextval()</I> function
    <I>before</I> inserting and then insert it explicitly. Using the
    example table in <A href="#4.15.1">4.15.1</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>

    Finally, you could use the <A href="#4.16"><SMALL>OID</SMALL></A>
    returned from the <SMALL>INSERT</SMALL> statement to look up the
    default value, though this is probably the least portable approach.
    In Perl, using DBI with Edmund Mergl's DBD::Pg module, the oid
    value is made available via <I>$sth-&gt;{pg_oid_status}</I> after
    <I>$sth-&gt;execute()</I>.

    <H4><A name="4.15.3">4.15.3</A>) Don't <I>currval()</I> and
    <I>nextval()</I> lead to a race condition with other users?</H4>

    <P>No. <i>currval</i>() returns the current value assigned by your
    backend, not by all users.</P>

    <H4><A name="4.15.4">4.15.4</A>) Why aren't my sequence numbers
    reused on transaction abort? Why are there gaps in the numbering of
    my sequence/SERIAL column?</H4>

    <P>To improve concurrency, sequence values are given out to running
    transactions as needed and are not locked until the transaction
    completes. This causes gaps in numbering from aborted
    transactions.</P>

    <H4><A name="4.16">4.16</A>) What is an <SMALL>OID</SMALL>? What is
    a <SMALL>TID</SMALL>?</H4>

    <P><SMALL>OID</SMALL>s are PostgreSQL's answer to unique row ids.
    Every row that is created in PostgreSQL gets a unique
    <SMALL>OID</SMALL>. All <SMALL>OID</SMALL>s generated during
    <I>initdb</I> are less than 16384 (from
    <I>include/access/transam.h</I>). All user-created
    <SMALL>OID</SMALL>s are equal to or greater than this. By default,
    all these <SMALL>OID</SMALL>s are unique not only within a table or
    database, but unique within the entire PostgreSQL installation.</P>

    <P>PostgreSQL uses <SMALL>OID</SMALL>s in its internal system
    tables to link rows between tables. These <SMALL>OID</SMALL>s can
    be used to identify specific user rows and used in joins. It is
    recommended you use column type <SMALL>OID</SMALL> to store
    <SMALL>OID</SMALL> values. You can create an index on the
    <SMALL>OID</SMALL> field for faster access.</P>

    <P>O<SMALL>ID</SMALL>s are assigned to all new rows from a central
    area that is used by all databases. If you want to change the
    <SMALL>OID</SMALL> to something else, or if you want to make a copy
    of the table, with the original <SMALL>OID</SMALL>s, there is no
    reason you can't do it:</P>
<PRE>
        CREATE TABLE new_table(old_oid oid, mycol int);
        SELECT old_oid, mycol INTO new FROM old;
        COPY new TO '/tmp/pgtable';
        DELETE FROM new;
        COPY new WITH OIDS FROM '/tmp/pgtable';
</PRE>
<!--
    CREATE TABLE new_table (mycol int);
    INSERT INTO new_table (oid, mycol) SELECT oid, mycol FROM old_table;
-->
    <P>O<SMALL>ID</SMALL>s are stored as 4-byte integers, and will
    overflow at 4 billion. No one has reported this ever happening, and
    we plan to have the limit removed before anyone does.</P>

    <P>T<SMALL>ID</SMALL>s are used to identify specific physical rows
    with block and offset values. T<SMALL>ID</SMALL>s change after rows
    are modified or reloaded. They are used by index entries to point
    to physical rows.</P>

    <H4><A name="4.17">4.17</A>) What is the meaning of some of the
    terms used in PostgreSQL?</H4>

    <P>Some of the source code and older documentation use terms that
    have more common usage. Here are some:</P>

    <UL>
      <LI>table, relation, class</LI>

      <LI>row, record, tuple</LI>

      <LI>column, field, attribute</LI>

      <LI>retrieve, select</LI>

      <LI>replace, update</LI>

      <LI>append, insert</LI>

      <LI><SMALL>OID</SMALL>, serial value</LI>

      <LI>portal, cursor</LI>

      <LI>range variable, table name, table alias</LI>
    </UL>

    <P>A list of general database terms can be found at: <A href=
    "http://hea-www.harvard.edu/MST/simul/software/docs/pkgs/pgsql/glossary/glossary.html">http://hea-www.harvard.edu/MST/simul/software/docs/pkgs/pgsql/glossary/glossary.html</A></P>

    <H4><A name="4.18">4.18</A>) Why do I get the error <I>"ERROR:
    Memory exhausted in AllocSetAlloc()"</I>?</H4>

    <P>You probably have run out of virtual memory on your system,
    or your kernel has a low limit for certain resources. Try this
    before starting <I>postmaster</I>:</P>
<PRE>
    ulimit -d 262144
    limit datasize 256m
</PRE>

    Depending on your shell, only one of these may succeed, but it will
    set your process data segment limit much higher and perhaps allow
    the query to complete. This command applies to the current process,
    and all subprocesses created after the command is run. If you are
    having a problem with the <SMALL>SQL</SMALL> client because the
    backend is returning too much data, try it before starting the
    client.

    <H4><A name="4.19">4.19</A>) How do I tell what PostgreSQL version
    I am running?</H4>

    <P>From <I>psql</I>, type <CODE>SELECT version();</CODE></P>

    <H4><A name="4.20">4.20</A>) Why does my large-object operations
    get <I>"invalid large obj descriptor"</I>?</H4>

    <P>You need to put <CODE>BEGIN WORK</CODE> and <CODE>COMMIT</CODE>
    around any use of a large object handle, that is, surrounding
    <CODE>lo_open</CODE> ... <CODE>lo_close.</CODE></P>

    <P>Currently PostgreSQL enforces the rule by closing large object
    handles at transaction commit. So the first attempt to do anything
    with the handle will draw <I>invalid large obj descriptor</I>. So
    code that used to work (at least most of the time) will now
    generate that error message if you fail to use a transaction.</P>

    <P>If you are using a client interface like <SMALL>ODBC</SMALL> you
    may need to set <CODE>auto-commit off.</CODE></P>

    <H4><A name="4.21">4.21</A>) How do I create a column that will
    default to the current time?</H4>

    <P>Use <I>CURRENT_TIMESTAMP</I>:</P>
<PRE>
<CODE>CREATE TABLE test (x int, modtime timestamp DEFAULT CURRENT_TIMESTAMP );
</CODE>
</PRE>

    <H4><A name="4.22">4.22</A>) Why are my subqueries using
    <CODE><SMALL>IN</SMALL></CODE> so slow?</H4>

    <P>Currently, we join subqueries to outer queries by sequentially
    scanning the result of the subquery for each row of the outer
    query. If the subquery returns only a few rows and the outer query
    returns many rows, <CODE><SMALL>IN</SMALL></CODE> is fastest.  To
    speed up other queries, replace <CODE>IN</CODE> with
    <CODE>EXISTS</CODE>:</P>
<PRE>    SELECT *
    FROM tab
    WHERE col IN (SELECT subcol FROM subtab);
</PRE>
    to:
<PRE>    SELECT *
    FROM tab
    WHERE EXISTS (SELECT subcol FROM subtab WHERE subcol = col);
</PRE>

    For this to be fast, <CODE>subcol</CODE> should be an indexed column.
    We hope to fix this limitation in a future release.

    <H4><A name="4.23">4.23</A>) How do I perform an outer join?</H4>

    <P>PostgreSQL supports outer joins using the SQL standard syntax.
    Here are two examples:</P>
<PRE>
    SELECT *
    FROM t1 LEFT OUTER JOIN t2 ON (t1.col = t2.col);
</PRE>
    or 
<PRE>
    SELECT *
    FROM t1 LEFT OUTER JOIN t2 USING (col);
</PRE>

    <P>These identical queries join t1.col to t2.col, and also return
    any unjoined rows in t1 (those with no match in t2). A
    <SMALL>RIGHT</SMALL> join would add unjoined rows of t2. A
    <SMALL>FULL</SMALL> join would return the matched rows plus all
    unjoined rows from t1 and t2. The word <SMALL>OUTER</SMALL> is
    optional and is assumed in <SMALL>LEFT</SMALL>,
    <SMALL>RIGHT</SMALL>, and <SMALL>FULL</SMALL> joins. Ordinary joins
    are called <SMALL>INNER</SMALL> joins.</P>

    <P>In previous releases, outer joins can be simulated using
    <SMALL>UNION</SMALL> and <SMALL>NOT IN</SMALL>. For example, when
    joining <I>tab1</I> and <I>tab2</I>, the following query does an
    <I>outer</I> join of the two tables:<BR>
    <BR>
    </P>
<PRE>
    SELECT tab1.col1, tab2.col2
    FROM tab1, tab2
    WHERE tab1.col1 = tab2.col1
    UNION ALL
    SELECT tab1.col1, NULL
    FROM tab1
    WHERE tab1.col1 NOT IN (SELECT tab2.col1 FROM tab2)
    ORDER BY col1
</PRE>

    <H4><A name="4.24">4.24</A>) How do I perform queries using
    multiple databases?</H4>

    <P>There is no way to query a database other than the current one.
    Because PostgreSQL loads database-specific system catalogs, it is
    uncertain how a cross-database query should even behave.</P>

    <P><I>/contrib/dblink</I> allows cross-database queries using
    function calls. Of course, a client can make simultaneous
    connections to different databases and merge the results on the
    client side.</P>

    <H4><A name="4.25">4.25</A>) How do I return multiple rows or
    columns from a function?</H4>

    <P>You can return result sets from PL/pgSQL functions using
    <I>refcursors</I>. See <A href=
    "http://www.PostgreSQL.org/idocs/index.php?plpgsql-cursors.html">
    http://www.PostgreSQL.org/idocs/index.php?plpgsql-cursors.html</A>,
    section 23.7.3.3.</P>

    <H4><A name="4.26">4.26</A>) Why can't I reliably create/drop
    temporary tables in PL/PgSQL functions?</H4>
    <P>PL/PgSQL caches function contents, and an unfortunate side effect
    is that if a PL/PgSQL function accesses a temporary table, and that
    table is later dropped and recreated, and the function called
    again, the function will fail because the cached function contents
    still point to the old temporary table. The solution is to use
    <SMALL>EXECUTE</SMALL> for temporary table access in PL/PgSQL. This
    will cause the query to be reparsed every time.</P>

    <H4><A name="4.27">4.27</A>) What replication options are available?
    </H4>
    <P>There are several master/slave replication options available.
    These allow only the master to make database changes and the slave
    can only do database reads. The bottom of <a
    href="http://gborg.PostgreSQL.org/genpage?replication_research">
    http://gborg.PostgreSQL.org/genpage?replication_research</a> lists
    them. A multi-master replication solution is being worked on at <a
    href="http://gborg.PostgreSQL.org/project/pgreplication/projdisplay.php">http://gborg.PostgreSQL.org/project/pgreplication/projdisplay.php</a>.</P>

    <H4><A name="4.28">4.28</A>) What encryption options are available?
    </H4>
    <UL>
    <LI><I>/contrib/pgcrypto</I> contains many encryption functions for
    use in <SMALL>SQL</SMALL> queries.</LI>
    <LI>The only way to encrypt transmission from the client to the
    server is by using <I>hostssl</I> in <I>pg_hba.conf</I>.</LI>
    <LI>Database user passwords are automatically encrypted when stored
    in version 7.3. In previous versions, you must enable the option
    <i>PASSWORD_ENCRYPTION</i> in <i>postgresql.conf</i>.</LI>
    <LI>The server can run using an encrypted file system.</LI>
    </UL>

    <HR>

    <H2 align="center">Extending PostgreSQL</H2>

    <H4><A name="5.1">5.1</A>) I wrote a user-defined function. When I
    run it in <I>psql</I>, why does it dump core?</H4>

    <P>The problem could be a number of things. Try testing your
    user-defined function in a stand-alone test program first.</P>

    <H4><A name="5.2">5.2</A>) How can I contribute some nifty new
    types and functions to PostgreSQL?</H4>

    <P>Send your extensions to the <I>pgsql-hackers</I> mailing list,
    and they will eventually end up in the <I>contrib/</I>
    subdirectory.</P>

    <H4><A name="5.3">5.3</A>) How do I write a C function to return a
    tuple?</H4>

    <P>This requires wizardry so extreme that the authors have never
    tried it, though in principle it can be done.</P>

    <H4><A name="5.4">5.4</A>) I have changed a source file. Why does
    the recompile not see the change?</H4>

    <P>The <I>Makefiles</I> do not have the proper dependencies for
    include files. You have to do a <I>make clean</I> and then another
    <I>make</I>. If you are using <SMALL>GCC</SMALL> you can use the
    <I>--enable-depend</I> option of <I>configure</I> to have the
    compiler compute the dependencies automatically.</P>
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