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

    <P>Last updated: Tue Oct 17 00:21:20 EDT 2000</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>

    <CENTER>
      <H2>General Questions</H2>
    </CENTER>
    <A href="#1.1">1.1</A>) What is PostgreSQL?<BR>
     <A href="#1.2">1.2</A>) What's 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>
     

    <CENTER>
      <H2>User Client Questions</H2>
    </CENTER>
    <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 use
    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>
     

    <CENTER>
      <H2>Administrative Questions</H2>
    </CENTER>
    <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 the <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 the <I>postmaster</I>,
    I get <I>IpcMemoryCreate</I> errors. Why?<BR>
     <A href="#3.4">3.4</A>) When I try to start the <I>postmaster</I>,
    I get <I>IpcSemaphoreCreate</I> errors. Why?<BR>
     <A href="#3.5">3.5</A>) How do I prevent other hosts from
    accessing my PostgreSQL database?<BR>
     <A href="#3.6">3.6</A>) Why can't I connect to my database from
    another machine?<BR>
     <A href="#3.7">3.7</A>) How do I tune the database engine for
    better performance?<BR>
     <A href="#3.8">3.8</A>) What debugging features are available?<BR>
     <A href="#3.9">3.9</A>) I get <I>"Sorry, too many clients"</I>
    when trying to connect. Why?<BR>
     <A href="#3.10">3.10</A>) What are the <I>pg_sorttempNNN.NN</I>
    files in my database directory?<BR>
     

    <CENTER>
      <H2>Operational Questions</H2>
    </CENTER>
    <A href="#4.1">4.1</A>) Why is the system confused about commas,
    decimal points, and date formats.<BR>
     <A href="#4.2">4.2</A>) What is the exact difference between
    binary cursors and normal cursors?<BR>
     <A href="#4.3">4.3</A>) How do I <SMALL>SELECT</SMALL> only the
    first few rows of a query?<BR>
     <A href="#4.4">4.4</A>) How do I get a list of tables or other
    things I can see in <I>psql?</I><BR>
     <A href="#4.5">4.5</A>) How do you remove a column from a
    table?<BR>
     <A href="#4.6">4.6</A>) What is the maximum size for a row, table,
    database?<BR>
     <A href="#4.7">4.7</A>) How much database disk space is required
    to store data from a typical text file?<BR>
     <A href="#4.8">4.8</A>) How do I find out what tables or indexes
    are defined in the database?<BR>
     <A href="#4.9">4.9</A>) My queries are slow or don't make use of
    the indexes. Why?<BR>
     <A href="#4.10">4.10</A>) How do I see how the query optimizer is
    evaluating my query?<BR>
     <A href="#4.11">4.11</A>) What is an R-tree index?<BR>
     <A href="#4.12">4.12</A>) What is the Genetic Query Optimizer?<BR>
     <A href="#4.13">4.13</A>) How do I perform regular expression
    searches and case-insensitive regular expression searches?<BR>
     <A href="#4.14">4.14</A>) In a query, how do I detect if a field
    is <SMALL>NULL</SMALL>?<BR>
     <A href="#4.15">4.15</A>) What is the difference between the
    various character types?<BR>
     <A href="#4.16.1">4.16.1</A>) How do I create a
    serial/auto-incrementing field?<BR>
     <A href="#4.16.2">4.16.2</A>) How do I get the value of a
    <SMALL>SERIAL</SMALL> insert?<BR>
     <A href="#4.16.3">4.16.3</A>) Don't <I>currval()</I> and
    <I>nextval()</I> lead to a race condition with other users?<BR>
     <A href="#4.17">4.17</A>) What is an <SMALL>OID</SMALL>? What is a
    <SMALL>TID</SMALL>?<BR>
     <A href="#4.18">4.18</A>) What is the meaning of some of the terms
    used in PostgreSQL?<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>) My large-object operations get
    <I>invalid large obj descriptor.</I> Why?<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>
     

    <CENTER>
      <H2>Extending PostgreSQL</H2>
    </CENTER>
    <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>

    <CENTER>
      <H2>General Questions</H2>
    </CENTER>

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

    <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 Internet
    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
    below 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>

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

    <H4><A name="1.2">1.2</A>) What's 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) 1996-2001, PostgreSQL Global Development
    Group Portions Copyright (c) 1994-6 Regents of the University of
    California</P>

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

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

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

    <H4><A name="1.3">1.3</A>) What 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>

    <H6>Client</H6>

    <P>It is possible to compile the <I>libpq</I> C library, psql, and
    other interfaces and binaries 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>win31.mak</I> is included in the distribution
    for making a Win32 <I>libpq</I> library and psql. PostgreSQL also
    communicates with <SMALL>ODBC</SMALL> clients.</P>

    <H6>Server</H6>

    <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 on our web site. We have no plan to do a native port to any
    Microsoft platform.</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 7.0.3.</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 is a PostgreSQL book available at <A href=
    "http://www.PostgreSQL.org/docs/awbook.html">http://www.PostgreSQL.org/docs/awbook.html</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://www.PostgreSQL.org/docs/todo.html">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 a nice tutorial at <A href=
    "http://w3.one.net/~jhoffman/sqltut.htm">http://w3.one.net/~jhoffman/sqltut.htm</A>
    and at <A href=
    "http://ourworld.compuserve.com/homepages/graeme_birchall/HTM_COOK.HTM">
    http://ourworld.compuserve.com/homepages/graeme_birchall/HTM_COOK.HTM.</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 2000AD, and before
    2000BC.</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>Fill out the "bug-template" file and send it to: <A href=
    "mailto:pgsql-bugs@PostgreSQL.org">pgsql-bugs@PostgreSQL.org</A></P>

    <P>Also check out our ftp site <A href=
    "ftp://ftp.PostgreSQL.org/pub">ftp://ftp.PostgreSQL.org/pub</A> to
    see if there is a more recent PostgreSQL version 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 don't 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 runs in two modes. Normal <I>fsync</I> mode
      flushes every completed transaction to disk, guaranteeing that if
      the OS crashes or loses power in the next few seconds, all your
      data is safely stored on disk. In this mode, we are slower than
      most commercial databases, partly because few of them do such
      conservative flushing to disk in their default modes. In
      <I>no-fsync</I> mode, we are usually faster than commercial
      databases, though in this mode, an OS crash could cause data
      corruption. We are working to provide an intermediate mode that
      suffers less performance overhead than full fsync mode, and will
      allow data integrity within 30 seconds of an OS crash.<BR>
      <BR>
       In comparison to MySQL or leaner database systems, we are slower
      on inserts/updates because we have transaction overhead. Of
      course, MySQL doesn't have any of the features mentioned in the
      <I>Features</I> section above. We are built for flexibility and
      features, though we continue to improve performance through
      profiling and source code analysis. 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>
       We handle each user connection by creating a Unix process.
      Backend processes share data buffers and locking information.
      With multiple CPU's, multiple backends can easily run on
      different CPU's.<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 list provides a large group of developers and
      users to help resolve any problems encountered. While we can not
      guarantee a fix, commercial <SMALL>DBMS</SMALL>'s don't 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 support FAQ item.)<BR>
      <BR>
      </DD>

      <DT><B>Price</B></DT>

      <DD>We are free for all use, both commercial and non-commercial.
      You can add our code to your product with no limitations, except
      those outlined in our BSD-style license stated above.<BR>
      <BR>
      </DD>
    </DL>
    <HR>

    <CENTER>
      <H2>User Client Questions</H2>
    </CENTER>

    <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. 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/users-lounge/docs/programmer/odbc.htm">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.webtools.com">http://www.webtools.com</A></P>

    <P>There is also one at <A href=
    "http://www.phone.net/home/mwm/hotlist/">http://www.phone.net/home/mwm/hotlist/.</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.</P>

    <P>A WWW gateway based on WDB using Perl can be downloaded from <A
    href=
    "http://www.eol.ists.ca/~dunlop/wdb-p95">http://www.eol.ists.ca/~dunlop/wdb-p95</A></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 <I>pgaccess,</I>
    which is shipped as part of the distribution. <I>Pgaccess</I> also
    has a report generator. The Web page is <A href=
    "http://www.flex.ro/pgaccess">http://www.flex.ro/pgaccess</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 available 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 (perl5)</LI>

      <LI>ODBC (odbc)</LI>

      <LI>Python (PyGreSQL)</LI>

      <LI>TCL (libpgtcl)</LI>

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

      <LI>Embedded <SMALL>HTML</SMALL> (<A href=
      "http://www.php.net">PHP from http://www.php.net</A>)</LI>
    </UL>
    <HR>

    <CENTER>
      <H2>Administrative Questions</H2>
    </CENTER>

    <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 the <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 the
    <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 the
    <I>postmaster.</I> For most systems, with default numbers of
    buffers and processes, you need a minimum of ~1MB.</P>

    <H4><A name="3.4">3.4</A>) When I try to start the
    <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 the
    <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>If the error message is something else, you might not have
    semaphore support configured in your kernel at all.</P>

    <H4><A name="3.5">3.5</A>) How do I prevent other hosts from
    accessing my PostgreSQL database?</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 the
    <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>

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

    <H4><A name="3.6">3.6</A>) Why can't I connect to my database from
    another machine?</H4>

    <P>The default configuration allows only unix domain socket
    connections from the local machine. To enable TCP/IP connections,
    make sure the <I>postmaster</I> has been started with the <I>-i</I>
    option, and add an appropriate host entry to the file
    <I>pgsql/data/pg_hba.conf</I>.</P>

    <H4><A name="3.7">3.7</A>) How do I tune the database engine for
    better performance?</H4>

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

    <P>If you are doing a lot of <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 indices when making large data changes.</P>

    <P>There are several tuning options. You can disable <I>fsync()</I>
    by starting the <I>postmaster</I> with a <I>-o -F</I> option. This
    will prevent <I>fsync()'s</I> 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've 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 (ie, 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="3.8">3.8</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()'s</I> 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 the <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 the <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 the <I>postmaster,</I> it is not
    running in an identical environment and locking/backend interaction
    problems may not be duplicated.</P>

    <P>If the <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 with the debugger and trace 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.</P>

    <H4><A name="3.9">3.9</A>) I get 'Sorry, too many clients' when
    trying to connect. Why?</H4>

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

    <P>In PostgreSQL 6.5 and up, the default limit is 32 processes. You
    can increase it by restarting the <I>postmaster</I> with a suitable
    <I>-N</I> value. With the default configuration you can set
    <I>-N</I> as large as 1024. If you need more, increase
    <SMALL>MAXBACKENDS</SMALL> in <I>include/config.h</I> and rebuild.
    You can set the default value of <I>-N</I> at configuration time,
    if you like, using <I>configure's</I> <I>--with-maxbackends</I>
    switch.</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="3.10">3.10</A>) What are the <I>pg_sorttempNNN.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>
    <HR>

    <CENTER>
      <H2>Operational Questions</H2>
    </CENTER>

    <H4><A name="4.1">4.1</A>) Why is system confused about commas,
    decimal points, and date formats.</H4>

    <P>Check your locale configuration. PostgreSQL uses the locale
    setting of the user that ran the <I>postmaster</I> process. There
    are postgres and psql <SMALL>SET</SMALL> commands to control the
    date format. Set those accordingly for your operating
    environment.</P>

    <H4><A name="4.2">4.2</A>) What is the exact difference between
    binary cursors and normal cursors?</H4>

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

    <H4><A name="4.3">4.3</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 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="4.4">4.4</A>) How do I get a list of tables or other
    things I can see in <I>psql?</I><BR>
    </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="4.5">4.5</A>) How do you remove a column from a
    table?</H4>

    <P>We do not support <SMALL>ALTER TABLE DROP COLUMN,</SMALL> but do
    this:</P>
<PRE>
    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;
</PRE>

    <H4><A name="4.6">4.6</A>) What is the maximum size for a row,
    table, database?</H4>

    <P>These are the limits:</P>
<PRE>
Maximum size for a database?             unlimited (60GB databases exist)
Maximum size for a table?                16 TB
Maximum size for a row?                  unlimited in 7.1 and later
Maximum size for a field?                1GB in 7.1 and later
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 16TB does not require large file
    support from the operating system. Large tables are stored as
    multiple 1GB files.</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="4.7">4.7</A>) How much database disk space is required
    to store data from a typical text file?<BR>
    </H4>

    <P>A PostgreSQL database may need six-and-a-half times the disk
    space required to store the data in a flat file.</P>

    <P>Consider a file of 300,000 lines with two integers on each line.
    The flat file is 2.4MB. The size of the PostgreSQL database file
    containing this data can be estimated at 14MB:</P>
<PRE>
    36 bytes: each row header (approximate)
   + 8 bytes: two int fields @ 4 bytes each
   + 4 bytes: pointer on page to tuple
   ----------------------------------------
    48 bytes per row

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

   8192 bytes per page
   -------------------   =  171 rows per database page (rounded up)
     48 bytes per row

   300000 data rows
   --------------------  =  1755 database pages
      171 rows per page

1755 database pages * 8192 bytes per page  =  14,376,960 bytes (14MB)
</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>

    <H4><A name="4.8">4.8</A>) How do I find out what tables or indexes
    are defined in the database?</H4>

    <P><I>psql</I> has a variety of backslash commands to show such
    information. Use \? to see them.</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="4.9">4.9</A>) My queries are slow or don't make use of
    the indexes. Why?</H4>

    <P>PostgreSQL does not automatically maintain statistics.
    V<SMALL>ACUUM</SMALL> must be run to update the statistics. After
    statistics are updated, the optimizer knows how many rows in the
    table, and can better decide if it should use indices. Note that
    the optimizer does not use indices in cases when the table is small
    because a sequential scan would be faster.</P>

    <P>For column-specific optimization statistics, use <SMALL>VACUUM
    ANALYZE.</SMALL> <SMALL>VACUUM ANALYZE</SMALL> is important for
    complex multijoin queries, so the optimizer can estimate the number
    of rows returned from each table, and choose the proper join order.
    The backend does not keep track of column statistics on its own, so
    <SMALL>VACUUM ANALYZE</SMALL> must be run to collect them
    periodically.</P>

    <P>Indexes are usually not used for <SMALL>ORDER BY</SMALL>
    operations: a sequential scan followed by an explicit sort is
    faster than an indexscan of all tuples of a large table, because it
    takes fewer disk accesses.</P>

    <P>When using wild-card operators such as <SMALL>LIKE</SMALL> or
    <I>~,</I> indices can only be used if the beginning of the search
    is anchored to the start of the string. So, to use indices,
    <SMALL>LIKE</SMALL> searches should not begin with <I>%,</I> and
    <I>~</I>(regular expression searches) should start with
    <I>^.</I></P>

    <H4><A name="4.10">4.10</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="4.11">4.11</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-tree's 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." Proc 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="4.12">4.12</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="4.13">4.13</A>) How do I perform regular expression
    searches and case-insensitive regular expression 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> in PostgreSQL 7.1 and later.</P>

    <H4><A name="4.14">4.14</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>IS NOT NULL.</P>

    <H4><A name="4.15">4.15</A>) What is the difference between the
    various character types?</H4>
<PRE>
Type            Internal Name   Notes
--------------------------------------------------
"char"          char            1 character
CHAR(#)         bpchar          blank padded to the specified fixed length
VARCHAR(#)      varchar         size specifies maximum length, no padding
TEXT            text            no specific upper limit on length
BYTEA           bytea           variable-length byte array (null-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>

    <H4><A name="4.16.1">4.16.1</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's -o</I> option or <SMALL>COPY WITH
    OIDS</SMALL> option to preserve the <SMALL>OID</SMALL>s. 

    <P><A href="http://www.PostgreSQL.org/docs/aw_pgsql_book">Numbering
    Rows.</A></P>

    <H4><A name="4.16.2">4.16.2</A>) How do I get the value of a
    <SMALL>SERIAL</SMALL> insert?</H4>

    <P>One approach is to 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="#4.16.1">4.16.1</A>, that might look like
    this:</P>
<PRE>
    $newSerialID = nextval('person_id_seq');
    INSERT INTO person (id, name) VALUES ($newSerialID, 'Blaise Pascal');
</PRE>
    You would then also have the new value stored in
    <CODE>$newSerialID</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>
    INSERT INTO person (name) VALUES ('Blaise Pascal');
    $newID = currval('person_id_seq');
</PRE>
    Finally, you could use the <A href="#4.17"><SMALL>OID</SMALL></A>
    returned from the <SMALL>INSERT</SMALL> statement to look up the
    default value, though this is probably the least portable approach.
    In Perl, using DBI with Edmund Mergl's DBD::Pg module, the oid
    value is made available via <I>$sth-&gt;{pg_oid_status} after
    $sth-&gt;execute().</I> 

    <H4><A name="4.16.3">4.16.3</A>) Don't <I>currval()</I> and
    <I>nextval()</I> lead to a race condition with other users?</H4>

    <P>No. This is handled by the backends.</P>

    <H4><A name="4.17">4.17</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>backend/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';
<!--
    CREATE TABLE new_table (mycol int);
    INSERT INTO new_table (oid, mycol) SELECT oid, mycol FROM old_table;
-->
</PRE>

    <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. Tids change after rows are modified
    or reloaded. They are used by index entries to point to physical
    rows.</P>

    <H4><A name="4.18">4.18</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://www.comptechnews.com/~reaster/dbdesign.html">http://www.comptechnews.com/~reaster/dbdesign.html</A></P>

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

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

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

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

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

    <P>Currently, we join subqueries to outer queries by sequentially
    scanning the result of the subquery for each row of the outer
    query. A workaround is to replace <CODE>IN</CODE> with
    <CODE>EXISTS</CODE>:</P>
<PRE>
<CODE>SELECT *
    FROM tab
    WHERE col1 IN (SELECT col2 FROM TAB2)
</CODE>
</PRE>
    to: 
<PRE>
<CODE>SELECT *
    FROM tab
    WHERE EXISTS (SELECT col2 FROM TAB2 WHERE col1 = col2)
</CODE>
</PRE>
    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?<BR>
    </H4>

    <P>PostgreSQL 7.1 and later 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>
    <HR>

    <CENTER>
      <H2>Extending PostgreSQL</H2>
    </CENTER>

    <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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