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<H1>Frequently Asked Questions (FAQ) for PostgreSQL</H1>
<P>Last updated: Sun Aug 25 21:05:29 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>
<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?</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 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>
<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 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 <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 <A href=
"http://www.postgresql.org/docs/faq-mswin.html">MS Windows FAQ</A>
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 7.2.2.</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.postgresql.org/books/">http://www.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 <A href=
"http://www.postgresql.org/bugs/bugs.php">PostgreSQL BugTool</A>
page, which gives guidelines and directions on how to submit a
bug.</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 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 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">support FAQ item</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
six years ago. 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=
"https://store.pgsql.com/shopping/index.php?id=1">
https://store.pgsql.com/shopping/index.php?id=1</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>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>
<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>
<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 >server.log 2>&1 &
</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_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>
<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 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>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.
<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 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>We do not support <SMALL>ALTER TABLE DROP COLUMN,</SMALL> but do
this:</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 filed
+ 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 like <SMALL>ILIKE</SMALL> and
<I>~*</I> can not be used. Instead, use functional
indexes, which are described later in this FAQ.</LI>
<LI>The default <I>C</I> local 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(#) 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-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()</SMALL> is best when storing strings that are
usually the same length. <SMALL>VARCHAR()</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>, that might look like
this in Perl:</P>
<PRE>
new_id = output of "SELECT nextval('person_id_seq')"
INSERT INTO person (id, name) VALUES (new_id, 'Blaise Pascal');
</PRE>
You would then also have the new value stored in
<CODE>new_id</CODE> for use in other queries (e.g., as a foreign key to the <CODE>person</CODE> table). Note that the name of the
automatically created <SMALL>SEQUENCE</SMALL> object will be named
<<I>table</I>>_<<I>serialcolumn</I>>_<I>seq</I>, where
<I>table</I> and <I>serialcolumn</I> are the names of your table
and your <SMALL>SERIAL</SMALL> column, respectively.
<P>Alternatively, you could retrieve the assigned
<SMALL>SERIAL</SMALL> value with the <I>currval</I>() function
<I>after</I> it was inserted by default, e.g.,</P>
<PRE>
INSERT INTO person (name) VALUES ('Blaise Pascal');
new_id = output of "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>$sth->{pg_oid_status}</I> after
<I>$sth->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. Currval() 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>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. 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://www.comptechnews.com/~reaster/dbdesign.html">http://www.comptechnews.com/~reaster/dbdesign.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. 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?</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 any database except the current one.
Because PostgreSQL loads database-specific system catalogs, it is
uncertain how a cross-database query should even behave.</P>
<P>Of course, a client can make simultaneous connections to
different databases and merge the information that way.</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://developer.postgresql.org/docs/postgres/plpgsql-cursors.html">
http://developer.postgresql.org/docs/postgres/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>
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.
<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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