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<TITLE>PostgreSQL FAQ</TITLE>
</HEAD>
<BODY BGCOLOR="#FFFFFF" TEXT="#000000" LINK="#FF0000" VLINK="#A00000" ALINK="#0000FF">
<H1>
Frequently Asked Questions (FAQ) for PostgreSQL
</H1>
<P>
Last updated: Thu Jul 27 12:52:35 EDT 2000
<P>
Current maintainer: Bruce Momjian (<A
HREF="mailto:pgman@candle.pha.pa.us">pgman@candle.pha.pa.us</A>)<BR><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>
Platform-specific questions are answered at <A
HREF="http://www.PostgreSQL.org/docs/">http://www.PostgreSQL.org/docs/</A>.<P>
<HR><P>
<H2><CENTER>General Questions</CENTER></H2>
<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 SQL?<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 DBMS's?<BR>
<H2><CENTER>User Client Questions</CENTER></H2>
<A HREF="#2.1">2.1</A>) Are there ODBC drivers for
PostgreSQL?<BR>
<A HREF="#2.2">2.2</A>) What tools are available for hooking
PostgreSQL to 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><CENTER>Administrative Questions</CENTER></H2>
<A HREF="#3.1">3.1</A>) Why does <I>initdb</I> fail?<BR>
<A HREF="#3.2">3.2</A>) How do I install PostgreSQL somewhere other than
<I>/usr/local/pgsql?</I><BR>
<A HREF="#3.3">3.3</A>) When I start the <i>postmaster</i>, I get a
<I>Bad System Call</I> or core dumped message. Why?<BR>
<A HREF="#3.4">3.4</A>) When I try to start the <i>postmaster</i>, I get
<I>IpcMemoryCreate</I> errors. Why?<BR>
<A HREF="#3.5">3.5</A>) When I try to start the <i>postmaster</i>, I get
<I>IpcSemaphoreCreate</I> errors. Why?<BR>
<A HREF="#3.6">3.6</A>) How do I prevent other hosts from accessing my
PostgreSQL database?<BR>
<A HREF="#3.7">3.7</A>) Why can't I connect to my database from
another machine?<BR>
<A HREF="#3.8">3.8</A>) Why can't I access the database as the
<I>root</I> user?<BR>
<A HREF="#3.9">3.9</A>) All my servers crash under concurrent
table access. Why?<BR>
<A HREF="#3.10">3.10</A>) How do I tune the database engine for
better performance?<BR>
<A HREF="#3.11">3.11</A>) What debugging features are available?<BR>
<A HREF="#3.12">3.12</A>) I get <I>"Sorry, too many clients"</I> when trying to
connect. Why?<BR>
<A HREF="#3.13">3.13</A>) What are the <I>pg_sorttempNNN.NN</I> files in my
database directory?<BR>
<H2><CENTER>Operational Questions</CENTER></H2>
<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 indices or
operations 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 Genetic Query Optimization?<BR>
<A HREF="#4.13">4.13</A>) How do I do 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 NULL?<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>) Why do I get the error <I>"FATAL: palloc
failure: memory exhausted?"</I><BR>
<A HREF="#4.20">4.20</A>) How do I tell what PostgreSQL version I
am running? <BR>
<A HREF="#4.21">4.21</A>) My large-object operations get <I>invalid
large obj descriptor.</I> Why?<BR>
<A HREF="#4.22">4.22</A>) How do I create a column that will default to the
current time?<BR>
<A HREF="#4.23">4.23</A>) Why are my subqueries using
<CODE><small>IN</small></CODE> so slow?<BR>
<A HREF="#4.24">4.24</A>) How do I do an <i>outer</i> join?<BR>
<H2><CENTER>Extending PostgreSQL</CENTER></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>) What does the message
<I>"NOTICE:PortalHeapMemoryFree: 0x402251d0 not in alloc set!"</I> mean?<BR>
<A HREF="#5.3">5.3</A>) How can I contribute some nifty new types and functions
to PostgreSQL?<BR>
<A HREF="#5.4">5.4</A>) How do I write a C function to return a
tuple?<BR>
<A HREF="#5.5">5.5</A>) I have changed a source file. Why does the
recompile not see the change?<BR>
<HR>
<H2><CENTER>General Questions</CENTER></H2>
<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 DBMS 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 SQL. PostgreSQL is
free and the complete source is available.<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>
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>
The original name of the software at Berkeley was Postgres. When SQL
functionality was added in 1995, its name was changed to Postgres95. The
name was changed at the end of 1996 to PostgreSQL.<P>
It is pronounced <I>Post-Gres-Q-L.</I>
<H4><A NAME="1.2">1.2</A>) What's the copyright on
PostgreSQL?</H4><P>
PostgreSQL is subject to the following COPYRIGHT.<P>
PostgreSQL Data Base Management System<P>
Portions copyright (c) 1996-2000, PostgreSQL, Inc
Portions Copyright (c) 1994-6 Regents of the University of California<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>
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>
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>
The authors have compiled and tested PostgreSQL on the following
platforms (some of these compiles require gcc):
<UL>
<LI> aix - IBM on AIX 3.2.5 or 4.x
<LI> alpha - DEC Alpha AXP on Digital Unix 2.0, 3.2, 4.0
<LI> BSD44_derived - OSs derived from 4.4-lite BSD (NetBSD, FreeBSD)
<LI> bsdi - BSD/OS 2.x, 3.x, 4.x
<LI> dgux - DG/UX 5.4R4.11
<LI> hpux - HP PA-RISC on HP-UX 9.*, 10.*
<LI> i386_solaris - i386 Solaris
<LI> irix5 - SGI MIPS on IRIX 5.3
<LI> linux - Intel i86
Alpha
SPARC
PPC
M68k
<LI> sco - SCO 3.2v5
Unixware
<LI> sparc_solaris - SUN SPARC on Solaris 2.4, 2.5, 2.5.1
<LI> sunos4 - SUN SPARC on SunOS 4.1.3
<LI> svr4 - Intel x86 on Intel SVR4 and MIPS
<LI> ultrix4 - DEC MIPS on Ultrix 4.4
</UL>
<P>
<H4><A NAME="1.4">1.4</A>) What non-unix ports are available?</H4><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.<P>
A file <I>win31.mak</I> is included in the distribution for making a
Win32 <I>libpq</I> library and psql.<P>
The database server is now working on Windows NT using the Cygnus
Unix/NT porting library. See <I>pgsql/doc/FAQ_NT</I> in the distribution.<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.
<H4><A NAME="1.6">1.6</A>) Where can I get support?</H4><P>
There is no support for PostgreSQL from the University of
California, Berkeley. It is maintained through volunteer effort.<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)
<PRE>
subscribe
end
</PRE><P>
to <A
HREF="mailto:pgsql-general-request@PostgreSQL.org">pgsql-general-request@PostgreSQL.org</A>.<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:
<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:<P>
<PRE>
subscribe
end
</PRE><P>
Additional mailing lists and information about PostgreSQL can be found
via the PostgreSQL WWW home page at:
<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>
Commercial support for PostgreSQL is available at <A
HREF="http://www.pgsql.com">http://www.pgsql.com/</A>.<P>
<H4><A NAME="1.7">1.7</A>) What is the latest release?</H4><P>
The latest release of PostgreSQL is version 7.0.2.<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 on-line at <A
HREF="http://www.PostgreSQL.org/docs/postgres">
http://www.PostgreSQL.org/docs/postgres</A>.
<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>
<I>psql</I> has some nice \d commands to show information about types,
operators, functions, aggregates, etc.<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 SQL-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 SQL?</H4><P>
The PostgreSQL book at <A
HREF="http://www.PostgreSQL.org/docs/awbook.html">
http://www.PostgreSQL.org/docs/awbook.html</A> teaches SQL.
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>
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>
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>
There are about a dozen people who have commit privileges to
the PostgreSQL CVS 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.
<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>
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
DBMS's?</H4><P>
There are several ways of measuring software: features, performance,
reliability, support, and price.<P>
<DL>
<DT> <B>Features</B>
<DD>
PostgreSQL has most features present in large commercial DBMS'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. We don't have outer
joins, but are working on them.<BR><BR>
<DT> <B>Performance</B>
<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>
<DT> <B>Reliability</B>
<DD>
We realize that a DBMS 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>
<DT> <B>Support</B>
<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 DBMS'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 DBMS's.
There is commercial per-incident support available for those who need
it. (See support FAQ item.)<BR><BR>
<DT> <B>Price</B>
<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>
</DL>
<HR>
<H2><CENTER>User Client Questions</CENTER></H2>
<P>
<H4><A NAME="2.1">2.1</A>) Are there ODBC drivers for PostgreSQL?</H4><P>
There are two ODBC drivers available, PsqlODBC and OpenLink ODBC.<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>
OpenLink ODBC can be gotten from <A HREF="http://www.openlinksw.com/">
http://www.openlinksw.com</A>. It works with their standard ODBC client
software so you'll have PostgreSQL ODBC available on every client
platform they support (Win, Mac, Unix, VMS).<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>
See also the <A HREF="http://www.PostgreSQL.org/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 hooking
PostgreSQL to 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>
There is also one at <A HREF="http://www.phone.net/home/mwm/hotlist/">
http://www.phone.net/home/mwm/hotlist/.</A><P>
For web integration, PHP is an excellent interface. It is at
<A HREF="http://www.php.net">http://www.php.net</A><P>
For complex cases, many use the Perl interface and CGI.pm.<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>
<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>
We also include <I>ecpg,</I> which is an embedded SQL query language interface for
C.
<H4><A NAME="2.4">2.4</A>) What languages are available to
communicate with PostgreSQL?</H4><P>
We have:
<UL>
<LI>C (libpq)
<LI>C++ (libpq++)
<LI>Embedded C (ecpg)
<LI>Java (jdbc)
<LI>Perl (perl5)
<LI>ODBC (odbc)
<LI>Python (PyGreSQL)
<LI>TCL (libpgtcl)
<LI>C Easy API (libpgeasy)
<LI>Embedded HTML (<A HREF="http://www.php.net">PHP from http://www.php.net</A>)
</UL><P>
<HR>
<H2><CENTER>Administrative Questions</CENTER></H2><P>
<H4><A NAME="3.1">3.1</A>) Why does <I>initdb</I> fail?</H4><P>
Try these:
<UL>
<LI> check that you don't have any of the previous version's binaries in
your path
<LI> check to see that you have the proper paths set
<LI> check that the <I>postgres</I> user owns the proper files
</UL><P>
<H4><A NAME="3.2">3.2</A>) How do I install PostgreSQL somewhere
other than <I>/usr/local/pgsql?</I></H4><P>
The simplest way is to specify the --prefix option when running <I>configure.</I>
If you forgot to do that, you can edit <I>Makefile.global</I> and change POSTGRESDIR
accordingly, or create a <I>Makefile.custom</I> and define POSTGRESDIR there.<P>
<H4><A NAME="3.3">3.3</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="3.4">3.4</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="3.5">3.5</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>
If the error message is something else, you might not have semaphore
support configured in your kernel at all.<P>
<H4><A NAME="3.6">3.6</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>
<H4><A NAME="3.7">3.7</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>.
<H4><A NAME="3.8">3.8</A>) Why can't I access the database as the <I>root</I>
user?</H4><P>
You should not create database users with user id 0 (root). They will be
unable to access the database. This is a security precaution because
of the ability of users to dynamically link object modules into the
database engine.<P>
<H4><A NAME="3.9">3.9</A>) All my servers crash under concurrent
table access. Why?</H4><P>
This problem can be caused by a kernel that is not configured to support
semaphores.<P>
<H4><A NAME="3.10">3.10</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>
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>
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>
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>
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>
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="3.11">3.11</A>) What debugging features are available?</H4><P>
PostgreSQL has several features that report status information that can
be valuable for debugging purposes.<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>
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:
<PRE>
cd /usr/local/pgsql
./bin/postmaster &gt;server.log 2&gt;&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>
If the <i>postmaster</i> is not running, you can actually run the
<I>postgres</I> backend from the command line, and type your SQL 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>
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>
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>
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="3.12">3.12</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>
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>
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>
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="3.13">3.13</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>
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>
<H2><CENTER>Operational Questions</CENTER></H2><P>
<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
SET 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 SELECT ... LIMIT....<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/psql.c.</I> It contains SQL 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:
<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><P>
<H4><A NAME="4.6">4.6</A>) What is the maximum size for a
row, table, database?</H4><P>
These are the limits:
<PRE>
Maximum size for a database? unlimited (60GB databases exist)
Maximum size for a table? unlimited on all operating systems
Maximum size for a row? 8k, configurable to 32k
Maximum number of rows in a table? unlimited
Maximum number of columns table? unlimited
Maximum number of indexes on a table? unlimited
</PRE>
Of course, these are not actually unlimited, but limited to available
disk space.<P>
To change the maximum row size, edit <I>include/config.h</I> and change
<SMALL>BLCKSZ.</SMALL> To use attributes larger than 8K, you can also
use the large object interface.<P>
The row length limit will be removed in 7.1.<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>
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:
<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 indices or
operations 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>
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>
For column-specific optimization statistics, use <SMALL>VACUUM
ANALYZE.</SMALL> <SMALL>VACUUM ANALYZE</SMALL> is important for complex
multi-join 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>
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>
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>
<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 efficient answer queries like select all points
within a bounding rectangle.<P>
The canonical paper that describes the original R-Tree design is:<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>
You can also find this paper in Stonebraker's "Readings in Database
Systems"<P>
Builtin 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 require 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 Genetic Query
Optimization?</H4><P>
The GEQO module speeds query
optimization when joining many tables by means of a Genetic
Algorithm (GA). It allows the handling of large join queries through
non-exhaustive search.<P>
<H4><A NAME="4.13">4.13</A>) How do I do 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. There is no
case-insensitive variant of the LIKE operator, but you can get the
effect of case-insensitive <SMALL>LIKE</SMALL> with this:
<PRE>
WHERE lower(textfield) LIKE lower(pattern)
</PRE>
<H4><A NAME="4.14">4.14</A>) In a query, how do I detect if a field
is NULL?</H4><P>
You test the column with IS NULL and 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 length limited only by maximum row length
BYTEA bytea variable-length array of bytes
</PRE><P>
You will see the internal name when examining system catalogs
and in some error messages.<P>
The last four types above are "varlena" types (i.e. the first four bytes
are the length, followed by the data). <I>char(#)</I> allocates the
maximum number of bytes no matter how much data is stored in the field.
<I>text, varchar(#),</I> and <I>bytea</I> all have variable length on the disk,
and because of this, there is a small performance penalty for using
them. Specifically, the penalty is for access to all columns after the
first column of this type.<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:
<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>
<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 SERIAL 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:
<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 SEQUENCE 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 SERIAL column, respectively.
<P>
Alternatively, you could retrieve the just-assigned SERIAL value with the <I>currval</I>() function <I>after</I> it was inserted by default, e.g.,
<PRE>
INSERT INTO person (name) VALUES ('Blaise Pascal');
$newID = currval('person_id_seq');
</PRE>
Finally, you could use the <A HREF="#4.17"><small>OID</small></A> returned from the
INSERT statement to lookup the default value, though this is probably
the least portable approach. In Perl, using DBI with Edmund Mergl's
DBD::Pg module, the oid value is made available via
<I>$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.
<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 or greater that 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>
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>
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:
<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>
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:
<UL>
<LI> table, relation, class
<LI> row, record, tuple
<LI> column, field, attribute
<LI> retrieve, select
<LI> replace, update
<LI> append, insert
<LI> <small>OID</small>, serial value
<LI> portal, cursor
<LI> range variable, table name, table alias
</UL><P>
<H4><A NAME="4.19">4.19</A>) Why do I get the error <I>"FATAL: palloc
failure: memory exhausted?"</I><BR></H4><P>
It is possible you have run out of virtual memory on your system, or
your kernel has a low limit for certain resources. Try this before
starting the <i>postmaster:</i>
<PRE>
ulimit -d 65536
limit datasize 64m
</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 SQL client because the backend is returning too much data, try
it before starting the client.<P>
<H4><A NAME="4.20">4.20</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="4.21">4.21</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>
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>
If you are using a client interface like ODBC you may need to set
<CODE>auto-commit off.</CODE><P>
<H4><A NAME="4.22">4.22</A>) How do I create a column that will default to the
current time?<BR></H4><P>
Use <i>now()</i>:
<CODE><PRE>
CREATE TABLE test (x int, modtime timestamp DEFAULT now() );
</PRE></CODE>
<P>
<H4><A NAME="4.23">4.23</A>) Why are my subqueries using
<CODE><small>IN</small></CODE> so slow?<BR></H4><P>
Currently, we join subqueries to outer queries by sequential 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>:
<CODE><PRE>
SELECT *
FROM tab
WHERE col1 IN (SELECT col2 FROM TAB2)
</PRE></CODE>
to:
<CODE><PRE>
SELECT *
FROM tab
WHERE EXISTS (SELECT col2 FROM TAB2 WHERE col1 = col2)
</PRE></CODE>
We hope to fix this limitation in a future release.
<H4><A NAME="4.24">4.24</A>) How do I do an <i>outer</i> join?<BR></H4><P>
PostgreSQL does not support outer joins in the current release. They 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:
<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 tab1.col1
</PRE>
<HR>
<H2><CENTER>Extending PostgreSQL</CENTER></H2><P>
<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.
<H4><A NAME="5.2">5.2</A>) What does the message
<I>"NOTICE:PortalHeapMemoryFree: 0x402251d0 not in alloc set!"</I> mean?</H4><P>
You are <I>pfree'ing</I> something that was not <I>palloc'ed.</I>
Beware of mixing <I>malloc/free</I> and <I>palloc/pfree.</I>
<H4><A NAME="5.3">5.3</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="5.4">5.4</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="5.5">5.5</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>.<P>
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