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11 <H1>Frequently Asked Questions (FAQ) for PostgreSQL</H1>
13 <P>Last updated: Tue Aug 31 23:28:02 EDT 2004</P>
15 <P>Current maintainer: Bruce Momjian (<A href=
16 "mailto:pgman@candle.pha.pa.us">pgman@candle.pha.pa.us</A>)<BR>
19 <P>The most recent version of this document can be viewed at <A href=
20 "http://www.PostgreSQL.org/docs/faqs/FAQ.html">http://www.PostgreSQL.org/docs/faqs/FAQ.html</A>.</P>
22 <P>Platform-specific questions are answered at <A href=
23 "http://www.PostgreSQL.org/docs/index.html">http://www.PostgreSQL.org/docs/index.html</A>.</P>
26 <H2 align="center">General Questions</H2>
27 <A href="#1.1">1.1</A>) What is PostgreSQL? How is it pronounced?<BR>
28 <A href="#1.2">1.2</A>) What is the copyright on PostgreSQL?<BR>
29 <A href="#1.3">1.3</A>) What Unix platforms does PostgreSQL run
31 <A href="#1.4">1.4</A>) What non-Unix ports are available?<BR>
32 <A href="#1.5">1.5</A>) Where can I get PostgreSQL?<BR>
33 <A href="#1.6">1.6</A>) Where can I get support?<BR>
34 <A href="#1.7">1.7</A>) What is the latest release?<BR>
35 <A href="#1.8">1.8</A>) What documentation is available?<BR>
36 <A href="#1.9">1.9</A>) How do I find out about known bugs or
38 <A href="#1.10">1.10</A>) How can I learn <SMALL>SQL</SMALL>?<BR>
39 <A href="#1.11">1.11</A>) Is PostgreSQL Y2K compliant?<BR>
40 <A href="#1.12">1.12</A>) How do I join the development team?<BR>
41 <A href="#1.13">1.13</A>) How do I submit a bug report?<BR>
42 <A href="#1.14">1.14</A>) How does PostgreSQL compare to other
43 <SMALL>DBMS</SMALL>s?<BR>
44 <A href="#1.15">1.15</A>) How can I financially assist
48 <H2 align="center">User Client Questions</H2>
49 <A href="#2.1">2.1</A>) Are there <SMALL>ODBC</SMALL> drivers for
51 <A href="#2.2">2.2</A>) What tools are available for using
52 PostgreSQL with Web pages?<BR>
53 <A href="#2.3">2.3</A>) Does PostgreSQL have a graphical user
55 <A href="#2.4">2.4</A>) What languages are available to
56 communicate with PostgreSQL?<BR>
59 <H2 align="center">Administrative Questions</H2>
60 <A href="#3.1">3.1</A>) How do I install PostgreSQL somewhere other
61 than <I>/usr/local/pgsql</I>?<BR>
62 <A href="#3.2">3.2</A>) When I start <I>postmaster</I>, I get a
63 <I>Bad System Call</I> or core dumped message. Why?<BR>
64 <A href="#3.3">3.3</A>) When I try to start <I>postmaster</I>, I
65 get <I>IpcMemoryCreate</I> errors. Why?<BR>
66 <A href="#3.4">3.4</A>) When I try to start <I>postmaster</I>, I
67 get <I>IpcSemaphoreCreate</I> errors. Why?<BR>
68 <A href="#3.5">3.5</A>) How do I control connections from other
70 <A href="#3.6">3.6</A>) How do I tune the database engine for
71 better performance?<BR>
72 <A href="#3.7">3.7</A>) What debugging features are available?<BR>
73 <A href="#3.8">3.8</A>) Why do I get <I>"Sorry, too many
74 clients"</I> when trying to connect?<BR>
75 <A href="#3.9">3.9</A>) What is in the <I>pgsql_tmp</I>
77 <A href="#3.10">3.10</A>) Why do I need to do a dump and restore
78 to upgrade PostgreSQL releases?<BR>
79 <A href="#3.11">3.11</A>) What computer hardware should I use?<BR>
82 <H2 align="center">Operational Questions</H2>
83 <A href="#4.1">4.1</A>) What is the difference between binary
84 cursors and normal cursors?<BR>
85 <A href="#4.2">4.2</A>) How do I <SMALL>SELECT</SMALL> only the
86 first few rows of a query? A random row?<BR>
87 <A href="#4.3">4.3</A>) How do I get a list of tables or other
88 things I can see in <I>psql</I>?<BR>
89 <A href="#4.4">4.4</A>) How do you remove a column from a
90 table, or change it's data type?<BR>
91 <A href="#4.5">4.5</A>) What is the maximum size for a row, a
92 table, and a database?<BR>
93 <A href="#4.6">4.6</A>) How much database disk space is required
94 to store data from a typical text file?<BR>
95 <A href="#4.7">4.7</A>) How do I find out what tables, indexes,
96 databases, and users are defined?<BR>
97 <A href="#4.8">4.8</A>) My queries are slow or don't make use of
99 <A href="#4.9">4.9</A>) How do I see how the query optimizer is
100 evaluating my query?<BR>
101 <A href="#4.10">4.10</A>) What is an R-tree index?<BR>
102 <A href="#4.11">4.11</A>) What is the Genetic Query Optimizer?<BR>
103 <A href="#4.12">4.12</A>) How do I perform regular expression
104 searches and case-insensitive regular expression searches? How do I
105 use an index for case-insensitive searches?<BR>
106 <A href="#4.13">4.13</A>) In a query, how do I detect if a field
107 is <SMALL>NULL</SMALL>?<BR>
108 <A href="#4.14">4.14</A>) What is the difference between the
109 various character types?<BR>
110 <A href="#4.15.1">4.15.1</A>) How do I create a
111 serial/auto-incrementing field?<BR>
112 <A href="#4.15.2">4.15.2</A>) How do I get the value of a
113 <SMALL>SERIAL</SMALL> insert?<BR>
114 <A href="#4.15.3">4.15.3</A>) Don't <I>currval()</I> and
115 <I>nextval()</I> lead to a race condition with other users?<BR>
116 <A href="#4.15.4">4.15.4</A>) Why aren't my sequence numbers
117 reused on transaction abort? Why are there gaps in the numbering of
118 my sequence/SERIAL column?<BR>
119 <A href="#4.16">4.16</A>) What is an <SMALL>OID</SMALL>? What is a
120 <SMALL>TID</SMALL>?<BR>
121 <A href="#4.17">4.17</A>) What is the meaning of some of the terms
122 used in PostgreSQL?<BR>
123 <A href="#4.18">4.18</A>) Why do I get the error <I>"ERROR: Memory
124 exhausted in AllocSetAlloc()"</I>?<BR>
125 <A href="#4.19">4.19</A>) How do I tell what PostgreSQL version I
127 <A href="#4.20">4.20</A>) Why does my large-object operations get
128 <I>"invalid large obj descriptor"</I>?<BR>
129 <A href="#4.21">4.21</A>) How do I create a column that will
130 default to the current time?<BR>
131 <A href="#4.22">4.22</A>) Why are my subqueries using
132 <CODE><SMALL>IN</SMALL></CODE> so slow?<BR>
133 <A href="#4.23">4.23</A>) How do I perform an outer join?<BR>
134 <A href="#4.24">4.24</A>) How do I perform queries using multiple
136 <A href="#4.25">4.25</A>) How do I return multiple rows or columns
138 <A href="#4.26">4.26</A>) Why can't I reliably create/drop
139 temporary tables in PL/PgSQL functions?<BR>
140 <A href="#4.27">4.27</A>) What replication options are available?<BR>
141 <A href="#4.28">4.28</A>) What encryption options are available?<BR>
144 <H2 align="center">Extending PostgreSQL</H2>
145 <A href="#5.1">5.1</A>) I wrote a user-defined function. When I run
146 it in <I>psql</I>, why does it dump core?<BR>
147 <A href="#5.2">5.2</A>) How can I contribute some nifty new types
148 and functions to PostgreSQL?<BR>
149 <A href="#5.3">5.3</A>) How do I write a C function to return a
151 <A href="#5.4">5.4</A>) I have changed a source file. Why does the
152 recompile not see the change?<BR>
156 <H2 align="center">General Questions</H2>
158 <H4><A name="1.1">1.1</A>) What is PostgreSQL? How is it pronounced?</H4>
160 <P>PostgreSQL is pronounced <I>Post-Gres-Q-L</I>. An audio file is
161 available at http://www.postgresql.org/postgresql.mp3 for those
162 would like to hear the pronunciation.
165 <P>PostgreSQL is an enhancement of the POSTGRES database management
166 system (and is still sometimes reffered to as simply "Postgres"),
167 a next-generation <SMALL>DBMS</SMALL> research prototype.
168 While PostgreSQL retains the powerful data model and rich data
169 types of POSTGRES, it replaces the PostQuel query language with an
170 extended subset of <SMALL>SQL</SMALL>. PostgreSQL is free and the
171 complete source is available.</P>
173 <P>PostgreSQL development is performed by a team of
174 developers who all subscribe to the PostgreSQL development mailing
175 list. The current coordinator is Marc G. Fournier (<A href=
176 "mailto:scrappy@PostgreSQL.org">scrappy@PostgreSQL.org</A>). (See
177 section <a href="#1.6">1.6</a> on how to join). This team is now
178 responsible for all development of PostgreSQL. It is a community
179 project and is not controlled by any company. To get involved, see
180 the developer's FAQ at <A href=
181 "http://www.PostgreSQL.org/docs/faqs/FAQ_DEV.html">http://www.PostgreSQL.org/docs/faqs/FAQ_DEV.html</A>
184 <P>The authors of PostgreSQL 1.01 were Andrew Yu and Jolly Chen.
185 Many others have contributed to the porting, testing, debugging,
186 and enhancement of the code. The original Postgres code, from which
187 PostgreSQL is derived, was the effort of many graduate students,
188 undergraduate students, and staff programmers working under the
189 direction of Professor Michael Stonebraker at the University of
190 California, Berkeley.</P>
192 <P>The original name of the software at Berkeley was Postgres. When
193 <SMALL>SQL</SMALL> functionality was added in 1995, its name was
194 changed to Postgres95. The name was changed at the end of 1996 to
197 <H4><A name="1.2">1.2</A>) What is the copyright on
200 <P>PostgreSQL is subject to the following COPYRIGHT:</P>
202 <P>PostgreSQL Data Base Management System</P>
204 <P>Portions copyright (c) 1996-2004, PostgreSQL Global Development
205 Group Portions Copyright (c) 1994-6 Regents of the University of
208 <P>Permission to use, copy, modify, and distribute this software
209 and its documentation for any purpose, without fee, and without a
210 written agreement is hereby granted, provided that the above
211 copyright notice and this paragraph and the following two
212 paragraphs appear in all copies.</P>
214 <P>IN NO EVENT SHALL THE UNIVERSITY OF CALIFORNIA BE LIABLE TO ANY
215 PARTY FOR DIRECT, INDIRECT, SPECIAL, INCIDENTAL, OR CONSEQUENTIAL
216 DAMAGES, INCLUDING LOST PROFITS, ARISING OUT OF THE USE OF THIS
217 SOFTWARE AND ITS DOCUMENTATION, EVEN IF THE UNIVERSITY OF
218 CALIFORNIA HAS BEEN ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.</P>
220 <P>THE UNIVERSITY OF CALIFORNIA SPECIFICALLY DISCLAIMS ANY
221 WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
222 OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE. THE
223 SOFTWARE PROVIDED HEREUNDER IS ON AN "AS IS" BASIS, AND THE
224 UNIVERSITY OF CALIFORNIA HAS NO OBLIGATIONS TO PROVIDE MAINTENANCE,
225 SUPPORT, UPDATES, ENHANCEMENTS, OR MODIFICATIONS.</P>
227 <P>The above is the BSD license, the classic open-source license.
228 It has no restrictions on how the source code may be used. We like
229 it and have no intention of changing it.</P>
231 <H4><A name="1.3">1.3</A>) What Unix platforms does PostgreSQL run
234 <P>In general, any modern Unix-compatible platform should be able to
235 run PostgreSQL. The platforms that had received explicit testing at
236 the time of release are listed in the installation
239 <H4><A name="1.4">1.4</A>) What non-Unix ports are available?</H4>
241 <P>Starting with version 8.0, PostgreSQL now runs natively on
242 Microsoft Windows NT-based operating systems like Win2000, WinXP,
243 and Win2003. A prepackaged installer is available at <a href=
244 "http://pgfoundry.org/projects/pginstaller">
245 http://pgfoundry.org/projects/pginstaller</a>.
247 <p>There is also a Novell Netware 6 port at
248 <a href="http://forge.novell.com">http://forge.novell.com</a>.</p>
250 <H4><A name="1.5">1.5</A>) Where can I get PostgreSQL?</H4>
252 <P>The primary anonymous ftp site for PostgreSQL is <A href=
253 "ftp://ftp.PostgreSQL.org/pub">ftp://ftp.PostgreSQL.org/pub</A>.
254 For mirror sites, see our main web site.</P>
256 <H4><A name="1.6">1.6</A>) Where can I get support?</H4>
258 <P>The main mailing list is: <A href=
259 "mailto:pgsql-general@PostgreSQL.org">pgsql-general@PostgreSQL.org</A>.
260 It is available for discussion of matters pertaining to PostgreSQL.
261 To subscribe, send mail with the following lines in the body (not
262 the subject line):</P>
269 "mailto:pgsql-general-request@PostgreSQL.org">pgsql-general-request@PostgreSQL.org</A>.</P>
271 <P>There is also a digest list available. To subscribe to this
272 list, send email to: <A href=
273 "mailto:pgsql-general-digest-request@PostgreSQL.org">pgsql-general-digest-request@PostgreSQL.org</A>
280 Digests are sent out to members of this list whenever the main list
281 has received around 30k of messages.
283 <P>The bugs mailing list is available. To subscribe to this list,
284 send email to <A href=
285 "mailto:pgsql-bugs-request@PostgreSQL.org">pgsql-bugs-request@PostgreSQL.org</A>
292 There is also a developers discussion mailing list available. To
293 subscribe to this list, send email to <A href=
294 "mailto:pgsql-hackers-request@PostgreSQL.org">pgsql-hackers-request@PostgreSQL.org</A>
301 <P>Additional mailing lists and information about PostgreSQL can be
302 found via the PostgreSQL WWW home page at:</P>
305 <A href="http://www.PostgreSQL.org">http://www.PostgreSQL.org</A>
308 <P>There is also an IRC channel on Freenode and EFNet,
309 channel <I>#PostgreSQL</I>. You can use the Unix command <CODE>irc -c
310 '#PostgreSQL' "$USER" irc.phoenix.net.</CODE> or <CODE>irc -c
311 '#PostgreSQL' "$USER" irc.freenode.net.</CODE></P>
313 <P>A list of commercial support companies is available at <A href=
314 "http://techdocs.postgresql.org/companies.php">http://techdocs.postgresql.org/companies.php</A>.</P>
316 <H4><A name="1.7">1.7</A>) What is the latest release?</H4>
318 <P>The latest release of PostgreSQL is version 7.4.5.</P>
320 <P>We plan to have major releases every six to eight months.</P>
322 <H4><A name="1.8">1.8</A>) What documentation is available?</H4>
324 <P>Several manuals, manual pages, and some small test examples are
325 included in the distribution. See the <I>/doc</I> directory. You
326 can also browse the manuals online at <A href=
327 "http://www.PostgreSQL.org/docs">http://www.PostgreSQL.org/docs</A>.</P>
329 <P>There are two PostgreSQL books available online at <A href=
330 "http://www.PostgreSQL.org/docs/awbook.html">http://www.PostgreSQL.org/docs/awbook.html</A>
332 "http://www.commandprompt.com/ppbook/">http://www.commandprompt.com/ppbook/</A>.
333 There is a list of PostgreSQL books available for purchase at <A
335 "http://techdocs.postgresql.org/techdocs/bookreviews.php">http://techdocs.PostgreSQL.org/techdocs/bookreviews.php</A>.
336 There is also a collection of PostgreSQL technical articles at <A
338 "http://techdocs.PostgreSQL.org/">http://techdocs.PostgreSQL.org/</A>.</P>
340 <P><I>psql</I> has some nice \d commands to show information about
341 types, operators, functions, aggregates, etc.</P>
343 <P>Our web site contains even more documentation.</P>
345 <H4><A name="1.9">1.9</A>) How do I find out about known bugs or
346 missing features?</H4>
348 <P>PostgreSQL supports an extended subset of <SMALL>SQL</SMALL>-92.
349 See our <A href="http://developer.PostgreSQL.org/todo.php">TODO</A>
350 list for known bugs, missing features, and future plans.</P>
352 <H4><A name="1.10">1.10</A>) How can I learn
353 <SMALL>SQL</SMALL>?</H4>
355 <P>The PostgreSQL book at <A href=
356 "http://www.PostgreSQL.org/docs/awbook.html">http://www.PostgreSQL.org/docs/awbook.html</A>
357 teaches <SMALL>SQL</SMALL>. There is another PostgreSQL book at <A
359 "http://www.commandprompt.com/ppbook/">http://www.commandprompt.com/ppbook.</A>
360 There is a nice tutorial at <A href=
361 "http://www.intermedia.net/support/sql/sqltut.shtm">http://www.intermedia.net/support/sql/sqltut.shtm,</A>
363 "http://ourworld.compuserve.com/homepages/graeme_birchall/HTM_COOK.HTM">
364 http://ourworld.compuserve.com/homepages/graeme_birchall/HTM_COOK.HTM,</A>
366 "http://sqlcourse.com/">http://sqlcourse.com.</A></P>
368 <P>Another one is "Teach Yourself SQL in 21 Days, Second Edition"
370 "http://members.tripod.com/er4ebus/sql/index.htm">http://members.tripod.com/er4ebus/sql/index.htm</A></P>
372 <P>Many of our users like <I>The Practical SQL Handbook</I>,
373 Bowman, Judith S., et al., Addison-Wesley. Others like <I>The
374 Complete Reference SQL</I>, Groff et al., McGraw-Hill.</P>
376 <H4><A name="1.11">1.11</A>) Is PostgreSQL Y2K compliant?</H4>
378 <P>Yes, we easily handle dates past the year 2000 AD, and before
381 <H4><A name="1.12">1.12</A>) How do I join the development
384 <P>First, download the latest source and read the PostgreSQL
385 Developers documentation on our web site, or in the distribution.
386 Second, subscribe to the <I>pgsql-hackers</I> and
387 <I>pgsql-patches</I> mailing lists. Third, submit high quality
388 patches to pgsql-patches.</P>
390 <P>There are about a dozen people who have commit privileges to the
391 PostgreSQL <SMALL>CVS</SMALL> archive. They each have submitted so
392 many high-quality patches that it was impossible for the existing
393 committers to keep up, and we had confidence that patches they
394 committed were of high quality.</P>
396 <H4><A name="1.13">1.13</A>) How do I submit a bug report?</H4>
398 <P>Please visit the PostgreSQL BugTool page at <A href=
399 "http://www.PostgreSQL.org/bugs/bugs.php">http://www.PostgreSQL.org/bugs/bugs.php</A>,
400 which gives guidelines and directions on how to submit a
403 <P>Also check out our ftp site <A href=
404 "ftp://ftp.PostgreSQL.org/pub">ftp://ftp.PostgreSQL.org/pub</A> to
405 see if there is a more recent PostgreSQL version or patches.</P>
407 <H4><A name="1.14">1.14</A>) How does PostgreSQL compare to other
408 <SMALL>DBMS</SMALL>s?</H4>
410 <P>There are several ways of measuring software: features,
411 performance, reliability, support, and price.</P>
414 <DT><B>Features</B></DT>
416 <DD>PostgreSQL has most features present in large commercial
417 <SMALL>DBMS</SMALL>s, like transactions, subselects, triggers,
418 views, foreign key referential integrity, and sophisticated
419 locking. We have some features they do not have, like
420 user-defined types, inheritance, rules, and multi-version
421 concurrency control to reduce lock contention.<BR>
425 <DT><B>Performance</B></DT>
427 <DD>PostgreSQL has performance similar to other commercial and
428 open source databases. it is faster for some things, slower for
429 others. In comparison to MySQL or leaner database systems, we are
430 faster for multiple users, complex queries, and a read/write query
431 load. MySQL is faster for simple SELECT queries done by a few users.
432 Of course, MySQL does not have most of the features mentioned in the
433 <I>Features</I> section above. We are built for reliability and
434 features, and we continue to improve performance in every
435 release. There is an interesting Web page comparing PostgreSQL to
436 MySQL at <A href="http://openacs.org/philosophy/why-not-mysql.html">
437 http://openacs.org/philosophy/why-not-mysql.html</A> Also, MySQL is
438 is a company that distributes its products via open source, and requires
439 a commercial license for close-source software, not an
440 open source development community like PostgreSQL.<BR>
445 <DT><B>Reliability</B></DT>
447 <DD>We realize that a <SMALL>DBMS</SMALL> must be reliable, or it
448 is worthless. We strive to release well-tested, stable code that
449 has a minimum of bugs. Each release has at least one month of
450 beta testing, and our release history shows that we can provide
451 stable, solid releases that are ready for production use. We
452 believe we compare favorably to other database software in this
457 <DT><B>Support</B></DT>
459 <DD>Our mailing lists provide contact with a large group of developers
460 and users to help resolve any problems encountered. While we cannot
461 guarantee a fix, commercial <SMALL>DBMS</SMALL>s do not always
462 supply a fix either. Direct access to developers, the user
463 community, manuals, and the source code often make PostgreSQL
464 support superior to other <SMALL>DBMS</SMALL>s. There is
465 commercial per-incident support available for those who need it.
466 (See <A href="#1.6">FAQ section 1.6</A>.)<BR>
470 <DT><B>Price</B></DT>
472 <DD>We are free for all use, both commercial and non-commercial.
473 You can add our code to your product with no limitations, except
474 those outlined in our BSD-style license stated above.<BR>
479 <H4><A name="1.15">1.15</A>) How can I financially assist
482 <P>PostgreSQL has had a first-class infrastructure since we started
483 in 1996. This is all thanks to Marc Fournier, who has created
484 and managed this infrastructure over the years.</P>
486 <P>Quality infrastructure is very important to an open-source
487 project. It prevents disruptions that can greatly delay forward
488 movement of the project.</P>
490 <P>Of course, this infrastructure is not cheap. There are a variety
491 of monthly and one-time expenses that are required to keep it
492 going. If you or your company has money it can donate to help fund
493 this effort, please go to <A href="http://store.pgsql.com/shopping/">http://store.pgsql.com/shopping/</A>
494 and make a donation.</P>
496 <P>Although the web page mentions PostgreSQL, Inc, the
497 "contributions" item is solely to support the PostgreSQL project
498 and does not fund any specific company. If you prefer, you can also
499 send a check to the contact address.</P>
502 <P>Also, if you have a success story about PostgreSQL, please submit
503 it to our advocacy site at <a href="http://advocacy.postgresql.org">
504 http://advocacy.postgresql.org</a>.</P>
507 <H2 align="center">User Client Questions</H2>
509 <H4><A name="2.1">2.1</A>) Are there <SMALL>ODBC</SMALL> drivers
512 <P>There are two <SMALL>ODBC</SMALL> drivers available, PsqlODBC
513 and OpenLink <SMALL>ODBC</SMALL>.</P>
515 <P>You can download PsqlODBC from <A href=
516 "http://gborg.postgresql.org/project/psqlodbc/projdisplay.php">
517 http://gborg.postgresql.org/project/psqlodbc/projdisplay.php</A>.</P>
519 <P>OpenLink <SMALL>ODBC</SMALL> can be gotten from <A href=
520 "http://www.openlinksw.com/">http://www.openlinksw.com</A>. It
521 works with their standard <SMALL>ODBC</SMALL> client software so
522 you'll have PostgreSQL <SMALL>ODBC</SMALL> available on every
523 client platform they support (Win, Mac, Unix, VMS).</P>
525 <P>They will probably be selling this product to people who need
526 commercial-quality support, but a freeware version will always be
527 available. Please send questions to <A href=
528 "mailto:postgres95@openlink.co.uk">postgres95@openlink.co.uk</A>.</P>
530 <H4><A name="2.2">2.2</A>) What tools are available for using
531 PostgreSQL with Web pages?</H4>
533 <P>A nice introduction to Database-backed Web pages can be seen at:
534 <A href="http://www.webreview.com">http://www.webreview.com</A></P>
536 <P>For Web integration, PHP is an excellent interface. It is at <A
537 href="http://www.php.net">http://www.php.net</A>.</P>
539 <P>For complex cases, many use the Perl interface and CGI.pm or mod_perl.</P>
541 <H4><A name="2.3">2.3</A>) Does PostgreSQL have a graphical user
544 <P>Yes, there are several graphical interfaces to PostgreSQL available.
545 These include PgAccess <a href="http://www.pgaccess.org">
546 http://www.pgaccess.org</a>), PgAdmin III (<a
547 href="http://www.pgadmin.org">http://www.pgadmin.org</a>, RHDB Admin (<a
548 href="http://sources.redhat.com/rhdb/">http://sources.redhat.com/rhdb/
549 </a>) and Rekall (<a href="http://www.thekompany.com/products/rekall/">
550 http://www.thekompany.com/products/rekall/</a>, proprietary). There is
551 also PhpPgAdmin (<a href="http://phppgadmin.sourceforge.net/">
552 http://phppgadmin.sourceforge.net/ </a>), a web-based interface to
555 <P>See <a href="http://techdocs.postgresql.org/guides/GUITools">http://techdocs.postgresql.org/guides/GUITools</a> for a more detailed list.</P>
557 <H4><A name="2.4">2.4</A>) What languages are able to communicate with
560 <P>Most popular programming languages contain an interface to
561 PostgreSQL. Check your programming language's list of extension
564 <P>The following interfaces are included in the PostgreSQL
570 <LI>Embedded C (ecpg)</LI>
574 <LI>Python (PyGreSQL)</LI>
576 <LI>TCL (libpgtcl)</LI>
579 <P>Additional interfaces are available at
580 <a href="http://gborg.postgresql.org">http://gborg.postgresql.org</A>
581 in the <I>Drivers/Interfaces</I> section.
585 <H2 align="center">Administrative Questions</H2>
587 <H4><A name="3.1">3.1</A>) How do I install PostgreSQL somewhere
588 other than <I>/usr/local/pgsql</I>?</H4>
590 <P>Specify the <I>--prefix</I> option when running
591 <I>configure</I>.</P>
593 <H4><A name="3.2">3.2</A>) When I start <I>postmaster</I>, I get a
594 <I>Bad System Call</I> or core dumped message. Why?</H4>
596 <P>It could be a variety of problems, but first check to see that
597 you have System V extensions installed in your kernel. PostgreSQL
598 requires kernel support for shared memory and semaphores.</P>
600 <H4><A name="3.3">3.3</A>) When I try to start <I>postmaster</I>, I
601 get <I>IpcMemoryCreate</I> errors. Why?</H4>
603 <P>You either do not have shared memory configured properly in your
604 kernel or you need to enlarge the shared memory available in the
605 kernel. The exact amount you need depends on your architecture and
606 how many buffers and backend processes you configure for
607 <I>postmaster</I>. For most systems, with default numbers of
608 buffers and processes, you need a minimum of ~1 MB. See the <A
610 "http://www.PostgreSQL.org/docs/view.php?version=current&idoc=1&file=kernel-resources.html">PostgreSQL
611 Administrator's Guide</A> for more detailed information about
612 shared memory and semaphores.</P>
614 <H4><A name="3.4">3.4</A>) When I try to start <I>postmaster</I>, I
615 get <I>IpcSemaphoreCreate</I> errors. Why?</H4>
617 <P>If the error message is <I>IpcSemaphoreCreate: semget failed (No
618 space left on device)</I> then your kernel is not configured with
619 enough semaphores. Postgres needs one semaphore per potential
620 backend process. A temporary solution is to start <I>postmaster</I>
621 with a smaller limit on the number of backend processes. Use
622 <I>-N</I> with a parameter less than the default of 32. A more
623 permanent solution is to increase your kernel's
624 <SMALL>SEMMNS</SMALL> and <SMALL>SEMMNI</SMALL> parameters.</P>
626 <P>Inoperative semaphores can also cause crashes during heavy
629 <P>If the error message is something else, you might not have
630 semaphore support configured in your kernel at all. See the
631 PostgreSQL Administrator's Guide for more detailed information
632 about shared memory and semaphores.</P>
634 <H4><A name="3.5">3.5</A>) How do I control connections from other
637 <P>By default, PostgreSQL only allows connections from the local
638 machine using Unix domain sockets or TCP/IP connections. Other
639 machines will not be able to connect unless you modify
640 listen_addresses in the postgresql.conf <B>and</B> enable
641 host-based authentication by modifying the file
642 <I>$PGDATA/pg_hba.conf</I> accordingly.</P>
644 <H4><A name="3.6">3.6</A>) How do I tune the database engine for
645 better performance?</H4>
647 <P>Certainly, indexes can speed up queries. The
648 <SMALL>EXPLAIN ANALYZE</SMALL> command allows you to see how PostgreSQL is
649 interpreting your query, and which indexes are being used.</P>
651 <P>If you are doing many <SMALL>INSERTs</SMALL>, consider doing
652 them in a large batch using the <SMALL>COPY</SMALL> command. This
653 is much faster than individual <SMALL>INSERTS</SMALL>. Second,
654 statements not in a <SMALL>BEGIN WORK/COMMIT</SMALL> transaction
655 block are considered to be in their own transaction. Consider
656 performing several statements in a single transaction block. This
657 reduces the transaction overhead. Also, consider dropping and
658 recreating indexes when making large data changes.</P>
660 <P>There are several tuning options. You can disable <I>fsync()</I>
661 by starting <I>postmaster</I> with a <I>-o -F</I> option. This will
662 prevent <I>fsync()</I>s from flushing to disk after every
665 <P>You can also use the <I>postmaster</I> <I>-B</I> option to
666 increase the number of shared memory buffers used by the backend
667 processes. If you make this parameter too high, the
668 <I>postmaster</I> may not start because you have exceeded your
669 kernel's limit on shared memory space. Each buffer is 8K and the
670 default is 64 buffers.</P>
672 <P>You can also use the backend <I>-S</I> option to increase the
673 maximum amount of memory used by the backend process for temporary
674 sorts. The <I>-S</I> value is measured in kilobytes, and the
675 default is 512 (i.e. 512K).</P>
677 <P>You can also use the <SMALL>CLUSTER</SMALL> command to group
678 data in tables to match an index. See the <SMALL>CLUSTER</SMALL>
679 manual page for more details.</P>
681 <H4><A name="3.7">3.7</A>) What debugging features are
684 <P>PostgreSQL has several features that report status information
685 that can be valuable for debugging purposes.</P>
687 <P>First, by running <I>configure</I> with the --enable-cassert
688 option, many <I>assert()</I>s monitor the progress of the backend
689 and halt the program when something unexpected occurs.</P>
691 <P>Both <I>postmaster</I> and <I>postgres</I> have several debug
692 options available. First, whenever you start <I>postmaster</I>,
693 make sure you send the standard output and error to a log file,
697 ./bin/postmaster >server.log 2>&1 &
700 <P>This will put a server.log file in the top-level PostgreSQL
701 directory. This file contains useful information about problems or
702 errors encountered by the server. <I>Postmaster</I> has a <I>-d</I>
703 option that allows even more detailed information to be reported.
704 The <I>-d</I> option takes a number that specifies the debug level.
705 Be warned that high debug level values generate large log
708 <P>If <I>postmaster</I> is not running, you can actually run the
709 <I>postgres</I> backend from the command line, and type your
710 <SMALL>SQL</SMALL> statement directly. This is recommended
711 <B>only</B> for debugging purposes. Note that a newline terminates
712 the query, not a semicolon. If you have compiled with debugging
713 symbols, you can use a debugger to see what is happening. Because
714 the backend was not started from <I>postmaster</I>, it is not
715 running in an identical environment and locking/backend interaction
716 problems may not be duplicated.</P>
718 <P>If <I>postmaster</I> is running, start <I>psql</I> in one
719 window, then find the <SMALL>PID</SMALL> of the <I>postgres</I>
720 process used by <I>psql</I>. Use a debugger to attach to the
721 <I>postgres</I> <SMALL>PID</SMALL>. You can set breakpoints in the
722 debugger and issue queries from <I>psql</I>. If you are debugging
723 <I>postgres</I> startup, you can set PGOPTIONS="-W n", then start
724 <I>psql</I>. This will cause startup to delay for <I>n</I> seconds
725 so you can attach to the process with the debugger, set any
726 breakpoints, and continue through the startup sequence.</P>
728 <P>The <I>postgres</I> program has <I>-s, -A</I>, and <I>-t</I>
729 options that can be very useful for debugging and performance
732 <P>You can also compile with profiling to see what functions are
733 taking execution time. The backend profile files will be deposited
734 in the <I>pgsql/data/base/dbname</I> directory. The client profile
735 file will be put in the client's current directory. Linux requires
736 a compile with <I>-DLINUX_PROFILE</I> for proper profiling.</P>
738 <H4><A name="3.8">3.8</A>) Why do I get <I>"Sorry, too many
739 clients"</I> when trying to connect?</H4>
741 <P>You need to increase <I>postmaster</I>'s limit on how many
742 concurrent backend processes it can start.</P>
744 <P>The default limit is 32 processes. You can increase it by
745 restarting <I>postmaster</I> with a suitable <I>-N</I> value or
746 modifying <I>postgresql.conf</I>.</P>
748 <P>Note that if you make <I>-N</I> larger than 32, you must also
749 increase <I>-B</I> beyond its default of 64; <I>-B</I> must be at
750 least twice <I>-N</I>, and probably should be more than that for
751 best performance. For large numbers of backend processes, you are
752 also likely to find that you need to increase various Unix kernel
753 configuration parameters. Things to check include the maximum size
754 of shared memory blocks, <SMALL>SHMMAX;</SMALL> the maximum number
755 of semaphores, <SMALL>SEMMNS</SMALL> and <SMALL>SEMMNI;</SMALL> the
756 maximum number of processes, <SMALL>NPROC;</SMALL> the maximum
757 number of processes per user, <SMALL>MAXUPRC;</SMALL> and the
758 maximum number of open files, <SMALL>NFILE</SMALL> and
759 <SMALL>NINODE</SMALL>. The reason that PostgreSQL has a limit on
760 the number of allowed backend processes is so your system won't run
761 out of resources.</P>
763 <H4><A name="3.9">3.9</A>) What is in the <I>pgsql_tmp</I> directory?</H4>
765 <P>This directory contains temporary files generated by the query
766 executor. For example, if a sort needs to be done to satisfy an
767 <SMALL>ORDER BY</SMALL> and the sort requires more space than the
768 backend's <I>-S</I> parameter allows, then temporary files are created
769 here to hold the extra data.</P>
771 <P>The temporary files are usually deleted automatically, but might
772 remain if a backend crashes during a sort. A stop and restart of the
773 <I>postmaster</I> will remove files from those directories.</P>
775 <H4><A name="3.10">3.10</A>) Why do I need to do a dump and restore
776 to upgrade between major PostgreSQL releases?</H4>
778 <P>The PostgreSQL team makes only small changes between minor releases,
779 so upgrading from 7.2 to 7.2.1 does not require a dump and restore.
780 However, major releases (e.g. from 7.2 to 7.3) often change the internal
781 format of system tables and data files. These changes are often complex,
782 so we don't maintain backward compatability for data files. A dump outputs
783 data in a generic format that can then be loaded in using the new internal
786 <P>In releases where the on-disk format does not change, the
787 <I>pg_upgrade</I> script can be used to upgrade without a dump/restore.
788 The release notes mention whether <I>pg_upgrade</I> is available for the
791 <H4><A name="3.11">3.11</A>) What computer hardware should I use?</H4>
793 <P>Because PC hardware is mostly compatible, people tend to believe that
794 all PC hardware is of equal quality. It is not. ECC RAM, SCSI, and
795 quality motherboards are more reliable and have better performance than
796 less expensive hardware. PostgreSQL will run on almost any hardware,
797 but if reliability and performance are important it is wise to
798 research your hardware options thoroughly. Our email lists can be used
799 to discuss hardware options and tradeoffs.</P>
803 <H2 align="center">Operational Questions</H2>
805 <H4><A name="4.1">4.1</A>) What is the difference between binary
806 cursors and normal cursors?</H4>
808 <P>See the <SMALL>DECLARE</SMALL> manual page for a
811 <H4><A name="4.2">4.2</A>) How do I <SMALL>SELECT</SMALL> only the
812 first few rows of a query? A random row?</H4>
814 <P>See the <SMALL>FETCH</SMALL> manual page, or use
815 <SMALL>SELECT</SMALL> ... <SMALL>LIMIT</SMALL>....</P>
817 <P>The entire query may have to be evaluated, even if you only want
818 the first few rows. Consider using a query that has an <SMALL>ORDER
819 BY</SMALL>. If there is an index that matches the <SMALL>ORDER
820 BY</SMALL>, PostgreSQL may be able to evaluate only the first few
821 records requested, or the entire query may have to be evaluated
822 until the desired rows have been generated.</P>
824 <P>To <SMALL>SELECT</SMALL> a random row, use:
832 <H4><A name="4.3">4.3</A>) How do I get a list of tables or other
833 things I can see in <I>psql</I>?</H4>
835 <P>Use the \dt command to see tables in <I>psql</I>. For a complete list of
836 commands inside psql you can use \?. Alternatively you can read the source
837 code for <I>psql</I> in file <I>pgsql/src/bin/psql/describe.c</I>, it
838 contains <SMALL>SQL</SMALL> commands that generate the output for
839 <I>psql</I>'s backslash commands. You can also start <I>psql</I> with the
840 <I>-E</I> option so it will print out the queries it uses to execute the
841 commands you give. PostgreSQL also provides an <SMALL>SQLi</SMALL> compliant
842 INFORMATION SCHEMA interface you can query to get information about the
845 <H4><A name="4.4">4.4</A>) How do you remove a column from a
846 table, or change its data type?</H4>
848 <P><SMALL>DROP COLUMN</SMALL> functionality was added in release 7.3 with
849 <SMALL>ALTER TABLE DROP COLUMN</SMALL>. In earlier versions,
853 LOCK TABLE old_table;
854 SELECT ... -- select all columns but the one you want to remove
857 DROP TABLE old_table;
858 ALTER TABLE new_table RENAME TO old_table;
862 <P>To change the data type of a column, do this:</P>
865 ALTER TABLE tab ADD COLUMN new_col <i>new_data_type</i>;
866 UPDATE tab SET new_col = CAST(old_col AS <i>new_data_type</i>);
867 ALTER TABLE tab DROP COLUMN old_col;
870 <P>You might then want to do <I>VACUUM FULL tab</I> to reclaim the
871 disk space used by the expired rows.</P>
873 <H4><A name="4.5">4.5</A>) What is the maximum size for a row, a
874 table, and a database?</H4>
876 <P>These are the limits:</P>
878 Maximum size for a database? unlimited (32 TB databases exist)
879 Maximum size for a table? 32 TB
880 Maximum size for a row? 1.6TB
881 Maximum size for a field? 1 GB
882 Maximum number of rows in a table? unlimited
883 Maximum number of columns in a table? 250-1600 depending on column types
884 Maximum number of indexes on a table? unlimited
887 Of course, these are not actually unlimited, but limited to
888 available disk space and memory/swap space. Performance may suffer
889 when these values get unusually large.
891 <P>The maximum table size of 32 TB does not require large file
892 support from the operating system. Large tables are stored as
893 multiple 1 GB files so file system size limits are not
896 <P>The maximum table size and maximum number of columns can be
897 quadrupled by increasing the default block size to 32k.</P>
899 <H4><A name="4.6">4.6</A>) How much database disk space is required
900 to store data from a typical text file?</H4>
902 <P>A PostgreSQL database may require up to five times the disk
903 space to store data from a text file.</P>
905 <P>As an example, consider a file of 100,000 lines with an integer
906 and text description on each line. Suppose the text string
907 avergages twenty bytes in length. The flat file would be 2.8 MB.
908 The size of the PostgreSQL database file containing this data can
909 be estimated as 6.4 MB:</P>
911 32 bytes: each row header (approximate)
912 24 bytes: one int field and one text field
913 + 4 bytes: pointer on page to tuple
914 ----------------------------------------
917 The data page size in PostgreSQL is 8192 bytes (8 KB), so:
920 ------------------- = 136 rows per database page (rounded down)
924 -------------------- = 735 database pages (rounded up)
927 735 database pages * 8192 bytes per page = 6,021,120 bytes (6 MB)
930 <P>Indexes do not require as much overhead, but do contain the data
931 that is being indexed, so they can be large also.</P>
933 <P><SMALL>NULL</SMALL>s are stored as bitmaps, so they
934 use very little space.</P>
936 <H4><A name="4.7">4.7</A>) How do I find out what tables, indexes,
937 databases, and users are defined?</H4>
939 <P><I>psql</I> has a variety of backslash commands to show such
940 information. Use \? to see them. There are also system tables
941 beginning with <I>pg_</I> that describe these too. Also, <I>psql
942 -l</I> will list all databases.</P>
944 <P>Also try the file <I>pgsql/src/tutorial/syscat.source</I>. It
945 illustrates many of the <SMALL>SELECT</SMALL>s needed to get
946 information from the database system tables.</P>
948 <H4><A name="4.8">4.8</A>) My queries are slow or don't make use of
949 the indexes. Why?</H4>
950 Indexes are not automatically used by every query. Indexes are only
951 used if the table is larger than a minimum size, and the query
952 selects only a small percentage of the rows in the table. This is
953 because the random disk access caused by an index scan can be
954 slower than a straight read through the table, or sequential scan.
956 <P>To determine if an index should be used, PostgreSQL must have
957 statistics about the table. These statistics are collected using
958 <SMALL>VACUUM ANALYZE</SMALL>, or simply <SMALL>ANALYZE</SMALL>.
959 Using statistics, the optimizer knows how many rows are in the
960 table, and can better determine if indexes should be used.
961 Statistics are also valuable in determining optimal join order and
962 join methods. Statistics collection should be performed
963 periodically as the contents of the table change.</P>
965 <P>Indexes are normally not used for <SMALL>ORDER BY</SMALL> or to
966 perform joins. A sequential scan followed by an explicit sort is
967 usually faster than an index scan of a large table.</P>
968 However, <SMALL>LIMIT</SMALL> combined with <SMALL>ORDER BY</SMALL>
969 often will use an index because only a small portion of the table
970 is returned. In fact, though MAX() and MIN() don't use indexes,
971 it is possible to retrieve such values using an index with ORDER BY
976 ORDER BY col [ DESC ]
980 <P>If you believe the optimizer is incorrect in choosing a
981 sequential scan, use <CODE>SET enable_seqscan TO 'off'</CODE> and
982 run tests to see if an index scan is indeed faster.</P>
984 <P>When using wild-card operators such as <SMALL>LIKE</SMALL> or
985 <I>~</I>, indexes can only be used in certain circumstances:</P>
987 <LI>The beginning of the search string must be anchored to the start
990 <LI><SMALL>LIKE</SMALL> patterns must not start with <I>%</I>.</LI>
991 <LI><I>~</I> (regular expression) patterns must start with
994 <LI>The search string can not start with a character class,
996 <LI>Case-insensitive searches such as <SMALL>ILIKE</SMALL> and
997 <I>~*</I> do not utilise indexes. Instead, use functional
998 indexes, which are described in section <a href="#4.12">4.12</a>.</LI>
999 <LI>The default <I>C</I> locale must be used during
1004 <P>In pre-8.0 releases, indexes often can not be used unless the data
1005 types exactly match the index's column types. This is particularly
1006 true of int2, int8, and numeric column indexes.</P>
1008 <H4><A name="4.9">4.9</A>) How do I see how the query optimizer is
1009 evaluating my query?</H4>
1011 <P>See the <SMALL>EXPLAIN</SMALL> manual page.</P>
1013 <H4><A name="4.10">4.10</A>) What is an R-tree index?</H4>
1015 <P>An R-tree index is used for indexing spatial data. A hash index
1016 can't handle range searches. A B-tree index only handles range
1017 searches in a single dimension. R-trees can handle
1018 multi-dimensional data. For example, if an R-tree index can be
1019 built on an attribute of type <I>point</I>, the system can more
1020 efficiently answer queries such as "select all points within a
1021 bounding rectangle."</P>
1023 <P>The canonical paper that describes the original R-tree design
1026 <P>Guttman, A. "R-trees: A Dynamic Index Structure for Spatial
1027 Searching." Proceedings of the 1984 ACM SIGMOD Int'l Conf on Mgmt
1030 <P>You can also find this paper in Stonebraker's "Readings in
1031 Database Systems".</P>
1033 <P>Built-in R-trees can handle polygons and boxes. In theory,
1034 R-trees can be extended to handle higher number of dimensions. In
1035 practice, extending R-trees requires a bit of work and we don't
1036 currently have any documentation on how to do it.</P>
1038 <H4><A name="4.11">4.11</A>) What is the Genetic Query
1041 <P>The <SMALL>GEQO</SMALL> module speeds query optimization when
1042 joining many tables by means of a Genetic Algorithm (GA). It allows
1043 the handling of large join queries through nonexhaustive
1046 <H4><A name="4.12">4.12</A>) How do I perform regular expression
1047 searches and case-insensitive regular expression searches? How do I
1048 use an index for case-insensitive searches?</H4>
1050 <P>The <I>~</I> operator does regular expression matching, and
1051 <I>~*</I> does case-insensitive regular expression matching. The
1052 case-insensitive variant of <SMALL>LIKE</SMALL> is called
1053 <SMALL>ILIKE</SMALL>.</P>
1055 <P>Case-insensitive equality comparisons are normally expressed
1060 WHERE lower(col) = 'abc';
1063 This will not use an standard index. However, if you create a
1064 functional index, it will be used:
1066 CREATE INDEX tabindex ON tab (lower(col));
1069 <H4><A name="4.13">4.13</A>) In a query, how do I detect if a field
1070 is <SMALL>NULL</SMALL>?</H4>
1072 <P>You test the column with <SMALL>IS NULL</SMALL> and <SMALL>IS
1073 NOT NULL</SMALL>.</P>
1075 <H4><A name="4.14">4.14</A>) What is the difference between the
1076 various character types?</H4>
1078 Type Internal Name Notes
1079 --------------------------------------------------
1080 VARCHAR(n) varchar size specifies maximum length, no padding
1081 CHAR(n) bpchar blank padded to the specified fixed length
1082 TEXT text no specific upper limit on length
1083 BYTEA bytea variable-length byte array (null-byte safe)
1084 "char" char one character
1087 <P>You will see the internal name when examining system catalogs
1088 and in some error messages.</P>
1090 <P>The first four types above are "varlena" types (i.e., the first
1091 four bytes on disk are the length, followed by the data). Thus the
1092 actual space used is slightly greater than the declared size.
1093 However, these data types are also subject to compression or being
1094 stored out-of-line by <SMALL>TOAST</SMALL>, so the space on disk
1095 might also be less than expected.</P>
1097 <SMALL>VARCHAR(n)</SMALL> is best when storing variable-length
1098 strings and it limits how long a string can be. <SMALL>TEXT</SMALL>
1099 is for strings of unlimited length, with a maximum of one gigabyte.
1100 <P><SMALL>CHAR(n)</SMALL> is for storing strings that are all the
1101 same length. <SMALL>CHAR(n)</SMALL> pads with blanks to the specified
1102 length, while <SMALL>VARCHAR(n)</SMALL> only stores the characters
1103 supplied. <SMALL>BYTEA</SMALL> is for storing binary data,
1104 particularly values that include <SMALL>NULL</SMALL> bytes. All the
1105 types described here have similar performance characteristics.</P>
1107 <H4><A name="4.15.1">4.15.1</A>) How do I create a
1108 serial/auto-incrementing field?</H4>
1110 <P>PostgreSQL supports a <SMALL>SERIAL</SMALL> data type. It
1111 auto-creates a sequence. For example,
1114 CREATE TABLE person (
1120 is automatically translated into this:
1122 CREATE SEQUENCE person_id_seq;
1123 CREATE TABLE person (
1124 id INT4 NOT NULL DEFAULT nextval('person_id_seq'),
1129 See the <I>create_sequence</I> manual page for more information
1130 about sequences. You can also use each row's <I>OID</I> field as a
1131 unique value. However, if you need to dump and reload the database,
1132 you need to use <I>pg_dump</I>'s <I>-o</I> option or <SMALL>COPY
1133 WITH OIDS</SMALL> option to preserve the <SMALL>OID</SMALL>s.
1135 <H4><A name="4.15.2">4.15.2</A>) How do I get the value of a
1136 <SMALL>SERIAL</SMALL> insert?</H4>
1138 <P>One approach is to retrieve the next <SMALL>SERIAL</SMALL> value
1139 from the sequence object with the <I>nextval()</I> function
1140 <I>before</I> inserting and then insert it explicitly. Using the
1141 example table in <A href="#4.15.1">4.15.1</A>, an example in a
1142 pseudo-language would look like this:</P>
1144 new_id = execute("SELECT nextval('person_id_seq')");
1145 execute("INSERT INTO person (id, name) VALUES (new_id, 'Blaise Pascal')");
1148 You would then also have the new value stored in
1149 <CODE>new_id</CODE> for use in other queries (e.g., as a foreign
1150 key to the <CODE>person</CODE> table). Note that the name of the
1151 automatically created <SMALL>SEQUENCE</SMALL> object will be named
1152 <<I>table</I>>_<<I>serialcolumn</I>>_<I>seq</I>, where
1153 <I>table</I> and <I>serialcolumn</I> are the names of your table
1154 and your <SMALL>SERIAL</SMALL> column, respectively.
1156 <P>Alternatively, you could retrieve the assigned
1157 <SMALL>SERIAL</SMALL> value with the <I>currval()</I> function
1158 <I>after</I> it was inserted by default, e.g.,</P>
1160 execute("INSERT INTO person (name) VALUES ('Blaise Pascal')");
1161 new_id = execute("SELECT currval('person_id_seq')");
1164 Finally, you could use the <A href="#4.16"><SMALL>OID</SMALL></A>
1165 returned from the <SMALL>INSERT</SMALL> statement to look up the
1166 default value, though this is probably the least portable approach,
1167 and the oid value will wrap around when it reaches 4 billion.
1168 In Perl, using DBI with Edmund Mergl's DBD::Pg module, the oid
1169 value is made available via <I>$sth->{pg_oid_status}</I> after
1170 <I>$sth->execute()</I>.
1172 <H4><A name="4.15.3">4.15.3</A>) Don't <I>currval()</I> and
1173 <I>nextval()</I> lead to a race condition with other users?</H4>
1175 <P>No. <I>currval()</I> returns the current value assigned by your
1176 backend, not by all users.</P>
1178 <H4><A name="4.15.4">4.15.4</A>) Why aren't my sequence numbers
1179 reused on transaction abort? Why are there gaps in the numbering of
1180 my sequence/SERIAL column?</H4>
1182 <P>To improve concurrency, sequence values are given out to running
1183 transactions as needed and are not locked until the transaction
1184 completes. This causes gaps in numbering from aborted
1187 <H4><A name="4.16">4.16</A>) What is an <SMALL>OID</SMALL>? What is
1188 a <SMALL>TID</SMALL>?</H4>
1190 <P><SMALL>OID</SMALL>s are PostgreSQL's answer to unique row ids.
1191 Every row that is created in PostgreSQL gets a unique
1192 <SMALL>OID</SMALL>. All <SMALL>OID</SMALL>s generated during
1193 <I>initdb</I> are less than 16384 (from
1194 <I>include/access/transam.h</I>). All user-created
1195 <SMALL>OID</SMALL>s are equal to or greater than this. By default,
1196 all these <SMALL>OID</SMALL>s are unique not only within a table or
1197 database, but unique within the entire PostgreSQL installation.</P>
1199 <P>PostgreSQL uses <SMALL>OID</SMALL>s in its internal system
1200 tables to link rows between tables. These <SMALL>OID</SMALL>s can
1201 be used to identify specific user rows and used in joins. It is
1202 recommended you use column type <SMALL>OID</SMALL> to store
1203 <SMALL>OID</SMALL> values. You can create an index on the
1204 <SMALL>OID</SMALL> field for faster access.</P>
1206 <P>O<SMALL>ID</SMALL>s are assigned to all new rows from a central
1207 area that is used by all databases. If you want to change the
1208 <SMALL>OID</SMALL> to something else, or if you want to make a copy
1209 of the table, with the original <SMALL>OID</SMALL>s, there is no
1210 reason you can't do it:</P>
1212 CREATE TABLE new_table(mycol int);
1213 SELECT oid AS old_oid, mycol INTO tmp_table FROM old_table;
1214 COPY tmp_table TO '/tmp/pgtable';
1215 COPY new_table WITH OIDS FROM '/tmp/pgtable';
1216 DROP TABLE tmp_table;
1218 <P>O<SMALL>ID</SMALL>s are stored as 4-byte integers, and will
1219 overflow at 4 billion. No one has reported this ever happening, and
1220 we plan to have the limit removed before anyone does.</P>
1222 <P>T<SMALL>ID</SMALL>s are used to identify specific physical rows
1223 with block and offset values. T<SMALL>ID</SMALL>s change after rows
1224 are modified or reloaded. They are used by index entries to point
1225 to physical rows.</P>
1227 <H4><A name="4.17">4.17</A>) What is the meaning of some of the
1228 terms used in PostgreSQL?</H4>
1230 <P>Some of the source code and older documentation use terms that
1231 have more common usage. Here are some:</P>
1234 <LI>table, relation, class</LI>
1236 <LI>row, record, tuple</LI>
1238 <LI>column, field, attribute</LI>
1240 <LI>retrieve, select</LI>
1242 <LI>replace, update</LI>
1244 <LI>append, insert</LI>
1246 <LI><SMALL>OID</SMALL>, serial value</LI>
1248 <LI>portal, cursor</LI>
1250 <LI>range variable, table name, table alias</LI>
1253 <P>A list of general database terms can be found at: <A href=
1254 "http://hea-www.harvard.edu/MST/simul/software/docs/pkgs/pgsql/glossary/glossary.html">http://hea-www.harvard.edu/MST/simul/software/docs/pkgs/pgsql/glossary/glossary.html</A></P>
1256 <H4><A name="4.18">4.18</A>) Why do I get the error <I>"ERROR:
1257 Memory exhausted in AllocSetAlloc()"</I>?</H4>
1259 <P>You probably have run out of virtual memory on your system,
1260 or your kernel has a low limit for certain resources. Try this
1261 before starting <I>postmaster</I>:</P>
1267 Depending on your shell, only one of these may succeed, but it will
1268 set your process data segment limit much higher and perhaps allow
1269 the query to complete. This command applies to the current process,
1270 and all subprocesses created after the command is run. If you are
1271 having a problem with the <SMALL>SQL</SMALL> client because the
1272 backend is returning too much data, try it before starting the
1275 <H4><A name="4.19">4.19</A>) How do I tell what PostgreSQL version
1278 <P>From <I>psql</I>, type <CODE>SELECT version();</CODE></P>
1280 <H4><A name="4.20">4.20</A>) Why does my large-object operations
1281 get <I>"invalid large obj descriptor"</I>?</H4>
1283 <P>You need to put <CODE>BEGIN WORK</CODE> and <CODE>COMMIT</CODE>
1284 around any use of a large object handle, that is, surrounding
1285 <CODE>lo_open</CODE> ... <CODE>lo_close.</CODE></P>
1287 <P>Currently PostgreSQL enforces the rule by closing large object
1288 handles at transaction commit. So the first attempt to do anything
1289 with the handle will draw <I>invalid large obj descriptor</I>. So
1290 code that used to work (at least most of the time) will now
1291 generate that error message if you fail to use a transaction.</P>
1293 <P>If you are using a client interface like <SMALL>ODBC</SMALL> you
1294 may need to set <CODE>auto-commit off.</CODE></P>
1296 <H4><A name="4.21">4.21</A>) How do I create a column that will
1297 default to the current time?</H4>
1299 <P>Use <I>CURRENT_TIMESTAMP</I>:</P>
1301 <CODE>CREATE TABLE test (x int, modtime timestamp DEFAULT CURRENT_TIMESTAMP );
1305 <H4><A name="4.22">4.22</A>) Why are my subqueries using
1306 <CODE><SMALL>IN</SMALL></CODE> so slow?</H4>
1308 <P>In versions prior to 7.4, subqueries were joined to outer queries
1309 by sequentially scanning the result of the subquery for each row of
1310 the outer query. If the subquery returns only a few rows and the outer
1311 query returns many rows, <CODE><SMALL>IN</SMALL></CODE> is fastest. To
1312 speed up other queries, replace <CODE>IN</CODE> with
1313 <CODE>EXISTS</CODE>:</P>
1316 WHERE col IN (SELECT subcol FROM subtab);
1321 WHERE EXISTS (SELECT subcol FROM subtab WHERE subcol = col);
1324 For this to be fast, <CODE>subcol</CODE> should be an indexed column.
1325 <P>In version 7.4 and later, <CODE>IN</CODE> actually uses the same
1326 sophisticated join techniques as normal queries, and is prefered
1327 to using <CODE>EXISTS</CODE>.
1329 <H4><A name="4.23">4.23</A>) How do I perform an outer join?</H4>
1331 <P>PostgreSQL supports outer joins using the SQL standard syntax.
1332 Here are two examples:</P>
1335 FROM t1 LEFT OUTER JOIN t2 ON (t1.col = t2.col);
1340 FROM t1 LEFT OUTER JOIN t2 USING (col);
1343 <P>These identical queries join t1.col to t2.col, and also return
1344 any unjoined rows in t1 (those with no match in t2). A
1345 <SMALL>RIGHT</SMALL> join would add unjoined rows of t2. A
1346 <SMALL>FULL</SMALL> join would return the matched rows plus all
1347 unjoined rows from t1 and t2. The word <SMALL>OUTER</SMALL> is
1348 optional and is assumed in <SMALL>LEFT</SMALL>,
1349 <SMALL>RIGHT</SMALL>, and <SMALL>FULL</SMALL> joins. Ordinary joins
1350 are called <SMALL>INNER</SMALL> joins.</P>
1352 <P>In previous releases, outer joins can be simulated using
1353 <SMALL>UNION</SMALL> and <SMALL>NOT IN</SMALL>. For example, when
1354 joining <I>tab1</I> and <I>tab2</I>, the following query does an
1355 <I>outer</I> join of the two tables:<BR>
1359 SELECT tab1.col1, tab2.col2
1361 WHERE tab1.col1 = tab2.col1
1363 SELECT tab1.col1, NULL
1365 WHERE tab1.col1 NOT IN (SELECT tab2.col1 FROM tab2)
1369 <H4><A name="4.24">4.24</A>) How do I perform queries using
1370 multiple databases?</H4>
1372 <P>There is no way to query a database other than the current one.
1373 Because PostgreSQL loads database-specific system catalogs, it is
1374 uncertain how a cross-database query should even behave.</P>
1376 <P><I>contrib/dblink</I> allows cross-database queries using
1377 function calls. Of course, a client can make simultaneous
1378 connections to different databases and merge the results on the
1381 <H4><A name="4.25">4.25</A>) How do I return multiple rows or
1382 columns from a function?</H4>
1384 <P>In 7.3, you can easily return multiple rows or columns from a
1386 <a href="http://techdocs.postgresql.org/guides/SetReturningFunctions">
1387 http://techdocs.postgresql.org/guides/SetReturningFunctions</a>.
1389 <H4><A name="4.26">4.26</A>) Why can't I reliably create/drop
1390 temporary tables in PL/PgSQL functions?</H4>
1391 <P>PL/PgSQL caches function contents, and an unfortunate side effect
1392 is that if a PL/PgSQL function accesses a temporary table, and that
1393 table is later dropped and recreated, and the function called
1394 again, the function will fail because the cached function contents
1395 still point to the old temporary table. The solution is to use
1396 <SMALL>EXECUTE</SMALL> for temporary table access in PL/PgSQL. This
1397 will cause the query to be reparsed every time.</P>
1399 <H4><A name="4.27">4.27</A>) What replication options are available?
1401 <P>There are several master/slave replication options available.
1402 These allow only the master to make database changes and the slave
1403 can only do database reads. The bottom of <a
1404 href="http://gborg.PostgreSQL.org/genpage?replication_research">
1405 http://gborg.PostgreSQL.org/genpage?replication_research</a> lists
1406 them. A multi-master replication solution is being worked on at <a
1407 href="http://gborg.PostgreSQL.org/project/pgreplication/projdisplay.php">http://gborg.PostgreSQL.org/project/pgreplication/projdisplay.php</a>.</P>
1409 <H4><A name="4.28">4.28</A>) What encryption options are available?
1412 <LI><I>contrib/pgcrypto</I> contains many encryption functions for
1413 use in <SMALL>SQL</SMALL> queries.</LI>
1414 <LI>To encrypt transmission from the client to the server, the server
1415 must have the <I>ssl</I> option set to <I>true</I> in <I>postgresql.conf,
1416 </I> and an applicable <I>host</I> or <I>hostssl</I> record must exist in
1417 <I>pg_hba.conf</I>, and the client <I>sslmode</I> must not be
1418 <I>disable.</I> (Note that it is also possible to use a third-party
1419 encrypted transport, such as stunnel or ssh, rather than PostgreSQL's
1420 native SSL connections.)
1421 <LI>Database user passwords are automatically encrypted when stored in
1422 version 7.3. In previous versions, you must enable the option
1423 <I>PASSWORD_ENCRYPTION</I> in <I>postgresql.conf</I>.</LI>
1424 <LI>The server can run using an encrypted file system.</LI>
1429 <H2 align="center">Extending PostgreSQL</H2>
1431 <H4><A name="5.1">5.1</A>) I wrote a user-defined function. When I
1432 run it in <I>psql</I>, why does it dump core?</H4>
1434 <P>The problem could be a number of things. Try testing your
1435 user-defined function in a stand-alone test program first.</P>
1437 <H4><A name="5.2">5.2</A>) How can I contribute some nifty new
1438 types and functions to PostgreSQL?</H4>
1440 <P>Send your extensions to the <I>pgsql-hackers</I> mailing list,
1441 and they will eventually end up in the <I>contrib/</I>
1444 <H4><A name="5.3">5.3</A>) How do I write a C function to return a
1447 <P>In versions of PostgreSQL beginning with 7.3, table-returning
1448 functions are fully supported in C, PL/PgSQL, and SQL. See the
1449 Programmer's Guide for more information. An example of a
1450 table-returning function defined in C can be found in
1451 <I>contrib/tablefunc</I>.</P>
1453 <H4><A name="5.4">5.4</A>) I have changed a source file. Why does
1454 the recompile not see the change?</H4>
1456 <P>The <I>Makefiles</I> do not have the proper dependencies for
1457 include files. You have to do a <I>make clean</I> and then another
1458 <I>make</I>. If you are using <SMALL>GCC</SMALL> you can use the
1459 <I>--enable-depend</I> option of <I>configure</I> to have the
1460 compiler compute the dependencies automatically.</P>
projects / postgresql / blob