1 <!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
4 <META name="generator" content="HTML Tidy, see www.w3.org">
5 <META http-equiv="Content-Type" content="text/html; charset=US-ASCII">
6 <TITLE>PostgreSQL FAQ</TITLE>
9 <BODY bgcolor="#ffffff" text="#000000" link="#ff0000" vlink="#a00000"
11 <H1>Frequently Asked Questions (FAQ) for PostgreSQL</H1>
13 <P>Last updated: Fri Jan 23 09:41:43 EST 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
21 "http://www.PostgreSQL.org/docs/faqs/FAQ.html">http://www.PostgreSQL.org/docs/faqs/FAQ.html</A>.</P>
23 <P>Platform-specific questions are answered at <A href=
24 "http://www.PostgreSQL.org/docs/index.html">http://www.PostgreSQL.org/docs/index.html</A>.</P>
27 <H2 align="center">General Questions</H2>
28 <A href="#1.1">1.1</A>) What is PostgreSQL? How is it pronounced?<BR>
29 <A href="#1.2">1.2</A>) What is the copyright on PostgreSQL?<BR>
30 <A href="#1.3">1.3</A>) What Unix platforms does PostgreSQL run
32 <A href="#1.4">1.4</A>) What non-Unix ports are available?<BR>
33 <A href="#1.5">1.5</A>) Where can I get PostgreSQL?<BR>
34 <A href="#1.6">1.6</A>) Where can I get support?<BR>
35 <A href="#1.7">1.7</A>) What is the latest release?<BR>
36 <A href="#1.8">1.8</A>) What documentation is available?<BR>
37 <A href="#1.9">1.9</A>) How do I find out about known bugs or
39 <A href="#1.10">1.10</A>) How can I learn <SMALL>SQL</SMALL>?<BR>
40 <A href="#1.11">1.11</A>) Is PostgreSQL Y2K compliant?<BR>
41 <A href="#1.12">1.12</A>) How do I join the development team?<BR>
42 <A href="#1.13">1.13</A>) How do I submit a bug report?<BR>
43 <A href="#1.14">1.14</A>) How does PostgreSQL compare to other
44 <SMALL>DBMS</SMALL>s?<BR>
45 <A href="#1.15">1.15</A>) How can I financially assist
49 <H2 align="center">User Client Questions</H2>
50 <A href="#2.1">2.1</A>) Are there <SMALL>ODBC</SMALL> drivers for
52 <A href="#2.2">2.2</A>) What tools are available for using
53 PostgreSQL with Web pages?<BR>
54 <A href="#2.3">2.3</A>) Does PostgreSQL have a graphical user
56 <A href="#2.4">2.4</A>) What languages are available to
57 communicate with PostgreSQL?<BR>
60 <H2 align="center">Administrative Questions</H2>
61 <A href="#3.1">3.1</A>) How do I install PostgreSQL somewhere other
62 than <I>/usr/local/pgsql</I>?<BR>
63 <A href="#3.2">3.2</A>) When I start <I>postmaster</I>, I get a
64 <I>Bad System Call</I> or core dumped message. Why?<BR>
65 <A href="#3.3">3.3</A>) When I try to start <I>postmaster</I>, I
66 get <I>IpcMemoryCreate</I> errors. Why?<BR>
67 <A href="#3.4">3.4</A>) When I try to start <I>postmaster</I>, I
68 get <I>IpcSemaphoreCreate</I> errors. Why?<BR>
69 <A href="#3.5">3.5</A>) How do I control connections from other
71 <A href="#3.6">3.6</A>) How do I tune the database engine for
72 better performance?<BR>
73 <A href="#3.7">3.7</A>) What debugging features are available?<BR>
74 <A href="#3.8">3.8</A>) Why do I get <I>"Sorry, too many
75 clients"</I> when trying to connect?<BR>
76 <A href="#3.9">3.9</A>) What is in the <I>pgsql_tmp</I>
78 <A href="#3.10">3.10</A>) Why do I need to do a dump and restore
79 to upgrade PostgreSQL releases?<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>.</P>
162 <P>PostgreSQL is an enhancement of the POSTGRES database management
163 system, a next-generation <SMALL>DBMS</SMALL> research prototype.
164 While PostgreSQL retains the powerful data model and rich data
165 types of POSTGRES, it replaces the PostQuel query language with an
166 extended subset of <SMALL>SQL</SMALL>. PostgreSQL is free and the
167 complete source is available.</P>
169 <P>PostgreSQL development is performed by a team of
170 developers who all subscribe to the PostgreSQL development mailing
171 list. The current coordinator is Marc G. Fournier (<A href=
172 "mailto:scrappy@PostgreSQL.org">scrappy@PostgreSQL.org</A>). (See
173 section <a href="#1.6">1.6</a> on how to join). This team is now
174 responsible for all development of PostgreSQL.</P>
176 <P>The authors of PostgreSQL 1.01 were Andrew Yu and Jolly Chen.
177 Many others have contributed to the porting, testing, debugging,
178 and enhancement of the code. The original Postgres code, from which
179 PostgreSQL is derived, was the effort of many graduate students,
180 undergraduate students, and staff programmers working under the
181 direction of Professor Michael Stonebraker at the University of
182 California, Berkeley.</P>
184 <P>The original name of the software at Berkeley was Postgres. When
185 <SMALL>SQL</SMALL> functionality was added in 1995, its name was
186 changed to Postgres95. The name was changed at the end of 1996 to
189 <H4><A name="1.2">1.2</A>) What is the copyright on
192 <P>PostgreSQL is subject to the following COPYRIGHT:</P>
194 <P>PostgreSQL Data Base Management System</P>
196 <P>Portions copyright (c) 1996-2002, PostgreSQL Global Development
197 Group Portions Copyright (c) 1994-6 Regents of the University of
200 <P>Permission to use, copy, modify, and distribute this software
201 and its documentation for any purpose, without fee, and without a
202 written agreement is hereby granted, provided that the above
203 copyright notice and this paragraph and the following two
204 paragraphs appear in all copies.</P>
206 <P>IN NO EVENT SHALL THE UNIVERSITY OF CALIFORNIA BE LIABLE TO ANY
207 PARTY FOR DIRECT, INDIRECT, SPECIAL, INCIDENTAL, OR CONSEQUENTIAL
208 DAMAGES, INCLUDING LOST PROFITS, ARISING OUT OF THE USE OF THIS
209 SOFTWARE AND ITS DOCUMENTATION, EVEN IF THE UNIVERSITY OF
210 CALIFORNIA HAS BEEN ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.</P>
212 <P>THE UNIVERSITY OF CALIFORNIA SPECIFICALLY DISCLAIMS ANY
213 WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
214 OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE. THE
215 SOFTWARE PROVIDED HEREUNDER IS ON AN "AS IS" BASIS, AND THE
216 UNIVERSITY OF CALIFORNIA HAS NO OBLIGATIONS TO PROVIDE MAINTENANCE,
217 SUPPORT, UPDATES, ENHANCEMENTS, OR MODIFICATIONS.</P>
219 <P>The above is the BSD license, the classic open-source license.
220 It has no restrictions on how the source code may be used. We like
221 it and have no intention of changing it.</P>
223 <H4><A name="1.3">1.3</A>) What Unix platforms does PostgreSQL run
226 <P>In general, a modern Unix-compatible platform should be able to
227 run PostgreSQL. The platforms that had received explicit testing at
228 the time of release are listed in the installation
231 <H4><A name="1.4">1.4</A>) What non-Unix ports are available?</H4>
233 <P><STRONG>Client</STRONG></P>
235 <P>It is possible to compile the <I>libpq</I> C library, psql, and
236 other interfaces and client applications to run on MS Windows platforms.
237 In this case, the client is running on MS Windows, and communicates
238 via TCP/IP to a server running on one of our supported Unix
239 platforms. A file <I>win32.mak</I> is included in the distribution
240 for making a Win32 <I>libpq</I> library and <I>psql</I>. PostgreSQL
241 also communicates with <SMALL>ODBC</SMALL> clients.</P>
243 <P><STRONG>Server</STRONG></P>
245 <P>The database server can run on Windows NT and Win2k using
246 Cygwin, the Cygnus Unix/NT porting library. See
247 <I>pgsql/doc/FAQ_MSWIN</I> in the distribution or the MS Windows FAQ
248 at <A href="http://www.PostgreSQL.org/docs/faqs/text/FAQ_MSWIN">
249 http://www.PostgreSQL.org/docs/faqs/text/FAQ_MSWIN</A>.</P>
251 <p>A native port to MS Win NT/2000/XP is currently being worked
252 on. For more details on the current status of PostgreSQL on Windows see
253 <a href="http://techdocs.postgresql.org/guides/Windows">
254 http://techdocs.postgresql.org/guides/Windows</a> and
255 <a href="http://momjian.postgresql.org/main/writings/pgsql/win32.html">
256 http://momjian.postgresql.org/main/writings/pgsql/win32.html</a>.</p>
258 <p>There is also a Novell Netware 6 port at
259 <a href="http://forge.novell.com">http://forge.novell.com</a>.</p>
261 <H4><A name="1.5">1.5</A>) Where can I get PostgreSQL?</H4>
263 <P>The primary anonymous ftp site for PostgreSQL is <A href=
264 "ftp://ftp.PostgreSQL.org/pub">ftp://ftp.PostgreSQL.org/pub</A>.
265 For mirror sites, see our main web site.</P>
267 <H4><A name="1.6">1.6</A>) Where can I get support?</H4>
269 <P>The main mailing list is: <A href=
270 "mailto:pgsql-general@PostgreSQL.org">pgsql-general@PostgreSQL.org</A>.
271 It is available for discussion of matters pertaining to PostgreSQL.
272 To subscribe, send mail with the following lines in the body (not
273 the subject line):</P>
280 "mailto:pgsql-general-request@PostgreSQL.org">pgsql-general-request@PostgreSQL.org</A>.</P>
282 <P>There is also a digest list available. To subscribe to this
283 list, send email to: <A href=
284 "mailto:pgsql-general-digest-request@PostgreSQL.org">pgsql-general-digest-request@PostgreSQL.org</A>
291 Digests are sent out to members of this list whenever the main list
292 has received around 30k of messages.
294 <P>The bugs mailing list is available. To subscribe to this list,
295 send email to <A href=
296 "mailto:pgsql-bugs-request@PostgreSQL.org">pgsql-bugs-request@PostgreSQL.org</A>
303 There is also a developers discussion mailing list available. To
304 subscribe to this list, send email to <A href=
305 "mailto:pgsql-hackers-request@PostgreSQL.org">pgsql-hackers-request@PostgreSQL.org</A>
312 <P>Additional mailing lists and information about PostgreSQL can be
313 found via the PostgreSQL WWW home page at:</P>
316 <A href="http://www.PostgreSQL.org">http://www.PostgreSQL.org</A>
319 <P>There is also an IRC channel on Freenode and EFNet,
320 channel <I>#PostgreSQL</I>. You can use the Unix command <CODE>irc -c
321 '#PostgreSQL' "$USER" irc.phoenix.net.</CODE> or <CODE>irc -c
322 '#PostgreSQL' "$USER" irc.freenode.net.</CODE></P>
324 <P>A list of commercial support companies is available at <A href=
325 "http://techdocs.postgresql.org/companies.php">http://techdocs.postgresql.org/companies.php</A>.</P>
327 <H4><A name="1.7">1.7</A>) What is the latest release?</H4>
329 <P>The latest release of PostgreSQL is version 7.4.1.</P>
331 <P>We plan to have major releases every six to eight months.</P>
333 <H4><A name="1.8">1.8</A>) What documentation is available?</H4>
335 <P>Several manuals, manual pages, and some small test examples are
336 included in the distribution. See the <I>/doc</I> directory. You
337 can also browse the manuals online at <A href=
338 "http://www.PostgreSQL.org/docs">http://www.PostgreSQL.org/docs</A>.</P>
340 <P>There are two PostgreSQL books available online at <A href=
341 "http://www.PostgreSQL.org/docs/awbook.html">http://www.PostgreSQL.org/docs/awbook.html</A>
343 "http://www.commandprompt.com/ppbook/">http://www.commandprompt.com/ppbook/</A>.
344 There is a list of PostgreSQL books available for purchase at <A
346 "http://techdocs.postgresql.org/techdocs/bookreviews.php">http://techdocs.PostgreSQL.org/techdocs/bookreviews.php</A>.
347 There is also a collection of PostgreSQL technical articles at <A
349 "http://techdocs.PostgreSQL.org/">http://techdocs.PostgreSQL.org/</A>.</P>
351 <P><I>psql</I> has some nice \d commands to show information about
352 types, operators, functions, aggregates, etc.</P>
354 <P>Our web site contains even more documentation.</P>
356 <H4><A name="1.9">1.9</A>) How do I find out about known bugs or
357 missing features?</H4>
359 <P>PostgreSQL supports an extended subset of <SMALL>SQL</SMALL>-92.
360 See our <A href="http://developer.PostgreSQL.org/todo.php">TODO</A>
361 list for known bugs, missing features, and future plans.</P>
363 <H4><A name="1.10">1.10</A>) How can I learn
364 <SMALL>SQL</SMALL>?</H4>
366 <P>The PostgreSQL book at <A href=
367 "http://www.PostgreSQL.org/docs/awbook.html">http://www.PostgreSQL.org/docs/awbook.html</A>
368 teaches <SMALL>SQL</SMALL>. There is another PostgreSQL book at <A
370 "http://www.commandprompt.com/ppbook/">http://www.commandprompt.com/ppbook.</A>
371 There is a nice tutorial at <A href=
372 "http://www.intermedia.net/support/sql/sqltut.shtm">http://www.intermedia.net/support/sql/sqltut.shtm,</A>
374 "http://ourworld.compuserve.com/homepages/graeme_birchall/HTM_COOK.HTM">
375 http://ourworld.compuserve.com/homepages/graeme_birchall/HTM_COOK.HTM,</A>
377 "http://sqlcourse.com/">http://sqlcourse.com.</A></P>
379 <P>Another one is "Teach Yourself SQL in 21 Days, Second Edition"
381 "http://members.tripod.com/er4ebus/sql/index.htm">http://members.tripod.com/er4ebus/sql/index.htm</A></P>
383 <P>Many of our users like <I>The Practical SQL Handbook</I>,
384 Bowman, Judith S., et al., Addison-Wesley. Others like <I>The
385 Complete Reference SQL</I>, Groff et al., McGraw-Hill.</P>
387 <H4><A name="1.11">1.11</A>) Is PostgreSQL Y2K compliant?</H4>
389 <P>Yes, we easily handle dates past the year 2000 AD, and before
392 <H4><A name="1.12">1.12</A>) How do I join the development
395 <P>First, download the latest source and read the PostgreSQL
396 Developers documentation on our web site, or in the distribution.
397 Second, subscribe to the <I>pgsql-hackers</I> and
398 <I>pgsql-patches</I> mailing lists. Third, submit high quality
399 patches to pgsql-patches.</P>
401 <P>There are about a dozen people who have commit privileges to the
402 PostgreSQL <SMALL>CVS</SMALL> archive. They each have submitted so
403 many high-quality patches that it was impossible for the existing
404 committers to keep up, and we had confidence that patches they
405 committed were of high quality.</P>
407 <H4><A name="1.13">1.13</A>) How do I submit a bug report?</H4>
409 <P>Please visit the PostgreSQL BugTool page at <A href=
410 "http://www.PostgreSQL.org/bugs/bugs.php">http://www.PostgreSQL.org/bugs/bugs.php</A>,
411 which gives guidelines and directions on how to submit a
414 <P>Also check out our ftp site <A href=
415 "ftp://ftp.PostgreSQL.org/pub">ftp://ftp.PostgreSQL.org/pub</A> to
416 see if there is a more recent PostgreSQL version or patches.</P>
418 <H4><A name="1.14">1.14</A>) How does PostgreSQL compare to other
419 <SMALL>DBMS</SMALL>s?</H4>
421 <P>There are several ways of measuring software: features,
422 performance, reliability, support, and price.</P>
425 <DT><B>Features</B></DT>
427 <DD>PostgreSQL has most features present in large commercial
428 <SMALL>DBMS</SMALL>s, like transactions, subselects, triggers,
429 views, foreign key referential integrity, and sophisticated
430 locking. We have some features they do not have, like
431 user-defined types, inheritance, rules, and multi-version
432 concurrency control to reduce lock contention.<BR>
436 <DT><B>Performance</B></DT>
438 <DD>PostgreSQL has performance similar to other commercial and
439 open source databases. it is faster for some things, slower for
440 others. In comparison to MySQL or leaner database systems, we are
441 faster for multiple users, complex queries, and a read/write query
442 load. MySQL is faster for simple SELECT queries done by a few users.
443 Of course, MySQL does not have most of the features mentioned in the
444 <I>Features</I> section above. We are built for reliability and
445 features, and we continue to improve performance in every
446 release. There is an interesting Web page comparing PostgreSQL to
447 MySQL at <A href="http://openacs.org/philosophy/why-not-mysql.html">
448 http://openacs.org/philosophy/why-not-mysql.html</A> Also, MySQL is
449 is a company that distributes its products via open source, and requires
450 a commercial license for close-source software, not an
451 open source development community like PostgreSQL.<BR>
456 <DT><B>Reliability</B></DT>
458 <DD>We realize that a <SMALL>DBMS</SMALL> must be reliable, or it
459 is worthless. We strive to release well-tested, stable code that
460 has a minimum of bugs. Each release has at least one month of
461 beta testing, and our release history shows that we can provide
462 stable, solid releases that are ready for production use. We
463 believe we compare favorably to other database software in this
468 <DT><B>Support</B></DT>
470 <DD>Our mailing lists provide contact with a large group of developers
471 and users to help resolve any problems encountered. While we cannot
472 guarantee a fix, commercial <SMALL>DBMS</SMALL>s do not always
473 supply a fix either. Direct access to developers, the user
474 community, manuals, and the source code often make PostgreSQL
475 support superior to other <SMALL>DBMS</SMALL>s. There is
476 commercial per-incident support available for those who need it.
477 (See <A href="#1.6">FAQ section 1.6</A>.)<BR>
481 <DT><B>Price</B></DT>
483 <DD>We are free for all use, both commercial and non-commercial.
484 You can add our code to your product with no limitations, except
485 those outlined in our BSD-style license stated above.<BR>
490 <H4><A name="1.15">1.15</A>) How can I financially assist
493 <P>PostgreSQL has had a first-class infrastructure since we started
494 in 1996. This is all thanks to Marc Fournier, who has created
495 and managed this infrastructure over the years.</P>
497 <P>Quality infrastructure is very important to an open-source
498 project. It prevents disruptions that can greatly delay forward
499 movement of the project.</P>
501 <P>Of course, this infrastructure is not cheap. There are a variety
502 of monthly and one-time expenses that are required to keep it
503 going. If you or your company has money it can donate to help fund
504 this effort, please go to <A href="http://store.pgsql.com/shopping/">http://store.pgsql.com/shopping/</A>
505 and make a donation.</P>
507 <P>Although the web page mentions PostgreSQL, Inc, the
508 "contributions" item is solely to support the PostgreSQL project
509 and does not fund any specific company. If you prefer, you can also
510 send a check to the contact address.</P>
513 <P>Also, if you have a success story about PostgreSQL, please submit
514 it to our advocacy site at <a href="http://advocacy.postgresql.org">
515 http://advocacy.postgresql.org</a>.</P>
518 <H2 align="center">User Client Questions</H2>
520 <H4><A name="2.1">2.1</A>) Are there <SMALL>ODBC</SMALL> drivers
523 <P>There are two <SMALL>ODBC</SMALL> drivers available, PsqlODBC
524 and OpenLink <SMALL>ODBC</SMALL>.</P>
526 <P>You can download PsqlODBC from <A href=
527 "http://gborg.postgresql.org/project/psqlodbc/projdisplay.php">
528 http://gborg.postgresql.org/project/psqlodbc/projdisplay.php</A>.</P>
530 <P>OpenLink <SMALL>ODBC</SMALL> can be gotten from <A href=
531 "http://www.openlinksw.com/">http://www.openlinksw.com</A>. It
532 works with their standard <SMALL>ODBC</SMALL> client software so
533 you'll have PostgreSQL <SMALL>ODBC</SMALL> available on every
534 client platform they support (Win, Mac, Unix, VMS).</P>
536 <P>They will probably be selling this product to people who need
537 commercial-quality support, but a freeware version will always be
538 available. Please send questions to <A href=
539 "mailto:postgres95@openlink.co.uk">postgres95@openlink.co.uk</A>.</P>
541 <H4><A name="2.2">2.2</A>) What tools are available for using
542 PostgreSQL with Web pages?</H4>
544 <P>A nice introduction to Database-backed Web pages can be seen at:
545 <A href="http://www.webreview.com">http://www.webreview.com</A></P>
547 <P>For Web integration, PHP is an excellent interface. It is at <A
548 href="http://www.php.net">http://www.php.net</A>.</P>
550 <P>For complex cases, many use the Perl interface and CGI.pm or mod_perl.</P>
552 <H4><A name="2.3">2.3</A>) Does PostgreSQL have a graphical user
555 <P>Yes, there are several graphical interfaces to PostgreSQL available.
556 These include PgAccess <a href="http://www.pgaccess.org">
557 http://www.pgaccess.org</a>), PgAdmin III (<a
558 href="http://www.pgadmin.org">http://www.pgadmin.org</a>, RHDB Admin (<a
559 href="http://sources.redhat.com/rhdb/">http://sources.redhat.com/rhdb/
560 </a>) and Rekall (<a href="http://www.thekompany.com/products/rekall/">
561 http://www.thekompany.com/products/rekall/</a>, proprietary). There is
562 also PHPPgAdmin (<a href="http://phppgadmin.sourceforge.net/">
563 http://phppgadmin.sourceforge.net/ </a>), a web-based interface to
566 <P>See <a href="http://techdocs.postgresql.org/guides/GUITools">http://techdocs.postgresql.org/guides/GUITools</a> for a more detailed list.</P>
568 <H4><A name="2.4">2.4</A>) What languages are able to communicate with
571 <P>Most popular programming languages contain an interface to
572 PostgreSQL. Check your programming language's list of extension
575 <P>The following interfaces are included in the PostgreSQL
581 <LI>Embedded C (ecpg)</LI>
585 <LI>Python (PyGreSQL)</LI>
587 <LI>TCL (libpgtcl)</LI>
590 <P>Additional interfaces are available at
591 <a href="http://gborg.postgresql.org">http://gborg.postgresql.org</A>
592 in the <I>Drivers/Interfaces</I> section.
596 <H2 align="center">Administrative Questions</H2>
598 <H4><A name="3.1">3.1</A>) How do I install PostgreSQL somewhere
599 other than <I>/usr/local/pgsql</I>?</H4>
601 <P>Specify the <I>--prefix</I> option when running
602 <I>configure</I>.</P>
604 <H4><A name="3.2">3.2</A>) When I start <I>postmaster</I>, I get a
605 <I>Bad System Call</I> or core dumped message. Why?</H4>
607 <P>It could be a variety of problems, but first check to see that
608 you have System V extensions installed in your kernel. PostgreSQL
609 requires kernel support for shared memory and semaphores.</P>
611 <H4><A name="3.3">3.3</A>) When I try to start <I>postmaster</I>, I
612 get <I>IpcMemoryCreate</I> errors. Why?</H4>
614 <P>You either do not have shared memory configured properly in your
615 kernel or you need to enlarge the shared memory available in the
616 kernel. The exact amount you need depends on your architecture and
617 how many buffers and backend processes you configure for
618 <I>postmaster</I>. For most systems, with default numbers of
619 buffers and processes, you need a minimum of ~1 MB. See the <A
621 "http://www.PostgreSQL.org/docs/view.php?version=current&idoc=1&file=kernel-resources.html">PostgreSQL
622 Administrator's Guide</A> for more detailed information about
623 shared memory and semaphores.</P>
625 <H4><A name="3.4">3.4</A>) When I try to start <I>postmaster</I>, I
626 get <I>IpcSemaphoreCreate</I> errors. Why?</H4>
628 <P>If the error message is <I>IpcSemaphoreCreate: semget failed (No
629 space left on device)</I> then your kernel is not configured with
630 enough semaphores. Postgres needs one semaphore per potential
631 backend process. A temporary solution is to start <I>postmaster</I>
632 with a smaller limit on the number of backend processes. Use
633 <I>-N</I> with a parameter less than the default of 32. A more
634 permanent solution is to increase your kernel's
635 <SMALL>SEMMNS</SMALL> and <SMALL>SEMMNI</SMALL> parameters.</P>
637 <P>Inoperative semaphores can also cause crashes during heavy
640 <P>If the error message is something else, you might not have
641 semaphore support configured in your kernel at all. See the
642 PostgreSQL Administrator's Guide for more detailed information
643 about shared memory and semaphores.</P>
645 <H4><A name="3.5">3.5</A>) How do I control connections from other
648 <P>By default, PostgreSQL only allows connections from the local
649 machine using Unix domain sockets. Other machines will not be able
650 to connect unless you add the <I>-i</I> flag to <I>postmaster</I>,
651 <B>and</B> enable host-based authentication by modifying the file
652 <I>$PGDATA/pg_hba.conf</I> accordingly. This will allow TCP/IP
655 <H4><A name="3.6">3.6</A>) How do I tune the database engine for
656 better performance?</H4>
658 <P>Certainly, indexes can speed up queries. The
659 <SMALL>EXPLAIN</SMALL> command allows you to see how PostgreSQL is
660 interpreting your query, and which indexes are being used.</P>
662 <P>If you are doing many <SMALL>INSERTs</SMALL>, consider doing
663 them in a large batch using the <SMALL>COPY</SMALL> command. This
664 is much faster than individual <SMALL>INSERTS</SMALL>. Second,
665 statements not in a <SMALL>BEGIN WORK/COMMIT</SMALL> transaction
666 block are considered to be in their own transaction. Consider
667 performing several statements in a single transaction block. This
668 reduces the transaction overhead. Also, consider dropping and
669 recreating indexes when making large data changes.</P>
671 <P>There are several tuning options. You can disable <I>fsync()</I>
672 by starting <I>postmaster</I> with a <I>-o -F</I> option. This will
673 prevent <I>fsync()</I>s from flushing to disk after every
676 <P>You can also use the <I>postmaster</I> <I>-B</I> option to
677 increase the number of shared memory buffers used by the backend
678 processes. If you make this parameter too high, the
679 <I>postmaster</I> may not start because you have exceeded your
680 kernel's limit on shared memory space. Each buffer is 8K and the
681 default is 64 buffers.</P>
683 <P>You can also use the backend <I>-S</I> option to increase the
684 maximum amount of memory used by the backend process for temporary
685 sorts. The <I>-S</I> value is measured in kilobytes, and the
686 default is 512 (i.e. 512K).</P>
688 <P>You can also use the <SMALL>CLUSTER</SMALL> command to group
689 data in tables to match an index. See the <SMALL>CLUSTER</SMALL>
690 manual page for more details.</P>
692 <H4><A name="3.7">3.7</A>) What debugging features are
695 <P>PostgreSQL has several features that report status information
696 that can be valuable for debugging purposes.</P>
698 <P>First, by running <I>configure</I> with the --enable-cassert
699 option, many <I>assert()</I>s monitor the progress of the backend
700 and halt the program when something unexpected occurs.</P>
702 <P>Both <I>postmaster</I> and <I>postgres</I> have several debug
703 options available. First, whenever you start <I>postmaster</I>,
704 make sure you send the standard output and error to a log file,
708 ./bin/postmaster >server.log 2>&1 &
711 <P>This will put a server.log file in the top-level PostgreSQL
712 directory. This file contains useful information about problems or
713 errors encountered by the server. <I>Postmaster</I> has a <I>-d</I>
714 option that allows even more detailed information to be reported.
715 The <I>-d</I> option takes a number that specifies the debug level.
716 Be warned that high debug level values generate large log
719 <P>If <I>postmaster</I> is not running, you can actually run the
720 <I>postgres</I> backend from the command line, and type your
721 <SMALL>SQL</SMALL> statement directly. This is recommended
722 <B>only</B> for debugging purposes. Note that a newline terminates
723 the query, not a semicolon. If you have compiled with debugging
724 symbols, you can use a debugger to see what is happening. Because
725 the backend was not started from <I>postmaster</I>, it is not
726 running in an identical environment and locking/backend interaction
727 problems may not be duplicated.</P>
729 <P>If <I>postmaster</I> is running, start <I>psql</I> in one
730 window, then find the <SMALL>PID</SMALL> of the <I>postgres</I>
731 process used by <I>psql</I>. Use a debugger to attach to the
732 <I>postgres</I> <SMALL>PID</SMALL>. You can set breakpoints in the
733 debugger and issue queries from <I>psql</I>. If you are debugging
734 <I>postgres</I> startup, you can set PGOPTIONS="-W n", then start
735 <I>psql</I>. This will cause startup to delay for <I>n</I> seconds
736 so you can attach to the process with the debugger, set any
737 breakpoints, and continue through the startup sequence.</P>
739 <P>The <I>postgres</I> program has <I>-s, -A</I>, and <I>-t</I>
740 options that can be very useful for debugging and performance
743 <P>You can also compile with profiling to see what functions are
744 taking execution time. The backend profile files will be deposited
745 in the <I>pgsql/data/base/dbname</I> directory. The client profile
746 file will be put in the client's current directory. Linux requires
747 a compile with <I>-DLINUX_PROFILE</I> for proper profiling.</P>
749 <H4><A name="3.8">3.8</A>) Why do I get <I>"Sorry, too many
750 clients"</I> when trying to connect?</H4>
752 <P>You need to increase <I>postmaster</I>'s limit on how many
753 concurrent backend processes it can start.</P>
755 <P>The default limit is 32 processes. You can increase it by
756 restarting <I>postmaster</I> with a suitable <I>-N</I> value or
757 modifying <I>postgresql.conf</I>.</P>
759 <P>Note that if you make <I>-N</I> larger than 32, you must also
760 increase <I>-B</I> beyond its default of 64; <I>-B</I> must be at
761 least twice <I>-N</I>, and probably should be more than that for
762 best performance. For large numbers of backend processes, you are
763 also likely to find that you need to increase various Unix kernel
764 configuration parameters. Things to check include the maximum size
765 of shared memory blocks, <SMALL>SHMMAX;</SMALL> the maximum number
766 of semaphores, <SMALL>SEMMNS</SMALL> and <SMALL>SEMMNI;</SMALL> the
767 maximum number of processes, <SMALL>NPROC;</SMALL> the maximum
768 number of processes per user, <SMALL>MAXUPRC;</SMALL> and the
769 maximum number of open files, <SMALL>NFILE</SMALL> and
770 <SMALL>NINODE</SMALL>. The reason that PostgreSQL has a limit on
771 the number of allowed backend processes is so your system won't run
772 out of resources.</P>
774 <H4><A name="3.9">3.9</A>) What is in the <I>pgsql_tmp</I> directory?</H4>
776 <P>This directory contains temporary files generated by the query
777 executor. For example, if a sort needs to be done to satisfy an
778 <SMALL>ORDER BY</SMALL> and the sort requires more space than the
779 backend's <I>-S</I> parameter allows, then temporary files are created
780 here to hold the extra data.</P>
782 <P>The temporary files are usually deleted automatically, but might
783 remain if a backend crashes during a sort. A stop and restart of the
784 <I>postmaster</I> will remove files from those directories.</P>
786 <H4><A name="3.10">3.10</A>) Why do I need to do a dump and restore
787 to upgrade between major PostgreSQL releases?</H4>
789 <P>The PostgreSQL team makes only small changes between minor releases,
790 so upgrading from 7.2 to 7.2.1 does not require a dump and restore.
791 However, major releases (e.g. from 7.2 to 7.3) often change the internal
792 format of system tables and data files. These changes are often complex,
793 so we don't maintain backward compatability for data files. A dump outputs
794 data in a generic format that can then be loaded in using the new internal
797 <P>In releases where the on-disk format does not change, the
798 <I>pg_upgrade</I> script can be used to upgrade without a dump/restore.
799 The release notes mention whether <I>pg_upgrade</I> is available for the
804 <H2 align="center">Operational Questions</H2>
806 <H4><A name="4.1">4.1</A>) What is the difference between binary
807 cursors and normal cursors?</H4>
809 <P>See the <SMALL>DECLARE</SMALL> manual page for a
812 <H4><A name="4.2">4.2</A>) How do I <SMALL>SELECT</SMALL> only the
813 first few rows of a query? A random row?</H4>
815 <P>See the <SMALL>FETCH</SMALL> manual page, or use
816 <SMALL>SELECT</SMALL> ... <SMALL>LIMIT</SMALL>....</P>
818 <P>The entire query may have to be evaluated, even if you only want
819 the first few rows. Consider using a query that has an <SMALL>ORDER
820 BY</SMALL>. If there is an index that matches the <SMALL>ORDER
821 BY</SMALL>, PostgreSQL may be able to evaluate only the first few
822 records requested, or the entire query may have to be evaluated
823 until the desired rows have been generated.</P>
825 <P>To <SMALL>SELECT</SMALL> a random row, use:
833 <H4><A name="4.3">4.3</A>) How do I get a list of tables or other
834 things I can see in <I>psql</I>?</H4>
836 <P>You can read the source code for <I>psql</I> in file
837 <I>pgsql/src/bin/psql/describe.c</I>. It contains
838 <SMALL>SQL</SMALL> commands that generate the output for psql's
839 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
841 execute the commands you give.</P>
843 <H4><A name="4.4">4.4</A>) How do you remove a column from a
844 table, or change its data type?</H4>
846 <P><SMALL>DROP COLUMN</SMALL> functionality was added in release 7.3 with
847 <SMALL>ALTER TABLE DROP COLUMN</SMALL>. In earlier versions,
851 LOCK TABLE old_table;
852 SELECT ... -- select all columns but the one you want to remove
855 DROP TABLE old_table;
856 ALTER TABLE new_table RENAME TO old_table;
860 <P>To change the data type of a column, do this:</P>
863 ALTER TABLE tab ADD COLUMN new_col <i>new_data_type</i>;
864 UPDATE tab SET new_col = CAST(old_col AS <i>new_data_type</i>);
865 ALTER TABLE tab DROP COLUMN old_col;
868 <P>You might then want to do <I>VACUUM FULL tab</I> to reclaim the
869 disk space used by the expired rows.</P>
871 <H4><A name="4.5">4.5</A>) What is the maximum size for a row, a
872 table, and a database?</H4>
874 <P>These are the limits:</P>
876 Maximum size for a database? unlimited (32 TB databases exist)
877 Maximum size for a table? 32 TB
878 Maximum size for a row? 1.6TB
879 Maximum size for a field? 1 GB
880 Maximum number of rows in a table? unlimited
881 Maximum number of columns in a table? 250-1600 depending on column types
882 Maximum number of indexes on a table? unlimited
885 Of course, these are not actually unlimited, but limited to
886 available disk space and memory/swap space. Performance may suffer
887 when these values get unusually large.
889 <P>The maximum table size of 32 TB does not require large file
890 support from the operating system. Large tables are stored as
891 multiple 1 GB files so file system size limits are not
894 <P>The maximum table size and maximum number of columns can be
895 quadrupled by increasing the default block size to 32k.</P>
897 <H4><A name="4.6">4.6</A>) How much database disk space is required
898 to store data from a typical text file?</H4>
900 <P>A PostgreSQL database may require up to five times the disk
901 space to store data from a text file.</P>
903 <P>As an example, consider a file of 100,000 lines with an integer
904 and text description on each line. Suppose the text string
905 avergages twenty bytes in length. The flat file would be 2.8 MB.
906 The size of the PostgreSQL database file containing this data can
907 be estimated as 6.4 MB:</P>
909 36 bytes: each row header (approximate)
910 24 bytes: one int field and one text field
911 + 4 bytes: pointer on page to tuple
912 ----------------------------------------
915 The data page size in PostgreSQL is 8192 bytes (8 KB), so:
918 ------------------- = 128 rows per database page (rounded down)
922 -------------------- = 782 database pages (rounded up)
925 782 database pages * 8192 bytes per page = 6,406,144 bytes (6.4 MB)
928 <P>Indexes do not require as much overhead, but do contain the data
929 that is being indexed, so they can be large also.</P>
931 <P><SMALL>NULL</SMALL>s are stored in bitmaps, so they
932 use very little space.</P>
934 <H4><A name="4.7">4.7</A>) How do I find out what tables, indexes,
935 databases, and users are defined?</H4>
937 <P><I>psql</I> has a variety of backslash commands to show such
938 information. Use \? to see them. There are also system tables
939 beginning with <I>pg_</I> that describe these too. Also, <I>psql
940 -l</I> will list all databases.</P>
942 <P>Also try the file <I>pgsql/src/tutorial/syscat.source</I>. It
943 illustrates many of the <SMALL>SELECT</SMALL>s needed to get
944 information from the database system tables.</P>
946 <H4><A name="4.8">4.8</A>) My queries are slow or don't make use of
947 the indexes. Why?</H4>
948 Indexes are not automatically used by every query. Indexes are only
949 used if the table is larger than a minimum size, and the query
950 selects only a small percentage of the rows in the table. This is
951 because the random disk access caused by an index scan can be
952 slower than a straight read through the table, or sequential scan.
954 <P>To determine if an index should be used, PostgreSQL must have
955 statistics about the table. These statistics are collected using
956 <SMALL>VACUUM ANALYZE</SMALL>, or simply <SMALL>ANALYZE</SMALL>.
957 Using statistics, the optimizer knows how many rows are in the
958 table, and can better determine if indexes should be used.
959 Statistics are also valuable in determining optimal join order and
960 join methods. Statistics collection should be performed
961 periodically as the contents of the table change.</P>
963 <P>Indexes are normally not used for <SMALL>ORDER BY</SMALL> or to
964 perform joins. A sequential scan followed by an explicit sort is
965 usually faster than an index scan of a large table.</P>
966 However, <SMALL>LIMIT</SMALL> combined with <SMALL>ORDER BY</SMALL>
967 often will use an index because only a small portion of the table
968 is returned. In fact, though MAX() and MIN() don't use indexes,
969 it is possible to retrieve such values using an index with ORDER BY
974 ORDER BY col [ DESC ]
978 <P>If you believe the optimizer is incorrect in choosing a
979 sequential scan, use <CODE>SET enable_seqscan TO 'off'</CODE> and
980 run tests to see if an index scan is indeed faster.</P>
982 <P>When using wild-card operators such as <SMALL>LIKE</SMALL> or
983 <I>~</I>, indexes can only be used in certain circumstances:</P>
985 <LI>The beginning of the search string must be anchored to the start
988 <LI><SMALL>LIKE</SMALL> patterns must not start with <I>%</I>.</LI>
989 <LI><I>~</I> (regular expression) patterns must start with
992 <LI>The search string can not start with a character class,
994 <LI>Case-insensitive searches such as <SMALL>ILIKE</SMALL> and
995 <I>~*</I> do not utilise indexes. Instead, use functional
996 indexes, which are described in section <a href="#4.12">4.12</a>.</LI>
997 <LI>The default <I>C</I> locale must be used during
1002 <H4><A name="4.9">4.9</A>) How do I see how the query optimizer is
1003 evaluating my query?</H4>
1005 <P>See the <SMALL>EXPLAIN</SMALL> manual page.</P>
1007 <H4><A name="4.10">4.10</A>) What is an R-tree index?</H4>
1009 <P>An R-tree index is used for indexing spatial data. A hash index
1010 can't handle range searches. A B-tree index only handles range
1011 searches in a single dimension. R-trees can handle
1012 multi-dimensional data. For example, if an R-tree index can be
1013 built on an attribute of type <I>point</I>, the system can more
1014 efficiently answer queries such as "select all points within a
1015 bounding rectangle."</P>
1017 <P>The canonical paper that describes the original R-tree design
1020 <P>Guttman, A. "R-trees: A Dynamic Index Structure for Spatial
1021 Searching." Proceedings of the 1984 ACM SIGMOD Int'l Conf on Mgmt
1024 <P>You can also find this paper in Stonebraker's "Readings in
1025 Database Systems".</P>
1027 <P>Built-in R-trees can handle polygons and boxes. In theory,
1028 R-trees can be extended to handle higher number of dimensions. In
1029 practice, extending R-trees requires a bit of work and we don't
1030 currently have any documentation on how to do it.</P>
1032 <H4><A name="4.11">4.11</A>) What is the Genetic Query
1035 <P>The <SMALL>GEQO</SMALL> module speeds query optimization when
1036 joining many tables by means of a Genetic Algorithm (GA). It allows
1037 the handling of large join queries through nonexhaustive
1040 <H4><A name="4.12">4.12</A>) How do I perform regular expression
1041 searches and case-insensitive regular expression searches? How do I
1042 use an index for case-insensitive searches?</H4>
1044 <P>The <I>~</I> operator does regular expression matching, and
1045 <I>~*</I> does case-insensitive regular expression matching. The
1046 case-insensitive variant of <SMALL>LIKE</SMALL> is called
1047 <SMALL>ILIKE</SMALL>.</P>
1049 <P>Case-insensitive equality comparisons are normally expressed
1054 WHERE lower(col) = 'abc';
1057 This will not use an standard index. However, if you create a
1058 functional index, it will be used:
1060 CREATE INDEX tabindex ON tab (lower(col));
1063 <H4><A name="4.13">4.13</A>) In a query, how do I detect if a field
1064 is <SMALL>NULL</SMALL>?</H4>
1066 <P>You test the column with <SMALL>IS NULL</SMALL> and <SMALL>IS
1067 NOT NULL</SMALL>.</P>
1069 <H4><A name="4.14">4.14</A>) What is the difference between the
1070 various character types?</H4>
1072 Type Internal Name Notes
1073 --------------------------------------------------
1074 VARCHAR(n) varchar size specifies maximum length, no padding
1075 CHAR(n) bpchar blank padded to the specified fixed length
1076 TEXT text no specific upper limit on length
1077 BYTEA bytea variable-length byte array (null-byte safe)
1078 "char" char one character
1081 <P>You will see the internal name when examining system catalogs
1082 and in some error messages.</P>
1084 <P>The first four types above are "varlena" types (i.e., the first
1085 four bytes on disk are the length, followed by the data). Thus the
1086 actual space used is slightly greater than the declared size.
1087 However, these data types are also subject to compression or being
1088 stored out-of-line by <SMALL>TOAST</SMALL>, so the space on disk
1089 might also be less than expected.</P>
1091 <SMALL>VARCHAR(n)</SMALL> is best when storing variable-length
1092 strings and it limits how long a string can be. <SMALL>TEXT</SMALL>
1093 is for strings of unlimited length, with a maximum of one gigabyte.
1094 <P><SMALL>CHAR(n)</SMALL> is for storing strings that are all the
1095 same length. <SMALL>CHAR(n)</SMALL> pads with blanks to the specified
1096 length, while <SMALL>VARCHAR(n)</SMALL> only stores the characters
1097 supplied. <SMALL>BYTEA</SMALL> is for storing binary data,
1098 particularly values that include <SMALL>NULL</SMALL> bytes. All the
1099 types described here have similar performance characteristics.</P>
1101 <H4><A name="4.15.1">4.15.1</A>) How do I create a
1102 serial/auto-incrementing field?</H4>
1104 <P>PostgreSQL supports a <SMALL>SERIAL</SMALL> data type. It
1105 auto-creates a sequence. For example,
1108 CREATE TABLE person (
1114 is automatically translated into this:
1116 CREATE SEQUENCE person_id_seq;
1117 CREATE TABLE person (
1118 id INT4 NOT NULL DEFAULT nextval('person_id_seq'),
1123 See the <I>create_sequence</I> manual page for more information
1124 about sequences. You can also use each row's <I>OID</I> field as a
1125 unique value. However, if you need to dump and reload the database,
1126 you need to use <I>pg_dump</I>'s <I>-o</I> option or <SMALL>COPY
1127 WITH OIDS</SMALL> option to preserve the <SMALL>OID</SMALL>s.
1129 <H4><A name="4.15.2">4.15.2</A>) How do I get the value of a
1130 <SMALL>SERIAL</SMALL> insert?</H4>
1132 <P>One approach is to retrieve the next <SMALL>SERIAL</SMALL> value
1133 from the sequence object with the <I>nextval()</I> function
1134 <I>before</I> inserting and then insert it explicitly. Using the
1135 example table in <A href="#4.15.1">4.15.1</A>, an example in a
1136 pseudo-language would look like this:</P>
1138 new_id = execute("SELECT nextval('person_id_seq')");
1139 execute("INSERT INTO person (id, name) VALUES (new_id, 'Blaise Pascal')");
1142 You would then also have the new value stored in
1143 <CODE>new_id</CODE> for use in other queries (e.g., as a foreign
1144 key to the <CODE>person</CODE> table). Note that the name of the
1145 automatically created <SMALL>SEQUENCE</SMALL> object will be named
1146 <<I>table</I>>_<<I>serialcolumn</I>>_<I>seq</I>, where
1147 <I>table</I> and <I>serialcolumn</I> are the names of your table
1148 and your <SMALL>SERIAL</SMALL> column, respectively.
1150 <P>Alternatively, you could retrieve the assigned
1151 <SMALL>SERIAL</SMALL> value with the <I>currval()</I> function
1152 <I>after</I> it was inserted by default, e.g.,</P>
1154 execute("INSERT INTO person (name) VALUES ('Blaise Pascal')");
1155 new_id = execute("SELECT currval('person_id_seq')");
1158 Finally, you could use the <A href="#4.16"><SMALL>OID</SMALL></A>
1159 returned from the <SMALL>INSERT</SMALL> statement to look up the
1160 default value, though this is probably the least portable approach,
1161 and the oid value will wrap around when it reaches 4 billion.
1162 In Perl, using DBI with Edmund Mergl's DBD::Pg module, the oid
1163 value is made available via <I>$sth->{pg_oid_status}</I> after
1164 <I>$sth->execute()</I>.
1166 <H4><A name="4.15.3">4.15.3</A>) Don't <I>currval()</I> and
1167 <I>nextval()</I> lead to a race condition with other users?</H4>
1169 <P>No. <I>currval()</I> returns the current value assigned by your
1170 backend, not by all users.</P>
1172 <H4><A name="4.15.4">4.15.4</A>) Why aren't my sequence numbers
1173 reused on transaction abort? Why are there gaps in the numbering of
1174 my sequence/SERIAL column?</H4>
1176 <P>To improve concurrency, sequence values are given out to running
1177 transactions as needed and are not locked until the transaction
1178 completes. This causes gaps in numbering from aborted
1181 <H4><A name="4.16">4.16</A>) What is an <SMALL>OID</SMALL>? What is
1182 a <SMALL>TID</SMALL>?</H4>
1184 <P><SMALL>OID</SMALL>s are PostgreSQL's answer to unique row ids.
1185 Every row that is created in PostgreSQL gets a unique
1186 <SMALL>OID</SMALL>. All <SMALL>OID</SMALL>s generated during
1187 <I>initdb</I> are less than 16384 (from
1188 <I>include/access/transam.h</I>). All user-created
1189 <SMALL>OID</SMALL>s are equal to or greater than this. By default,
1190 all these <SMALL>OID</SMALL>s are unique not only within a table or
1191 database, but unique within the entire PostgreSQL installation.</P>
1193 <P>PostgreSQL uses <SMALL>OID</SMALL>s in its internal system
1194 tables to link rows between tables. These <SMALL>OID</SMALL>s can
1195 be used to identify specific user rows and used in joins. It is
1196 recommended you use column type <SMALL>OID</SMALL> to store
1197 <SMALL>OID</SMALL> values. You can create an index on the
1198 <SMALL>OID</SMALL> field for faster access.</P>
1200 <P>O<SMALL>ID</SMALL>s are assigned to all new rows from a central
1201 area that is used by all databases. If you want to change the
1202 <SMALL>OID</SMALL> to something else, or if you want to make a copy
1203 of the table, with the original <SMALL>OID</SMALL>s, there is no
1204 reason you can't do it:</P>
1206 CREATE TABLE new_table(old_oid oid, mycol int);
1207 SELECT old_oid, mycol INTO new FROM old;
1208 COPY new TO '/tmp/pgtable';
1210 COPY new WITH OIDS FROM '/tmp/pgtable';
1213 CREATE TABLE new_table (mycol int);
1214 INSERT INTO new_table (oid, mycol) SELECT oid, mycol FROM old_table;
1216 <P>O<SMALL>ID</SMALL>s are stored as 4-byte integers, and will
1217 overflow at 4 billion. No one has reported this ever happening, and
1218 we plan to have the limit removed before anyone does.</P>
1220 <P>T<SMALL>ID</SMALL>s are used to identify specific physical rows
1221 with block and offset values. T<SMALL>ID</SMALL>s change after rows
1222 are modified or reloaded. They are used by index entries to point
1223 to physical rows.</P>
1225 <H4><A name="4.17">4.17</A>) What is the meaning of some of the
1226 terms used in PostgreSQL?</H4>
1228 <P>Some of the source code and older documentation use terms that
1229 have more common usage. Here are some:</P>
1232 <LI>table, relation, class</LI>
1234 <LI>row, record, tuple</LI>
1236 <LI>column, field, attribute</LI>
1238 <LI>retrieve, select</LI>
1240 <LI>replace, update</LI>
1242 <LI>append, insert</LI>
1244 <LI><SMALL>OID</SMALL>, serial value</LI>
1246 <LI>portal, cursor</LI>
1248 <LI>range variable, table name, table alias</LI>
1251 <P>A list of general database terms can be found at: <A href=
1252 "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>
1254 <H4><A name="4.18">4.18</A>) Why do I get the error <I>"ERROR:
1255 Memory exhausted in AllocSetAlloc()"</I>?</H4>
1257 <P>You probably have run out of virtual memory on your system,
1258 or your kernel has a low limit for certain resources. Try this
1259 before starting <I>postmaster</I>:</P>
1265 Depending on your shell, only one of these may succeed, but it will
1266 set your process data segment limit much higher and perhaps allow
1267 the query to complete. This command applies to the current process,
1268 and all subprocesses created after the command is run. If you are
1269 having a problem with the <SMALL>SQL</SMALL> client because the
1270 backend is returning too much data, try it before starting the
1273 <H4><A name="4.19">4.19</A>) How do I tell what PostgreSQL version
1276 <P>From <I>psql</I>, type <CODE>SELECT version();</CODE></P>
1278 <H4><A name="4.20">4.20</A>) Why does my large-object operations
1279 get <I>"invalid large obj descriptor"</I>?</H4>
1281 <P>You need to put <CODE>BEGIN WORK</CODE> and <CODE>COMMIT</CODE>
1282 around any use of a large object handle, that is, surrounding
1283 <CODE>lo_open</CODE> ... <CODE>lo_close.</CODE></P>
1285 <P>Currently PostgreSQL enforces the rule by closing large object
1286 handles at transaction commit. So the first attempt to do anything
1287 with the handle will draw <I>invalid large obj descriptor</I>. So
1288 code that used to work (at least most of the time) will now
1289 generate that error message if you fail to use a transaction.</P>
1291 <P>If you are using a client interface like <SMALL>ODBC</SMALL> you
1292 may need to set <CODE>auto-commit off.</CODE></P>
1294 <H4><A name="4.21">4.21</A>) How do I create a column that will
1295 default to the current time?</H4>
1297 <P>Use <I>CURRENT_TIMESTAMP</I>:</P>
1299 <CODE>CREATE TABLE test (x int, modtime timestamp DEFAULT CURRENT_TIMESTAMP );
1303 <H4><A name="4.22">4.22</A>) Why are my subqueries using
1304 <CODE><SMALL>IN</SMALL></CODE> so slow?</H4>
1306 <P>In versions prior to 7.4, subqueries were joined to outer queries
1307 by sequentially scanning the result of the subquery for each row of
1308 the outer query. If the subquery returns only a few rows and the outer
1309 query returns many rows, <CODE><SMALL>IN</SMALL></CODE> is fastest. To
1310 speed up other queries, replace <CODE>IN</CODE> with
1311 <CODE>EXISTS</CODE>:</P>
1314 WHERE col IN (SELECT subcol FROM subtab);
1319 WHERE EXISTS (SELECT subcol FROM subtab WHERE subcol = col);
1322 For this to be fast, <CODE>subcol</CODE> should be an indexed column.
1323 <P>In version 7.4 and later, <CODE>IN</CODE> actually uses the same
1324 sophisticated join techniques as normal queries, and is prefered
1325 to using <CODE>EXISTS</CODE>.
1327 <H4><A name="4.23">4.23</A>) How do I perform an outer join?</H4>
1329 <P>PostgreSQL supports outer joins using the SQL standard syntax.
1330 Here are two examples:</P>
1333 FROM t1 LEFT OUTER JOIN t2 ON (t1.col = t2.col);
1338 FROM t1 LEFT OUTER JOIN t2 USING (col);
1341 <P>These identical queries join t1.col to t2.col, and also return
1342 any unjoined rows in t1 (those with no match in t2). A
1343 <SMALL>RIGHT</SMALL> join would add unjoined rows of t2. A
1344 <SMALL>FULL</SMALL> join would return the matched rows plus all
1345 unjoined rows from t1 and t2. The word <SMALL>OUTER</SMALL> is
1346 optional and is assumed in <SMALL>LEFT</SMALL>,
1347 <SMALL>RIGHT</SMALL>, and <SMALL>FULL</SMALL> joins. Ordinary joins
1348 are called <SMALL>INNER</SMALL> joins.</P>
1350 <P>In previous releases, outer joins can be simulated using
1351 <SMALL>UNION</SMALL> and <SMALL>NOT IN</SMALL>. For example, when
1352 joining <I>tab1</I> and <I>tab2</I>, the following query does an
1353 <I>outer</I> join of the two tables:<BR>
1357 SELECT tab1.col1, tab2.col2
1359 WHERE tab1.col1 = tab2.col1
1361 SELECT tab1.col1, NULL
1363 WHERE tab1.col1 NOT IN (SELECT tab2.col1 FROM tab2)
1367 <H4><A name="4.24">4.24</A>) How do I perform queries using
1368 multiple databases?</H4>
1370 <P>There is no way to query a database other than the current one.
1371 Because PostgreSQL loads database-specific system catalogs, it is
1372 uncertain how a cross-database query should even behave.</P>
1374 <P><I>contrib/dblink</I> allows cross-database queries using
1375 function calls. Of course, a client can make simultaneous
1376 connections to different databases and merge the results on the
1379 <H4><A name="4.25">4.25</A>) How do I return multiple rows or
1380 columns from a function?</H4>
1382 <P>In 7.3, you can easily return multiple rows or columns from a
1384 <a href="http://techdocs.postgresql.org/guides/SetReturningFunctions">
1385 http://techdocs.postgresql.org/guides/SetReturningFunctions</a>.
1387 <H4><A name="4.26">4.26</A>) Why can't I reliably create/drop
1388 temporary tables in PL/PgSQL functions?</H4>
1389 <P>PL/PgSQL caches function contents, and an unfortunate side effect
1390 is that if a PL/PgSQL function accesses a temporary table, and that
1391 table is later dropped and recreated, and the function called
1392 again, the function will fail because the cached function contents
1393 still point to the old temporary table. The solution is to use
1394 <SMALL>EXECUTE</SMALL> for temporary table access in PL/PgSQL. This
1395 will cause the query to be reparsed every time.</P>
1397 <H4><A name="4.27">4.27</A>) What replication options are available?
1399 <P>There are several master/slave replication options available.
1400 These allow only the master to make database changes and the slave
1401 can only do database reads. The bottom of <a
1402 href="http://gborg.PostgreSQL.org/genpage?replication_research">
1403 http://gborg.PostgreSQL.org/genpage?replication_research</a> lists
1404 them. A multi-master replication solution is being worked on at <a
1405 href="http://gborg.PostgreSQL.org/project/pgreplication/projdisplay.php">http://gborg.PostgreSQL.org/project/pgreplication/projdisplay.php</a>.</P>
1407 <H4><A name="4.28">4.28</A>) What encryption options are available?
1410 <LI><I>contrib/pgcrypto</I> contains many encryption functions for
1411 use in <SMALL>SQL</SMALL> queries.</LI>
1412 <LI>To encrypt transmission from the client to the server, the server
1413 must have the <I>ssl</I> option set to <I>true</I> in <I>postgresql.conf,
1414 </I> and an applicable <I>host</I> or <I>hostssl</I> record must exist in
1415 <I>pg_hba.conf</I>, and the client <I>sslmode</I> must not be
1416 <I>disable.</I> (Note that it is also possible to use a third-party
1417 encrypted transport, such as stunnel or ssh, rather than PostgreSQL's
1418 native SSL connections.)
1419 <LI>Database user passwords are automatically encrypted when stored in
1420 version 7.3. In previous versions, you must enable the option
1421 <I>PASSWORD_ENCRYPTION</I> in <I>postgresql.conf</I>.</LI>
1422 <LI>The server can run using an encrypted file system.</LI>
1427 <H2 align="center">Extending PostgreSQL</H2>
1429 <H4><A name="5.1">5.1</A>) I wrote a user-defined function. When I
1430 run it in <I>psql</I>, why does it dump core?</H4>
1432 <P>The problem could be a number of things. Try testing your
1433 user-defined function in a stand-alone test program first.</P>
1435 <H4><A name="5.2">5.2</A>) How can I contribute some nifty new
1436 types and functions to PostgreSQL?</H4>
1438 <P>Send your extensions to the <I>pgsql-hackers</I> mailing list,
1439 and they will eventually end up in the <I>contrib/</I>
1442 <H4><A name="5.3">5.3</A>) How do I write a C function to return a
1445 <P>In versions of PostgreSQL beginning with 7.3, table-returning
1446 functions are fully supported in C, PL/PgSQL, and SQL. See the
1447 Programmer's Guide for more information. An example of a
1448 table-returning function defined in C can be found in
1449 <I>contrib/tablefunc</I>.</P>
1451 <H4><A name="5.4">5.4</A>) I have changed a source file. Why does
1452 the recompile not see the change?</H4>
1454 <P>The <I>Makefiles</I> do not have the proper dependencies for
1455 include files. You have to do a <I>make clean</I> and then another
1456 <I>make</I>. If you are using <SMALL>GCC</SMALL> you can use the
1457 <I>--enable-depend</I> option of <I>configure</I> to have the
1458 compiler compute the dependencies automatically.</P>
projects / postgresql / blob