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: Sat Nov 29 23:54:03 EST 2003</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 EFNet and Freenode,
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.</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, not an
450 open source development community like PostgreSQL.<BR>
455 <DT><B>Reliability</B></DT>
457 <DD>We realize that a <SMALL>DBMS</SMALL> must be reliable, or it
458 is worthless. We strive to release well-tested, stable code that
459 has a minimum of bugs. Each release has at least one month of
460 beta testing, and our release history shows that we can provide
461 stable, solid releases that are ready for production use. We
462 believe we compare favorably to other database software in this
467 <DT><B>Support</B></DT>
469 <DD>Our mailing lists provide contact with a large group of developers
470 and users to help resolve any problems encountered. While we cannot
471 guarantee a fix, commercial <SMALL>DBMS</SMALL>s do not always
472 supply a fix either. Direct access to developers, the user
473 community, manuals, and the source code often make PostgreSQL
474 support superior to other <SMALL>DBMS</SMALL>s. There is
475 commercial per-incident support available for those who need it.
476 (See <A href="#1.6">FAQ section 1.6</A>.)<BR>
480 <DT><B>Price</B></DT>
482 <DD>We are free for all use, both commercial and non-commercial.
483 You can add our code to your product with no limitations, except
484 those outlined in our BSD-style license stated above.<BR>
489 <H4><A name="1.15">1.15</A>) How can I financially assist
492 <P>PostgreSQL has had a first-class infrastructure since we started
493 in 1996. This is all thanks to Marc Fournier, who has created
494 and managed this infrastructure over the years.</P>
496 <P>Quality infrastructure is very important to an open-source
497 project. It prevents disruptions that can greatly delay forward
498 movement of the project.</P>
500 <P>Of course, this infrastructure is not cheap. There are a variety
501 of monthly and one-time expenses that are required to keep it
502 going. If you or your company has money it can donate to help fund
503 this effort, please go to <A href="http://store.pgsql.com/shopping/">http://store.pgsql.com/shopping/</A>
504 and make a donation.</P>
506 <P>Although the web page mentions PostgreSQL, Inc, the
507 "contributions" item is solely to support the PostgreSQL project
508 and does not fund any specific company. If you prefer, you can also
509 send a check to the contact address.</P>
512 <P>Also, if you have a success story about PostgreSQL, please submit
513 it to our advocacy site at <a href="http://advocacy.postgresql.org">
514 http://advocacy.postgresql.org</a>.</P>
517 <H2 align="center">User Client Questions</H2>
519 <H4><A name="2.1">2.1</A>) Are there <SMALL>ODBC</SMALL> drivers
522 <P>There are two <SMALL>ODBC</SMALL> drivers available, PsqlODBC
523 and OpenLink <SMALL>ODBC</SMALL>.</P>
525 <P>You can download PsqlODBC from <A href=
526 "http://gborg.postgresql.org/project/psqlodbc/projdisplay.php">
527 http://gborg.postgresql.org/project/psqlodbc/projdisplay.php</A>.</P>
529 <P>OpenLink <SMALL>ODBC</SMALL> can be gotten from <A href=
530 "http://www.openlinksw.com/">http://www.openlinksw.com</A>. It
531 works with their standard <SMALL>ODBC</SMALL> client software so
532 you'll have PostgreSQL <SMALL>ODBC</SMALL> available on every
533 client platform they support (Win, Mac, Unix, VMS).</P>
535 <P>They will probably be selling this product to people who need
536 commercial-quality support, but a freeware version will always be
537 available. Please send questions to <A href=
538 "mailto:postgres95@openlink.co.uk">postgres95@openlink.co.uk</A>.</P>
540 <H4><A name="2.2">2.2</A>) What tools are available for using
541 PostgreSQL with Web pages?</H4>
543 <P>A nice introduction to Database-backed Web pages can be seen at:
544 <A href="http://www.webreview.com">http://www.webreview.com</A></P>
546 <P>For Web integration, PHP is an excellent interface. It is at <A
547 href="http://www.php.net">http://www.php.net</A>.</P>
549 <P>For complex cases, many use the Perl interface and CGI.pm or mod_perl.</P>
551 <H4><A name="2.3">2.3</A>) Does PostgreSQL have a graphical user
554 <P>Yes, there are several graphical interfaces to PostgreSQL available.
555 These include PgAccess <a href="http://www.pgaccess.org">
556 http://www.pgaccess.org</a>), PgAdmin III (<a
557 href="http://www.pgadmin.org">http://www.pgadmin.org</a>, RHDB Admin (<a
558 href="http://sources.redhat.com/rhdb/">http://sources.redhat.com/rhdb/
559 </a>) and Rekall (<a href="http://www.thekompany.com/products/rekall/">
560 http://www.thekompany.com/products/rekall/</a>, proprietary). There is
561 also PHPPgAdmin (<a href="http://phppgadmin.sourceforge.net/">
562 http://phppgadmin.sourceforge.net/ </a>), a web-based interface to
565 <P>See <a href="http://techdocs.postgresql.org/guides/GUITools">http://techdocs.postgresql.org/guides/GUITools</a> for a more detailed list.</P>
567 <H4><A name="2.4">2.4</A>) What languages are able to communicate with
570 <P>Most popular programming languages contain an interface to
571 PostgreSQL. Check your programming language's list of extension
574 <P>The following interfaces are included in the PostgreSQL
580 <LI>Embedded C (ecpg)</LI>
584 <LI>Python (PyGreSQL)</LI>
586 <LI>TCL (libpgtcl)</LI>
589 <P>Additional interfaces are available at
590 <a href="http://gborg.postgresql.org">http://gborg.postgresql.org</A>
591 in the <I>Drivers/Interfaces</I> section.
595 <H2 align="center">Administrative Questions</H2>
597 <H4><A name="3.1">3.1</A>) How do I install PostgreSQL somewhere
598 other than <I>/usr/local/pgsql</I>?</H4>
600 <P>Specify the <I>--prefix</I> option when running
601 <I>configure</I>.</P>
603 <H4><A name="3.2">3.2</A>) When I start <I>postmaster</I>, I get a
604 <I>Bad System Call</I> or core dumped message. Why?</H4>
606 <P>It could be a variety of problems, but first check to see that
607 you have System V extensions installed in your kernel. PostgreSQL
608 requires kernel support for shared memory and semaphores.</P>
610 <H4><A name="3.3">3.3</A>) When I try to start <I>postmaster</I>, I
611 get <I>IpcMemoryCreate</I> errors. Why?</H4>
613 <P>You either do not have shared memory configured properly in your
614 kernel or you need to enlarge the shared memory available in the
615 kernel. The exact amount you need depends on your architecture and
616 how many buffers and backend processes you configure for
617 <I>postmaster</I>. For most systems, with default numbers of
618 buffers and processes, you need a minimum of ~1 MB. See the <A
620 "http://www.PostgreSQL.org/docs/view.php?version=current&idoc=1&file=kernel-resources.html">PostgreSQL
621 Administrator's Guide</A> for more detailed information about
622 shared memory and semaphores.</P>
624 <H4><A name="3.4">3.4</A>) When I try to start <I>postmaster</I>, I
625 get <I>IpcSemaphoreCreate</I> errors. Why?</H4>
627 <P>If the error message is <I>IpcSemaphoreCreate: semget failed (No
628 space left on device)</I> then your kernel is not configured with
629 enough semaphores. Postgres needs one semaphore per potential
630 backend process. A temporary solution is to start <I>postmaster</I>
631 with a smaller limit on the number of backend processes. Use
632 <I>-N</I> with a parameter less than the default of 32. A more
633 permanent solution is to increase your kernel's
634 <SMALL>SEMMNS</SMALL> and <SMALL>SEMMNI</SMALL> parameters.</P>
636 <P>Inoperative semaphores can also cause crashes during heavy
639 <P>If the error message is something else, you might not have
640 semaphore support configured in your kernel at all. See the
641 PostgreSQL Administrator's Guide for more detailed information
642 about shared memory and semaphores.</P>
644 <H4><A name="3.5">3.5</A>) How do I control connections from other
647 <P>By default, PostgreSQL only allows connections from the local
648 machine using Unix domain sockets. Other machines will not be able
649 to connect unless you add the <I>-i</I> flag to <I>postmaster</I>,
650 <B>and</B> enable host-based authentication by modifying the file
651 <I>$PGDATA/pg_hba.conf</I> accordingly. This will allow TCP/IP
654 <H4><A name="3.6">3.6</A>) How do I tune the database engine for
655 better performance?</H4>
657 <P>Certainly, indexes can speed up queries. The
658 <SMALL>EXPLAIN</SMALL> command allows you to see how PostgreSQL is
659 interpreting your query, and which indexes are being used.</P>
661 <P>If you are doing many <SMALL>INSERTs</SMALL>, consider doing
662 them in a large batch using the <SMALL>COPY</SMALL> command. This
663 is much faster than individual <SMALL>INSERTS</SMALL>. Second,
664 statements not in a <SMALL>BEGIN WORK/COMMIT</SMALL> transaction
665 block are considered to be in their own transaction. Consider
666 performing several statements in a single transaction block. This
667 reduces the transaction overhead. Also, consider dropping and
668 recreating indexes when making large data changes.</P>
670 <P>There are several tuning options. You can disable <I>fsync()</I>
671 by starting <I>postmaster</I> with a <I>-o -F</I> option. This will
672 prevent <I>fsync()</I>s from flushing to disk after every
675 <P>You can also use the <I>postmaster</I> <I>-B</I> option to
676 increase the number of shared memory buffers used by the backend
677 processes. If you make this parameter too high, the
678 <I>postmaster</I> may not start because you have exceeded your
679 kernel's limit on shared memory space. Each buffer is 8K and the
680 default is 64 buffers.</P>
682 <P>You can also use the backend <I>-S</I> option to increase the
683 maximum amount of memory used by the backend process for temporary
684 sorts. The <I>-S</I> value is measured in kilobytes, and the
685 default is 512 (i.e. 512K).</P>
687 <P>You can also use the <SMALL>CLUSTER</SMALL> command to group
688 data in tables to match an index. See the <SMALL>CLUSTER</SMALL>
689 manual page for more details.</P>
691 <H4><A name="3.7">3.7</A>) What debugging features are
694 <P>PostgreSQL has several features that report status information
695 that can be valuable for debugging purposes.</P>
697 <P>First, by running <I>configure</I> with the --enable-cassert
698 option, many <I>assert()</I>s monitor the progress of the backend
699 and halt the program when something unexpected occurs.</P>
701 <P>Both <I>postmaster</I> and <I>postgres</I> have several debug
702 options available. First, whenever you start <I>postmaster</I>,
703 make sure you send the standard output and error to a log file,
707 ./bin/postmaster >server.log 2>&1 &
710 <P>This will put a server.log file in the top-level PostgreSQL
711 directory. This file contains useful information about problems or
712 errors encountered by the server. <I>Postmaster</I> has a <I>-d</I>
713 option that allows even more detailed information to be reported.
714 The <I>-d</I> option takes a number that specifies the debug level.
715 Be warned that high debug level values generate large log
718 <P>If <I>postmaster</I> is not running, you can actually run the
719 <I>postgres</I> backend from the command line, and type your
720 <SMALL>SQL</SMALL> statement directly. This is recommended
721 <B>only</B> for debugging purposes. Note that a newline terminates
722 the query, not a semicolon. If you have compiled with debugging
723 symbols, you can use a debugger to see what is happening. Because
724 the backend was not started from <I>postmaster</I>, it is not
725 running in an identical environment and locking/backend interaction
726 problems may not be duplicated.</P>
728 <P>If <I>postmaster</I> is running, start <I>psql</I> in one
729 window, then find the <SMALL>PID</SMALL> of the <I>postgres</I>
730 process used by <I>psql</I>. Use a debugger to attach to the
731 <I>postgres</I> <SMALL>PID</SMALL>. You can set breakpoints in the
732 debugger and issue queries from <I>psql</I>. If you are debugging
733 <I>postgres</I> startup, you can set PGOPTIONS="-W n", then start
734 <I>psql</I>. This will cause startup to delay for <I>n</I> seconds
735 so you can attach to the process with the debugger, set any
736 breakpoints, and continue through the startup sequence.</P>
738 <P>The <I>postgres</I> program has <I>-s, -A</I>, and <I>-t</I>
739 options that can be very useful for debugging and performance
742 <P>You can also compile with profiling to see what functions are
743 taking execution time. The backend profile files will be deposited
744 in the <I>pgsql/data/base/dbname</I> directory. The client profile
745 file will be put in the client's current directory. Linux requires
746 a compile with <I>-DLINUX_PROFILE</I> for proper profiling.</P>
748 <H4><A name="3.8">3.8</A>) Why do I get <I>"Sorry, too many
749 clients"</I> when trying to connect?</H4>
751 <P>You need to increase <I>postmaster</I>'s limit on how many
752 concurrent backend processes it can start.</P>
754 <P>The default limit is 32 processes. You can increase it by
755 restarting <I>postmaster</I> with a suitable <I>-N</I> value or
756 modifying <I>postgresql.conf</I>.</P>
758 <P>Note that if you make <I>-N</I> larger than 32, you must also
759 increase <I>-B</I> beyond its default of 64; <I>-B</I> must be at
760 least twice <I>-N</I>, and probably should be more than that for
761 best performance. For large numbers of backend processes, you are
762 also likely to find that you need to increase various Unix kernel
763 configuration parameters. Things to check include the maximum size
764 of shared memory blocks, <SMALL>SHMMAX;</SMALL> the maximum number
765 of semaphores, <SMALL>SEMMNS</SMALL> and <SMALL>SEMMNI;</SMALL> the
766 maximum number of processes, <SMALL>NPROC;</SMALL> the maximum
767 number of processes per user, <SMALL>MAXUPRC;</SMALL> and the
768 maximum number of open files, <SMALL>NFILE</SMALL> and
769 <SMALL>NINODE</SMALL>. The reason that PostgreSQL has a limit on
770 the number of allowed backend processes is so your system won't run
771 out of resources.</P>
773 <H4><A name="3.9">3.9</A>) What is in the <I>pgsql_tmp</I> directory?</H4>
775 <P>This directory contains temporary files generated by the query
776 executor. For example, if a sort needs to be done to satisfy an
777 <SMALL>ORDER BY</SMALL> and the sort requires more space than the
778 backend's <I>-S</I> parameter allows, then temporary files are created
779 here to hold the extra data.</P>
781 <P>The temporary files are usually deleted automatically, but might
782 remain if a backend crashes during a sort. A stop and restart of the
783 <I>postmaster</I> will remove files from those directories.</P>
785 <H4><A name="3.10">3.10</A>) Why do I need to do a dump and restore
786 to upgrade between major PostgreSQL releases?</H4>
788 <P>The PostgreSQL team makes only small changes between minor releases,
789 so upgrading from 7.2 to 7.2.1 does not require a dump and restore.
790 However, major releases (e.g. from 7.2 to 7.3) often change the internal
791 format of system tables and data files. These changes are often complex,
792 so we don't maintain backward compatability for data files. A dump outputs
793 data in a generic format that can then be loaded in using the new internal
796 <P>In releases where the on-disk format does not change, the
797 <I>pg_upgrade</I> script can be used to upgrade without a dump/restore.
798 The release notes mention whether <I>pg_upgrade</I> is available for the
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>You can read the source code for <I>psql</I> in file
836 <I>pgsql/src/bin/psql/describe.c</I>. It contains
837 <SMALL>SQL</SMALL> commands that generate the output for psql's
838 backslash commands. You can also start <I>psql</I> with the
839 <I>-E</I> option so it will print out the queries it uses to
840 execute the commands you give.</P>
842 <H4><A name="4.4">4.4</A>) How do you remove a column from a
843 table, or change its data type?</H4>
845 <P><SMALL>DROP COLUMN</SMALL> functionality was added in release 7.3 with
846 <SMALL>ALTER TABLE DROP COLUMN</SMALL>. In earlier versions,
850 LOCK TABLE old_table;
851 SELECT ... -- select all columns but the one you want to remove
854 DROP TABLE old_table;
855 ALTER TABLE new_table RENAME TO old_table;
859 <P>To change the data type of a column, do this:</P>
862 ALTER TABLE tab ADD COLUMN new_col <i>new_data_type</i>;
863 UPDATE tab SET new_col = CAST(old_col AS <i>new_data_type</i>);
864 ALTER TABLE tab DROP COLUMN old_col;
867 <P>You might then want to do <I>VACUUM FULL tab</I> to reclaim the
868 disk space used by the expired rows.</P>
870 <H4><A name="4.5">4.5</A>) What is the maximum size for a row, a
871 table, and a database?</H4>
873 <P>These are the limits:</P>
875 Maximum size for a database? unlimited (32 TB databases exist)
876 Maximum size for a table? 32 TB
877 Maximum size for a row? 1.6TB
878 Maximum size for a field? 1 GB
879 Maximum number of rows in a table? unlimited
880 Maximum number of columns in a table? 250-1600 depending on column types
881 Maximum number of indexes on a table? unlimited
884 Of course, these are not actually unlimited, but limited to
885 available disk space and memory/swap space. Performance may suffer
886 when these values get unusually large.
888 <P>The maximum table size of 32 TB does not require large file
889 support from the operating system. Large tables are stored as
890 multiple 1 GB files so file system size limits are not
893 <P>The maximum table size and maximum number of columns can be
894 quadrupled by increasing the default block size to 32k.</P>
896 <H4><A name="4.6">4.6</A>) How much database disk space is required
897 to store data from a typical text file?</H4>
899 <P>A PostgreSQL database may require up to five times the disk
900 space to store data from a text file.</P>
902 <P>As an example, consider a file of 100,000 lines with an integer
903 and text description on each line. Suppose the text string
904 avergages twenty bytes in length. The flat file would be 2.8 MB.
905 The size of the PostgreSQL database file containing this data can
906 be estimated as 6.4 MB:</P>
908 36 bytes: each row header (approximate)
909 24 bytes: one int field and one text field
910 + 4 bytes: pointer on page to tuple
911 ----------------------------------------
914 The data page size in PostgreSQL is 8192 bytes (8 KB), so:
917 ------------------- = 128 rows per database page (rounded down)
921 -------------------- = 782 database pages (rounded up)
924 782 database pages * 8192 bytes per page = 6,406,144 bytes (6.4 MB)
927 <P>Indexes do not require as much overhead, but do contain the data
928 that is being indexed, so they can be large also.</P>
930 <P><SMALL>NULL</SMALL>s are stored in bitmaps, so they
931 use very little space.</P>
933 <H4><A name="4.7">4.7</A>) How do I find out what tables, indexes,
934 databases, and users are defined?</H4>
936 <P><I>psql</I> has a variety of backslash commands to show such
937 information. Use \? to see them. There are also system tables
938 beginning with <I>pg_</I> that describe these too. Also, <I>psql
939 -l</I> will list all databases.</P>
941 <P>Also try the file <I>pgsql/src/tutorial/syscat.source</I>. It
942 illustrates many of the <SMALL>SELECT</SMALL>s needed to get
943 information from the database system tables.</P>
945 <H4><A name="4.8">4.8</A>) My queries are slow or don't make use of
946 the indexes. Why?</H4>
947 Indexes are not automatically used by every query. Indexes are only
948 used if the table is larger than a minimum size, and the query
949 selects only a small percentage of the rows in the table. This is
950 because the random disk access caused by an index scan can be
951 slower than a straight read through the table, or sequential scan.
953 <P>To determine if an index should be used, PostgreSQL must have
954 statistics about the table. These statistics are collected using
955 <SMALL>VACUUM ANALYZE</SMALL>, or simply <SMALL>ANALYZE</SMALL>.
956 Using statistics, the optimizer knows how many rows are in the
957 table, and can better determine if indexes should be used.
958 Statistics are also valuable in determining optimal join order and
959 join methods. Statistics collection should be performed
960 periodically as the contents of the table change.</P>
962 <P>Indexes are normally not used for <SMALL>ORDER BY</SMALL> or to
963 perform joins. A sequential scan followed by an explicit sort is
964 usually faster than an index scan of a large table.</P>
965 However, <SMALL>LIMIT</SMALL> combined with <SMALL>ORDER BY</SMALL>
966 often will use an index because only a small portion of the table
967 is returned. In fact, though MAX() and MIN() don't use indexes,
968 it is possible to retrieve such values using an index with ORDER BY
973 ORDER BY col [ DESC ]
977 <P>If you believe the optimizer is incorrect in choosing a
978 sequential scan, use <CODE>SET enable_seqscan TO 'off'</CODE> and
979 run tests to see if an index scan is indeed faster.</P>
981 <P>When using wild-card operators such as <SMALL>LIKE</SMALL> or
982 <I>~</I>, indexes can only be used in certain circumstances:</P>
984 <LI>The beginning of the search string must be anchored to the start
987 <LI><SMALL>LIKE</SMALL> patterns must not start with <I>%</I>.</LI>
988 <LI><I>~</I> (regular expression) patterns must start with
991 <LI>The search string can not start with a character class,
993 <LI>Case-insensitive searches such as <SMALL>ILIKE</SMALL> and
994 <I>~*</I> do not utilise indexes. Instead, use functional
995 indexes, which are described in section <a href="#4.12">4.12</a>.</LI>
996 <LI>The default <I>C</I> locale must be used during
1001 <H4><A name="4.9">4.9</A>) How do I see how the query optimizer is
1002 evaluating my query?</H4>
1004 <P>See the <SMALL>EXPLAIN</SMALL> manual page.</P>
1006 <H4><A name="4.10">4.10</A>) What is an R-tree index?</H4>
1008 <P>An R-tree index is used for indexing spatial data. A hash index
1009 can't handle range searches. A B-tree index only handles range
1010 searches in a single dimension. R-trees can handle
1011 multi-dimensional data. For example, if an R-tree index can be
1012 built on an attribute of type <I>point</I>, the system can more
1013 efficiently answer queries such as "select all points within a
1014 bounding rectangle."</P>
1016 <P>The canonical paper that describes the original R-tree design
1019 <P>Guttman, A. "R-trees: A Dynamic Index Structure for Spatial
1020 Searching." Proceedings of the 1984 ACM SIGMOD Int'l Conf on Mgmt
1023 <P>You can also find this paper in Stonebraker's "Readings in
1024 Database Systems".</P>
1026 <P>Built-in R-trees can handle polygons and boxes. In theory,
1027 R-trees can be extended to handle higher number of dimensions. In
1028 practice, extending R-trees requires a bit of work and we don't
1029 currently have any documentation on how to do it.</P>
1031 <H4><A name="4.11">4.11</A>) What is the Genetic Query
1034 <P>The <SMALL>GEQO</SMALL> module speeds query optimization when
1035 joining many tables by means of a Genetic Algorithm (GA). It allows
1036 the handling of large join queries through nonexhaustive
1039 <H4><A name="4.12">4.12</A>) How do I perform regular expression
1040 searches and case-insensitive regular expression searches? How do I
1041 use an index for case-insensitive searches?</H4>
1043 <P>The <I>~</I> operator does regular expression matching, and
1044 <I>~*</I> does case-insensitive regular expression matching. The
1045 case-insensitive variant of <SMALL>LIKE</SMALL> is called
1046 <SMALL>ILIKE</SMALL>.</P>
1048 <P>Case-insensitive equality comparisons are normally expressed
1053 WHERE lower(col) = 'abc';
1056 This will not use an standard index. However, if you create a
1057 functional index, it will be used:
1059 CREATE INDEX tabindex ON tab (lower(col));
1062 <H4><A name="4.13">4.13</A>) In a query, how do I detect if a field
1063 is <SMALL>NULL</SMALL>?</H4>
1065 <P>You test the column with <SMALL>IS NULL</SMALL> and <SMALL>IS
1066 NOT NULL</SMALL>.</P>
1068 <H4><A name="4.14">4.14</A>) What is the difference between the
1069 various character types?</H4>
1071 Type Internal Name Notes
1072 --------------------------------------------------
1073 VARCHAR(n) varchar size specifies maximum length, no padding
1074 CHAR(n) bpchar blank padded to the specified fixed length
1075 TEXT text no specific upper limit on length
1076 BYTEA bytea variable-length byte array (null-byte safe)
1077 "char" char one character
1080 <P>You will see the internal name when examining system catalogs
1081 and in some error messages.</P>
1083 <P>The first four types above are "varlena" types (i.e., the first
1084 four bytes on disk are the length, followed by the data). Thus the
1085 actual space used is slightly greater than the declared size.
1086 However, these data types are also subject to compression or being
1087 stored out-of-line by <SMALL>TOAST</SMALL>, so the space on disk
1088 might also be less than expected.</P>
1090 <SMALL>VARCHAR(n)</SMALL> is best when storing variable-length
1091 strings and it limits how long a string can be. <SMALL>TEXT</SMALL>
1092 is for strings of unlimited length, with a maximum of one gigabyte.
1093 <P><SMALL>CHAR(n)</SMALL> is for storing strings that are all the
1094 same length. <SMALL>CHAR(n)</SMALL> pads with blanks to the specified
1095 length, while <SMALL>VARCHAR(n)</SMALL> only stores the characters
1096 supplied. <SMALL>BYTEA</SMALL> is for storing binary data,
1097 particularly values that include <SMALL>NULL</SMALL> bytes. All the
1098 types described here have similar performance characteristics.</P>
1100 <H4><A name="4.15.1">4.15.1</A>) How do I create a
1101 serial/auto-incrementing field?</H4>
1103 <P>PostgreSQL supports a <SMALL>SERIAL</SMALL> data type. It
1104 auto-creates a sequence. For example,
1107 CREATE TABLE person (
1113 is automatically translated into this:
1115 CREATE SEQUENCE person_id_seq;
1116 CREATE TABLE person (
1117 id INT4 NOT NULL DEFAULT nextval('person_id_seq'),
1122 See the <I>create_sequence</I> manual page for more information
1123 about sequences. You can also use each row's <I>OID</I> field as a
1124 unique value. However, if you need to dump and reload the database,
1125 you need to use <I>pg_dump</I>'s <I>-o</I> option or <SMALL>COPY
1126 WITH OIDS</SMALL> option to preserve the <SMALL>OID</SMALL>s.
1128 <H4><A name="4.15.2">4.15.2</A>) How do I get the value of a
1129 <SMALL>SERIAL</SMALL> insert?</H4>
1131 <P>One approach is to retrieve the next <SMALL>SERIAL</SMALL> value
1132 from the sequence object with the <I>nextval()</I> function
1133 <I>before</I> inserting and then insert it explicitly. Using the
1134 example table in <A href="#4.15.1">4.15.1</A>, an example in a
1135 pseudo-language would look like this:</P>
1137 new_id = execute("SELECT nextval('person_id_seq')");
1138 execute("INSERT INTO person (id, name) VALUES (new_id, 'Blaise Pascal')");
1141 You would then also have the new value stored in
1142 <CODE>new_id</CODE> for use in other queries (e.g., as a foreign
1143 key to the <CODE>person</CODE> table). Note that the name of the
1144 automatically created <SMALL>SEQUENCE</SMALL> object will be named
1145 <<I>table</I>>_<<I>serialcolumn</I>>_<I>seq</I>, where
1146 <I>table</I> and <I>serialcolumn</I> are the names of your table
1147 and your <SMALL>SERIAL</SMALL> column, respectively.
1149 <P>Alternatively, you could retrieve the assigned
1150 <SMALL>SERIAL</SMALL> value with the <I>currval()</I> function
1151 <I>after</I> it was inserted by default, e.g.,</P>
1153 execute("INSERT INTO person (name) VALUES ('Blaise Pascal')");
1154 new_id = execute("SELECT currval('person_id_seq')");
1157 Finally, you could use the <A href="#4.16"><SMALL>OID</SMALL></A>
1158 returned from the <SMALL>INSERT</SMALL> statement to look up the
1159 default value, though this is probably the least portable approach,
1160 and the oid value will wrap around when it reaches 4 billion.
1161 In Perl, using DBI with Edmund Mergl's DBD::Pg module, the oid
1162 value is made available via <I>$sth->{pg_oid_status}</I> after
1163 <I>$sth->execute()</I>.
1165 <H4><A name="4.15.3">4.15.3</A>) Don't <I>currval()</I> and
1166 <I>nextval()</I> lead to a race condition with other users?</H4>
1168 <P>No. <I>currval()</I> returns the current value assigned by your
1169 backend, not by all users.</P>
1171 <H4><A name="4.15.4">4.15.4</A>) Why aren't my sequence numbers
1172 reused on transaction abort? Why are there gaps in the numbering of
1173 my sequence/SERIAL column?</H4>
1175 <P>To improve concurrency, sequence values are given out to running
1176 transactions as needed and are not locked until the transaction
1177 completes. This causes gaps in numbering from aborted
1180 <H4><A name="4.16">4.16</A>) What is an <SMALL>OID</SMALL>? What is
1181 a <SMALL>TID</SMALL>?</H4>
1183 <P><SMALL>OID</SMALL>s are PostgreSQL's answer to unique row ids.
1184 Every row that is created in PostgreSQL gets a unique
1185 <SMALL>OID</SMALL>. All <SMALL>OID</SMALL>s generated during
1186 <I>initdb</I> are less than 16384 (from
1187 <I>include/access/transam.h</I>). All user-created
1188 <SMALL>OID</SMALL>s are equal to or greater than this. By default,
1189 all these <SMALL>OID</SMALL>s are unique not only within a table or
1190 database, but unique within the entire PostgreSQL installation.</P>
1192 <P>PostgreSQL uses <SMALL>OID</SMALL>s in its internal system
1193 tables to link rows between tables. These <SMALL>OID</SMALL>s can
1194 be used to identify specific user rows and used in joins. It is
1195 recommended you use column type <SMALL>OID</SMALL> to store
1196 <SMALL>OID</SMALL> values. You can create an index on the
1197 <SMALL>OID</SMALL> field for faster access.</P>
1199 <P>O<SMALL>ID</SMALL>s are assigned to all new rows from a central
1200 area that is used by all databases. If you want to change the
1201 <SMALL>OID</SMALL> to something else, or if you want to make a copy
1202 of the table, with the original <SMALL>OID</SMALL>s, there is no
1203 reason you can't do it:</P>
1205 CREATE TABLE new_table(old_oid oid, mycol int);
1206 SELECT old_oid, mycol INTO new FROM old;
1207 COPY new TO '/tmp/pgtable';
1209 COPY new WITH OIDS FROM '/tmp/pgtable';
1212 CREATE TABLE new_table (mycol int);
1213 INSERT INTO new_table (oid, mycol) SELECT oid, mycol FROM old_table;
1215 <P>O<SMALL>ID</SMALL>s are stored as 4-byte integers, and will
1216 overflow at 4 billion. No one has reported this ever happening, and
1217 we plan to have the limit removed before anyone does.</P>
1219 <P>T<SMALL>ID</SMALL>s are used to identify specific physical rows
1220 with block and offset values. T<SMALL>ID</SMALL>s change after rows
1221 are modified or reloaded. They are used by index entries to point
1222 to physical rows.</P>
1224 <H4><A name="4.17">4.17</A>) What is the meaning of some of the
1225 terms used in PostgreSQL?</H4>
1227 <P>Some of the source code and older documentation use terms that
1228 have more common usage. Here are some:</P>
1231 <LI>table, relation, class</LI>
1233 <LI>row, record, tuple</LI>
1235 <LI>column, field, attribute</LI>
1237 <LI>retrieve, select</LI>
1239 <LI>replace, update</LI>
1241 <LI>append, insert</LI>
1243 <LI><SMALL>OID</SMALL>, serial value</LI>
1245 <LI>portal, cursor</LI>
1247 <LI>range variable, table name, table alias</LI>
1250 <P>A list of general database terms can be found at: <A href=
1251 "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>
1253 <H4><A name="4.18">4.18</A>) Why do I get the error <I>"ERROR:
1254 Memory exhausted in AllocSetAlloc()"</I>?</H4>
1256 <P>You probably have run out of virtual memory on your system,
1257 or your kernel has a low limit for certain resources. Try this
1258 before starting <I>postmaster</I>:</P>
1264 Depending on your shell, only one of these may succeed, but it will
1265 set your process data segment limit much higher and perhaps allow
1266 the query to complete. This command applies to the current process,
1267 and all subprocesses created after the command is run. If you are
1268 having a problem with the <SMALL>SQL</SMALL> client because the
1269 backend is returning too much data, try it before starting the
1272 <H4><A name="4.19">4.19</A>) How do I tell what PostgreSQL version
1275 <P>From <I>psql</I>, type <CODE>SELECT version();</CODE></P>
1277 <H4><A name="4.20">4.20</A>) Why does my large-object operations
1278 get <I>"invalid large obj descriptor"</I>?</H4>
1280 <P>You need to put <CODE>BEGIN WORK</CODE> and <CODE>COMMIT</CODE>
1281 around any use of a large object handle, that is, surrounding
1282 <CODE>lo_open</CODE> ... <CODE>lo_close.</CODE></P>
1284 <P>Currently PostgreSQL enforces the rule by closing large object
1285 handles at transaction commit. So the first attempt to do anything
1286 with the handle will draw <I>invalid large obj descriptor</I>. So
1287 code that used to work (at least most of the time) will now
1288 generate that error message if you fail to use a transaction.</P>
1290 <P>If you are using a client interface like <SMALL>ODBC</SMALL> you
1291 may need to set <CODE>auto-commit off.</CODE></P>
1293 <H4><A name="4.21">4.21</A>) How do I create a column that will
1294 default to the current time?</H4>
1296 <P>Use <I>CURRENT_TIMESTAMP</I>:</P>
1298 <CODE>CREATE TABLE test (x int, modtime timestamp DEFAULT CURRENT_TIMESTAMP );
1302 <H4><A name="4.22">4.22</A>) Why are my subqueries using
1303 <CODE><SMALL>IN</SMALL></CODE> so slow?</H4>
1305 <P>In versions prior to 7.4, subqueries were joined to outer queries
1306 by sequentially scanning the result of the subquery for each row of
1307 the outer query. If the subquery returns only a few rows and the outer
1308 query returns many rows, <CODE><SMALL>IN</SMALL></CODE> is fastest. To
1309 speed up other queries, replace <CODE>IN</CODE> with
1310 <CODE>EXISTS</CODE>:</P>
1313 WHERE col IN (SELECT subcol FROM subtab);
1318 WHERE EXISTS (SELECT subcol FROM subtab WHERE subcol = col);
1321 For this to be fast, <CODE>subcol</CODE> should be an indexed column.
1322 <P>In version 7.4 and later, <CODE>IN</CODE> actually uses the same
1323 sophisticated join techniques as normal queries, and is prefered
1324 to using <CODE>EXISTS</CODE>.
1326 <H4><A name="4.23">4.23</A>) How do I perform an outer join?</H4>
1328 <P>PostgreSQL supports outer joins using the SQL standard syntax.
1329 Here are two examples:</P>
1332 FROM t1 LEFT OUTER JOIN t2 ON (t1.col = t2.col);
1337 FROM t1 LEFT OUTER JOIN t2 USING (col);
1340 <P>These identical queries join t1.col to t2.col, and also return
1341 any unjoined rows in t1 (those with no match in t2). A
1342 <SMALL>RIGHT</SMALL> join would add unjoined rows of t2. A
1343 <SMALL>FULL</SMALL> join would return the matched rows plus all
1344 unjoined rows from t1 and t2. The word <SMALL>OUTER</SMALL> is
1345 optional and is assumed in <SMALL>LEFT</SMALL>,
1346 <SMALL>RIGHT</SMALL>, and <SMALL>FULL</SMALL> joins. Ordinary joins
1347 are called <SMALL>INNER</SMALL> joins.</P>
1349 <P>In previous releases, outer joins can be simulated using
1350 <SMALL>UNION</SMALL> and <SMALL>NOT IN</SMALL>. For example, when
1351 joining <I>tab1</I> and <I>tab2</I>, the following query does an
1352 <I>outer</I> join of the two tables:<BR>
1356 SELECT tab1.col1, tab2.col2
1358 WHERE tab1.col1 = tab2.col1
1360 SELECT tab1.col1, NULL
1362 WHERE tab1.col1 NOT IN (SELECT tab2.col1 FROM tab2)
1366 <H4><A name="4.24">4.24</A>) How do I perform queries using
1367 multiple databases?</H4>
1369 <P>There is no way to query a database other than the current one.
1370 Because PostgreSQL loads database-specific system catalogs, it is
1371 uncertain how a cross-database query should even behave.</P>
1373 <P><I>contrib/dblink</I> allows cross-database queries using
1374 function calls. Of course, a client can make simultaneous
1375 connections to different databases and merge the results on the
1378 <H4><A name="4.25">4.25</A>) How do I return multiple rows or
1379 columns from a function?</H4>
1381 <P>In 7.3, you can easily return multiple rows or columns from a
1383 <a href="http://techdocs.postgresql.org/guides/SetReturningFunctions">
1384 http://techdocs.postgresql.org/guides/SetReturningFunctions</a>.
1386 <H4><A name="4.26">4.26</A>) Why can't I reliably create/drop
1387 temporary tables in PL/PgSQL functions?</H4>
1388 <P>PL/PgSQL caches function contents, and an unfortunate side effect
1389 is that if a PL/PgSQL function accesses a temporary table, and that
1390 table is later dropped and recreated, and the function called
1391 again, the function will fail because the cached function contents
1392 still point to the old temporary table. The solution is to use
1393 <SMALL>EXECUTE</SMALL> for temporary table access in PL/PgSQL. This
1394 will cause the query to be reparsed every time.</P>
1396 <H4><A name="4.27">4.27</A>) What replication options are available?
1398 <P>There are several master/slave replication options available.
1399 These allow only the master to make database changes and the slave
1400 can only do database reads. The bottom of <a
1401 href="http://gborg.PostgreSQL.org/genpage?replication_research">
1402 http://gborg.PostgreSQL.org/genpage?replication_research</a> lists
1403 them. A multi-master replication solution is being worked on at <a
1404 href="http://gborg.PostgreSQL.org/project/pgreplication/projdisplay.php">http://gborg.PostgreSQL.org/project/pgreplication/projdisplay.php</a>.</P>
1406 <H4><A name="4.28">4.28</A>) What encryption options are available?
1409 <LI><I>contrib/pgcrypto</I> contains many encryption functions for
1410 use in <SMALL>SQL</SMALL> queries.</LI>
1411 <LI>To encrypt transmission from the client to the server, the server
1412 must have the <I>ssl</I> option set to <I>true</I> in <I>postgresql.conf,
1413 </I> and an applicable <I>host</I> or <I>hostssl</I> record must exist in
1414 <I>pg_hba.conf</I>, and the client <I>sslmode</I> must not be
1415 <I>disable.</I> (Note that it is also possible to use a third-party
1416 encrypted transport, such as stunnel or ssh, rather than PostgreSQL's
1417 native SSL connections.)
1418 <LI>Database user passwords are automatically encrypted when stored in
1419 version 7.3. In previous versions, you must enable the option
1420 <I>PASSWORD_ENCRYPTION</I> in <I>postgresql.conf</I>.</LI>
1421 <LI>The server can run using an encrypted file system.</LI>
1426 <H2 align="center">Extending PostgreSQL</H2>
1428 <H4><A name="5.1">5.1</A>) I wrote a user-defined function. When I
1429 run it in <I>psql</I>, why does it dump core?</H4>
1431 <P>The problem could be a number of things. Try testing your
1432 user-defined function in a stand-alone test program first.</P>
1434 <H4><A name="5.2">5.2</A>) How can I contribute some nifty new
1435 types and functions to PostgreSQL?</H4>
1437 <P>Send your extensions to the <I>pgsql-hackers</I> mailing list,
1438 and they will eventually end up in the <I>contrib/</I>
1441 <H4><A name="5.3">5.3</A>) How do I write a C function to return a
1444 <P>In versions of PostgreSQL beginning with 7.3, table-returning
1445 functions are fully supported in C, PL/PgSQL, and SQL. See the
1446 Programmer's Guide for more information. An example of a
1447 table-returning function defined in C can be found in
1448 <I>contrib/tablefunc</I>.</P>
1450 <H4><A name="5.4">5.4</A>) I have changed a source file. Why does
1451 the recompile not see the change?</H4>
1453 <P>The <I>Makefiles</I> do not have the proper dependencies for
1454 include files. You have to do a <I>make clean</I> and then another
1455 <I>make</I>. If you are using <SMALL>GCC</SMALL> you can use the
1456 <I>--enable-depend</I> option of <I>configure</I> to have the
1457 compiler compute the dependencies automatically.</P>
projects / postgresql / blob