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11 <H1>Frequently Asked Questions (FAQ) for PostgreSQL</H1>
13 <P>Last updated: Fri Mar 12 08:51:11 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 href=
20 "http://www.PostgreSQL.org/docs/faqs/FAQ.html">http://www.PostgreSQL.org/docs/faqs/FAQ.html</A>.</P>
22 <P>Platform-specific questions are answered at <A href=
23 "http://www.PostgreSQL.org/docs/index.html">http://www.PostgreSQL.org/docs/index.html</A>.</P>
26 <H2 align="center">General Questions</H2>
27 <A href="#1.1">1.1</A>) What is PostgreSQL? How is it pronounced?<BR>
28 <A href="#1.2">1.2</A>) What is the copyright on PostgreSQL?<BR>
29 <A href="#1.3">1.3</A>) What Unix platforms does PostgreSQL run
31 <A href="#1.4">1.4</A>) What non-Unix ports are available?<BR>
32 <A href="#1.5">1.5</A>) Where can I get PostgreSQL?<BR>
33 <A href="#1.6">1.6</A>) Where can I get support?<BR>
34 <A href="#1.7">1.7</A>) What is the latest release?<BR>
35 <A href="#1.8">1.8</A>) What documentation is available?<BR>
36 <A href="#1.9">1.9</A>) How do I find out about known bugs or
38 <A href="#1.10">1.10</A>) How can I learn <SMALL>SQL</SMALL>?<BR>
39 <A href="#1.11">1.11</A>) Is PostgreSQL Y2K compliant?<BR>
40 <A href="#1.12">1.12</A>) How do I join the development team?<BR>
41 <A href="#1.13">1.13</A>) How do I submit a bug report?<BR>
42 <A href="#1.14">1.14</A>) How does PostgreSQL compare to other
43 <SMALL>DBMS</SMALL>s?<BR>
44 <A href="#1.15">1.15</A>) How can I financially assist
48 <H2 align="center">User Client Questions</H2>
49 <A href="#2.1">2.1</A>) Are there <SMALL>ODBC</SMALL> drivers for
51 <A href="#2.2">2.2</A>) What tools are available for using
52 PostgreSQL with Web pages?<BR>
53 <A href="#2.3">2.3</A>) Does PostgreSQL have a graphical user
55 <A href="#2.4">2.4</A>) What languages are available to
56 communicate with PostgreSQL?<BR>
59 <H2 align="center">Administrative Questions</H2>
60 <A href="#3.1">3.1</A>) How do I install PostgreSQL somewhere other
61 than <I>/usr/local/pgsql</I>?<BR>
62 <A href="#3.2">3.2</A>) When I start <I>postmaster</I>, I get a
63 <I>Bad System Call</I> or core dumped message. Why?<BR>
64 <A href="#3.3">3.3</A>) When I try to start <I>postmaster</I>, I
65 get <I>IpcMemoryCreate</I> errors. Why?<BR>
66 <A href="#3.4">3.4</A>) When I try to start <I>postmaster</I>, I
67 get <I>IpcSemaphoreCreate</I> errors. Why?<BR>
68 <A href="#3.5">3.5</A>) How do I control connections from other
70 <A href="#3.6">3.6</A>) How do I tune the database engine for
71 better performance?<BR>
72 <A href="#3.7">3.7</A>) What debugging features are available?<BR>
73 <A href="#3.8">3.8</A>) Why do I get <I>"Sorry, too many
74 clients"</I> when trying to connect?<BR>
75 <A href="#3.9">3.9</A>) What is in the <I>pgsql_tmp</I>
77 <A href="#3.10">3.10</A>) Why do I need to do a dump and restore
78 to upgrade PostgreSQL releases?<BR>
81 <H2 align="center">Operational Questions</H2>
82 <A href="#4.1">4.1</A>) What is the difference between binary
83 cursors and normal cursors?<BR>
84 <A href="#4.2">4.2</A>) How do I <SMALL>SELECT</SMALL> only the
85 first few rows of a query? A random row?<BR>
86 <A href="#4.3">4.3</A>) How do I get a list of tables or other
87 things I can see in <I>psql</I>?<BR>
88 <A href="#4.4">4.4</A>) How do you remove a column from a
89 table, or change it's data type?<BR>
90 <A href="#4.5">4.5</A>) What is the maximum size for a row, a
91 table, and a database?<BR>
92 <A href="#4.6">4.6</A>) How much database disk space is required
93 to store data from a typical text file?<BR>
94 <A href="#4.7">4.7</A>) How do I find out what tables, indexes,
95 databases, and users are defined?<BR>
96 <A href="#4.8">4.8</A>) My queries are slow or don't make use of
98 <A href="#4.9">4.9</A>) How do I see how the query optimizer is
99 evaluating my query?<BR>
100 <A href="#4.10">4.10</A>) What is an R-tree index?<BR>
101 <A href="#4.11">4.11</A>) What is the Genetic Query Optimizer?<BR>
102 <A href="#4.12">4.12</A>) How do I perform regular expression
103 searches and case-insensitive regular expression searches? How do I
104 use an index for case-insensitive searches?<BR>
105 <A href="#4.13">4.13</A>) In a query, how do I detect if a field
106 is <SMALL>NULL</SMALL>?<BR>
107 <A href="#4.14">4.14</A>) What is the difference between the
108 various character types?<BR>
109 <A href="#4.15.1">4.15.1</A>) How do I create a
110 serial/auto-incrementing field?<BR>
111 <A href="#4.15.2">4.15.2</A>) How do I get the value of a
112 <SMALL>SERIAL</SMALL> insert?<BR>
113 <A href="#4.15.3">4.15.3</A>) Don't <I>currval()</I> and
114 <I>nextval()</I> lead to a race condition with other users?<BR>
115 <A href="#4.15.4">4.15.4</A>) Why aren't my sequence numbers
116 reused on transaction abort? Why are there gaps in the numbering of
117 my sequence/SERIAL column?<BR>
118 <A href="#4.16">4.16</A>) What is an <SMALL>OID</SMALL>? What is a
119 <SMALL>TID</SMALL>?<BR>
120 <A href="#4.17">4.17</A>) What is the meaning of some of the terms
121 used in PostgreSQL?<BR>
122 <A href="#4.18">4.18</A>) Why do I get the error <I>"ERROR: Memory
123 exhausted in AllocSetAlloc()"</I>?<BR>
124 <A href="#4.19">4.19</A>) How do I tell what PostgreSQL version I
126 <A href="#4.20">4.20</A>) Why does my large-object operations get
127 <I>"invalid large obj descriptor"</I>?<BR>
128 <A href="#4.21">4.21</A>) How do I create a column that will
129 default to the current time?<BR>
130 <A href="#4.22">4.22</A>) Why are my subqueries using
131 <CODE><SMALL>IN</SMALL></CODE> so slow?<BR>
132 <A href="#4.23">4.23</A>) How do I perform an outer join?<BR>
133 <A href="#4.24">4.24</A>) How do I perform queries using multiple
135 <A href="#4.25">4.25</A>) How do I return multiple rows or columns
137 <A href="#4.26">4.26</A>) Why can't I reliably create/drop
138 temporary tables in PL/PgSQL functions?<BR>
139 <A href="#4.27">4.27</A>) What replication options are available?<BR>
140 <A href="#4.28">4.28</A>) What encryption options are available?<BR>
143 <H2 align="center">Extending PostgreSQL</H2>
144 <A href="#5.1">5.1</A>) I wrote a user-defined function. When I run
145 it in <I>psql</I>, why does it dump core?<BR>
146 <A href="#5.2">5.2</A>) How can I contribute some nifty new types
147 and functions to PostgreSQL?<BR>
148 <A href="#5.3">5.3</A>) How do I write a C function to return a
150 <A href="#5.4">5.4</A>) I have changed a source file. Why does the
151 recompile not see the change?<BR>
155 <H2 align="center">General Questions</H2>
157 <H4><A name="1.1">1.1</A>) What is PostgreSQL? How is it pronounced?</H4>
159 <P>PostgreSQL is pronounced <I>Post-Gres-Q-L</I>. The name "Postgres" is
160 also used in conversation.</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. It is a community
175 project and is not controlled by any company. To get involved, see
176 the developer's FAQ,<A href=
177 "http://www.PostgreSQL.org/docs/faqs/FAQ_DEV.html">http://www.PostgreSQL.org/docs/faqs/FAQ_DEV.html</A>
180 <P>The authors of PostgreSQL 1.01 were Andrew Yu and Jolly Chen.
181 Many others have contributed to the porting, testing, debugging,
182 and enhancement of the code. The original Postgres code, from which
183 PostgreSQL is derived, was the effort of many graduate students,
184 undergraduate students, and staff programmers working under the
185 direction of Professor Michael Stonebraker at the University of
186 California, Berkeley.</P>
188 <P>The original name of the software at Berkeley was Postgres. When
189 <SMALL>SQL</SMALL> functionality was added in 1995, its name was
190 changed to Postgres95. The name was changed at the end of 1996 to
193 <H4><A name="1.2">1.2</A>) What is the copyright on
196 <P>PostgreSQL is subject to the following COPYRIGHT:</P>
198 <P>PostgreSQL Data Base Management System</P>
200 <P>Portions copyright (c) 1996-2002, PostgreSQL Global Development
201 Group Portions Copyright (c) 1994-6 Regents of the University of
204 <P>Permission to use, copy, modify, and distribute this software
205 and its documentation for any purpose, without fee, and without a
206 written agreement is hereby granted, provided that the above
207 copyright notice and this paragraph and the following two
208 paragraphs appear in all copies.</P>
210 <P>IN NO EVENT SHALL THE UNIVERSITY OF CALIFORNIA BE LIABLE TO ANY
211 PARTY FOR DIRECT, INDIRECT, SPECIAL, INCIDENTAL, OR CONSEQUENTIAL
212 DAMAGES, INCLUDING LOST PROFITS, ARISING OUT OF THE USE OF THIS
213 SOFTWARE AND ITS DOCUMENTATION, EVEN IF THE UNIVERSITY OF
214 CALIFORNIA HAS BEEN ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.</P>
216 <P>THE UNIVERSITY OF CALIFORNIA SPECIFICALLY DISCLAIMS ANY
217 WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
218 OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE. THE
219 SOFTWARE PROVIDED HEREUNDER IS ON AN "AS IS" BASIS, AND THE
220 UNIVERSITY OF CALIFORNIA HAS NO OBLIGATIONS TO PROVIDE MAINTENANCE,
221 SUPPORT, UPDATES, ENHANCEMENTS, OR MODIFICATIONS.</P>
223 <P>The above is the BSD license, the classic open-source license.
224 It has no restrictions on how the source code may be used. We like
225 it and have no intention of changing it.</P>
227 <H4><A name="1.3">1.3</A>) What Unix platforms does PostgreSQL run
230 <P>In general, a modern Unix-compatible platform should be able to
231 run PostgreSQL. The platforms that had received explicit testing at
232 the time of release are listed in the installation
235 <H4><A name="1.4">1.4</A>) What non-Unix ports are available?</H4>
237 <P><STRONG>Client</STRONG></P>
239 <P>It is possible to compile the <I>libpq</I> C library, psql, and
240 other interfaces and client applications to run on MS Windows platforms.
241 In this case, the client is running on MS Windows, and communicates
242 via TCP/IP to a server running on one of our supported Unix
243 platforms. A file <I>win32.mak</I> is included in the distribution
244 for making a Win32 <I>libpq</I> library and <I>psql</I>. PostgreSQL
245 also communicates with <SMALL>ODBC</SMALL> clients.</P>
247 <P><STRONG>Server</STRONG></P>
249 <P>The database server can run on Windows NT and Win2k using
250 Cygwin, the Cygnus Unix/NT porting library. See
251 <I>pgsql/doc/FAQ_MSWIN</I> in the distribution or the MS Windows FAQ
252 at <A href="http://www.PostgreSQL.org/docs/faqs/text/FAQ_MSWIN">
253 http://www.PostgreSQL.org/docs/faqs/text/FAQ_MSWIN</A>.</P>
255 <p>A native port to MS Win NT/2000/XP is currently being worked
256 on. For more details on the current status of PostgreSQL on Windows see
257 <a href="http://techdocs.postgresql.org/guides/Windows">
258 http://techdocs.postgresql.org/guides/Windows</a> and
259 <a href="http://momjian.postgresql.org/main/writings/pgsql/win32.html">
260 http://momjian.postgresql.org/main/writings/pgsql/win32.html</a>.</p>
262 <p>There is also a Novell Netware 6 port at
263 <a href="http://forge.novell.com">http://forge.novell.com</a>.</p>
265 <H4><A name="1.5">1.5</A>) Where can I get PostgreSQL?</H4>
267 <P>The primary anonymous ftp site for PostgreSQL is <A href=
268 "ftp://ftp.PostgreSQL.org/pub">ftp://ftp.PostgreSQL.org/pub</A>.
269 For mirror sites, see our main web site.</P>
271 <H4><A name="1.6">1.6</A>) Where can I get support?</H4>
273 <P>The main mailing list is: <A href=
274 "mailto:pgsql-general@PostgreSQL.org">pgsql-general@PostgreSQL.org</A>.
275 It is available for discussion of matters pertaining to PostgreSQL.
276 To subscribe, send mail with the following lines in the body (not
277 the subject line):</P>
284 "mailto:pgsql-general-request@PostgreSQL.org">pgsql-general-request@PostgreSQL.org</A>.</P>
286 <P>There is also a digest list available. To subscribe to this
287 list, send email to: <A href=
288 "mailto:pgsql-general-digest-request@PostgreSQL.org">pgsql-general-digest-request@PostgreSQL.org</A>
295 Digests are sent out to members of this list whenever the main list
296 has received around 30k of messages.
298 <P>The bugs mailing list is available. To subscribe to this list,
299 send email to <A href=
300 "mailto:pgsql-bugs-request@PostgreSQL.org">pgsql-bugs-request@PostgreSQL.org</A>
307 There is also a developers discussion mailing list available. To
308 subscribe to this list, send email to <A href=
309 "mailto:pgsql-hackers-request@PostgreSQL.org">pgsql-hackers-request@PostgreSQL.org</A>
316 <P>Additional mailing lists and information about PostgreSQL can be
317 found via the PostgreSQL WWW home page at:</P>
320 <A href="http://www.PostgreSQL.org">http://www.PostgreSQL.org</A>
323 <P>There is also an IRC channel on Freenode and EFNet,
324 channel <I>#PostgreSQL</I>. You can use the Unix command <CODE>irc -c
325 '#PostgreSQL' "$USER" irc.phoenix.net.</CODE> or <CODE>irc -c
326 '#PostgreSQL' "$USER" irc.freenode.net.</CODE></P>
328 <P>A list of commercial support companies is available at <A href=
329 "http://techdocs.postgresql.org/companies.php">http://techdocs.postgresql.org/companies.php</A>.</P>
331 <H4><A name="1.7">1.7</A>) What is the latest release?</H4>
333 <P>The latest release of PostgreSQL is version 7.4.2.</P>
335 <P>We plan to have major releases every six to eight months.</P>
337 <H4><A name="1.8">1.8</A>) What documentation is available?</H4>
339 <P>Several manuals, manual pages, and some small test examples are
340 included in the distribution. See the <I>/doc</I> directory. You
341 can also browse the manuals online at <A href=
342 "http://www.PostgreSQL.org/docs">http://www.PostgreSQL.org/docs</A>.</P>
344 <P>There are two PostgreSQL books available online at <A href=
345 "http://www.PostgreSQL.org/docs/awbook.html">http://www.PostgreSQL.org/docs/awbook.html</A>
347 "http://www.commandprompt.com/ppbook/">http://www.commandprompt.com/ppbook/</A>.
348 There is a list of PostgreSQL books available for purchase at <A
350 "http://techdocs.postgresql.org/techdocs/bookreviews.php">http://techdocs.PostgreSQL.org/techdocs/bookreviews.php</A>.
351 There is also a collection of PostgreSQL technical articles at <A
353 "http://techdocs.PostgreSQL.org/">http://techdocs.PostgreSQL.org/</A>.</P>
355 <P><I>psql</I> has some nice \d commands to show information about
356 types, operators, functions, aggregates, etc.</P>
358 <P>Our web site contains even more documentation.</P>
360 <H4><A name="1.9">1.9</A>) How do I find out about known bugs or
361 missing features?</H4>
363 <P>PostgreSQL supports an extended subset of <SMALL>SQL</SMALL>-92.
364 See our <A href="http://developer.PostgreSQL.org/todo.php">TODO</A>
365 list for known bugs, missing features, and future plans.</P>
367 <H4><A name="1.10">1.10</A>) How can I learn
368 <SMALL>SQL</SMALL>?</H4>
370 <P>The PostgreSQL book at <A href=
371 "http://www.PostgreSQL.org/docs/awbook.html">http://www.PostgreSQL.org/docs/awbook.html</A>
372 teaches <SMALL>SQL</SMALL>. There is another PostgreSQL book at <A
374 "http://www.commandprompt.com/ppbook/">http://www.commandprompt.com/ppbook.</A>
375 There is a nice tutorial at <A href=
376 "http://www.intermedia.net/support/sql/sqltut.shtm">http://www.intermedia.net/support/sql/sqltut.shtm,</A>
378 "http://ourworld.compuserve.com/homepages/graeme_birchall/HTM_COOK.HTM">
379 http://ourworld.compuserve.com/homepages/graeme_birchall/HTM_COOK.HTM,</A>
381 "http://sqlcourse.com/">http://sqlcourse.com.</A></P>
383 <P>Another one is "Teach Yourself SQL in 21 Days, Second Edition"
385 "http://members.tripod.com/er4ebus/sql/index.htm">http://members.tripod.com/er4ebus/sql/index.htm</A></P>
387 <P>Many of our users like <I>The Practical SQL Handbook</I>,
388 Bowman, Judith S., et al., Addison-Wesley. Others like <I>The
389 Complete Reference SQL</I>, Groff et al., McGraw-Hill.</P>
391 <H4><A name="1.11">1.11</A>) Is PostgreSQL Y2K compliant?</H4>
393 <P>Yes, we easily handle dates past the year 2000 AD, and before
396 <H4><A name="1.12">1.12</A>) How do I join the development
399 <P>First, download the latest source and read the PostgreSQL
400 Developers documentation on our web site, or in the distribution.
401 Second, subscribe to the <I>pgsql-hackers</I> and
402 <I>pgsql-patches</I> mailing lists. Third, submit high quality
403 patches to pgsql-patches.</P>
405 <P>There are about a dozen people who have commit privileges to the
406 PostgreSQL <SMALL>CVS</SMALL> archive. They each have submitted so
407 many high-quality patches that it was impossible for the existing
408 committers to keep up, and we had confidence that patches they
409 committed were of high quality.</P>
411 <H4><A name="1.13">1.13</A>) How do I submit a bug report?</H4>
413 <P>Please visit the PostgreSQL BugTool page at <A href=
414 "http://www.PostgreSQL.org/bugs/bugs.php">http://www.PostgreSQL.org/bugs/bugs.php</A>,
415 which gives guidelines and directions on how to submit a
418 <P>Also check out our ftp site <A href=
419 "ftp://ftp.PostgreSQL.org/pub">ftp://ftp.PostgreSQL.org/pub</A> to
420 see if there is a more recent PostgreSQL version or patches.</P>
422 <H4><A name="1.14">1.14</A>) How does PostgreSQL compare to other
423 <SMALL>DBMS</SMALL>s?</H4>
425 <P>There are several ways of measuring software: features,
426 performance, reliability, support, and price.</P>
429 <DT><B>Features</B></DT>
431 <DD>PostgreSQL has most features present in large commercial
432 <SMALL>DBMS</SMALL>s, like transactions, subselects, triggers,
433 views, foreign key referential integrity, and sophisticated
434 locking. We have some features they do not have, like
435 user-defined types, inheritance, rules, and multi-version
436 concurrency control to reduce lock contention.<BR>
440 <DT><B>Performance</B></DT>
442 <DD>PostgreSQL has performance similar to other commercial and
443 open source databases. it is faster for some things, slower for
444 others. In comparison to MySQL or leaner database systems, we are
445 faster for multiple users, complex queries, and a read/write query
446 load. MySQL is faster for simple SELECT queries done by a few users.
447 Of course, MySQL does not have most of the features mentioned in the
448 <I>Features</I> section above. We are built for reliability and
449 features, and we continue to improve performance in every
450 release. There is an interesting Web page comparing PostgreSQL to
451 MySQL at <A href="http://openacs.org/philosophy/why-not-mysql.html">
452 http://openacs.org/philosophy/why-not-mysql.html</A> Also, MySQL is
453 is a company that distributes its products via open source, and requires
454 a commercial license for close-source software, not an
455 open source development community like PostgreSQL.<BR>
460 <DT><B>Reliability</B></DT>
462 <DD>We realize that a <SMALL>DBMS</SMALL> must be reliable, or it
463 is worthless. We strive to release well-tested, stable code that
464 has a minimum of bugs. Each release has at least one month of
465 beta testing, and our release history shows that we can provide
466 stable, solid releases that are ready for production use. We
467 believe we compare favorably to other database software in this
472 <DT><B>Support</B></DT>
474 <DD>Our mailing lists provide contact with a large group of developers
475 and users to help resolve any problems encountered. While we cannot
476 guarantee a fix, commercial <SMALL>DBMS</SMALL>s do not always
477 supply a fix either. Direct access to developers, the user
478 community, manuals, and the source code often make PostgreSQL
479 support superior to other <SMALL>DBMS</SMALL>s. There is
480 commercial per-incident support available for those who need it.
481 (See <A href="#1.6">FAQ section 1.6</A>.)<BR>
485 <DT><B>Price</B></DT>
487 <DD>We are free for all use, both commercial and non-commercial.
488 You can add our code to your product with no limitations, except
489 those outlined in our BSD-style license stated above.<BR>
494 <H4><A name="1.15">1.15</A>) How can I financially assist
497 <P>PostgreSQL has had a first-class infrastructure since we started
498 in 1996. This is all thanks to Marc Fournier, who has created
499 and managed this infrastructure over the years.</P>
501 <P>Quality infrastructure is very important to an open-source
502 project. It prevents disruptions that can greatly delay forward
503 movement of the project.</P>
505 <P>Of course, this infrastructure is not cheap. There are a variety
506 of monthly and one-time expenses that are required to keep it
507 going. If you or your company has money it can donate to help fund
508 this effort, please go to <A href="http://store.pgsql.com/shopping/">http://store.pgsql.com/shopping/</A>
509 and make a donation.</P>
511 <P>Although the web page mentions PostgreSQL, Inc, the
512 "contributions" item is solely to support the PostgreSQL project
513 and does not fund any specific company. If you prefer, you can also
514 send a check to the contact address.</P>
517 <P>Also, if you have a success story about PostgreSQL, please submit
518 it to our advocacy site at <a href="http://advocacy.postgresql.org">
519 http://advocacy.postgresql.org</a>.</P>
522 <H2 align="center">User Client Questions</H2>
524 <H4><A name="2.1">2.1</A>) Are there <SMALL>ODBC</SMALL> drivers
527 <P>There are two <SMALL>ODBC</SMALL> drivers available, PsqlODBC
528 and OpenLink <SMALL>ODBC</SMALL>.</P>
530 <P>You can download PsqlODBC from <A href=
531 "http://gborg.postgresql.org/project/psqlodbc/projdisplay.php">
532 http://gborg.postgresql.org/project/psqlodbc/projdisplay.php</A>.</P>
534 <P>OpenLink <SMALL>ODBC</SMALL> can be gotten from <A href=
535 "http://www.openlinksw.com/">http://www.openlinksw.com</A>. It
536 works with their standard <SMALL>ODBC</SMALL> client software so
537 you'll have PostgreSQL <SMALL>ODBC</SMALL> available on every
538 client platform they support (Win, Mac, Unix, VMS).</P>
540 <P>They will probably be selling this product to people who need
541 commercial-quality support, but a freeware version will always be
542 available. Please send questions to <A href=
543 "mailto:postgres95@openlink.co.uk">postgres95@openlink.co.uk</A>.</P>
545 <H4><A name="2.2">2.2</A>) What tools are available for using
546 PostgreSQL with Web pages?</H4>
548 <P>A nice introduction to Database-backed Web pages can be seen at:
549 <A href="http://www.webreview.com">http://www.webreview.com</A></P>
551 <P>For Web integration, PHP is an excellent interface. It is at <A
552 href="http://www.php.net">http://www.php.net</A>.</P>
554 <P>For complex cases, many use the Perl interface and CGI.pm or mod_perl.</P>
556 <H4><A name="2.3">2.3</A>) Does PostgreSQL have a graphical user
559 <P>Yes, there are several graphical interfaces to PostgreSQL available.
560 These include PgAccess <a href="http://www.pgaccess.org">
561 http://www.pgaccess.org</a>), PgAdmin III (<a
562 href="http://www.pgadmin.org">http://www.pgadmin.org</a>, RHDB Admin (<a
563 href="http://sources.redhat.com/rhdb/">http://sources.redhat.com/rhdb/
564 </a>) and Rekall (<a href="http://www.thekompany.com/products/rekall/">
565 http://www.thekompany.com/products/rekall/</a>, proprietary). There is
566 also PHPPgAdmin (<a href="http://phppgadmin.sourceforge.net/">
567 http://phppgadmin.sourceforge.net/ </a>), a web-based interface to
570 <P>See <a href="http://techdocs.postgresql.org/guides/GUITools">http://techdocs.postgresql.org/guides/GUITools</a> for a more detailed list.</P>
572 <H4><A name="2.4">2.4</A>) What languages are able to communicate with
575 <P>Most popular programming languages contain an interface to
576 PostgreSQL. Check your programming language's list of extension
579 <P>The following interfaces are included in the PostgreSQL
585 <LI>Embedded C (ecpg)</LI>
589 <LI>Python (PyGreSQL)</LI>
591 <LI>TCL (libpgtcl)</LI>
594 <P>Additional interfaces are available at
595 <a href="http://gborg.postgresql.org">http://gborg.postgresql.org</A>
596 in the <I>Drivers/Interfaces</I> section.
600 <H2 align="center">Administrative Questions</H2>
602 <H4><A name="3.1">3.1</A>) How do I install PostgreSQL somewhere
603 other than <I>/usr/local/pgsql</I>?</H4>
605 <P>Specify the <I>--prefix</I> option when running
606 <I>configure</I>.</P>
608 <H4><A name="3.2">3.2</A>) When I start <I>postmaster</I>, I get a
609 <I>Bad System Call</I> or core dumped message. Why?</H4>
611 <P>It could be a variety of problems, but first check to see that
612 you have System V extensions installed in your kernel. PostgreSQL
613 requires kernel support for shared memory and semaphores.</P>
615 <H4><A name="3.3">3.3</A>) When I try to start <I>postmaster</I>, I
616 get <I>IpcMemoryCreate</I> errors. Why?</H4>
618 <P>You either do not have shared memory configured properly in your
619 kernel or you need to enlarge the shared memory available in the
620 kernel. The exact amount you need depends on your architecture and
621 how many buffers and backend processes you configure for
622 <I>postmaster</I>. For most systems, with default numbers of
623 buffers and processes, you need a minimum of ~1 MB. See the <A
625 "http://www.PostgreSQL.org/docs/view.php?version=current&idoc=1&file=kernel-resources.html">PostgreSQL
626 Administrator's Guide</A> for more detailed information about
627 shared memory and semaphores.</P>
629 <H4><A name="3.4">3.4</A>) When I try to start <I>postmaster</I>, I
630 get <I>IpcSemaphoreCreate</I> errors. Why?</H4>
632 <P>If the error message is <I>IpcSemaphoreCreate: semget failed (No
633 space left on device)</I> then your kernel is not configured with
634 enough semaphores. Postgres needs one semaphore per potential
635 backend process. A temporary solution is to start <I>postmaster</I>
636 with a smaller limit on the number of backend processes. Use
637 <I>-N</I> with a parameter less than the default of 32. A more
638 permanent solution is to increase your kernel's
639 <SMALL>SEMMNS</SMALL> and <SMALL>SEMMNI</SMALL> parameters.</P>
641 <P>Inoperative semaphores can also cause crashes during heavy
644 <P>If the error message is something else, you might not have
645 semaphore support configured in your kernel at all. See the
646 PostgreSQL Administrator's Guide for more detailed information
647 about shared memory and semaphores.</P>
649 <H4><A name="3.5">3.5</A>) How do I control connections from other
652 <P>By default, PostgreSQL only allows connections from the local
653 machine using Unix domain sockets. Other machines will not be able
654 to connect unless you add the <I>-i</I> flag to <I>postmaster</I>,
655 <B>and</B> enable host-based authentication by modifying the file
656 <I>$PGDATA/pg_hba.conf</I> accordingly. This will allow TCP/IP
659 <H4><A name="3.6">3.6</A>) How do I tune the database engine for
660 better performance?</H4>
662 <P>Certainly, indexes can speed up queries. The
663 <SMALL>EXPLAIN</SMALL> command allows you to see how PostgreSQL is
664 interpreting your query, and which indexes are being used.</P>
666 <P>If you are doing many <SMALL>INSERTs</SMALL>, consider doing
667 them in a large batch using the <SMALL>COPY</SMALL> command. This
668 is much faster than individual <SMALL>INSERTS</SMALL>. Second,
669 statements not in a <SMALL>BEGIN WORK/COMMIT</SMALL> transaction
670 block are considered to be in their own transaction. Consider
671 performing several statements in a single transaction block. This
672 reduces the transaction overhead. Also, consider dropping and
673 recreating indexes when making large data changes.</P>
675 <P>There are several tuning options. You can disable <I>fsync()</I>
676 by starting <I>postmaster</I> with a <I>-o -F</I> option. This will
677 prevent <I>fsync()</I>s from flushing to disk after every
680 <P>You can also use the <I>postmaster</I> <I>-B</I> option to
681 increase the number of shared memory buffers used by the backend
682 processes. If you make this parameter too high, the
683 <I>postmaster</I> may not start because you have exceeded your
684 kernel's limit on shared memory space. Each buffer is 8K and the
685 default is 64 buffers.</P>
687 <P>You can also use the backend <I>-S</I> option to increase the
688 maximum amount of memory used by the backend process for temporary
689 sorts. The <I>-S</I> value is measured in kilobytes, and the
690 default is 512 (i.e. 512K).</P>
692 <P>You can also use the <SMALL>CLUSTER</SMALL> command to group
693 data in tables to match an index. See the <SMALL>CLUSTER</SMALL>
694 manual page for more details.</P>
696 <H4><A name="3.7">3.7</A>) What debugging features are
699 <P>PostgreSQL has several features that report status information
700 that can be valuable for debugging purposes.</P>
702 <P>First, by running <I>configure</I> with the --enable-cassert
703 option, many <I>assert()</I>s monitor the progress of the backend
704 and halt the program when something unexpected occurs.</P>
706 <P>Both <I>postmaster</I> and <I>postgres</I> have several debug
707 options available. First, whenever you start <I>postmaster</I>,
708 make sure you send the standard output and error to a log file,
712 ./bin/postmaster >server.log 2>&1 &
715 <P>This will put a server.log file in the top-level PostgreSQL
716 directory. This file contains useful information about problems or
717 errors encountered by the server. <I>Postmaster</I> has a <I>-d</I>
718 option that allows even more detailed information to be reported.
719 The <I>-d</I> option takes a number that specifies the debug level.
720 Be warned that high debug level values generate large log
723 <P>If <I>postmaster</I> is not running, you can actually run the
724 <I>postgres</I> backend from the command line, and type your
725 <SMALL>SQL</SMALL> statement directly. This is recommended
726 <B>only</B> for debugging purposes. Note that a newline terminates
727 the query, not a semicolon. If you have compiled with debugging
728 symbols, you can use a debugger to see what is happening. Because
729 the backend was not started from <I>postmaster</I>, it is not
730 running in an identical environment and locking/backend interaction
731 problems may not be duplicated.</P>
733 <P>If <I>postmaster</I> is running, start <I>psql</I> in one
734 window, then find the <SMALL>PID</SMALL> of the <I>postgres</I>
735 process used by <I>psql</I>. Use a debugger to attach to the
736 <I>postgres</I> <SMALL>PID</SMALL>. You can set breakpoints in the
737 debugger and issue queries from <I>psql</I>. If you are debugging
738 <I>postgres</I> startup, you can set PGOPTIONS="-W n", then start
739 <I>psql</I>. This will cause startup to delay for <I>n</I> seconds
740 so you can attach to the process with the debugger, set any
741 breakpoints, and continue through the startup sequence.</P>
743 <P>The <I>postgres</I> program has <I>-s, -A</I>, and <I>-t</I>
744 options that can be very useful for debugging and performance
747 <P>You can also compile with profiling to see what functions are
748 taking execution time. The backend profile files will be deposited
749 in the <I>pgsql/data/base/dbname</I> directory. The client profile
750 file will be put in the client's current directory. Linux requires
751 a compile with <I>-DLINUX_PROFILE</I> for proper profiling.</P>
753 <H4><A name="3.8">3.8</A>) Why do I get <I>"Sorry, too many
754 clients"</I> when trying to connect?</H4>
756 <P>You need to increase <I>postmaster</I>'s limit on how many
757 concurrent backend processes it can start.</P>
759 <P>The default limit is 32 processes. You can increase it by
760 restarting <I>postmaster</I> with a suitable <I>-N</I> value or
761 modifying <I>postgresql.conf</I>.</P>
763 <P>Note that if you make <I>-N</I> larger than 32, you must also
764 increase <I>-B</I> beyond its default of 64; <I>-B</I> must be at
765 least twice <I>-N</I>, and probably should be more than that for
766 best performance. For large numbers of backend processes, you are
767 also likely to find that you need to increase various Unix kernel
768 configuration parameters. Things to check include the maximum size
769 of shared memory blocks, <SMALL>SHMMAX;</SMALL> the maximum number
770 of semaphores, <SMALL>SEMMNS</SMALL> and <SMALL>SEMMNI;</SMALL> the
771 maximum number of processes, <SMALL>NPROC;</SMALL> the maximum
772 number of processes per user, <SMALL>MAXUPRC;</SMALL> and the
773 maximum number of open files, <SMALL>NFILE</SMALL> and
774 <SMALL>NINODE</SMALL>. The reason that PostgreSQL has a limit on
775 the number of allowed backend processes is so your system won't run
776 out of resources.</P>
778 <H4><A name="3.9">3.9</A>) What is in the <I>pgsql_tmp</I> directory?</H4>
780 <P>This directory contains temporary files generated by the query
781 executor. For example, if a sort needs to be done to satisfy an
782 <SMALL>ORDER BY</SMALL> and the sort requires more space than the
783 backend's <I>-S</I> parameter allows, then temporary files are created
784 here to hold the extra data.</P>
786 <P>The temporary files are usually deleted automatically, but might
787 remain if a backend crashes during a sort. A stop and restart of the
788 <I>postmaster</I> will remove files from those directories.</P>
790 <H4><A name="3.10">3.10</A>) Why do I need to do a dump and restore
791 to upgrade between major PostgreSQL releases?</H4>
793 <P>The PostgreSQL team makes only small changes between minor releases,
794 so upgrading from 7.2 to 7.2.1 does not require a dump and restore.
795 However, major releases (e.g. from 7.2 to 7.3) often change the internal
796 format of system tables and data files. These changes are often complex,
797 so we don't maintain backward compatability for data files. A dump outputs
798 data in a generic format that can then be loaded in using the new internal
801 <P>In releases where the on-disk format does not change, the
802 <I>pg_upgrade</I> script can be used to upgrade without a dump/restore.
803 The release notes mention whether <I>pg_upgrade</I> is available for the
808 <H2 align="center">Operational Questions</H2>
810 <H4><A name="4.1">4.1</A>) What is the difference between binary
811 cursors and normal cursors?</H4>
813 <P>See the <SMALL>DECLARE</SMALL> manual page for a
816 <H4><A name="4.2">4.2</A>) How do I <SMALL>SELECT</SMALL> only the
817 first few rows of a query? A random row?</H4>
819 <P>See the <SMALL>FETCH</SMALL> manual page, or use
820 <SMALL>SELECT</SMALL> ... <SMALL>LIMIT</SMALL>....</P>
822 <P>The entire query may have to be evaluated, even if you only want
823 the first few rows. Consider using a query that has an <SMALL>ORDER
824 BY</SMALL>. If there is an index that matches the <SMALL>ORDER
825 BY</SMALL>, PostgreSQL may be able to evaluate only the first few
826 records requested, or the entire query may have to be evaluated
827 until the desired rows have been generated.</P>
829 <P>To <SMALL>SELECT</SMALL> a random row, use:
837 <H4><A name="4.3">4.3</A>) How do I get a list of tables or other
838 things I can see in <I>psql</I>?</H4>
840 <P>You can read the source code for <I>psql</I> in file
841 <I>pgsql/src/bin/psql/describe.c</I>. It contains
842 <SMALL>SQL</SMALL> commands that generate the output for psql's
843 backslash commands. You can also start <I>psql</I> with the
844 <I>-E</I> option so it will print out the queries it uses to
845 execute the commands you give.</P>
847 <H4><A name="4.4">4.4</A>) How do you remove a column from a
848 table, or change its data type?</H4>
850 <P><SMALL>DROP COLUMN</SMALL> functionality was added in release 7.3 with
851 <SMALL>ALTER TABLE DROP COLUMN</SMALL>. In earlier versions,
855 LOCK TABLE old_table;
856 SELECT ... -- select all columns but the one you want to remove
859 DROP TABLE old_table;
860 ALTER TABLE new_table RENAME TO old_table;
864 <P>To change the data type of a column, do this:</P>
867 ALTER TABLE tab ADD COLUMN new_col <i>new_data_type</i>;
868 UPDATE tab SET new_col = CAST(old_col AS <i>new_data_type</i>);
869 ALTER TABLE tab DROP COLUMN old_col;
872 <P>You might then want to do <I>VACUUM FULL tab</I> to reclaim the
873 disk space used by the expired rows.</P>
875 <H4><A name="4.5">4.5</A>) What is the maximum size for a row, a
876 table, and a database?</H4>
878 <P>These are the limits:</P>
880 Maximum size for a database? unlimited (32 TB databases exist)
881 Maximum size for a table? 32 TB
882 Maximum size for a row? 1.6TB
883 Maximum size for a field? 1 GB
884 Maximum number of rows in a table? unlimited
885 Maximum number of columns in a table? 250-1600 depending on column types
886 Maximum number of indexes on a table? unlimited
889 Of course, these are not actually unlimited, but limited to
890 available disk space and memory/swap space. Performance may suffer
891 when these values get unusually large.
893 <P>The maximum table size of 32 TB does not require large file
894 support from the operating system. Large tables are stored as
895 multiple 1 GB files so file system size limits are not
898 <P>The maximum table size and maximum number of columns can be
899 quadrupled by increasing the default block size to 32k.</P>
901 <H4><A name="4.6">4.6</A>) How much database disk space is required
902 to store data from a typical text file?</H4>
904 <P>A PostgreSQL database may require up to five times the disk
905 space to store data from a text file.</P>
907 <P>As an example, consider a file of 100,000 lines with an integer
908 and text description on each line. Suppose the text string
909 avergages twenty bytes in length. The flat file would be 2.8 MB.
910 The size of the PostgreSQL database file containing this data can
911 be estimated as 6.4 MB:</P>
913 36 bytes: each row header (approximate)
914 24 bytes: one int field and one text field
915 + 4 bytes: pointer on page to tuple
916 ----------------------------------------
919 The data page size in PostgreSQL is 8192 bytes (8 KB), so:
922 ------------------- = 128 rows per database page (rounded down)
926 -------------------- = 782 database pages (rounded up)
929 782 database pages * 8192 bytes per page = 6,406,144 bytes (6.4 MB)
932 <P>Indexes do not require as much overhead, but do contain the data
933 that is being indexed, so they can be large also.</P>
935 <P><SMALL>NULL</SMALL>s are stored as bitmaps, so they
936 use very little space.</P>
938 <H4><A name="4.7">4.7</A>) How do I find out what tables, indexes,
939 databases, and users are defined?</H4>
941 <P><I>psql</I> has a variety of backslash commands to show such
942 information. Use \? to see them. There are also system tables
943 beginning with <I>pg_</I> that describe these too. Also, <I>psql
944 -l</I> will list all databases.</P>
946 <P>Also try the file <I>pgsql/src/tutorial/syscat.source</I>. It
947 illustrates many of the <SMALL>SELECT</SMALL>s needed to get
948 information from the database system tables.</P>
950 <H4><A name="4.8">4.8</A>) My queries are slow or don't make use of
951 the indexes. Why?</H4>
952 Indexes are not automatically used by every query. Indexes are only
953 used if the table is larger than a minimum size, and the query
954 selects only a small percentage of the rows in the table. This is
955 because the random disk access caused by an index scan can be
956 slower than a straight read through the table, or sequential scan.
958 <P>To determine if an index should be used, PostgreSQL must have
959 statistics about the table. These statistics are collected using
960 <SMALL>VACUUM ANALYZE</SMALL>, or simply <SMALL>ANALYZE</SMALL>.
961 Using statistics, the optimizer knows how many rows are in the
962 table, and can better determine if indexes should be used.
963 Statistics are also valuable in determining optimal join order and
964 join methods. Statistics collection should be performed
965 periodically as the contents of the table change.</P>
967 <P>Indexes are normally not used for <SMALL>ORDER BY</SMALL> or to
968 perform joins. A sequential scan followed by an explicit sort is
969 usually faster than an index scan of a large table.</P>
970 However, <SMALL>LIMIT</SMALL> combined with <SMALL>ORDER BY</SMALL>
971 often will use an index because only a small portion of the table
972 is returned. In fact, though MAX() and MIN() don't use indexes,
973 it is possible to retrieve such values using an index with ORDER BY
978 ORDER BY col [ DESC ]
982 <P>If you believe the optimizer is incorrect in choosing a
983 sequential scan, use <CODE>SET enable_seqscan TO 'off'</CODE> and
984 run tests to see if an index scan is indeed faster.</P>
986 <P>When using wild-card operators such as <SMALL>LIKE</SMALL> or
987 <I>~</I>, indexes can only be used in certain circumstances:</P>
989 <LI>The beginning of the search string must be anchored to the start
992 <LI><SMALL>LIKE</SMALL> patterns must not start with <I>%</I>.</LI>
993 <LI><I>~</I> (regular expression) patterns must start with
996 <LI>The search string can not start with a character class,
998 <LI>Case-insensitive searches such as <SMALL>ILIKE</SMALL> and
999 <I>~*</I> do not utilise indexes. Instead, use functional
1000 indexes, which are described in section <a href="#4.12">4.12</a>.</LI>
1001 <LI>The default <I>C</I> locale must be used during
1006 <H4><A name="4.9">4.9</A>) How do I see how the query optimizer is
1007 evaluating my query?</H4>
1009 <P>See the <SMALL>EXPLAIN</SMALL> manual page.</P>
1011 <H4><A name="4.10">4.10</A>) What is an R-tree index?</H4>
1013 <P>An R-tree index is used for indexing spatial data. A hash index
1014 can't handle range searches. A B-tree index only handles range
1015 searches in a single dimension. R-trees can handle
1016 multi-dimensional data. For example, if an R-tree index can be
1017 built on an attribute of type <I>point</I>, the system can more
1018 efficiently answer queries such as "select all points within a
1019 bounding rectangle."</P>
1021 <P>The canonical paper that describes the original R-tree design
1024 <P>Guttman, A. "R-trees: A Dynamic Index Structure for Spatial
1025 Searching." Proceedings of the 1984 ACM SIGMOD Int'l Conf on Mgmt
1028 <P>You can also find this paper in Stonebraker's "Readings in
1029 Database Systems".</P>
1031 <P>Built-in R-trees can handle polygons and boxes. In theory,
1032 R-trees can be extended to handle higher number of dimensions. In
1033 practice, extending R-trees requires a bit of work and we don't
1034 currently have any documentation on how to do it.</P>
1036 <H4><A name="4.11">4.11</A>) What is the Genetic Query
1039 <P>The <SMALL>GEQO</SMALL> module speeds query optimization when
1040 joining many tables by means of a Genetic Algorithm (GA). It allows
1041 the handling of large join queries through nonexhaustive
1044 <H4><A name="4.12">4.12</A>) How do I perform regular expression
1045 searches and case-insensitive regular expression searches? How do I
1046 use an index for case-insensitive searches?</H4>
1048 <P>The <I>~</I> operator does regular expression matching, and
1049 <I>~*</I> does case-insensitive regular expression matching. The
1050 case-insensitive variant of <SMALL>LIKE</SMALL> is called
1051 <SMALL>ILIKE</SMALL>.</P>
1053 <P>Case-insensitive equality comparisons are normally expressed
1058 WHERE lower(col) = 'abc';
1061 This will not use an standard index. However, if you create a
1062 functional index, it will be used:
1064 CREATE INDEX tabindex ON tab (lower(col));
1067 <H4><A name="4.13">4.13</A>) In a query, how do I detect if a field
1068 is <SMALL>NULL</SMALL>?</H4>
1070 <P>You test the column with <SMALL>IS NULL</SMALL> and <SMALL>IS
1071 NOT NULL</SMALL>.</P>
1073 <H4><A name="4.14">4.14</A>) What is the difference between the
1074 various character types?</H4>
1076 Type Internal Name Notes
1077 --------------------------------------------------
1078 VARCHAR(n) varchar size specifies maximum length, no padding
1079 CHAR(n) bpchar blank padded to the specified fixed length
1080 TEXT text no specific upper limit on length
1081 BYTEA bytea variable-length byte array (null-byte safe)
1082 "char" char one character
1085 <P>You will see the internal name when examining system catalogs
1086 and in some error messages.</P>
1088 <P>The first four types above are "varlena" types (i.e., the first
1089 four bytes on disk are the length, followed by the data). Thus the
1090 actual space used is slightly greater than the declared size.
1091 However, these data types are also subject to compression or being
1092 stored out-of-line by <SMALL>TOAST</SMALL>, so the space on disk
1093 might also be less than expected.</P>
1095 <SMALL>VARCHAR(n)</SMALL> is best when storing variable-length
1096 strings and it limits how long a string can be. <SMALL>TEXT</SMALL>
1097 is for strings of unlimited length, with a maximum of one gigabyte.
1098 <P><SMALL>CHAR(n)</SMALL> is for storing strings that are all the
1099 same length. <SMALL>CHAR(n)</SMALL> pads with blanks to the specified
1100 length, while <SMALL>VARCHAR(n)</SMALL> only stores the characters
1101 supplied. <SMALL>BYTEA</SMALL> is for storing binary data,
1102 particularly values that include <SMALL>NULL</SMALL> bytes. All the
1103 types described here have similar performance characteristics.</P>
1105 <H4><A name="4.15.1">4.15.1</A>) How do I create a
1106 serial/auto-incrementing field?</H4>
1108 <P>PostgreSQL supports a <SMALL>SERIAL</SMALL> data type. It
1109 auto-creates a sequence. For example,
1112 CREATE TABLE person (
1118 is automatically translated into this:
1120 CREATE SEQUENCE person_id_seq;
1121 CREATE TABLE person (
1122 id INT4 NOT NULL DEFAULT nextval('person_id_seq'),
1127 See the <I>create_sequence</I> manual page for more information
1128 about sequences. You can also use each row's <I>OID</I> field as a
1129 unique value. However, if you need to dump and reload the database,
1130 you need to use <I>pg_dump</I>'s <I>-o</I> option or <SMALL>COPY
1131 WITH OIDS</SMALL> option to preserve the <SMALL>OID</SMALL>s.
1133 <H4><A name="4.15.2">4.15.2</A>) How do I get the value of a
1134 <SMALL>SERIAL</SMALL> insert?</H4>
1136 <P>One approach is to retrieve the next <SMALL>SERIAL</SMALL> value
1137 from the sequence object with the <I>nextval()</I> function
1138 <I>before</I> inserting and then insert it explicitly. Using the
1139 example table in <A href="#4.15.1">4.15.1</A>, an example in a
1140 pseudo-language would look like this:</P>
1142 new_id = execute("SELECT nextval('person_id_seq')");
1143 execute("INSERT INTO person (id, name) VALUES (new_id, 'Blaise Pascal')");
1146 You would then also have the new value stored in
1147 <CODE>new_id</CODE> for use in other queries (e.g., as a foreign
1148 key to the <CODE>person</CODE> table). Note that the name of the
1149 automatically created <SMALL>SEQUENCE</SMALL> object will be named
1150 <<I>table</I>>_<<I>serialcolumn</I>>_<I>seq</I>, where
1151 <I>table</I> and <I>serialcolumn</I> are the names of your table
1152 and your <SMALL>SERIAL</SMALL> column, respectively.
1154 <P>Alternatively, you could retrieve the assigned
1155 <SMALL>SERIAL</SMALL> value with the <I>currval()</I> function
1156 <I>after</I> it was inserted by default, e.g.,</P>
1158 execute("INSERT INTO person (name) VALUES ('Blaise Pascal')");
1159 new_id = execute("SELECT currval('person_id_seq')");
1162 Finally, you could use the <A href="#4.16"><SMALL>OID</SMALL></A>
1163 returned from the <SMALL>INSERT</SMALL> statement to look up the
1164 default value, though this is probably the least portable approach,
1165 and the oid value will wrap around when it reaches 4 billion.
1166 In Perl, using DBI with Edmund Mergl's DBD::Pg module, the oid
1167 value is made available via <I>$sth->{pg_oid_status}</I> after
1168 <I>$sth->execute()</I>.
1170 <H4><A name="4.15.3">4.15.3</A>) Don't <I>currval()</I> and
1171 <I>nextval()</I> lead to a race condition with other users?</H4>
1173 <P>No. <I>currval()</I> returns the current value assigned by your
1174 backend, not by all users.</P>
1176 <H4><A name="4.15.4">4.15.4</A>) Why aren't my sequence numbers
1177 reused on transaction abort? Why are there gaps in the numbering of
1178 my sequence/SERIAL column?</H4>
1180 <P>To improve concurrency, sequence values are given out to running
1181 transactions as needed and are not locked until the transaction
1182 completes. This causes gaps in numbering from aborted
1185 <H4><A name="4.16">4.16</A>) What is an <SMALL>OID</SMALL>? What is
1186 a <SMALL>TID</SMALL>?</H4>
1188 <P><SMALL>OID</SMALL>s are PostgreSQL's answer to unique row ids.
1189 Every row that is created in PostgreSQL gets a unique
1190 <SMALL>OID</SMALL>. All <SMALL>OID</SMALL>s generated during
1191 <I>initdb</I> are less than 16384 (from
1192 <I>include/access/transam.h</I>). All user-created
1193 <SMALL>OID</SMALL>s are equal to or greater than this. By default,
1194 all these <SMALL>OID</SMALL>s are unique not only within a table or
1195 database, but unique within the entire PostgreSQL installation.</P>
1197 <P>PostgreSQL uses <SMALL>OID</SMALL>s in its internal system
1198 tables to link rows between tables. These <SMALL>OID</SMALL>s can
1199 be used to identify specific user rows and used in joins. It is
1200 recommended you use column type <SMALL>OID</SMALL> to store
1201 <SMALL>OID</SMALL> values. You can create an index on the
1202 <SMALL>OID</SMALL> field for faster access.</P>
1204 <P>O<SMALL>ID</SMALL>s are assigned to all new rows from a central
1205 area that is used by all databases. If you want to change the
1206 <SMALL>OID</SMALL> to something else, or if you want to make a copy
1207 of the table, with the original <SMALL>OID</SMALL>s, there is no
1208 reason you can't do it:</P>
1210 CREATE TABLE new_table(old_oid oid, mycol int);
1211 SELECT old_oid, mycol INTO new FROM old;
1212 COPY new TO '/tmp/pgtable';
1214 COPY new WITH OIDS FROM '/tmp/pgtable';
1217 CREATE TABLE new_table (mycol int);
1218 INSERT INTO new_table (oid, mycol) SELECT oid, mycol FROM old_table;
1220 <P>O<SMALL>ID</SMALL>s are stored as 4-byte integers, and will
1221 overflow at 4 billion. No one has reported this ever happening, and
1222 we plan to have the limit removed before anyone does.</P>
1224 <P>T<SMALL>ID</SMALL>s are used to identify specific physical rows
1225 with block and offset values. T<SMALL>ID</SMALL>s change after rows
1226 are modified or reloaded. They are used by index entries to point
1227 to physical rows.</P>
1229 <H4><A name="4.17">4.17</A>) What is the meaning of some of the
1230 terms used in PostgreSQL?</H4>
1232 <P>Some of the source code and older documentation use terms that
1233 have more common usage. Here are some:</P>
1236 <LI>table, relation, class</LI>
1238 <LI>row, record, tuple</LI>
1240 <LI>column, field, attribute</LI>
1242 <LI>retrieve, select</LI>
1244 <LI>replace, update</LI>
1246 <LI>append, insert</LI>
1248 <LI><SMALL>OID</SMALL>, serial value</LI>
1250 <LI>portal, cursor</LI>
1252 <LI>range variable, table name, table alias</LI>
1255 <P>A list of general database terms can be found at: <A href=
1256 "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>
1258 <H4><A name="4.18">4.18</A>) Why do I get the error <I>"ERROR:
1259 Memory exhausted in AllocSetAlloc()"</I>?</H4>
1261 <P>You probably have run out of virtual memory on your system,
1262 or your kernel has a low limit for certain resources. Try this
1263 before starting <I>postmaster</I>:</P>
1269 Depending on your shell, only one of these may succeed, but it will
1270 set your process data segment limit much higher and perhaps allow
1271 the query to complete. This command applies to the current process,
1272 and all subprocesses created after the command is run. If you are
1273 having a problem with the <SMALL>SQL</SMALL> client because the
1274 backend is returning too much data, try it before starting the
1277 <H4><A name="4.19">4.19</A>) How do I tell what PostgreSQL version
1280 <P>From <I>psql</I>, type <CODE>SELECT version();</CODE></P>
1282 <H4><A name="4.20">4.20</A>) Why does my large-object operations
1283 get <I>"invalid large obj descriptor"</I>?</H4>
1285 <P>You need to put <CODE>BEGIN WORK</CODE> and <CODE>COMMIT</CODE>
1286 around any use of a large object handle, that is, surrounding
1287 <CODE>lo_open</CODE> ... <CODE>lo_close.</CODE></P>
1289 <P>Currently PostgreSQL enforces the rule by closing large object
1290 handles at transaction commit. So the first attempt to do anything
1291 with the handle will draw <I>invalid large obj descriptor</I>. So
1292 code that used to work (at least most of the time) will now
1293 generate that error message if you fail to use a transaction.</P>
1295 <P>If you are using a client interface like <SMALL>ODBC</SMALL> you
1296 may need to set <CODE>auto-commit off.</CODE></P>
1298 <H4><A name="4.21">4.21</A>) How do I create a column that will
1299 default to the current time?</H4>
1301 <P>Use <I>CURRENT_TIMESTAMP</I>:</P>
1303 <CODE>CREATE TABLE test (x int, modtime timestamp DEFAULT CURRENT_TIMESTAMP );
1307 <H4><A name="4.22">4.22</A>) Why are my subqueries using
1308 <CODE><SMALL>IN</SMALL></CODE> so slow?</H4>
1310 <P>In versions prior to 7.4, subqueries were joined to outer queries
1311 by sequentially scanning the result of the subquery for each row of
1312 the outer query. If the subquery returns only a few rows and the outer
1313 query returns many rows, <CODE><SMALL>IN</SMALL></CODE> is fastest. To
1314 speed up other queries, replace <CODE>IN</CODE> with
1315 <CODE>EXISTS</CODE>:</P>
1318 WHERE col IN (SELECT subcol FROM subtab);
1323 WHERE EXISTS (SELECT subcol FROM subtab WHERE subcol = col);
1326 For this to be fast, <CODE>subcol</CODE> should be an indexed column.
1327 <P>In version 7.4 and later, <CODE>IN</CODE> actually uses the same
1328 sophisticated join techniques as normal queries, and is prefered
1329 to using <CODE>EXISTS</CODE>.
1331 <H4><A name="4.23">4.23</A>) How do I perform an outer join?</H4>
1333 <P>PostgreSQL supports outer joins using the SQL standard syntax.
1334 Here are two examples:</P>
1337 FROM t1 LEFT OUTER JOIN t2 ON (t1.col = t2.col);
1342 FROM t1 LEFT OUTER JOIN t2 USING (col);
1345 <P>These identical queries join t1.col to t2.col, and also return
1346 any unjoined rows in t1 (those with no match in t2). A
1347 <SMALL>RIGHT</SMALL> join would add unjoined rows of t2. A
1348 <SMALL>FULL</SMALL> join would return the matched rows plus all
1349 unjoined rows from t1 and t2. The word <SMALL>OUTER</SMALL> is
1350 optional and is assumed in <SMALL>LEFT</SMALL>,
1351 <SMALL>RIGHT</SMALL>, and <SMALL>FULL</SMALL> joins. Ordinary joins
1352 are called <SMALL>INNER</SMALL> joins.</P>
1354 <P>In previous releases, outer joins can be simulated using
1355 <SMALL>UNION</SMALL> and <SMALL>NOT IN</SMALL>. For example, when
1356 joining <I>tab1</I> and <I>tab2</I>, the following query does an
1357 <I>outer</I> join of the two tables:<BR>
1361 SELECT tab1.col1, tab2.col2
1363 WHERE tab1.col1 = tab2.col1
1365 SELECT tab1.col1, NULL
1367 WHERE tab1.col1 NOT IN (SELECT tab2.col1 FROM tab2)
1371 <H4><A name="4.24">4.24</A>) How do I perform queries using
1372 multiple databases?</H4>
1374 <P>There is no way to query a database other than the current one.
1375 Because PostgreSQL loads database-specific system catalogs, it is
1376 uncertain how a cross-database query should even behave.</P>
1378 <P><I>contrib/dblink</I> allows cross-database queries using
1379 function calls. Of course, a client can make simultaneous
1380 connections to different databases and merge the results on the
1383 <H4><A name="4.25">4.25</A>) How do I return multiple rows or
1384 columns from a function?</H4>
1386 <P>In 7.3, you can easily return multiple rows or columns from a
1388 <a href="http://techdocs.postgresql.org/guides/SetReturningFunctions">
1389 http://techdocs.postgresql.org/guides/SetReturningFunctions</a>.
1391 <H4><A name="4.26">4.26</A>) Why can't I reliably create/drop
1392 temporary tables in PL/PgSQL functions?</H4>
1393 <P>PL/PgSQL caches function contents, and an unfortunate side effect
1394 is that if a PL/PgSQL function accesses a temporary table, and that
1395 table is later dropped and recreated, and the function called
1396 again, the function will fail because the cached function contents
1397 still point to the old temporary table. The solution is to use
1398 <SMALL>EXECUTE</SMALL> for temporary table access in PL/PgSQL. This
1399 will cause the query to be reparsed every time.</P>
1401 <H4><A name="4.27">4.27</A>) What replication options are available?
1403 <P>There are several master/slave replication options available.
1404 These allow only the master to make database changes and the slave
1405 can only do database reads. The bottom of <a
1406 href="http://gborg.PostgreSQL.org/genpage?replication_research">
1407 http://gborg.PostgreSQL.org/genpage?replication_research</a> lists
1408 them. A multi-master replication solution is being worked on at <a
1409 href="http://gborg.PostgreSQL.org/project/pgreplication/projdisplay.php">http://gborg.PostgreSQL.org/project/pgreplication/projdisplay.php</a>.</P>
1411 <H4><A name="4.28">4.28</A>) What encryption options are available?
1414 <LI><I>contrib/pgcrypto</I> contains many encryption functions for
1415 use in <SMALL>SQL</SMALL> queries.</LI>
1416 <LI>To encrypt transmission from the client to the server, the server
1417 must have the <I>ssl</I> option set to <I>true</I> in <I>postgresql.conf,
1418 </I> and an applicable <I>host</I> or <I>hostssl</I> record must exist in
1419 <I>pg_hba.conf</I>, and the client <I>sslmode</I> must not be
1420 <I>disable.</I> (Note that it is also possible to use a third-party
1421 encrypted transport, such as stunnel or ssh, rather than PostgreSQL's
1422 native SSL connections.)
1423 <LI>Database user passwords are automatically encrypted when stored in
1424 version 7.3. In previous versions, you must enable the option
1425 <I>PASSWORD_ENCRYPTION</I> in <I>postgresql.conf</I>.</LI>
1426 <LI>The server can run using an encrypted file system.</LI>
1431 <H2 align="center">Extending PostgreSQL</H2>
1433 <H4><A name="5.1">5.1</A>) I wrote a user-defined function. When I
1434 run it in <I>psql</I>, why does it dump core?</H4>
1436 <P>The problem could be a number of things. Try testing your
1437 user-defined function in a stand-alone test program first.</P>
1439 <H4><A name="5.2">5.2</A>) How can I contribute some nifty new
1440 types and functions to PostgreSQL?</H4>
1442 <P>Send your extensions to the <I>pgsql-hackers</I> mailing list,
1443 and they will eventually end up in the <I>contrib/</I>
1446 <H4><A name="5.3">5.3</A>) How do I write a C function to return a
1449 <P>In versions of PostgreSQL beginning with 7.3, table-returning
1450 functions are fully supported in C, PL/PgSQL, and SQL. See the
1451 Programmer's Guide for more information. An example of a
1452 table-returning function defined in C can be found in
1453 <I>contrib/tablefunc</I>.</P>
1455 <H4><A name="5.4">5.4</A>) I have changed a source file. Why does
1456 the recompile not see the change?</H4>
1458 <P>The <I>Makefiles</I> do not have the proper dependencies for
1459 include files. You have to do a <I>make clean</I> and then another
1460 <I>make</I>. If you are using <SMALL>GCC</SMALL> you can use the
1461 <I>--enable-depend</I> option of <I>configure</I> to have the
1462 compiler compute the dependencies automatically.</P>
projects / postgresql / blob