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15 <H1>Frequently Asked Questions (FAQ) for PostgreSQL</H1>
17 <P>Last updated: Thu Jul 11 12:35:53 EDT 2002</P>
19 <P>Current maintainer: Bruce Momjian (<A href=
20 "mailto:pgman@candle.pha.pa.us">pgman@candle.pha.pa.us</A>)<BR>
23 <P>The most recent version of this document can be viewed at <A
25 "http://www.Postgresql.org/docs/faq-english.html">http://www.PostgreSQL.org/docs/faq-english.html</A>.</P>
27 <P>Platform-specific questions are answered at <A href=
28 "http://www.PostgreSQL.org/users-lounge/docs/faq.html">http://www.PostgreSQL.org/users-lounge/docs/faq.html</A>.</P>
31 <H2 align="center">General Questions</H2>
32 <A href="#1.1">1.1</A>) What is PostgreSQL? How is it
34 <A href="#1.2">1.2</A>) What is the copyright on PostgreSQL?<BR>
35 <A href="#1.3">1.3</A>) What Unix platforms does PostgreSQL run
37 <A href="#1.4">1.4</A>) What non-Unix ports are available?<BR>
38 <A href="#1.5">1.5</A>) Where can I get PostgreSQL?<BR>
39 <A href="#1.6">1.6</A>) Where can I get support?<BR>
40 <A href="#1.7">1.7</A>) What is the latest release?<BR>
41 <A href="#1.8">1.8</A>) What documentation is available?<BR>
42 <A href="#1.9">1.9</A>) How do I find out about known bugs or
44 <A href="#1.10">1.10</A>) How can I learn <SMALL>SQL</SMALL>?<BR>
45 <A href="#1.11">1.11</A>) Is PostgreSQL Y2K compliant?<BR>
46 <A href="#1.12">1.12</A>) How do I join the development team?<BR>
47 <A href="#1.13">1.13</A>) How do I submit a bug report?<BR>
48 <A href="#1.14">1.14</A>) How does PostgreSQL compare to other
49 <SMALL>DBMS</SMALL>s?<BR>
50 <A href="#1.15">1.15</A>) How can I financially assist
54 <H2 align="center">User Client Questions</H2>
55 <A href="#2.1">2.1</A>) Are there <SMALL>ODBC</SMALL> drivers for
57 <A href="#2.2">2.2</A>) What tools are available for using
58 PostgreSQL with Web pages?<BR>
59 <A href="#2.3">2.3</A>) Does PostgreSQL have a graphical user
60 interface? A report generator? An embedded query language
62 <A href="#2.4">2.4</A>) What languages are available to
63 communicate with PostgreSQL?<BR>
66 <H2 align="center">Administrative Questions</H2>
67 <A href="#3.1">3.1</A>) How do I install PostgreSQL somewhere other
68 than <I>/usr/local/pgsql</I>?<BR>
69 <A href="#3.2">3.2</A>) When I start <I>postmaster</I>, I get a
70 <I>Bad System Call</I> or core dumped message. Why?<BR>
71 <A href="#3.3">3.3</A>) When I try to start <I>postmaster</I>, I
72 get <I>IpcMemoryCreate</I> errors. Why?<BR>
73 <A href="#3.4">3.4</A>) When I try to start <I>postmaster</I>, I
74 get <I>IpcSemaphoreCreate</I> errors. Why?<BR>
75 <A href="#3.5">3.5</A>) How do I control connections from other
77 <A href="#3.6">3.6</A>) How do I tune the database engine for
78 better performance?<BR>
79 <A href="#3.7">3.7</A>) What debugging features are available?<BR>
80 <A href="#3.8">3.8</A>) Why do I get <I>"Sorry, too many
81 clients"</I> when trying to connect?<BR>
82 <A href="#3.9">3.9</A>) What are the <I>pg_sorttempNNN.NN</I>
83 files in my database directory?<BR>
84 <A href="#3.10">3.10</A>) Why do I need to do a dump and restore
85 to upgrade PostgreSQL?<BR>
88 <H2 align="center">Operational Questions</H2>
89 <A href="#4.1">4.1</A>) What is the difference between binary
90 cursors and normal cursors?<BR>
91 <A href="#4.2">4.2</A>) How do I <SMALL>SELECT</SMALL> only the
92 first few rows of a query?<BR>
93 <A href="#4.3">4.3</A>) How do I get a list of tables or other
94 things I can see in <I>psql</I>?<BR>
95 <A href="#4.4">4.4</A>) How do you remove a column from a
97 <A href="#4.5">4.5</A>) What is the maximum size for a row, a
98 table, and a database?<BR>
99 <A href="#4.6">4.6</A>) How much database disk space is required
100 to store data from a typical text file?<BR>
101 <A href="#4.7">4.7</A>) How do I find out what tables, indexes,
102 databases, and users are defined?<BR>
103 <A href="#4.8">4.8</A>) My queries are slow or don't make use of
104 the indexes. Why?<BR>
105 <A href="#4.9">4.9</A>) How do I see how the query optimizer is
106 evaluating my query?<BR>
107 <A href="#4.10">4.10</A>) What is an R-tree index?<BR>
108 <A href="#4.11">4.11</A>) What is the Genetic Query Optimizer?<BR>
109 <A href="#4.12">4.12</A>) How do I perform regular expression
110 searches and case-insensitive regular expression searches? How do I
111 use an index for case-insensitive searches?<BR>
112 <A href="#4.13">4.13</A>) In a query, how do I detect if a field
113 is <SMALL>NULL</SMALL>?<BR>
114 <A href="#4.14">4.14</A>) What is the difference between the
115 various character types?<BR>
116 <A href="#4.15.1">4.15.1</A>) How do I create a
117 serial/auto-incrementing field?<BR>
118 <A href="#4.15.2">4.15.2</A>) How do I get the value of a
119 <SMALL>SERIAL</SMALL> insert?<BR>
120 <A href="#4.15.3">4.15.3</A>) Don't <I>currval()</I> and
121 <I>nextval()</I> lead to a race condition with other users?<BR>
122 <A href="#4.15.4">4.15.4</A>) Why aren't my sequence numbers
123 reused on transaction abort? Why are there gaps in the numbering of
124 my sequence/SERIAL column?<BR>
125 <A href="#4.16">4.16</A>) What is an <SMALL>OID</SMALL>? What is a
126 <SMALL>TID</SMALL>?<BR>
127 <A href="#4.17">4.17</A>) What is the meaning of some of the terms
128 used in PostgreSQL?<BR>
129 <A href="#4.18">4.18</A>) Why do I get the error <I>"ERROR: Memory
130 exhausted in AllocSetAlloc()"</I>?<BR>
131 <A href="#4.19">4.19</A>) How do I tell what PostgreSQL version I
133 <A href="#4.20">4.20</A>) Why does my large-object operations get
134 <I>"invalid large obj descriptor"</I>?<BR>
135 <A href="#4.21">4.21</A>) How do I create a column that will
136 default to the current time?<BR>
137 <A href="#4.22">4.22</A>) Why are my subqueries using
138 <CODE><SMALL>IN</SMALL></CODE> so slow?<BR>
139 <A href="#4.23">4.23</A>) How do I perform an outer join?<BR>
140 <A href="#4.24">4.24</A>) How do I perform queries using multiple
142 <A href="#4.25">4.25</A>) How do I return multiple rows or columns
144 <A href="#4.26">4.26</A>) Why can't I reliably create/drop
145 temporary tables in PL/PgSQL functions?<BR>
148 <H2 align="center">Extending PostgreSQL</H2>
149 <A href="#5.1">5.1</A>) I wrote a user-defined function. When I run
150 it in <I>psql</I>, why does it dump core?<BR>
151 <A href="#5.2">5.2</A>) How can I contribute some nifty new types
152 and functions to PostgreSQL?<BR>
153 <A href="#5.3">5.3</A>) How do I write a C function to return a
155 <A href="#5.4">5.4</A>) I have changed a source file. Why does the
156 recompile not see the change?<BR>
160 <H2 align="center">General Questions</H2>
162 <H4><A name="1.1">1.1</A>) What is PostgreSQL?</H4>
164 <P>PostgreSQL is pronounced <I>Post-Gres-Q-L</I>.</P>
166 <P>PostgreSQL is an enhancement of the POSTGRES database management
167 system, a next-generation <SMALL>DBMS</SMALL> research prototype.
168 While PostgreSQL retains the powerful data model and rich data
169 types of POSTGRES, it replaces the PostQuel query language with an
170 extended subset of <SMALL>SQL</SMALL>. PostgreSQL is free and the
171 complete source is available.</P>
173 <P>PostgreSQL development is performed by a team of Internet
174 developers who all subscribe to the PostgreSQL development mailing
175 list. The current coordinator is Marc G. Fournier (<A href=
176 "mailto:scrappy@PostgreSQL.org">scrappy@PostgreSQL.org</A>). (See
177 below on how to join). This team is now responsible for all
178 development of PostgreSQL.</P>
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 binaries to run on MS Windows platforms. In
241 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>win31.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 <A href=
252 "http://www.postgresql.org/docs/faq-mswin.html">MS Windows FAQ</A>
253 on our web site. We have no plan to do a native port to any
254 Microsoft platform.</P>
256 <H4><A name="1.5">1.5</A>) Where can I get PostgreSQL?</H4>
258 <P>The primary anonymous ftp site for PostgreSQL is <A href=
259 "ftp://ftp.PostgreSQL.org/pub">ftp://ftp.PostgreSQL.org/pub</A>.
260 For mirror sites, see our main web site.</P>
262 <H4><A name="1.6">1.6</A>) Where can I get support?</H4>
264 <P>The main mailing list is: <A href=
265 "mailto:pgsql-general@PostgreSQL.org">pgsql-general@PostgreSQL.org</A>.
266 It is available for discussion of matters pertaining to PostgreSQL.
267 To subscribe, send mail with the following lines in the body (not
268 the subject line):</P>
275 "mailto:pgsql-general-request@PostgreSQL.org">pgsql-general-request@PostgreSQL.org</A>.</P>
277 <P>There is also a digest list available. To subscribe to this
278 list, send email to: <A href=
279 "mailto:pgsql-general-digest-request@PostgreSQL.org">pgsql-general-digest-request@PostgreSQL.org</A>
286 Digests are sent out to members of this list whenever the main list
287 has received around 30k of messages.
289 <P>The bugs mailing list is available. To subscribe to this list,
290 send email to <A href=
291 "mailto:pgsql-bugs-request@PostgreSQL.org">pgsql-bugs-request@PostgreSQL.org</A>
298 There is also a developers discussion mailing list available. To
299 subscribe to this list, send email to <A href=
300 "mailto:pgsql-hackers-request@PostgreSQL.org">pgsql-hackers-request@PostgreSQL.org</A>
307 <P>Additional mailing lists and information about PostgreSQL can be
308 found via the PostgreSQL WWW home page at:</P>
311 <A href="http://www.PostgreSQL.org">http://www.PostgreSQL.org</A>
314 <P>There is also an IRC channel on EFNet, channel
315 <I>#PostgreSQL</I>. I use the Unix command <CODE>irc -c
316 '#PostgreSQL' "$USER" irc.phoenix.net.</CODE></P>
318 <P>A list of commercial support companies is available at <A href=
319 "http://www.postgresql.org/users-lounge/commercial-support.html">http://www.postgresql.org/users-lounge/commercial-support.html</A>.</P>
321 <H4><A name="1.7">1.7</A>) What is the latest release?</H4>
323 <P>The latest release of PostgreSQL is version 7.2.1.</P>
325 <P>We plan to have major releases every four months.</P>
327 <H4><A name="1.8">1.8</A>) What documentation is available?</H4>
329 <P>Several manuals, manual pages, and some small test examples are
330 included in the distribution. See the <I>/doc</I> directory. You
331 can also browse the manual online at <A href=
332 "http://www.PostgreSQL.org/users-lounge/docs/">http://www.PostgreSQL.org/users-lounge/docs/</A>.</P>
334 <P>There are two PostgreSQL books available online at <A href=
335 "http://www.PostgreSQL.org/docs/awbook.html">http://www.PostgreSQL.org/docs/awbook.html</A>
337 "http://www.commandprompt.com/ppbook/">http://www.commandprompt.com/ppbook/</A>.
338 There is a list of PostgreSQL books available for purchase at <A
340 "http://www.postgresql.org/books/">http://www.postgresql.org/books/</A>.
341 There is also a collection of PostgreSQL technical articles at <A
343 "http://techdocs.postgresql.org/">http://techdocs.postgresql.org/</A>.</P>
345 <P><I>psql</I> has some nice \d commands to show information about
346 types, operators, functions, aggregates, etc.</P>
348 <P>Our web site contains even more documentation.</P>
350 <H4><A name="1.9">1.9</A>) How do I find out about known bugs or
351 missing features?</H4>
353 <P>PostgreSQL supports an extended subset of <SMALL>SQL</SMALL>-92.
354 See our <A href="http://developer.PostgreSQL.org/todo.php">TODO</A>
355 list for known bugs, missing features, and future plans.</P>
357 <H4><A name="1.10">1.10</A>) How can I learn
358 <SMALL>SQL</SMALL>?</H4>
360 <P>The PostgreSQL book at <A href=
361 "http://www.PostgreSQL.org/docs/awbook.html">http://www.PostgreSQL.org/docs/awbook.html</A>
362 teaches <SMALL>SQL</SMALL>. There is another PostgreSQL book at <A
364 "http://www.commandprompt.com/ppbook/">http://www.commandprompt.com/ppbook.</A>
365 There is a nice tutorial at <A href=
366 "http://www.intermedia.net/support/sql/sqltut.shtm">http://www.intermedia.net/support/sql/sqltut.shtm,</A>
368 "http://ourworld.compuserve.com/homepages/graeme_birchall/HTM_COOK.HTM">
369 http://ourworld.compuserve.com/homepages/graeme_birchall/HTM_COOK.HTM,</A>
371 "http://sqlcourse.com/">http://sqlcourse.com.</A></P>
373 <P>Another one is "Teach Yourself SQL in 21 Days, Second Edition"
375 "http://members.tripod.com/er4ebus/sql/index.htm">http://members.tripod.com/er4ebus/sql/index.htm</A></P>
377 <P>Many of our users like <I>The Practical SQL Handbook</I>,
378 Bowman, Judith S., et al., Addison-Wesley. Others like <I>The
379 Complete Reference SQL</I>, Groff et al., McGraw-Hill.</P>
381 <H4><A name="1.11">1.11</A>) Is PostgreSQL Y2K compliant?</H4>
383 <P>Yes, we easily handle dates past the year 2000 AD, and before
386 <H4><A name="1.12">1.12</A>) How do I join the development
389 <P>First, download the latest source and read the PostgreSQL
390 Developers documentation on our web site, or in the distribution.
391 Second, subscribe to the <I>pgsql-hackers</I> and
392 <I>pgsql-patches</I> mailing lists. Third, submit high quality
393 patches to pgsql-patches.</P>
395 <P>There are about a dozen people who have commit privileges to the
396 PostgreSQL <SMALL>CVS</SMALL> archive. They each have submitted so
397 many high-quality patches that it was impossible for the existing
398 committers to keep up, and we had confidence that patches they
399 committed were of high quality.</P>
401 <H4><A name="1.13">1.13</A>) How do I submit a bug report?</H4>
403 <P>Please visit the <A href=
404 "http://www.postgresql.org/bugs/bugs.php">PostgreSQL BugTool</A>
405 page, which gives guidelines and directions on how to submit a
408 <P>Also check out our ftp site <A href=
409 "ftp://ftp.PostgreSQL.org/pub">ftp://ftp.PostgreSQL.org/pub</A> to
410 see if there is a more recent PostgreSQL version or patches.</P>
412 <H4><A name="1.14">1.14</A>) How does PostgreSQL compare to other
413 <SMALL>DBMS</SMALL>s?</H4>
415 <P>There are several ways of measuring software: features,
416 performance, reliability, support, and price.</P>
419 <DT><B>Features</B></DT>
421 <DD>PostgreSQL has most features present in large commercial
422 <SMALL>DBMS</SMALL>s, like transactions, subselects, triggers,
423 views, foreign key referential integrity, and sophisticated
424 locking. We have some features they do not have, like
425 user-defined types, inheritance, rules, and multi-version
426 concurrency control to reduce lock contention.<BR>
430 <DT><B>Performance</B></DT>
432 <DD>PostgreSQL has performance similar to other commercial and
433 open source databases. it is faster for some things, slower for
434 others. In comparison to MySQL or leaner database systems, we are
435 slower on inserts/updates because of transaction overhead. Of
436 course, MySQL does not have any of the features mentioned in the
437 <I>Features</I> section above. We are built for reliability and
438 features, though we continue to improve performance in every
439 release. There is an interesting Web page comparing PostgreSQL to
440 MySQL at <A href= "http://openacs.org/why-not-mysql.html">
442 http://openacs.org/why-not-mysql.html</A><BR>
447 <DT><B>Reliability</B></DT>
449 <DD>We realize that a <SMALL>DBMS</SMALL> must be reliable, or it
450 is worthless. We strive to release well-tested, stable code that
451 has a minimum of bugs. Each release has at least one month of
452 beta testing, and our release history shows that we can provide
453 stable, solid releases that are ready for production use. We
454 believe we compare favorably to other database software in this
459 <DT><B>Support</B></DT>
461 <DD>Our mailing list provides a large group of developers and
462 users to help resolve any problems encountered. While we can not
463 guarantee a fix, commercial <SMALL>DBMS</SMALL>s do not always
464 supply a fix either. Direct access to developers, the user
465 community, manuals, and the source code often make PostgreSQL
466 support superior to other <SMALL>DBMS</SMALL>s. There is
467 commercial per-incident support available for those who need it.
468 (See <A href="#1.6">support FAQ item</A>.)<BR>
472 <DT><B>Price</B></DT>
474 <DD>We are free for all use, both commercial and non-commercial.
475 You can add our code to your product with no limitations, except
476 those outlined in our BSD-style license stated above.<BR>
481 <H4><A name="1.15">1.15</A>) How can I financially assist
484 <P>PostgreSQL has had a first-class infrastructure since we started
485 six years ago. This is all thanks to Marc Fournier, who has created
486 and managed this infrastructure over the years.</P>
488 <P>Quality infrastructure is very important to an open-source
489 project. It prevents disruptions that can greatly delay forward
490 movement of the project.</P>
492 <P>Of course, this infrastructure is not cheap. There are a variety
493 of monthly and one-time expenses that are required to keep it
494 going. If you or your company has money it can donate to help fund
495 this effort, please go to <A href=
496 "http://www.pgsql.com/pg_goodies">http://www.pgsql.com/pg_goodies</A>
497 and make a donation.</P>
499 <P>Although the web page mentions PostgreSQL, Inc, the
500 "contributions" item is solely to support the PostgreSQL project
501 and does not fund any specific company. If you prefer, you can also
502 send a check to the contact address.</P>
505 <H2 align="center">User Client Questions</H2>
507 <H4><A name="2.1">2.1</A>) Are there <SMALL>ODBC</SMALL> drivers
510 <P>There are two <SMALL>ODBC</SMALL> drivers available, PsqlODBC
511 and OpenLink <SMALL>ODBC</SMALL>.</P>
513 <P>PsqlODBC is included in the distribution. More information about
514 it can be gotten from <A href=
515 "ftp://ftp.PostgreSQL.org/pub/odbc/">ftp://ftp.PostgreSQL.org/pub/odbc/</A>.</P>
517 <P>OpenLink <SMALL>ODBC</SMALL> can be gotten from <A href=
518 "http://www.openlinksw.com/">http://www.openlinksw.com</A>. It
519 works with their standard <SMALL>ODBC</SMALL> client software so
520 you'll have PostgreSQL <SMALL>ODBC</SMALL> available on every
521 client platform they support (Win, Mac, Unix, VMS).</P>
523 <P>They will probably be selling this product to people who need
524 commercial-quality support, but a freeware version will always be
525 available. Please send questions to <A href=
526 "mailto:postgres95@openlink.co.uk">postgres95@openlink.co.uk</A>.</P>
528 <P>See also the <A href=
529 "http://www.postgresql.org/devel-corner/docs/programmer/odbc.html">ODBC
530 chapter of the Programmer's Guide</A>.</P>
532 <H4><A name="2.2">2.2</A>) What tools are available for using
533 PostgreSQL with Web pages?</H4>
535 <P>A nice introduction to Database-backed Web pages can be seen at:
536 <A href="http://www.webreview.com">http://www.webreview.com</A></P>
538 <P>There is also one at <A href=
539 "http://www.phone.net/home/mwm/hotlist/">http://www.phone.net/home/mwm/hotlist/.</A></P>
541 <P>For Web integration, PHP is an excellent interface. It is at <A
542 href="http://www.php.net">http://www.php.net</A>.</P>
544 <P>For complex cases, many use the Perl interface and CGI.pm.</P>
546 <H4><A name="2.3">2.3</A>) Does PostgreSQL have a graphical user
547 interface? A report generator? An embedded query language
550 <P>We have a nice graphical user interface called <I>pgaccess</I>,
551 which is shipped as part of the distribution. <I>pgaccess</I> also
552 has a report generator. The Web page is <A href=
553 "http://www.flex.ro/pgaccess">http://www.flex.ro/pgaccess</A></P>
555 <P>We also include <I>ecpg</I>, which is an embedded SQL query
556 language interface for C.</P>
558 <H4><A name="2.4">2.4</A>) What languages are available to
559 communicate with PostgreSQL?</H4>
566 <LI>C++ (libpq++)</LI>
568 <LI>Embedded C (ecpg)</LI>
572 <LI>Perl (perl5)</LI>
576 <LI>Python (PyGreSQL)</LI>
578 <LI>TCL (libpgtcl)</LI>
580 <LI>C Easy API (libpgeasy)</LI>
582 <LI>Embedded <SMALL>HTML</SMALL> (<A href=
583 "http://www.php.net">PHP from http://www.php.net</A>)</LI>
587 <H2 align="center">Administrative Questions</H2>
589 <H4><A name="3.1">3.1</A>) How do I install PostgreSQL somewhere
590 other than <I>/usr/local/pgsql</I>?</H4>
592 <P>Specify the <I>--prefix</I> option when running
593 <I>configure</I>.</P>
595 <H4><A name="3.2">3.2</A>) When I start <I>postmaster</I>, I get a
596 <I>Bad System Call</I> or core dumped message. Why?</H4>
598 <P>It could be a variety of problems, but first check to see that
599 you have System V extensions installed in your kernel. PostgreSQL
600 requires kernel support for shared memory and semaphores.</P>
602 <H4><A name="3.3">3.3</A>) When I try to start <I>postmaster</I>, I
603 get <I>IpcMemoryCreate</I> errors. Why?</H4>
605 <P>You either do not have shared memory configured properly in your
606 kernel or you need to enlarge the shared memory available in the
607 kernel. The exact amount you need depends on your architecture and
608 how many buffers and backend processes you configure for
609 <I>postmaster</I>. For most systems, with default numbers of
610 buffers and processes, you need a minimum of ~1 MB. See the <A
612 "http://www.postgresql.org/idocs/index.php?kernel-resources.html">PostgreSQL
613 Administrator's Guide</A> for more detailed information about
614 shared memory and semaphores.</P>
616 <H4><A name="3.4">3.4</A>) When I try to start <I>postmaster</I>, I
617 get <I>IpcSemaphoreCreate</I> errors. Why?</H4>
619 <P>If the error message is <I>IpcSemaphoreCreate: semget failed (No
620 space left on device)</I> then your kernel is not configured with
621 enough semaphores. Postgres needs one semaphore per potential
622 backend process. A temporary solution is to start <I>postmaster</I>
623 with a smaller limit on the number of backend processes. Use
624 <I>-N</I> with a parameter less than the default of 32. A more
625 permanent solution is to increase your kernel's
626 <SMALL>SEMMNS</SMALL> and <SMALL>SEMMNI</SMALL> parameters.</P>
628 <P>Inoperative semaphores can also cause crashes during heavy
631 <P>If the error message is something else, you might not have
632 semaphore support configured in your kernel at all. See the
633 PostgreSQL Administrator's Guide for more detailed information
634 about shared memory and semaphores.</P>
636 <H4><A name="3.5">3.5</A>) How do I control connections from other
639 <P>By default, PostgreSQL only allows connections from the local
640 machine using Unix domain sockets. Other machines will not be able
641 to connect unless you add the <I>-i</I> flag to <I>postmaster</I>,
642 <B>and</B> enable host-based authentication by modifying the file
643 <I>$PGDATA/pg_hba.conf</I> accordingly. This will allow TCP/IP
646 <H4><A name="3.6">3.6</A>) How do I tune the database engine for
647 better performance?</H4>
649 <P>Certainly, indexes can speed up queries. The
650 <SMALL>EXPLAIN</SMALL> command allows you to see how PostgreSQL is
651 interpreting your query, and which indexes are being used.</P>
653 <P>If you are doing many <SMALL>INSERTs</SMALL>, consider doing
654 them in a large batch using the <SMALL>COPY</SMALL> command. This
655 is much faster than individual <SMALL>INSERTS.</SMALL> Second,
656 statements not in a <SMALL>BEGIN WORK/COMMIT</SMALL> transaction
657 block are considered to be in their own transaction. Consider
658 performing several statements in a single transaction block. This
659 reduces the transaction overhead. Also, consider dropping and
660 recreating indexes when making large data changes.</P>
662 <P>There are several tuning options. You can disable <I>fsync()</I>
663 by starting <I>postmaster</I> with a <I>-o -F</I> option. This will
664 prevent <I>fsync()</I>s from flushing to disk after every
667 <P>You can also use the <I>postmaster</I> <I>-B</I> option to
668 increase the number of shared memory buffers used by the backend
669 processes. If you make this parameter too high, the
670 <I>postmaster</I> may not start because you have exceeded your
671 kernel's limit on shared memory space. Each buffer is 8K and the
672 default is 64 buffers.</P>
674 <P>You can also use the backend <I>-S</I> option to increase the
675 maximum amount of memory used by the backend process for temporary
676 sorts. The <I>-S</I> value is measured in kilobytes, and the
677 default is 512 (i.e. 512K).</P>
679 <P>You can also use the <SMALL>CLUSTER</SMALL> command to group
680 data in tables to match an index. See the <SMALL>CLUSTER</SMALL>
681 manual page for more details.</P>
683 <H4><A name="3.7">3.7</A>) What debugging features are
686 <P>PostgreSQL has several features that report status information
687 that can be valuable for debugging purposes.</P>
689 <P>First, by running <I>configure</I> with the --enable-cassert
690 option, many <I>assert()</I>s monitor the progress of the backend
691 and halt the program when something unexpected occurs.</P>
693 <P>Both <I>postmaster</I> and <I>postgres</I> have several debug
694 options available. First, whenever you start <I>postmaster</I>,
695 make sure you send the standard output and error to a log file,
699 ./bin/postmaster >server.log 2>&1 &
702 <P>This will put a server.log file in the top-level PostgreSQL
703 directory. This file contains useful information about problems or
704 errors encountered by the server. <I>Postmaster</I> has a <I>-d</I>
705 option that allows even more detailed information to be reported.
706 The <I>-d</I> option takes a number that specifies the debug level.
707 Be warned that high debug level values generate large log
710 <P>If <I>postmaster</I> is not running, you can actually run the
711 <I>postgres</I> backend from the command line, and type your
712 <SMALL>SQL</SMALL> statement directly. This is recommended
713 <B>only</B> for debugging purposes. Note that a newline terminates
714 the query, not a semicolon. If you have compiled with debugging
715 symbols, you can use a debugger to see what is happening. Because
716 the backend was not started from <I>postmaster</I>, it is not
717 running in an identical environment and locking/backend interaction
718 problems may not be duplicated.</P>
720 <P>If <I>postmaster</I> is running, start <I>psql</I> in one
721 window, then find the <SMALL>PID</SMALL> of the <I>postgres</I>
722 process used by <I>psql</I>. Use a debugger to attach to the
723 <I>postgres</I> <SMALL>PID.</SMALL> You can set breakpoints in the
724 debugger and issue queries from <I>psql</I>. If you are debugging
725 <I>postgres</I> startup, you can set PGOPTIONS="-W n", then start
726 <I>psql</I>. This will cause startup to delay for <I>n</I> seconds
727 so you can attach to the process with the debugger, set any
728 breakpoints, and continue through the startup sequence.</P>
730 <P>The <I>postgres</I> program has <I>-s, -A</I>, and <I>-t</I>
731 options that can be very useful for debugging and performance
734 <P>You can also compile with profiling to see what functions are
735 taking execution time. The backend profile files will be deposited
736 in the <I>pgsql/data/base/dbname</I> directory. The client profile
737 file will be put in the client's current directory. Linux requires
738 a compile with <I>-DLINUX_PROFILE</I> for proper profiling.</P>
740 <H4><A name="3.8">3.8</A>) Why do I get <I>"Sorry, too many
741 clients"</I> when trying to connect?</H4>
743 <P>You need to increase <I>postmaster</I>'s limit on how many
744 concurrent backend processes it can start.</P>
746 <P>The default limit is 32 processes. You can increase it by
747 restarting <I>postmaster</I> with a suitable <I>-N</I> value or
748 modifying <I>postgresql.conf</I>.</P>
750 <P>Note that if you make <I>-N</I> larger than 32, you must also
751 increase <I>-B</I> beyond its default of 64; <I>-B</I> must be at
752 least twice <I>-N</I>, and probably should be more than that for
753 best performance. For large numbers of backend processes, you are
754 also likely to find that you need to increase various Unix kernel
755 configuration parameters. Things to check include the maximum size
756 of shared memory blocks, <SMALL>SHMMAX;</SMALL> the maximum number
757 of semaphores, <SMALL>SEMMNS</SMALL> and <SMALL>SEMMNI;</SMALL> the
758 maximum number of processes, <SMALL>NPROC;</SMALL> the maximum
759 number of processes per user, <SMALL>MAXUPRC;</SMALL> and the
760 maximum number of open files, <SMALL>NFILE</SMALL> and
761 <SMALL>NINODE.</SMALL> The reason that PostgreSQL has a limit on
762 the number of allowed backend processes is so your system won't run
763 out of resources.</P>
765 <P>In PostgreSQL versions prior to 6.5, the maximum number of
766 backends was 64, and changing it required a rebuild after altering
767 the MaxBackendId constant in
768 <I>include/storage/sinvaladt.h</I>.</P>
770 <H4><A name="3.9">3.9</A>) What are the <I>pg_sorttempNNN.NN</I>
771 files in my database directory?</H4>
773 <P>They are temporary files generated by the query executor. For
774 example, if a sort needs to be done to satisfy an <SMALL>ORDER
775 BY,</SMALL> and the sort requires more space than the backend's
776 <I>-S</I> parameter allows, then temporary files are created to
777 hold the extra data.</P>
779 <P>The temporary files should be deleted automatically, but might
780 not if a backend crashes during a sort. If you have no backends
781 running at the time, it is safe to delete the pg_tempNNN.NN
784 <H4><A name="3.10">3.10</A>) Why do I need to do a dump and restore
785 to upgrade PostgreSQL?</H4>
787 <P>The PostgreSQL team tries very heard to maintain compatability across
788 minor releases. So upgrading from 7.2 to 7.2.1 does not require a dump a
789 restore. However, new features are continuously being adding and
790 sometimes this requires new fields to be added to system tables.
792 <P>These changes may be across many tables and so maintaining backward
793 compatability would be quite difficult. Thus, restoring from a dump is
794 required to make everything work.
796 <P>Note that the actual on-disk file format does not change very often,
797 a feature the pg_upgrade script uses quite successfully. There the dump
798 is used create the necessary information in the system tables. The data
799 files are then just copied across. This method is not as guarenteed as
800 the dump/restore method but when it works it can make upgrades very
805 <H2 align="center">Operational Questions</H2>
807 <H4><A name="4.1">4.1</A>) What is the difference between binary
808 cursors and normal cursors?</H4>
810 <P>See the <SMALL>DECLARE</SMALL> manual page for a
813 <H4><A name="4.2">4.2</A>) How do I <SMALL>SELECT</SMALL> only the
814 first few rows of a query?</H4>
816 <P>See the <SMALL>FETCH</SMALL> manual page, or use
817 <SMALL>SELECT</SMALL> ... <SMALL>LIMIT</SMALL>....</P>
819 <P>The entire query may have to be evaluated, even if you only want
820 the first few rows. Consider a query that has an <SMALL>ORDER
821 BY.</SMALL> If there is an index that matches the <SMALL>ORDER
822 BY</SMALL>, PostgreSQL may be able to evaluate only the first few
823 records requested, or the entire query may have to be evaluated
824 until the desired rows have been generated.</P>
826 <H4><A name="4.3">4.3</A>) How do I get a list of tables or other
827 things I can see in <I>psql</I>?</H4>
829 <P>You can read the source code for <I>psql</I> in file
830 <I>pgsql/src/bin/psql/describe.c</I>. It contains
831 <SMALL>SQL</SMALL> commands that generate the output for psql's
832 backslash commands. You can also start <I>psql</I> with the
833 <I>-E</I> option so it will print out the queries it uses to
834 execute the commands you give.</P>
836 <H4><A name="4.4">4.4</A>) How do you remove a column from a
839 <P>We do not support <SMALL>ALTER TABLE DROP COLUMN,</SMALL> but do
843 LOCK TABLE old_table;
844 SELECT ... -- select all columns but the one you want to remove
847 DROP TABLE old_table;
848 ALTER TABLE new_table RENAME TO old_table;
852 <H4><A name="4.5">4.5</A>) What is the maximum size for a row, a
853 table, and a database?</H4>
855 <P>These are the limits:</P>
857 Maximum size for a database? unlimited (500 GB databases exist)
858 Maximum size for a table? 16 TB
859 Maximum size for a row? 1.6TB
860 Maximum size for a field? 1 GB
861 Maximum number of rows in a table? unlimited
862 Maximum number of columns in a table? 250-1600 depending on column types
863 Maximum number of indexes on a table? unlimited
866 Of course, these are not actually unlimited, but limited to
867 available disk space and memory/swap space. Performance may suffer
868 when these values get unusually large.
870 <P>The maximum table size of 16 TB does not require large file
871 support from the operating system. Large tables are stored as
872 multiple 1 GB files so file system size limits are not
875 <P>The maximum table size and maximum number of columns can be
876 increased if the default block size is increased to 32k.</P>
878 <H4><A name="4.6">4.6</A>) How much database disk space is required
879 to store data from a typical text file?</H4>
881 <P>A PostgreSQL database may require up to five times the disk
882 space to store data from a text file.</P>
884 <P>As an example, consider a file of 100,000 lines with an integer
885 and text description on each line. Suppose the text string
886 avergages twenty bytes in length. The flat file would be 2.8 MB.
887 The size of the PostgreSQL database file containing this data can
888 be estimated as 6.4 MB:</P>
890 36 bytes: each row header (approximate)
891 24 bytes: one int field and one text filed
892 + 4 bytes: pointer on page to tuple
893 ----------------------------------------
896 The data page size in PostgreSQL is 8192 bytes (8 KB), so:
899 ------------------- = 128 rows per database page (rounded down)
903 -------------------- = 782 database pages (rounded up)
906 782 database pages * 8192 bytes per page = 6,406,144 bytes (6.4 MB)
909 <P>Indexes do not require as much overhead, but do contain the data
910 that is being indexed, so they can be large also.</P>
912 <P><SMALL>NULL</SMALL>s are stored in bitmaps, so they
913 use very little space.</P>
915 <H4><A name="4.7">4.7</A>) How do I find out what tables, indexes,
916 databases, and users are defined?</H4>
918 <P><I>psql</I> has a variety of backslash commands to show such
919 information. Use \? to see them. There are also system tables
920 beginning with <I>pg_</I> that describe these too. Also, <I>psql
921 -l</I> will list all databases.</P>
923 <P>Also try the file <I>pgsql/src/tutorial/syscat.source</I>. It
924 illustrates many of the <SMALL>SELECT</SMALL>s needed to get
925 information from the database system tables.</P>
927 <H4><A name="4.8">4.8</A>) My queries are slow or don't make use of
928 the indexes. Why?</H4>
929 Indexes are not automatically used by every query. Indexes are only
930 used if the table is larger than a minimum size, and the query
931 selects only a small percentage of the rows in the table. This is
932 because the random disk access caused by an index scan can be
933 slower than a straight read through the table, or sequential scan.
935 <P>To determine if an index should be used, PostgreSQL must have
936 statistics about the table. These statistics are collected using
937 <SMALL>VACUUM ANALYZE</SMALL>, or simply <SMALL>ANALYZE</SMALL>.
938 Using statistics, the optimizer knows how many rows are in the
939 table, and can better determine if indexes should be used.
940 Statistics are also valuable in determining optimal join order and
941 join methods. Statistics collection should be performed
942 periodically as the contents of the table change.</P>
944 <P>Indexes are normally not used for <SMALL>ORDER BY</SMALL> or to
945 perform joins. A sequential scan followed by an explicit sort is
946 usually faster than an index scan of a large table.</P>
947 However, <SMALL>LIMIT</SMALL> combined with <SMALL>ORDER BY</SMALL>
948 often will use an index because only a small portion of the table
949 is returned. In fact, though MAX() and MIN() don't use indexes,
950 it is possible to retrieve such values using an index with ORDER BY
955 ORDER BY col [ DESC ]
959 <P>When using wild-card operators such as <SMALL>LIKE</SMALL> or
960 <I>~</I>, indexes can only be used in certain circumstances:
962 <LI>The beginning of the search string must be anchored to the start
963 of the string, i.e.:</LI>
965 <LI><SMALL>LIKE</SMALL> patterns must not start with <I>%.</I></LI>
966 <LI><I>~</I> (regular expression) patterns must start with
969 <LI>The search string can not start with a character class,
971 <LI>Case-insensitive searches like <SMALL>ILIKE</SMALL> and
972 <I>~*</I> can not be used. Instead, use functional
973 indexes, which are described later in this FAQ.</LI>
974 <LI>The default <I>C</I> local must be used during
979 <H4><A name="4.9">4.9</A>) How do I see how the query optimizer is
980 evaluating my query?</H4>
982 <P>See the <SMALL>EXPLAIN</SMALL> manual page.</P>
984 <H4><A name="4.10">4.10</A>) What is an R-tree index?</H4>
986 <P>An R-tree index is used for indexing spatial data. A hash index
987 can't handle range searches. A B-tree index only handles range
988 searches in a single dimension. R-trees can handle
989 multi-dimensional data. For example, if an R-tree index can be
990 built on an attribute of type <I>point</I>, the system can more
991 efficiently answer queries such as "select all points within a
992 bounding rectangle."</P>
994 <P>The canonical paper that describes the original R-tree design
997 <P>Guttman, A. "R-trees: A Dynamic Index Structure for Spatial
998 Searching." Proceedings of the 1984 ACM SIGMOD Int'l Conf on Mgmt
1001 <P>You can also find this paper in Stonebraker's "Readings in
1002 Database Systems".</P>
1004 <P>Built-in R-trees can handle polygons and boxes. In theory,
1005 R-trees can be extended to handle higher number of dimensions. In
1006 practice, extending R-trees requires a bit of work and we don't
1007 currently have any documentation on how to do it.</P>
1009 <H4><A name="4.11">4.11</A>) What is the Genetic Query
1012 <P>The <SMALL>GEQO</SMALL> module speeds query optimization when
1013 joining many tables by means of a Genetic Algorithm (GA). It allows
1014 the handling of large join queries through nonexhaustive
1017 <H4><A name="4.12">4.12</A>) How do I perform regular expression
1018 searches and case-insensitive regular expression searches? How do I
1019 use an index for case-insensitive searches?</H4>
1021 <P>The <I>~</I> operator does regular expression matching, and
1022 <I>~*</I> does case-insensitive regular expression matching. The
1023 case-insensitive variant of <SMALL>LIKE</SMALL> is called
1024 <SMALL>ILIKE</SMALL> in PostgreSQL 7.1 and later.</P>
1026 <P>Case-insensitive equality comparisons are normally expressed
1031 WHERE lower(col) = 'abc'
1034 This will not use an standard index. However, if you create a
1035 functional index, it will be used:
1037 CREATE INDEX tabindex on tab (lower(col));
1040 <H4><A name="4.13">4.13</A>) In a query, how do I detect if a field
1041 is <SMALL>NULL</SMALL>?</H4>
1043 <P>You test the column with <SMALL>IS NULL</SMALL> and <SMALL>IS
1044 NOT NULL</SMALL>.</P>
1046 <H4><A name="4.14">4.14</A>) What is the difference between the
1047 various character types?</H4>
1049 Type Internal Name Notes
1050 --------------------------------------------------
1051 "char" char 1 character
1052 CHAR(#) bpchar blank padded to the specified fixed length
1053 VARCHAR(#) varchar size specifies maximum length, no padding
1054 TEXT text no specific upper limit on length
1055 BYTEA bytea variable-length byte array (null-byte safe)
1058 <P>You will see the internal name when examining system catalogs
1059 and in some error messages.</P>
1061 <P>The last four types above are "varlena" types (i.e., the first
1062 four bytes on disk are the length, followed by the data). Thus the
1063 actual space used is slightly greater than the declared size.
1064 However, these data types are also subject to compression or being
1065 stored out-of-line by <SMALL>TOAST</SMALL>, so the space on disk
1066 might also be less than expected.</P>
1068 <P><SMALL>CHAR()</SMALL> is best when storing strings that are
1069 usually the same length. <SMALL>VARCHAR()</SMALL> is best when
1070 storing variable-length strings but it limits how long a string can
1071 be. <SMALL>TEXT</SMALL> is for strings of unlimited length, maximum
1072 1 gigabyte. <SMALL>BYTEA</SMALL> is for storing binary data,
1073 particularly values that include <SMALL>NULL</SMALL> bytes.</P>
1075 <H4><A name="4.15.1">4.15.1</A>) How do I create a
1076 serial/auto-incrementing field?</H4>
1078 <P>PostgreSQL supports a <SMALL>SERIAL</SMALL> data type. It
1079 auto-creates a sequence and index on the column. For example,
1082 CREATE TABLE person (
1088 is automatically translated into this:
1090 CREATE SEQUENCE person_id_seq;
1091 CREATE TABLE person (
1092 id INT4 NOT NULL DEFAULT nextval('person_id_seq'),
1095 CREATE UNIQUE INDEX person_id_key ON person ( id );
1098 See the <I>create_sequence</I> manual page for more information
1099 about sequences. You can also use each row's <I>OID</I> field as a
1100 unique value. However, if you need to dump and reload the database,
1101 you need to use <I>pg_dump</I>'s <I>-o</I> option or <SMALL>COPY
1102 WITH OIDS</SMALL> option to preserve the <SMALL>OID</SMALL>s.
1104 <H4><A name="4.15.2">4.15.2</A>) How do I get the value of a
1105 <SMALL>SERIAL</SMALL> insert?</H4>
1107 <P>One approach is to retrieve the next <SMALL>SERIAL</SMALL> value
1108 from the sequence object with the <I>nextval()</I> function
1109 <I>before</I> inserting and then insert it explicitly. Using the
1110 example table in <A href="#4.15.1">4.15.1</A>, that might look like
1113 new_id = output of "SELECT nextval('person_id_seq')"
1114 INSERT INTO person (id, name) VALUES (new_id, 'Blaise Pascal');
1117 You would then also have the new value stored in
1118 <CODE>new_id</CODE> for use in other queries (e.g., as a foreign
1119 key to the <CODE>person</CODE> table). Note that the name of the
1120 automatically created <SMALL>SEQUENCE</SMALL> object will be named
1121 <<I>table</I>>_<<I>serialcolumn</I>>_<I>seq</I>, where
1122 <I>table</I> and <I>serialcolumn</I> are the names of your table
1123 and your <SMALL>SERIAL</SMALL> column, respectively.
1125 <P>Alternatively, you could retrieve the assigned
1126 <SMALL>SERIAL</SMALL> value with the <I>currval</I>() function
1127 <I>after</I> it was inserted by default, e.g.,</P>
1129 INSERT INTO person (name) VALUES ('Blaise Pascal');
1130 new_id = output of "SELECT currval('person_id_seq')";
1133 Finally, you could use the <A href="#4.16"><SMALL>OID</SMALL></A>
1134 returned from the <SMALL>INSERT</SMALL> statement to look up the
1135 default value, though this is probably the least portable approach.
1136 In Perl, using DBI with Edmund Mergl's DBD::Pg module, the oid
1137 value is made available via <I>$sth->{pg_oid_status}</I> after
1138 <I>$sth->execute()</I>.
1140 <H4><A name="4.15.3">4.15.3</A>) Don't <I>currval()</I> and
1141 <I>nextval()</I> lead to a race condition with other users?</H4>
1143 <P>No. Currval() returns the current value assigned by your
1144 backend, not by all users.</P>
1146 <H4><A name="4.15.4">4.15.4</A>) Why aren't my sequence numbers
1147 reused on transaction abort? Why are there gaps in the numbering of
1148 my sequence/SERIAL column?</H4>
1150 <P>To improve concurrency, sequence values are given out to running
1151 transactions as needed and are not locked until the transaction
1152 completes. This causes gaps in numbering from aborted
1155 <H4><A name="4.16">4.16</A>) What is an <SMALL>OID</SMALL>? What is
1156 a <SMALL>TID</SMALL>?</H4>
1158 <P><SMALL>OID</SMALL>s are PostgreSQL's answer to unique row ids.
1159 Every row that is created in PostgreSQL gets a unique
1160 <SMALL>OID</SMALL>. All <SMALL>OID</SMALL>s generated during
1161 <I>initdb</I> are less than 16384 (from
1162 <I>backend/access/transam.h</I>). All user-created
1163 <SMALL>OID</SMALL>s are equal to or greater than this. By default,
1164 all these <SMALL>OID</SMALL>s are unique not only within a table or
1165 database, but unique within the entire PostgreSQL installation.</P>
1167 <P>PostgreSQL uses <SMALL>OID</SMALL>s in its internal system
1168 tables to link rows between tables. These <SMALL>OID</SMALL>s can
1169 be used to identify specific user rows and used in joins. It is
1170 recommended you use column type <SMALL>OID</SMALL> to store
1171 <SMALL>OID</SMALL> values. You can create an index on the
1172 <SMALL>OID</SMALL> field for faster access.</P>
1174 <P>O<SMALL>ID</SMALL>s are assigned to all new rows from a central
1175 area that is used by all databases. If you want to change the
1176 <SMALL>OID</SMALL> to something else, or if you want to make a copy
1177 of the table, with the original <SMALL>OID</SMALL>s, there is no
1178 reason you can't do it:</P>
1180 CREATE TABLE new_table(old_oid oid, mycol int);
1181 SELECT old_oid, mycol INTO new FROM old;
1182 COPY new TO '/tmp/pgtable';
1184 COPY new WITH OIDS FROM '/tmp/pgtable';
1186 CREATE TABLE new_table (mycol int);
1187 INSERT INTO new_table (oid, mycol) SELECT oid, mycol FROM old_table;
1191 <P>O<SMALL>ID</SMALL>s are stored as 4-byte integers, and will
1192 overflow at 4 billion. No one has reported this ever happening, and
1193 we plan to have the limit removed before anyone does.</P>
1195 <P>T<SMALL>ID</SMALL>s are used to identify specific physical rows
1196 with block and offset values. T<SMALL>ID</SMALL>s change after rows
1197 are modified or reloaded. They are used by index entries to point
1198 to physical rows.</P>
1200 <H4><A name="4.17">4.17</A>) What is the meaning of some of the
1201 terms used in PostgreSQL?</H4>
1203 <P>Some of the source code and older documentation use terms that
1204 have more common usage. Here are some:</P>
1207 <LI>table, relation, class</LI>
1209 <LI>row, record, tuple</LI>
1211 <LI>column, field, attribute</LI>
1213 <LI>retrieve, select</LI>
1215 <LI>replace, update</LI>
1217 <LI>append, insert</LI>
1219 <LI><SMALL>OID</SMALL>, serial value</LI>
1221 <LI>portal, cursor</LI>
1223 <LI>range variable, table name, table alias</LI>
1226 <P>A list of general database terms can be found at: <A href=
1227 "http://www.comptechnews.com/~reaster/dbdesign.html">http://www.comptechnews.com/~reaster/dbdesign.html</A></P>
1229 <H4><A name="4.18">4.18</A>) Why do I get the error <I>"ERROR:
1230 Memory exhausted in AllocSetAlloc()"</I>?</H4>
1232 <P>If you are running a version older than 7.1, an upgrade may fix
1233 the problem. Also it is possible you have run out of virtual memory
1234 on your system, or your kernel has a low limit for certain
1235 resources. Try this before starting <I>postmaster</I>:</P>
1241 Depending on your shell, only one of these may succeed, but it will
1242 set your process data segment limit much higher and perhaps allow
1243 the query to complete. This command applies to the current process,
1244 and all subprocesses created after the command is run. If you are
1245 having a problem with the <SMALL>SQL</SMALL> client because the
1246 backend is returning too much data, try it before starting the
1249 <H4><A name="4.19">4.19</A>) How do I tell what PostgreSQL version
1252 <P>From <I>psql</I>, type <CODE>select version();</CODE></P>
1254 <H4><A name="4.20">4.20</A>) Why does my large-object operations
1255 get <I>"invalid large obj descriptor"</I>?</H4>
1257 <P>You need to put <CODE>BEGIN WORK</CODE> and <CODE>COMMIT</CODE>
1258 around any use of a large object handle, that is, surrounding
1259 <CODE>lo_open</CODE> ... <CODE>lo_close.</CODE></P>
1261 <P>Currently PostgreSQL enforces the rule by closing large object
1262 handles at transaction commit. So the first attempt to do anything
1263 with the handle will draw <I>invalid large obj descriptor</I>. So
1264 code that used to work (at least most of the time) will now
1265 generate that error message if you fail to use a transaction.</P>
1267 <P>If you are using a client interface like <SMALL>ODBC</SMALL> you
1268 may need to set <CODE>auto-commit off.</CODE></P>
1270 <H4><A name="4.21">4.21</A>) How do I create a column that will
1271 default to the current time?</H4>
1273 <P>Use <I>CURRENT_TIMESTAMP</I>:</P>
1275 <CODE>CREATE TABLE test (x int, modtime timestamp DEFAULT CURRENT_TIMESTAMP );
1279 <H4><A name="4.22">4.22</A>) Why are my subqueries using
1280 <CODE><SMALL>IN</SMALL></CODE> so slow?</H4>
1282 <P>Currently, we join subqueries to outer queries by sequentially
1283 scanning the result of the subquery for each row of the outer
1284 query. A workaround is to replace <CODE>IN</CODE> with
1285 <CODE>EXISTS</CODE>:</P>
1289 WHERE col1 IN (SELECT col2 FROM TAB2)
1296 WHERE EXISTS (SELECT col2 FROM TAB2 WHERE col1 = col2)
1300 We hope to fix this limitation in a future release.
1302 <H4><A name="4.23">4.23</A>) How do I perform an outer join?</H4>
1304 <P>PostgreSQL 7.1 and later supports outer joins using the SQL
1305 standard syntax. Here are two examples:</P>
1308 FROM t1 LEFT OUTER JOIN t2 ON (t1.col = t2.col);
1313 FROM t1 LEFT OUTER JOIN t2 USING (col);
1316 <P>These identical queries join t1.col to t2.col, and also return
1317 any unjoined rows in t1 (those with no match in t2). A
1318 <SMALL>RIGHT</SMALL> join would add unjoined rows of t2. A
1319 <SMALL>FULL</SMALL> join would return the matched rows plus all
1320 unjoined rows from t1 and t2. The word <SMALL>OUTER</SMALL> is
1321 optional and is assumed in <SMALL>LEFT</SMALL>,
1322 <SMALL>RIGHT</SMALL>, and <SMALL>FULL</SMALL> joins. Ordinary joins
1323 are called <SMALL>INNER</SMALL> joins.</P>
1325 <P>In previous releases, outer joins can be simulated using
1326 <SMALL>UNION</SMALL> and <SMALL>NOT IN</SMALL>. For example, when
1327 joining <I>tab1</I> and <I>tab2</I>, the following query does an
1328 <I>outer</I> join of the two tables:<BR>
1332 SELECT tab1.col1, tab2.col2
1334 WHERE tab1.col1 = tab2.col1
1336 SELECT tab1.col1, NULL
1338 WHERE tab1.col1 NOT IN (SELECT tab2.col1 FROM tab2)
1342 <H4><A name="4.24">4.24</A>) How do I perform queries using
1343 multiple databases?</H4>
1345 <P>There is no way to query any database except the current one.
1346 Because PostgreSQL loads database-specific system catalogs, it is
1347 uncertain how a cross-database query should even behave.</P>
1349 <P>Of course, a client can make simultaneous connections to
1350 different databases and merge the information that way.</P>
1352 <H4><A name="4.25">4.25</A>) How do I return multiple rows or
1353 columns from a function?</H4>
1355 <P>You can return result sets from PL/pgSQL functions using
1356 <I>refcursors</I>. See <A href=
1357 "http://developer.postgresql.org/docs/postgres/plpgsql-cursors.html">
1358 http://developer.postgresql.org/docs/postgres/plpgsql-cursors.html,</A>
1359 section 23.7.3.3.</P>
1361 <H4><A name="4.26">4.26</A>) Why can't I reliably create/drop
1362 temporary tables in PL/PgSQL functions?</H4>
1363 PL/PgSQL caches function contents, and an unfortunate side effect
1364 is that if a PL/PgSQL function accesses a temporary table, and that
1365 table is later dropped and recreated, and the function called
1366 again, the function will fail because the cached function contents
1367 still point to the old temporary table. The solution is to use
1368 <SMALL>EXECUTE</SMALL> for temporary table access in PL/PgSQL. This
1369 will cause the query to be reparsed every time.
1373 <H2 align="center">Extending PostgreSQL</H2>
1375 <H4><A name="5.1">5.1</A>) I wrote a user-defined function. When I
1376 run it in <I>psql</I>, why does it dump core?</H4>
1378 <P>The problem could be a number of things. Try testing your
1379 user-defined function in a stand-alone test program first.</P>
1381 <H4><A name="5.2">5.2</A>) How can I contribute some nifty new
1382 types and functions to PostgreSQL?</H4>
1384 <P>Send your extensions to the <I>pgsql-hackers</I> mailing list,
1385 and they will eventually end up in the <I>contrib/</I>
1388 <H4><A name="5.3">5.3</A>) How do I write a C function to return a
1391 <P>This requires wizardry so extreme that the authors have never
1392 tried it, though in principle it can be done.</P>
1394 <H4><A name="5.4">5.4</A>) I have changed a source file. Why does
1395 the recompile not see the change?</H4>
1397 <P>The <I>Makefiles</I> do not have the proper dependencies for
1398 include files. You have to do a <I>make clean</I> and then another
1399 <I>make</I>. If you are using <SMALL>GCC</SMALL> you can use the
1400 <I>--enable-depend</I> option of <I>configure</I> to have the
1401 compiler compute the dependencies automatically.</P>
projects / postgresql / blob