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15 <H1>Frequently Asked Questions (FAQ) for PostgreSQL</H1>
17 <P>Last updated: Wed Oct 9 23:14:53 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 releases?<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 7.2.3.</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 "https://store.pgsql.com/shopping/index.php?id=1">
497 https://store.pgsql.com/shopping/index.php?id=1</A>
498 and make a donation.</P>
500 <P>Although the web page mentions PostgreSQL, Inc, the
501 "contributions" item is solely to support the PostgreSQL project
502 and does not fund any specific company. If you prefer, you can also
503 send a check to the contact address.</P>
506 <H2 align="center">User Client Questions</H2>
508 <H4><A name="2.1">2.1</A>) Are there <SMALL>ODBC</SMALL> drivers
511 <P>There are two <SMALL>ODBC</SMALL> drivers available, PsqlODBC
512 and OpenLink <SMALL>ODBC</SMALL>.</P>
514 <P>PsqlODBC is included in the distribution. More information about
515 it can be gotten from <A href=
516 "ftp://ftp.PostgreSQL.org/pub/odbc/">ftp://ftp.PostgreSQL.org/pub/odbc/</A>.</P>
518 <P>OpenLink <SMALL>ODBC</SMALL> can be gotten from <A href=
519 "http://www.openlinksw.com/">http://www.openlinksw.com</A>. It
520 works with their standard <SMALL>ODBC</SMALL> client software so
521 you'll have PostgreSQL <SMALL>ODBC</SMALL> available on every
522 client platform they support (Win, Mac, Unix, VMS).</P>
524 <P>They will probably be selling this product to people who need
525 commercial-quality support, but a freeware version will always be
526 available. Please send questions to <A href=
527 "mailto:postgres95@openlink.co.uk">postgres95@openlink.co.uk</A>.</P>
529 <P>See also the <A href=
530 "http://www.postgresql.org/devel-corner/docs/programmer/odbc.html">ODBC
531 chapter of the Programmer's Guide</A>.</P>
533 <H4><A name="2.2">2.2</A>) What tools are available for using
534 PostgreSQL with Web pages?</H4>
536 <P>A nice introduction to Database-backed Web pages can be seen at:
537 <A href="http://www.webreview.com">http://www.webreview.com</A></P>
539 <P>There is also one at <A href=
540 "http://www.phone.net/home/mwm/hotlist/">http://www.phone.net/home/mwm/hotlist/.</A></P>
542 <P>For Web integration, PHP is an excellent interface. It is at <A
543 href="http://www.php.net">http://www.php.net</A>.</P>
545 <P>For complex cases, many use the Perl interface and CGI.pm.</P>
547 <H4><A name="2.3">2.3</A>) Does PostgreSQL have a graphical user
548 interface? A report generator? An embedded query language
551 <P>We have a nice graphical user interface called <I>pgaccess</I>,
552 which is shipped as part of the distribution. <I>pgaccess</I> also
553 has a report generator. The Web page is <A href=
554 "http://www.flex.ro/pgaccess">http://www.flex.ro/pgaccess</A></P>
556 <P>We also include <I>ecpg</I>, which is an embedded SQL query
557 language interface for C.</P>
559 <H4><A name="2.4">2.4</A>) What languages are available to
560 communicate with PostgreSQL?</H4>
567 <LI>C++ (libpq++)</LI>
569 <LI>Embedded C (ecpg)</LI>
573 <LI>Perl (perl5)</LI>
577 <LI>Python (PyGreSQL)</LI>
579 <LI>TCL (libpgtcl)</LI>
581 <LI>C Easy API (libpgeasy)</LI>
583 <LI>Embedded <SMALL>HTML</SMALL> (<A href=
584 "http://www.php.net">PHP from http://www.php.net</A>)</LI>
586 <P>Additional interfaces are available at <a
587 href="http://www.postgresql.org/interfaces.html">
588 http://www.postgresql.org/interfaces.html.</A>
592 <H2 align="center">Administrative Questions</H2>
594 <H4><A name="3.1">3.1</A>) How do I install PostgreSQL somewhere
595 other than <I>/usr/local/pgsql</I>?</H4>
597 <P>Specify the <I>--prefix</I> option when running
598 <I>configure</I>.</P>
600 <H4><A name="3.2">3.2</A>) When I start <I>postmaster</I>, I get a
601 <I>Bad System Call</I> or core dumped message. Why?</H4>
603 <P>It could be a variety of problems, but first check to see that
604 you have System V extensions installed in your kernel. PostgreSQL
605 requires kernel support for shared memory and semaphores.</P>
607 <H4><A name="3.3">3.3</A>) When I try to start <I>postmaster</I>, I
608 get <I>IpcMemoryCreate</I> errors. Why?</H4>
610 <P>You either do not have shared memory configured properly in your
611 kernel or you need to enlarge the shared memory available in the
612 kernel. The exact amount you need depends on your architecture and
613 how many buffers and backend processes you configure for
614 <I>postmaster</I>. For most systems, with default numbers of
615 buffers and processes, you need a minimum of ~1 MB. See the <A
617 "http://www.postgresql.org/idocs/index.php?kernel-resources.html">PostgreSQL
618 Administrator's Guide</A> for more detailed information about
619 shared memory and semaphores.</P>
621 <H4><A name="3.4">3.4</A>) When I try to start <I>postmaster</I>, I
622 get <I>IpcSemaphoreCreate</I> errors. Why?</H4>
624 <P>If the error message is <I>IpcSemaphoreCreate: semget failed (No
625 space left on device)</I> then your kernel is not configured with
626 enough semaphores. Postgres needs one semaphore per potential
627 backend process. A temporary solution is to start <I>postmaster</I>
628 with a smaller limit on the number of backend processes. Use
629 <I>-N</I> with a parameter less than the default of 32. A more
630 permanent solution is to increase your kernel's
631 <SMALL>SEMMNS</SMALL> and <SMALL>SEMMNI</SMALL> parameters.</P>
633 <P>Inoperative semaphores can also cause crashes during heavy
636 <P>If the error message is something else, you might not have
637 semaphore support configured in your kernel at all. See the
638 PostgreSQL Administrator's Guide for more detailed information
639 about shared memory and semaphores.</P>
641 <H4><A name="3.5">3.5</A>) How do I control connections from other
644 <P>By default, PostgreSQL only allows connections from the local
645 machine using Unix domain sockets. Other machines will not be able
646 to connect unless you add the <I>-i</I> flag to <I>postmaster</I>,
647 <B>and</B> enable host-based authentication by modifying the file
648 <I>$PGDATA/pg_hba.conf</I> accordingly. This will allow TCP/IP
651 <H4><A name="3.6">3.6</A>) How do I tune the database engine for
652 better performance?</H4>
654 <P>Certainly, indexes can speed up queries. The
655 <SMALL>EXPLAIN</SMALL> command allows you to see how PostgreSQL is
656 interpreting your query, and which indexes are being used.</P>
658 <P>If you are doing many <SMALL>INSERTs</SMALL>, consider doing
659 them in a large batch using the <SMALL>COPY</SMALL> command. This
660 is much faster than individual <SMALL>INSERTS.</SMALL> Second,
661 statements not in a <SMALL>BEGIN WORK/COMMIT</SMALL> transaction
662 block are considered to be in their own transaction. Consider
663 performing several statements in a single transaction block. This
664 reduces the transaction overhead. Also, consider dropping and
665 recreating indexes when making large data changes.</P>
667 <P>There are several tuning options. You can disable <I>fsync()</I>
668 by starting <I>postmaster</I> with a <I>-o -F</I> option. This will
669 prevent <I>fsync()</I>s from flushing to disk after every
672 <P>You can also use the <I>postmaster</I> <I>-B</I> option to
673 increase the number of shared memory buffers used by the backend
674 processes. If you make this parameter too high, the
675 <I>postmaster</I> may not start because you have exceeded your
676 kernel's limit on shared memory space. Each buffer is 8K and the
677 default is 64 buffers.</P>
679 <P>You can also use the backend <I>-S</I> option to increase the
680 maximum amount of memory used by the backend process for temporary
681 sorts. The <I>-S</I> value is measured in kilobytes, and the
682 default is 512 (i.e. 512K).</P>
684 <P>You can also use the <SMALL>CLUSTER</SMALL> command to group
685 data in tables to match an index. See the <SMALL>CLUSTER</SMALL>
686 manual page for more details.</P>
688 <H4><A name="3.7">3.7</A>) What debugging features are
691 <P>PostgreSQL has several features that report status information
692 that can be valuable for debugging purposes.</P>
694 <P>First, by running <I>configure</I> with the --enable-cassert
695 option, many <I>assert()</I>s monitor the progress of the backend
696 and halt the program when something unexpected occurs.</P>
698 <P>Both <I>postmaster</I> and <I>postgres</I> have several debug
699 options available. First, whenever you start <I>postmaster</I>,
700 make sure you send the standard output and error to a log file,
704 ./bin/postmaster >server.log 2>&1 &
707 <P>This will put a server.log file in the top-level PostgreSQL
708 directory. This file contains useful information about problems or
709 errors encountered by the server. <I>Postmaster</I> has a <I>-d</I>
710 option that allows even more detailed information to be reported.
711 The <I>-d</I> option takes a number that specifies the debug level.
712 Be warned that high debug level values generate large log
715 <P>If <I>postmaster</I> is not running, you can actually run the
716 <I>postgres</I> backend from the command line, and type your
717 <SMALL>SQL</SMALL> statement directly. This is recommended
718 <B>only</B> for debugging purposes. Note that a newline terminates
719 the query, not a semicolon. If you have compiled with debugging
720 symbols, you can use a debugger to see what is happening. Because
721 the backend was not started from <I>postmaster</I>, it is not
722 running in an identical environment and locking/backend interaction
723 problems may not be duplicated.</P>
725 <P>If <I>postmaster</I> is running, start <I>psql</I> in one
726 window, then find the <SMALL>PID</SMALL> of the <I>postgres</I>
727 process used by <I>psql</I>. Use a debugger to attach to the
728 <I>postgres</I> <SMALL>PID.</SMALL> You can set breakpoints in the
729 debugger and issue queries from <I>psql</I>. If you are debugging
730 <I>postgres</I> startup, you can set PGOPTIONS="-W n", then start
731 <I>psql</I>. This will cause startup to delay for <I>n</I> seconds
732 so you can attach to the process with the debugger, set any
733 breakpoints, and continue through the startup sequence.</P>
735 <P>The <I>postgres</I> program has <I>-s, -A</I>, and <I>-t</I>
736 options that can be very useful for debugging and performance
739 <P>You can also compile with profiling to see what functions are
740 taking execution time. The backend profile files will be deposited
741 in the <I>pgsql/data/base/dbname</I> directory. The client profile
742 file will be put in the client's current directory. Linux requires
743 a compile with <I>-DLINUX_PROFILE</I> for proper profiling.</P>
745 <H4><A name="3.8">3.8</A>) Why do I get <I>"Sorry, too many
746 clients"</I> when trying to connect?</H4>
748 <P>You need to increase <I>postmaster</I>'s limit on how many
749 concurrent backend processes it can start.</P>
751 <P>The default limit is 32 processes. You can increase it by
752 restarting <I>postmaster</I> with a suitable <I>-N</I> value or
753 modifying <I>postgresql.conf</I>.</P>
755 <P>Note that if you make <I>-N</I> larger than 32, you must also
756 increase <I>-B</I> beyond its default of 64; <I>-B</I> must be at
757 least twice <I>-N</I>, and probably should be more than that for
758 best performance. For large numbers of backend processes, you are
759 also likely to find that you need to increase various Unix kernel
760 configuration parameters. Things to check include the maximum size
761 of shared memory blocks, <SMALL>SHMMAX;</SMALL> the maximum number
762 of semaphores, <SMALL>SEMMNS</SMALL> and <SMALL>SEMMNI;</SMALL> the
763 maximum number of processes, <SMALL>NPROC;</SMALL> the maximum
764 number of processes per user, <SMALL>MAXUPRC;</SMALL> and the
765 maximum number of open files, <SMALL>NFILE</SMALL> and
766 <SMALL>NINODE.</SMALL> The reason that PostgreSQL has a limit on
767 the number of allowed backend processes is so your system won't run
768 out of resources.</P>
770 <P>In PostgreSQL versions prior to 6.5, the maximum number of
771 backends was 64, and changing it required a rebuild after altering
772 the MaxBackendId constant in
773 <I>include/storage/sinvaladt.h</I>.</P>
775 <H4><A name="3.9">3.9</A>) What are the <I>pg_sorttempNNN.NN</I>
776 files in my database directory?</H4>
778 <P>They are temporary files generated by the query executor. For
779 example, if a sort needs to be done to satisfy an <SMALL>ORDER
780 BY,</SMALL> and the sort requires more space than the backend's
781 <I>-S</I> parameter allows, then temporary files are created to
782 hold the extra data.</P>
784 <P>The temporary files should be deleted automatically, but might
785 not if a backend crashes during a sort. If you have no backends
786 running at the time, it is safe to delete the pg_tempNNN.NN
789 <H4><A name="3.10">3.10</A>) Why do I need to do a dump and restore
790 to upgrade between major PostgreSQL releases?</H4>
792 <P>The PostgreSQL team makes only small changes between minor releases,
793 so upgrading from 7.2 to 7.2.1 does not require a dump and restore.
794 However, major releases often change the internal format of system
795 tables and data files. These changes are often complex, so we don't
796 maintain backward compatability for data files. A dump outputs data
797 in a generic format that can then be loaded in using the new internal
800 <P>In releases where the on-disk format does not change, the
801 <i>pg_upgrade</i> script can be used to upgrade without a dump/restore.
802 The release notes mention whether <i>pg_upgrade</i> is available for the
807 <H2 align="center">Operational Questions</H2>
809 <H4><A name="4.1">4.1</A>) What is the difference between binary
810 cursors and normal cursors?</H4>
812 <P>See the <SMALL>DECLARE</SMALL> manual page for a
815 <H4><A name="4.2">4.2</A>) How do I <SMALL>SELECT</SMALL> only the
816 first few rows of a query?</H4>
818 <P>See the <SMALL>FETCH</SMALL> manual page, or use
819 <SMALL>SELECT</SMALL> ... <SMALL>LIMIT</SMALL>....</P>
821 <P>The entire query may have to be evaluated, even if you only want
822 the first few rows. Consider a query that has an <SMALL>ORDER
823 BY.</SMALL> If there is an index that matches the <SMALL>ORDER
824 BY</SMALL>, PostgreSQL may be able to evaluate only the first few
825 records requested, or the entire query may have to be evaluated
826 until the desired rows have been generated.</P>
828 <H4><A name="4.3">4.3</A>) How do I get a list of tables or other
829 things I can see in <I>psql</I>?</H4>
831 <P>You can read the source code for <I>psql</I> in file
832 <I>pgsql/src/bin/psql/describe.c</I>. It contains
833 <SMALL>SQL</SMALL> commands that generate the output for psql's
834 backslash commands. You can also start <I>psql</I> with the
835 <I>-E</I> option so it will print out the queries it uses to
836 execute the commands you give.</P>
838 <H4><A name="4.4">4.4</A>) How do you remove a column from a
841 <P>We do not support <SMALL>ALTER TABLE DROP COLUMN,</SMALL> but do
845 LOCK TABLE old_table;
846 SELECT ... -- select all columns but the one you want to remove
849 DROP TABLE old_table;
850 ALTER TABLE new_table RENAME TO old_table;
854 <H4><A name="4.5">4.5</A>) What is the maximum size for a row, a
855 table, and a database?</H4>
857 <P>These are the limits:</P>
859 Maximum size for a database? unlimited (1 TB databases exist)
860 Maximum size for a table? 16 TB
861 Maximum size for a row? 1.6TB
862 Maximum size for a field? 1 GB
863 Maximum number of rows in a table? unlimited
864 Maximum number of columns in a table? 250-1600 depending on column types
865 Maximum number of indexes on a table? unlimited
868 Of course, these are not actually unlimited, but limited to
869 available disk space and memory/swap space. Performance may suffer
870 when these values get unusually large.
872 <P>The maximum table size of 16 TB does not require large file
873 support from the operating system. Large tables are stored as
874 multiple 1 GB files so file system size limits are not
877 <P>The maximum table size and maximum number of columns can be
878 increased if the default block size is increased to 32k.</P>
880 <H4><A name="4.6">4.6</A>) How much database disk space is required
881 to store data from a typical text file?</H4>
883 <P>A PostgreSQL database may require up to five times the disk
884 space to store data from a text file.</P>
886 <P>As an example, consider a file of 100,000 lines with an integer
887 and text description on each line. Suppose the text string
888 avergages twenty bytes in length. The flat file would be 2.8 MB.
889 The size of the PostgreSQL database file containing this data can
890 be estimated as 6.4 MB:</P>
892 36 bytes: each row header (approximate)
893 24 bytes: one int field and one text filed
894 + 4 bytes: pointer on page to tuple
895 ----------------------------------------
898 The data page size in PostgreSQL is 8192 bytes (8 KB), so:
901 ------------------- = 128 rows per database page (rounded down)
905 -------------------- = 782 database pages (rounded up)
908 782 database pages * 8192 bytes per page = 6,406,144 bytes (6.4 MB)
911 <P>Indexes do not require as much overhead, but do contain the data
912 that is being indexed, so they can be large also.</P>
914 <P><SMALL>NULL</SMALL>s are stored in bitmaps, so they
915 use very little space.</P>
917 <H4><A name="4.7">4.7</A>) How do I find out what tables, indexes,
918 databases, and users are defined?</H4>
920 <P><I>psql</I> has a variety of backslash commands to show such
921 information. Use \? to see them. There are also system tables
922 beginning with <I>pg_</I> that describe these too. Also, <I>psql
923 -l</I> will list all databases.</P>
925 <P>Also try the file <I>pgsql/src/tutorial/syscat.source</I>. It
926 illustrates many of the <SMALL>SELECT</SMALL>s needed to get
927 information from the database system tables.</P>
929 <H4><A name="4.8">4.8</A>) My queries are slow or don't make use of
930 the indexes. Why?</H4>
931 Indexes are not automatically used by every query. Indexes are only
932 used if the table is larger than a minimum size, and the query
933 selects only a small percentage of the rows in the table. This is
934 because the random disk access caused by an index scan can be
935 slower than a straight read through the table, or sequential scan.
937 <P>To determine if an index should be used, PostgreSQL must have
938 statistics about the table. These statistics are collected using
939 <SMALL>VACUUM ANALYZE</SMALL>, or simply <SMALL>ANALYZE</SMALL>.
940 Using statistics, the optimizer knows how many rows are in the
941 table, and can better determine if indexes should be used.
942 Statistics are also valuable in determining optimal join order and
943 join methods. Statistics collection should be performed
944 periodically as the contents of the table change.</P>
946 <P>Indexes are normally not used for <SMALL>ORDER BY</SMALL> or to
947 perform joins. A sequential scan followed by an explicit sort is
948 usually faster than an index scan of a large table.</P>
949 However, <SMALL>LIMIT</SMALL> combined with <SMALL>ORDER BY</SMALL>
950 often will use an index because only a small portion of the table
951 is returned. In fact, though MAX() and MIN() don't use indexes,
952 it is possible to retrieve such values using an index with ORDER BY
957 ORDER BY col [ DESC ]
961 <P>When using wild-card operators such as <SMALL>LIKE</SMALL> or
962 <I>~</I>, indexes can only be used in certain circumstances:
964 <LI>The beginning of the search string must be anchored to the start
965 of the string, i.e.:</LI>
967 <LI><SMALL>LIKE</SMALL> patterns must not start with <I>%.</I></LI>
968 <LI><I>~</I> (regular expression) patterns must start with
971 <LI>The search string can not start with a character class,
973 <LI>Case-insensitive searches like <SMALL>ILIKE</SMALL> and
974 <I>~*</I> can not be used. Instead, use functional
975 indexes, which are described later in this FAQ.</LI>
976 <LI>The default <I>C</I> local must be used during
981 <H4><A name="4.9">4.9</A>) How do I see how the query optimizer is
982 evaluating my query?</H4>
984 <P>See the <SMALL>EXPLAIN</SMALL> manual page.</P>
986 <H4><A name="4.10">4.10</A>) What is an R-tree index?</H4>
988 <P>An R-tree index is used for indexing spatial data. A hash index
989 can't handle range searches. A B-tree index only handles range
990 searches in a single dimension. R-trees can handle
991 multi-dimensional data. For example, if an R-tree index can be
992 built on an attribute of type <I>point</I>, the system can more
993 efficiently answer queries such as "select all points within a
994 bounding rectangle."</P>
996 <P>The canonical paper that describes the original R-tree design
999 <P>Guttman, A. "R-trees: A Dynamic Index Structure for Spatial
1000 Searching." Proceedings of the 1984 ACM SIGMOD Int'l Conf on Mgmt
1003 <P>You can also find this paper in Stonebraker's "Readings in
1004 Database Systems".</P>
1006 <P>Built-in R-trees can handle polygons and boxes. In theory,
1007 R-trees can be extended to handle higher number of dimensions. In
1008 practice, extending R-trees requires a bit of work and we don't
1009 currently have any documentation on how to do it.</P>
1011 <H4><A name="4.11">4.11</A>) What is the Genetic Query
1014 <P>The <SMALL>GEQO</SMALL> module speeds query optimization when
1015 joining many tables by means of a Genetic Algorithm (GA). It allows
1016 the handling of large join queries through nonexhaustive
1019 <H4><A name="4.12">4.12</A>) How do I perform regular expression
1020 searches and case-insensitive regular expression searches? How do I
1021 use an index for case-insensitive searches?</H4>
1023 <P>The <I>~</I> operator does regular expression matching, and
1024 <I>~*</I> does case-insensitive regular expression matching. The
1025 case-insensitive variant of <SMALL>LIKE</SMALL> is called
1026 <SMALL>ILIKE</SMALL>.</P>
1028 <P>Case-insensitive equality comparisons are normally expressed
1033 WHERE lower(col) = 'abc'
1036 This will not use an standard index. However, if you create a
1037 functional index, it will be used:
1039 CREATE INDEX tabindex on tab (lower(col));
1042 <H4><A name="4.13">4.13</A>) In a query, how do I detect if a field
1043 is <SMALL>NULL</SMALL>?</H4>
1045 <P>You test the column with <SMALL>IS NULL</SMALL> and <SMALL>IS
1046 NOT NULL</SMALL>.</P>
1048 <H4><A name="4.14">4.14</A>) What is the difference between the
1049 various character types?</H4>
1051 Type Internal Name Notes
1052 --------------------------------------------------
1053 "char" char 1 character
1054 CHAR(#) bpchar blank padded to the specified fixed length
1055 VARCHAR(#) varchar size specifies maximum length, no padding
1056 TEXT text no specific upper limit on length
1057 BYTEA bytea variable-length byte array (null-byte safe)
1060 <P>You will see the internal name when examining system catalogs
1061 and in some error messages.</P>
1063 <P>The last four types above are "varlena" types (i.e., the first
1064 four bytes on disk are the length, followed by the data). Thus the
1065 actual space used is slightly greater than the declared size.
1066 However, these data types are also subject to compression or being
1067 stored out-of-line by <SMALL>TOAST</SMALL>, so the space on disk
1068 might also be less than expected.</P>
1070 <P><SMALL>CHAR()</SMALL> is best when storing strings that are
1071 usually the same length. <SMALL>VARCHAR()</SMALL> is best when
1072 storing variable-length strings but it limits how long a string can
1073 be. <SMALL>TEXT</SMALL> is for strings of unlimited length, maximum
1074 1 gigabyte. <SMALL>BYTEA</SMALL> is for storing binary data,
1075 particularly values that include <SMALL>NULL</SMALL> bytes.</P>
1077 <H4><A name="4.15.1">4.15.1</A>) How do I create a
1078 serial/auto-incrementing field?</H4>
1080 <P>PostgreSQL supports a <SMALL>SERIAL</SMALL> data type. It
1081 auto-creates a sequence and index on the column. For example,
1084 CREATE TABLE person (
1090 is automatically translated into this:
1092 CREATE SEQUENCE person_id_seq;
1093 CREATE TABLE person (
1094 id INT4 NOT NULL DEFAULT nextval('person_id_seq'),
1097 CREATE UNIQUE INDEX person_id_key ON person ( id );
1100 See the <I>create_sequence</I> manual page for more information
1101 about sequences. You can also use each row's <I>OID</I> field as a
1102 unique value. However, if you need to dump and reload the database,
1103 you need to use <I>pg_dump</I>'s <I>-o</I> option or <SMALL>COPY
1104 WITH OIDS</SMALL> option to preserve the <SMALL>OID</SMALL>s.
1106 <H4><A name="4.15.2">4.15.2</A>) How do I get the value of a
1107 <SMALL>SERIAL</SMALL> insert?</H4>
1109 <P>One approach is to retrieve the next <SMALL>SERIAL</SMALL> value
1110 from the sequence object with the <I>nextval()</I> function
1111 <I>before</I> inserting and then insert it explicitly. Using the
1112 example table in <A href="#4.15.1">4.15.1</A>, that might look like
1115 new_id = output of "SELECT nextval('person_id_seq')"
1116 INSERT INTO person (id, name) VALUES (new_id, 'Blaise Pascal');
1119 You would then also have the new value stored in
1120 <CODE>new_id</CODE> for use in other queries (e.g., as a foreign
1121 key to the <CODE>person</CODE> table). Note that the name of the
1122 automatically created <SMALL>SEQUENCE</SMALL> object will be named
1123 <<I>table</I>>_<<I>serialcolumn</I>>_<I>seq</I>, where
1124 <I>table</I> and <I>serialcolumn</I> are the names of your table
1125 and your <SMALL>SERIAL</SMALL> column, respectively.
1127 <P>Alternatively, you could retrieve the assigned
1128 <SMALL>SERIAL</SMALL> value with the <I>currval</I>() function
1129 <I>after</I> it was inserted by default, e.g.,</P>
1131 INSERT INTO person (name) VALUES ('Blaise Pascal');
1132 new_id = output of "SELECT currval('person_id_seq')";
1135 Finally, you could use the <A href="#4.16"><SMALL>OID</SMALL></A>
1136 returned from the <SMALL>INSERT</SMALL> statement to look up the
1137 default value, though this is probably the least portable approach.
1138 In Perl, using DBI with Edmund Mergl's DBD::Pg module, the oid
1139 value is made available via <I>$sth->{pg_oid_status}</I> after
1140 <I>$sth->execute()</I>.
1142 <H4><A name="4.15.3">4.15.3</A>) Don't <I>currval()</I> and
1143 <I>nextval()</I> lead to a race condition with other users?</H4>
1145 <P>No. Currval() returns the current value assigned by your
1146 backend, not by all users.</P>
1148 <H4><A name="4.15.4">4.15.4</A>) Why aren't my sequence numbers
1149 reused on transaction abort? Why are there gaps in the numbering of
1150 my sequence/SERIAL column?</H4>
1152 <P>To improve concurrency, sequence values are given out to running
1153 transactions as needed and are not locked until the transaction
1154 completes. This causes gaps in numbering from aborted
1157 <H4><A name="4.16">4.16</A>) What is an <SMALL>OID</SMALL>? What is
1158 a <SMALL>TID</SMALL>?</H4>
1160 <P><SMALL>OID</SMALL>s are PostgreSQL's answer to unique row ids.
1161 Every row that is created in PostgreSQL gets a unique
1162 <SMALL>OID</SMALL>. All <SMALL>OID</SMALL>s generated during
1163 <I>initdb</I> are less than 16384 (from
1164 <I>backend/access/transam.h</I>). All user-created
1165 <SMALL>OID</SMALL>s are equal to or greater than this. By default,
1166 all these <SMALL>OID</SMALL>s are unique not only within a table or
1167 database, but unique within the entire PostgreSQL installation.</P>
1169 <P>PostgreSQL uses <SMALL>OID</SMALL>s in its internal system
1170 tables to link rows between tables. These <SMALL>OID</SMALL>s can
1171 be used to identify specific user rows and used in joins. It is
1172 recommended you use column type <SMALL>OID</SMALL> to store
1173 <SMALL>OID</SMALL> values. You can create an index on the
1174 <SMALL>OID</SMALL> field for faster access.</P>
1176 <P>O<SMALL>ID</SMALL>s are assigned to all new rows from a central
1177 area that is used by all databases. If you want to change the
1178 <SMALL>OID</SMALL> to something else, or if you want to make a copy
1179 of the table, with the original <SMALL>OID</SMALL>s, there is no
1180 reason you can't do it:</P>
1182 CREATE TABLE new_table(old_oid oid, mycol int);
1183 SELECT old_oid, mycol INTO new FROM old;
1184 COPY new TO '/tmp/pgtable';
1186 COPY new WITH OIDS FROM '/tmp/pgtable';
1188 CREATE TABLE new_table (mycol int);
1189 INSERT INTO new_table (oid, mycol) SELECT oid, mycol FROM old_table;
1193 <P>O<SMALL>ID</SMALL>s are stored as 4-byte integers, and will
1194 overflow at 4 billion. No one has reported this ever happening, and
1195 we plan to have the limit removed before anyone does.</P>
1197 <P>T<SMALL>ID</SMALL>s are used to identify specific physical rows
1198 with block and offset values. T<SMALL>ID</SMALL>s change after rows
1199 are modified or reloaded. They are used by index entries to point
1200 to physical rows.</P>
1202 <H4><A name="4.17">4.17</A>) What is the meaning of some of the
1203 terms used in PostgreSQL?</H4>
1205 <P>Some of the source code and older documentation use terms that
1206 have more common usage. Here are some:</P>
1209 <LI>table, relation, class</LI>
1211 <LI>row, record, tuple</LI>
1213 <LI>column, field, attribute</LI>
1215 <LI>retrieve, select</LI>
1217 <LI>replace, update</LI>
1219 <LI>append, insert</LI>
1221 <LI><SMALL>OID</SMALL>, serial value</LI>
1223 <LI>portal, cursor</LI>
1225 <LI>range variable, table name, table alias</LI>
1228 <P>A list of general database terms can be found at: <A href=
1229 "http://www.comptechnews.com/~reaster/dbdesign.html">http://www.comptechnews.com/~reaster/dbdesign.html</A></P>
1231 <H4><A name="4.18">4.18</A>) Why do I get the error <I>"ERROR:
1232 Memory exhausted in AllocSetAlloc()"</I>?</H4>
1234 <P>You probably have run out of virtual memory on your system,
1235 or your kernel has a low limit for certain resources. Try this
1236 before starting <I>postmaster</I>:</P>
1242 Depending on your shell, only one of these may succeed, but it will
1243 set your process data segment limit much higher and perhaps allow
1244 the query to complete. This command applies to the current process,
1245 and all subprocesses created after the command is run. If you are
1246 having a problem with the <SMALL>SQL</SMALL> client because the
1247 backend is returning too much data, try it before starting the
1250 <H4><A name="4.19">4.19</A>) How do I tell what PostgreSQL version
1253 <P>From <I>psql</I>, type <CODE>select version();</CODE></P>
1255 <H4><A name="4.20">4.20</A>) Why does my large-object operations
1256 get <I>"invalid large obj descriptor"</I>?</H4>
1258 <P>You need to put <CODE>BEGIN WORK</CODE> and <CODE>COMMIT</CODE>
1259 around any use of a large object handle, that is, surrounding
1260 <CODE>lo_open</CODE> ... <CODE>lo_close.</CODE></P>
1262 <P>Currently PostgreSQL enforces the rule by closing large object
1263 handles at transaction commit. So the first attempt to do anything
1264 with the handle will draw <I>invalid large obj descriptor</I>. So
1265 code that used to work (at least most of the time) will now
1266 generate that error message if you fail to use a transaction.</P>
1268 <P>If you are using a client interface like <SMALL>ODBC</SMALL> you
1269 may need to set <CODE>auto-commit off.</CODE></P>
1271 <H4><A name="4.21">4.21</A>) How do I create a column that will
1272 default to the current time?</H4>
1274 <P>Use <I>CURRENT_TIMESTAMP</I>:</P>
1276 <CODE>CREATE TABLE test (x int, modtime timestamp DEFAULT CURRENT_TIMESTAMP );
1280 <H4><A name="4.22">4.22</A>) Why are my subqueries using
1281 <CODE><SMALL>IN</SMALL></CODE> so slow?</H4>
1283 <P>Currently, we join subqueries to outer queries by sequentially
1284 scanning the result of the subquery for each row of the outer
1285 query. If the subquery returns only a few rows and the outer query
1286 returns many rows, <CODE><SMALL>IN</SMALL></CODE> is fastest. To
1287 speed up other queries, replace <CODE>IN</CODE> with
1288 <CODE>EXISTS</CODE>:</P>
1292 WHERE col IN (SELECT subcol FROM subtab)
1299 WHERE EXISTS (SELECT subcol FROM subtab WHERE subcol = col)
1303 For this to be fast, <CODE>subcol</CODE> should be an indexed column.
1304 We hope to fix this limitation in a future release.
1306 <H4><A name="4.23">4.23</A>) How do I perform an outer join?</H4>
1308 <P>PostgreSQL supports outer joins using the SQL standard syntax.
1309 Here are two examples:</P>
1312 FROM t1 LEFT OUTER JOIN t2 ON (t1.col = t2.col);
1317 FROM t1 LEFT OUTER JOIN t2 USING (col);
1320 <P>These identical queries join t1.col to t2.col, and also return
1321 any unjoined rows in t1 (those with no match in t2). A
1322 <SMALL>RIGHT</SMALL> join would add unjoined rows of t2. A
1323 <SMALL>FULL</SMALL> join would return the matched rows plus all
1324 unjoined rows from t1 and t2. The word <SMALL>OUTER</SMALL> is
1325 optional and is assumed in <SMALL>LEFT</SMALL>,
1326 <SMALL>RIGHT</SMALL>, and <SMALL>FULL</SMALL> joins. Ordinary joins
1327 are called <SMALL>INNER</SMALL> joins.</P>
1329 <P>In previous releases, outer joins can be simulated using
1330 <SMALL>UNION</SMALL> and <SMALL>NOT IN</SMALL>. For example, when
1331 joining <I>tab1</I> and <I>tab2</I>, the following query does an
1332 <I>outer</I> join of the two tables:<BR>
1336 SELECT tab1.col1, tab2.col2
1338 WHERE tab1.col1 = tab2.col1
1340 SELECT tab1.col1, NULL
1342 WHERE tab1.col1 NOT IN (SELECT tab2.col1 FROM tab2)
1346 <H4><A name="4.24">4.24</A>) How do I perform queries using
1347 multiple databases?</H4>
1349 <P>There is no way to query any database except the current one.
1350 Because PostgreSQL loads database-specific system catalogs, it is
1351 uncertain how a cross-database query should even behave.</P>
1353 <P>Of course, a client can make simultaneous connections to
1354 different databases and merge the information that way.</P>
1356 <H4><A name="4.25">4.25</A>) How do I return multiple rows or
1357 columns from a function?</H4>
1359 <P>You can return result sets from PL/pgSQL functions using
1360 <I>refcursors</I>. See <A href=
1361 "http://developer.postgresql.org/docs/postgres/plpgsql-cursors.html">
1362 http://developer.postgresql.org/docs/postgres/plpgsql-cursors.html,</A>
1363 section 23.7.3.3.</P>
1365 <H4><A name="4.26">4.26</A>) Why can't I reliably create/drop
1366 temporary tables in PL/PgSQL functions?</H4>
1367 PL/PgSQL caches function contents, and an unfortunate side effect
1368 is that if a PL/PgSQL function accesses a temporary table, and that
1369 table is later dropped and recreated, and the function called
1370 again, the function will fail because the cached function contents
1371 still point to the old temporary table. The solution is to use
1372 <SMALL>EXECUTE</SMALL> for temporary table access in PL/PgSQL. This
1373 will cause the query to be reparsed every time.
1377 <H2 align="center">Extending PostgreSQL</H2>
1379 <H4><A name="5.1">5.1</A>) I wrote a user-defined function. When I
1380 run it in <I>psql</I>, why does it dump core?</H4>
1382 <P>The problem could be a number of things. Try testing your
1383 user-defined function in a stand-alone test program first.</P>
1385 <H4><A name="5.2">5.2</A>) How can I contribute some nifty new
1386 types and functions to PostgreSQL?</H4>
1388 <P>Send your extensions to the <I>pgsql-hackers</I> mailing list,
1389 and they will eventually end up in the <I>contrib/</I>
1392 <H4><A name="5.3">5.3</A>) How do I write a C function to return a
1395 <P>This requires wizardry so extreme that the authors have never
1396 tried it, though in principle it can be done.</P>
1398 <H4><A name="5.4">5.4</A>) I have changed a source file. Why does
1399 the recompile not see the change?</H4>
1401 <P>The <I>Makefiles</I> do not have the proper dependencies for
1402 include files. You have to do a <I>make clean</I> and then another
1403 <I>make</I>. If you are using <SMALL>GCC</SMALL> you can use the
1404 <I>--enable-depend</I> option of <I>configure</I> to have the
1405 compiler compute the dependencies automatically.</P>
projects / postgresql / blob