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11 <H1>Frequently Asked Questions (FAQ) for PostgreSQL</H1>
13 <P>Last updated: Mon Mar 17 11:32:59 EST 2003</P>
15 <P>Current maintainer: Bruce Momjian (<A href=
16 "mailto:pgman@candle.pha.pa.us">pgman@candle.pha.pa.us</A>)<BR>
19 <P>The most recent version of this document can be viewed at <A
21 "http://www.PostgreSQL.org/docs/faqs/FAQ.html">http://www.PostgreSQL.org/docs/faqs/FAQ.html</A>.</P>
23 <P>Platform-specific questions are answered at <A href=
24 "http://www.PostgreSQL.org/users-lounge/docs/index.html">http://www.PostgreSQL.org/docs/index.html</A>.</P>
27 <H2 align="center">General Questions</H2>
28 <A href="#1.1">1.1</A>) What is PostgreSQL? How is it pronounced?<BR>
29 <A href="#1.2">1.2</A>) What is the copyright on PostgreSQL?<BR>
30 <A href="#1.3">1.3</A>) What Unix platforms does PostgreSQL run
32 <A href="#1.4">1.4</A>) What non-Unix ports are available?<BR>
33 <A href="#1.5">1.5</A>) Where can I get PostgreSQL?<BR>
34 <A href="#1.6">1.6</A>) Where can I get support?<BR>
35 <A href="#1.7">1.7</A>) What is the latest release?<BR>
36 <A href="#1.8">1.8</A>) What documentation is available?<BR>
37 <A href="#1.9">1.9</A>) How do I find out about known bugs or
39 <A href="#1.10">1.10</A>) How can I learn <SMALL>SQL</SMALL>?<BR>
40 <A href="#1.11">1.11</A>) Is PostgreSQL Y2K compliant?<BR>
41 <A href="#1.12">1.12</A>) How do I join the development team?<BR>
42 <A href="#1.13">1.13</A>) How do I submit a bug report?<BR>
43 <A href="#1.14">1.14</A>) How does PostgreSQL compare to other
44 <SMALL>DBMS</SMALL>s?<BR>
45 <A href="#1.15">1.15</A>) How can I financially assist
49 <H2 align="center">User Client Questions</H2>
50 <A href="#2.1">2.1</A>) Are there <SMALL>ODBC</SMALL> drivers for
52 <A href="#2.2">2.2</A>) What tools are available for using
53 PostgreSQL with Web pages?<BR>
54 <A href="#2.3">2.3</A>) Does PostgreSQL have a graphical user
56 <A href="#2.4">2.4</A>) What languages are available to
57 communicate with PostgreSQL?<BR>
60 <H2 align="center">Administrative Questions</H2>
61 <A href="#3.1">3.1</A>) How do I install PostgreSQL somewhere other
62 than <I>/usr/local/pgsql</I>?<BR>
63 <A href="#3.2">3.2</A>) When I start <I>postmaster</I>, I get a
64 <I>Bad System Call</I> or core dumped message. Why?<BR>
65 <A href="#3.3">3.3</A>) When I try to start <I>postmaster</I>, I
66 get <I>IpcMemoryCreate</I> errors. Why?<BR>
67 <A href="#3.4">3.4</A>) When I try to start <I>postmaster</I>, I
68 get <I>IpcSemaphoreCreate</I> errors. Why?<BR>
69 <A href="#3.5">3.5</A>) How do I control connections from other
71 <A href="#3.6">3.6</A>) How do I tune the database engine for
72 better performance?<BR>
73 <A href="#3.7">3.7</A>) What debugging features are available?<BR>
74 <A href="#3.8">3.8</A>) Why do I get <I>"Sorry, too many
75 clients"</I> when trying to connect?<BR>
76 <A href="#3.9">3.9</A>) What is in the <I>pgsql_tmp</I>
78 <A href="#3.10">3.10</A>) Why do I need to do a dump and restore
79 to upgrade PostgreSQL releases?<BR>
82 <H2 align="center">Operational Questions</H2>
83 <A href="#4.1">4.1</A>) What is the difference between binary
84 cursors and normal cursors?<BR>
85 <A href="#4.2">4.2</A>) How do I <SMALL>SELECT</SMALL> only the
86 first few rows of a query?<BR>
87 <A href="#4.3">4.3</A>) How do I get a list of tables or other
88 things I can see in <I>psql</I>?<BR>
89 <A href="#4.4">4.4</A>) How do you remove a column from a
91 <A href="#4.5">4.5</A>) What is the maximum size for a row, a
92 table, and a database?<BR>
93 <A href="#4.6">4.6</A>) How much database disk space is required
94 to store data from a typical text file?<BR>
95 <A href="#4.7">4.7</A>) How do I find out what tables, indexes,
96 databases, and users are defined?<BR>
97 <A href="#4.8">4.8</A>) My queries are slow or don't make use of
99 <A href="#4.9">4.9</A>) How do I see how the query optimizer is
100 evaluating my query?<BR>
101 <A href="#4.10">4.10</A>) What is an R-tree index?<BR>
102 <A href="#4.11">4.11</A>) What is the Genetic Query Optimizer?<BR>
103 <A href="#4.12">4.12</A>) How do I perform regular expression
104 searches and case-insensitive regular expression searches? How do I
105 use an index for case-insensitive searches?<BR>
106 <A href="#4.13">4.13</A>) In a query, how do I detect if a field
107 is <SMALL>NULL</SMALL>?<BR>
108 <A href="#4.14">4.14</A>) What is the difference between the
109 various character types?<BR>
110 <A href="#4.15.1">4.15.1</A>) How do I create a
111 serial/auto-incrementing field?<BR>
112 <A href="#4.15.2">4.15.2</A>) How do I get the value of a
113 <SMALL>SERIAL</SMALL> insert?<BR>
114 <A href="#4.15.3">4.15.3</A>) Don't <I>currval()</I> and
115 <I>nextval()</I> lead to a race condition with other users?<BR>
116 <A href="#4.15.4">4.15.4</A>) Why aren't my sequence numbers
117 reused on transaction abort? Why are there gaps in the numbering of
118 my sequence/SERIAL column?<BR>
119 <A href="#4.16">4.16</A>) What is an <SMALL>OID</SMALL>? What is a
120 <SMALL>TID</SMALL>?<BR>
121 <A href="#4.17">4.17</A>) What is the meaning of some of the terms
122 used in PostgreSQL?<BR>
123 <A href="#4.18">4.18</A>) Why do I get the error <I>"ERROR: Memory
124 exhausted in AllocSetAlloc()"</I>?<BR>
125 <A href="#4.19">4.19</A>) How do I tell what PostgreSQL version I
127 <A href="#4.20">4.20</A>) Why does my large-object operations get
128 <I>"invalid large obj descriptor"</I>?<BR>
129 <A href="#4.21">4.21</A>) How do I create a column that will
130 default to the current time?<BR>
131 <A href="#4.22">4.22</A>) Why are my subqueries using
132 <CODE><SMALL>IN</SMALL></CODE> so slow?<BR>
133 <A href="#4.23">4.23</A>) How do I perform an outer join?<BR>
134 <A href="#4.24">4.24</A>) How do I perform queries using multiple
136 <A href="#4.25">4.25</A>) How do I return multiple rows or columns
138 <A href="#4.26">4.26</A>) Why can't I reliably create/drop
139 temporary tables in PL/PgSQL functions?<BR>
140 <A href="#4.27">4.27</A>) What replication options are available?<BR>
141 <A href="#4.28">4.28</A>) What encryption options are available?<BR>
144 <H2 align="center">Extending PostgreSQL</H2>
145 <A href="#5.1">5.1</A>) I wrote a user-defined function. When I run
146 it in <I>psql</I>, why does it dump core?<BR>
147 <A href="#5.2">5.2</A>) How can I contribute some nifty new types
148 and functions to PostgreSQL?<BR>
149 <A href="#5.3">5.3</A>) How do I write a C function to return a
151 <A href="#5.4">5.4</A>) I have changed a source file. Why does the
152 recompile not see the change?<BR>
156 <H2 align="center">General Questions</H2>
158 <H4><A name="1.1">1.1</A>) What is PostgreSQL? How is it pronounced?</H4>
160 <P>PostgreSQL is pronounced <I>Post-Gres-Q-L</I>.</P>
162 <P>PostgreSQL is an enhancement of the POSTGRES database management
163 system, a next-generation <SMALL>DBMS</SMALL> research prototype.
164 While PostgreSQL retains the powerful data model and rich data
165 types of POSTGRES, it replaces the PostQuel query language with an
166 extended subset of <SMALL>SQL</SMALL>. PostgreSQL is free and the
167 complete source is available.</P>
169 <P>PostgreSQL development is performed by a team of
170 developers who all subscribe to the PostgreSQL development mailing
171 list. The current coordinator is Marc G. Fournier (<A href=
172 "mailto:scrappy@PostgreSQL.org">scrappy@PostgreSQL.org</A>). (See
173 section <a href="#1.6">1.6</a> on how to join). This team is now
174 responsible for all development of PostgreSQL.</P>
176 <P>The authors of PostgreSQL 1.01 were Andrew Yu and Jolly Chen.
177 Many others have contributed to the porting, testing, debugging,
178 and enhancement of the code. The original Postgres code, from which
179 PostgreSQL is derived, was the effort of many graduate students,
180 undergraduate students, and staff programmers working under the
181 direction of Professor Michael Stonebraker at the University of
182 California, Berkeley.</P>
184 <P>The original name of the software at Berkeley was Postgres. When
185 <SMALL>SQL</SMALL> functionality was added in 1995, its name was
186 changed to Postgres95. The name was changed at the end of 1996 to
189 <H4><A name="1.2">1.2</A>) What is the copyright on
192 <P>PostgreSQL is subject to the following COPYRIGHT:</P>
194 <P>PostgreSQL Data Base Management System</P>
196 <P>Portions copyright (c) 1996-2002, PostgreSQL Global Development
197 Group Portions Copyright (c) 1994-6 Regents of the University of
200 <P>Permission to use, copy, modify, and distribute this software
201 and its documentation for any purpose, without fee, and without a
202 written agreement is hereby granted, provided that the above
203 copyright notice and this paragraph and the following two
204 paragraphs appear in all copies.</P>
206 <P>IN NO EVENT SHALL THE UNIVERSITY OF CALIFORNIA BE LIABLE TO ANY
207 PARTY FOR DIRECT, INDIRECT, SPECIAL, INCIDENTAL, OR CONSEQUENTIAL
208 DAMAGES, INCLUDING LOST PROFITS, ARISING OUT OF THE USE OF THIS
209 SOFTWARE AND ITS DOCUMENTATION, EVEN IF THE UNIVERSITY OF
210 CALIFORNIA HAS BEEN ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.</P>
212 <P>THE UNIVERSITY OF CALIFORNIA SPECIFICALLY DISCLAIMS ANY
213 WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
214 OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE. THE
215 SOFTWARE PROVIDED HEREUNDER IS ON AN "AS IS" BASIS, AND THE
216 UNIVERSITY OF CALIFORNIA HAS NO OBLIGATIONS TO PROVIDE MAINTENANCE,
217 SUPPORT, UPDATES, ENHANCEMENTS, OR MODIFICATIONS.</P>
219 <P>The above is the BSD license, the classic open-source license.
220 It has no restrictions on how the source code may be used. We like
221 it and have no intention of changing it.</P>
223 <H4><A name="1.3">1.3</A>) What Unix platforms does PostgreSQL run
226 <P>In general, a modern Unix-compatible platform should be able to
227 run PostgreSQL. The platforms that had received explicit testing at
228 the time of release are listed in the installation
231 <H4><A name="1.4">1.4</A>) What non-Unix ports are available?</H4>
233 <P><STRONG>Client</STRONG></P>
235 <P>It is possible to compile the <I>libpq</I> C library, psql, and
236 other interfaces and client applications to run on MS Windows platforms.
237 In this case, the client is running on MS Windows, and communicates
238 via TCP/IP to a server running on one of our supported Unix
239 platforms. A file <I>win32.mak</I> is included in the distribution
240 for making a Win32 <I>libpq</I> library and <I>psql</I>. PostgreSQL
241 also communicates with <SMALL>ODBC</SMALL> clients.</P>
243 <P><STRONG>Server</STRONG></P>
245 <P>The database server can run on Windows NT and Win2k using
246 Cygwin, the Cygnus Unix/NT porting library. See
247 <I>pgsql/doc/FAQ_MSWIN</I> in the distribution or the MS Windows FAQ
248 at <A href="http://www.PostgreSQL.org/docs/faqs/faq-mswin.html">
249 http://www.PostgreSQL.org/docs/faqs/faq-mswin.html</A>.</P>
251 <p>A native port to MS Win NT/2000/XP is currently being worked
254 <H4><A name="1.5">1.5</A>) Where can I get PostgreSQL?</H4>
256 <P>The primary anonymous ftp site for PostgreSQL is <A href=
257 "ftp://ftp.PostgreSQL.org/pub">ftp://ftp.PostgreSQL.org/pub</A>.
258 For mirror sites, see our main web site.</P>
260 <H4><A name="1.6">1.6</A>) Where can I get support?</H4>
262 <P>The main mailing list is: <A href=
263 "mailto:pgsql-general@PostgreSQL.org">pgsql-general@PostgreSQL.org</A>.
264 It is available for discussion of matters pertaining to PostgreSQL.
265 To subscribe, send mail with the following lines in the body (not
266 the subject line):</P>
273 "mailto:pgsql-general-request@PostgreSQL.org">pgsql-general-request@PostgreSQL.org</A>.</P>
275 <P>There is also a digest list available. To subscribe to this
276 list, send email to: <A href=
277 "mailto:pgsql-general-digest-request@PostgreSQL.org">pgsql-general-digest-request@PostgreSQL.org</A>
284 Digests are sent out to members of this list whenever the main list
285 has received around 30k of messages.
287 <P>The bugs mailing list is available. To subscribe to this list,
288 send email to <A href=
289 "mailto:pgsql-bugs-request@PostgreSQL.org">pgsql-bugs-request@PostgreSQL.org</A>
296 There is also a developers discussion mailing list available. To
297 subscribe to this list, send email to <A href=
298 "mailto:pgsql-hackers-request@PostgreSQL.org">pgsql-hackers-request@PostgreSQL.org</A>
305 <P>Additional mailing lists and information about PostgreSQL can be
306 found via the PostgreSQL WWW home page at:</P>
309 <A href="http://www.PostgreSQL.org">http://www.PostgreSQL.org</A>
312 <P>There is also an IRC channel on EFNet, channel
313 <I>#PostgreSQL</I>. I use the Unix command <CODE>irc -c
314 '#PostgreSQL' "$USER" irc.phoenix.net.</CODE></P>
316 <P>A list of commercial support companies is available at <A href=
317 "http://www.ca.PostgreSQL.org/users-lounge/commercial-support.html">http://www.ca.PostgreSQL.org/users-lounge/commercial-support.html</A>.</P>
319 <H4><A name="1.7">1.7</A>) What is the latest release?</H4>
321 <P>The latest release of PostgreSQL is version 7.3.2.</P>
323 <P>We plan to have major releases every four months.</P>
325 <H4><A name="1.8">1.8</A>) What documentation is available?</H4>
327 <P>Several manuals, manual pages, and some small test examples are
328 included in the distribution. See the <I>/doc</I> directory. You
329 can also browse the manual online at <A href=
330 "http://www.ca.PostgreSQL.org/users-lounge/docs/">http://www.ca.PostgreSQL.org/users-lounge/docs/</A>.</P>
332 <P>There are two PostgreSQL books available online at <A href=
333 "http://www.PostgreSQL.org/docs/awbook.html">http://www.PostgreSQL.org/docs/awbook.html</A>
335 "http://www.commandprompt.com/ppbook/">http://www.commandprompt.com/ppbook/</A>.
336 There is a list of PostgreSQL books available for purchase at <A
338 "http://www.ca.PostgreSQL.org/books/">http://www.ca.PostgreSQL.org/books/</A>.
339 There is also a collection of PostgreSQL technical articles at <A
341 "http://techdocs.PostgreSQL.org/">http://techdocs.PostgreSQL.org/</A>.</P>
343 <P><I>psql</I> has some nice \d commands to show information about
344 types, operators, functions, aggregates, etc.</P>
346 <P>Our web site contains even more documentation.</P>
348 <H4><A name="1.9">1.9</A>) How do I find out about known bugs or
349 missing features?</H4>
351 <P>PostgreSQL supports an extended subset of <SMALL>SQL</SMALL>-92.
352 See our <A href="http://developer.PostgreSQL.org/todo.php">TODO</A>
353 list for known bugs, missing features, and future plans.</P>
355 <H4><A name="1.10">1.10</A>) How can I learn
356 <SMALL>SQL</SMALL>?</H4>
358 <P>The PostgreSQL book at <A href=
359 "http://www.PostgreSQL.org/docs/awbook.html">http://www.PostgreSQL.org/docs/awbook.html</A>
360 teaches <SMALL>SQL</SMALL>. There is another PostgreSQL book at <A
362 "http://www.commandprompt.com/ppbook/">http://www.commandprompt.com/ppbook.</A>
363 There is a nice tutorial at <A href=
364 "http://www.intermedia.net/support/sql/sqltut.shtm">http://www.intermedia.net/support/sql/sqltut.shtm,</A>
366 "http://ourworld.compuserve.com/homepages/graeme_birchall/HTM_COOK.HTM">
367 http://ourworld.compuserve.com/homepages/graeme_birchall/HTM_COOK.HTM,</A>
369 "http://sqlcourse.com/">http://sqlcourse.com.</A></P>
371 <P>Another one is "Teach Yourself SQL in 21 Days, Second Edition"
373 "http://members.tripod.com/er4ebus/sql/index.htm">http://members.tripod.com/er4ebus/sql/index.htm</A></P>
375 <P>Many of our users like <I>The Practical SQL Handbook</I>,
376 Bowman, Judith S., et al., Addison-Wesley. Others like <I>The
377 Complete Reference SQL</I>, Groff et al., McGraw-Hill.</P>
379 <H4><A name="1.11">1.11</A>) Is PostgreSQL Y2K compliant?</H4>
381 <P>Yes, we easily handle dates past the year 2000 AD, and before
384 <H4><A name="1.12">1.12</A>) How do I join the development
387 <P>First, download the latest source and read the PostgreSQL
388 Developers documentation on our web site, or in the distribution.
389 Second, subscribe to the <I>pgsql-hackers</I> and
390 <I>pgsql-patches</I> mailing lists. Third, submit high quality
391 patches to pgsql-patches.</P>
393 <P>There are about a dozen people who have commit privileges to the
394 PostgreSQL <SMALL>CVS</SMALL> archive. They each have submitted so
395 many high-quality patches that it was impossible for the existing
396 committers to keep up, and we had confidence that patches they
397 committed were of high quality.</P>
399 <H4><A name="1.13">1.13</A>) How do I submit a bug report?</H4>
401 <P>Please visit the PostgreSQL BugTool page at <A href=
402 "http://www.PostgreSQL.org/bugs/bugs.php">http://www.PostgreSQL.org/bugs/bugs.php</A>,
403 which gives guidelines and directions on how to submit a
406 <P>Also check out our ftp site <A href=
407 "ftp://ftp.PostgreSQL.org/pub">ftp://ftp.PostgreSQL.org/pub</A> to
408 see if there is a more recent PostgreSQL version or patches.</P>
410 <H4><A name="1.14">1.14</A>) How does PostgreSQL compare to other
411 <SMALL>DBMS</SMALL>s?</H4>
413 <P>There are several ways of measuring software: features,
414 performance, reliability, support, and price.</P>
417 <DT><B>Features</B></DT>
419 <DD>PostgreSQL has most features present in large commercial
420 <SMALL>DBMS</SMALL>s, like transactions, subselects, triggers,
421 views, foreign key referential integrity, and sophisticated
422 locking. We have some features they do not have, like
423 user-defined types, inheritance, rules, and multi-version
424 concurrency control to reduce lock contention.<BR>
428 <DT><B>Performance</B></DT>
430 <DD>PostgreSQL has performance similar to other commercial and
431 open source databases. it is faster for some things, slower for
432 others. In comparison to MySQL or leaner database systems, we are
433 slower on inserts/updates because of transaction overhead. Of
434 course, MySQL does not have any of the features mentioned in the
435 <I>Features</I> section above. We are built for reliability and
436 features, though we continue to improve performance in every
437 release. There is an interesting Web page comparing PostgreSQL to
438 MySQL at <A href="http://openacs.org/philosophy/why-not-mysql.html">
439 http://openacs.org/philosophy/why-not-mysql.html</A><BR>
444 <DT><B>Reliability</B></DT>
446 <DD>We realize that a <SMALL>DBMS</SMALL> must be reliable, or it
447 is worthless. We strive to release well-tested, stable code that
448 has a minimum of bugs. Each release has at least one month of
449 beta testing, and our release history shows that we can provide
450 stable, solid releases that are ready for production use. We
451 believe we compare favorably to other database software in this
456 <DT><B>Support</B></DT>
458 <DD>Our mailing lists provide contact with a large group of developers
459 and users to help resolve any problems encountered. While we cannot
460 guarantee a fix, commercial <SMALL>DBMS</SMALL>s do not always
461 supply a fix either. Direct access to developers, the user
462 community, manuals, and the source code often make PostgreSQL
463 support superior to other <SMALL>DBMS</SMALL>s. There is
464 commercial per-incident support available for those who need it.
465 (See <A href="#1.6">FAQ section 1.6</A>.)<BR>
469 <DT><B>Price</B></DT>
471 <DD>We are free for all use, both commercial and non-commercial.
472 You can add our code to your product with no limitations, except
473 those outlined in our BSD-style license stated above.<BR>
478 <H4><A name="1.15">1.15</A>) How can I financially assist
481 <P>PostgreSQL has had a first-class infrastructure since we started
482 in 1996. This is all thanks to Marc Fournier, who has created
483 and managed this infrastructure over the years.</P>
485 <P>Quality infrastructure is very important to an open-source
486 project. It prevents disruptions that can greatly delay forward
487 movement of the project.</P>
489 <P>Of course, this infrastructure is not cheap. There are a variety
490 of monthly and one-time expenses that are required to keep it
491 going. If you or your company has money it can donate to help fund
492 this effort, please go to <A href="http://store.pgsql.com/shopping/">http://store.pgsql.com/shopping/</A>
493 and make a donation.</P>
495 <P>Although the web page mentions PostgreSQL, Inc, the
496 "contributions" item is solely to support the PostgreSQL project
497 and does not fund any specific company. If you prefer, you can also
498 send a check to the contact address.</P>
501 <H2 align="center">User Client Questions</H2>
503 <H4><A name="2.1">2.1</A>) Are there <SMALL>ODBC</SMALL> drivers
506 <P>There are two <SMALL>ODBC</SMALL> drivers available, PsqlODBC
507 and OpenLink <SMALL>ODBC</SMALL>.</P>
509 <P>You can download PsqlODBC from <A href=
510 "http://gborg.postgresql.org/project/psqlodbc/projdisplay.php">
511 http://gborg.postgresql.org/project/psqlodbc/projdisplay.php</A>.</P>
513 <P>OpenLink <SMALL>ODBC</SMALL> can be gotten from <A href=
514 "http://www.openlinksw.com/">http://www.openlinksw.com</A>. It
515 works with their standard <SMALL>ODBC</SMALL> client software so
516 you'll have PostgreSQL <SMALL>ODBC</SMALL> available on every
517 client platform they support (Win, Mac, Unix, VMS).</P>
519 <P>They will probably be selling this product to people who need
520 commercial-quality support, but a freeware version will always be
521 available. Please send questions to <A href=
522 "mailto:postgres95@openlink.co.uk">postgres95@openlink.co.uk</A>.</P>
524 <H4><A name="2.2">2.2</A>) What tools are available for using
525 PostgreSQL with Web pages?</H4>
527 <P>A nice introduction to Database-backed Web pages can be seen at:
528 <A href="http://www.webreview.com">http://www.webreview.com</A></P>
530 <P>For Web integration, PHP is an excellent interface. It is at <A
531 href="http://www.php.net">http://www.php.net</A>.</P>
533 <P>For complex cases, many use the Perl interface and CGI.pm or mod_perl.</P>
535 <H4><A name="2.3">2.3</A>) Does PostgreSQL have a graphical user
538 <P>Yes, there are several graphical interfaces to PostgreSQL available.
539 These include PgAccess <a href="http://www.pgaccess.org">
540 http://www.pgaccess.org</a>), PgAdmin II (<a
541 href="http://www.pgadmin.org">http://www.pgadmin.org</a>,
542 Win32-only), RHDB Admin (<a
543 href="http://sources.redhat.com/rhdb/">http://sources.redhat.com/rhdb/
544 </a>) and Rekall (<a href="http://www.thekompany.com/products/rekall/">
545 http://www.thekompany.com/products/rekall/</a>, proprietary). There is
546 also PHPPgAdmin (<a href="http://phppgadmin.sourceforge.net/">
547 http://phppgadmin.sourceforge.net/ </a>), a web-based interface to
550 <P>See <a href="http://techdocs.postgresql.org/guides/GUITools">http://techdocs.postgresql.org/guides/GUITools</a> for a more detailed list.</P>
552 <H4><A name="2.4">2.4</A>) What languages are able to communicate with
555 <P>Most popular programming languages contain an interface to
556 PostgreSQL. Check your programming language's list of extension
559 <P>The following interfaces are included in the PostgreSQL
565 <LI>Embedded C (ecpg)</LI>
569 <LI>Python (PyGreSQL)</LI>
571 <LI>TCL (libpgtcl)</LI>
574 <P>Additional interfaces are available at
575 <a href="http://gborg.postgresql.org">http://gborg.postgresql.org</A>
576 in the <I>Drivers/Interfaces</I> section.
580 <H2 align="center">Administrative Questions</H2>
582 <H4><A name="3.1">3.1</A>) How do I install PostgreSQL somewhere
583 other than <I>/usr/local/pgsql</I>?</H4>
585 <P>Specify the <I>--prefix</I> option when running
586 <I>configure</I>.</P>
588 <H4><A name="3.2">3.2</A>) When I start <I>postmaster</I>, I get a
589 <I>Bad System Call</I> or core dumped message. Why?</H4>
591 <P>It could be a variety of problems, but first check to see that
592 you have System V extensions installed in your kernel. PostgreSQL
593 requires kernel support for shared memory and semaphores.</P>
595 <H4><A name="3.3">3.3</A>) When I try to start <I>postmaster</I>, I
596 get <I>IpcMemoryCreate</I> errors. Why?</H4>
598 <P>You either do not have shared memory configured properly in your
599 kernel or you need to enlarge the shared memory available in the
600 kernel. The exact amount you need depends on your architecture and
601 how many buffers and backend processes you configure for
602 <I>postmaster</I>. For most systems, with default numbers of
603 buffers and processes, you need a minimum of ~1 MB. See the <A
605 "http://www.postgresql.org/idocs/index.php?kernel-resources.html">PostgreSQL
606 Administrator's Guide</A> for more detailed information about
607 shared memory and semaphores.</P>
609 <H4><A name="3.4">3.4</A>) When I try to start <I>postmaster</I>, I
610 get <I>IpcSemaphoreCreate</I> errors. Why?</H4>
612 <P>If the error message is <I>IpcSemaphoreCreate: semget failed (No
613 space left on device)</I> then your kernel is not configured with
614 enough semaphores. Postgres needs one semaphore per potential
615 backend process. A temporary solution is to start <I>postmaster</I>
616 with a smaller limit on the number of backend processes. Use
617 <I>-N</I> with a parameter less than the default of 32. A more
618 permanent solution is to increase your kernel's
619 <SMALL>SEMMNS</SMALL> and <SMALL>SEMMNI</SMALL> parameters.</P>
621 <P>Inoperative semaphores can also cause crashes during heavy
624 <P>If the error message is something else, you might not have
625 semaphore support configured in your kernel at all. See the
626 PostgreSQL Administrator's Guide for more detailed information
627 about shared memory and semaphores.</P>
629 <H4><A name="3.5">3.5</A>) How do I control connections from other
632 <P>By default, PostgreSQL only allows connections from the local
633 machine using Unix domain sockets. Other machines will not be able
634 to connect unless you add the <I>-i</I> flag to <I>postmaster</I>,
635 <B>and</B> enable host-based authentication by modifying the file
636 <I>$PGDATA/pg_hba.conf</I> accordingly. This will allow TCP/IP
639 <H4><A name="3.6">3.6</A>) How do I tune the database engine for
640 better performance?</H4>
642 <P>Certainly, indexes can speed up queries. The
643 <SMALL>EXPLAIN</SMALL> command allows you to see how PostgreSQL is
644 interpreting your query, and which indexes are being used.</P>
646 <P>If you are doing many <SMALL>INSERTs</SMALL>, consider doing
647 them in a large batch using the <SMALL>COPY</SMALL> command. This
648 is much faster than individual <SMALL>INSERTS</SMALL>. Second,
649 statements not in a <SMALL>BEGIN WORK/COMMIT</SMALL> transaction
650 block are considered to be in their own transaction. Consider
651 performing several statements in a single transaction block. This
652 reduces the transaction overhead. Also, consider dropping and
653 recreating indexes when making large data changes.</P>
655 <P>There are several tuning options. You can disable <I>fsync()</I>
656 by starting <I>postmaster</I> with a <I>-o -F</I> option. This will
657 prevent <I>fsync()</I>s from flushing to disk after every
660 <P>You can also use the <I>postmaster</I> <I>-B</I> option to
661 increase the number of shared memory buffers used by the backend
662 processes. If you make this parameter too high, the
663 <I>postmaster</I> may not start because you have exceeded your
664 kernel's limit on shared memory space. Each buffer is 8K and the
665 default is 64 buffers.</P>
667 <P>You can also use the backend <I>-S</I> option to increase the
668 maximum amount of memory used by the backend process for temporary
669 sorts. The <I>-S</I> value is measured in kilobytes, and the
670 default is 512 (i.e. 512K).</P>
672 <P>You can also use the <SMALL>CLUSTER</SMALL> command to group
673 data in tables to match an index. See the <SMALL>CLUSTER</SMALL>
674 manual page for more details.</P>
676 <H4><A name="3.7">3.7</A>) What debugging features are
679 <P>PostgreSQL has several features that report status information
680 that can be valuable for debugging purposes.</P>
682 <P>First, by running <I>configure</I> with the --enable-cassert
683 option, many <I>assert()</I>s monitor the progress of the backend
684 and halt the program when something unexpected occurs.</P>
686 <P>Both <I>postmaster</I> and <I>postgres</I> have several debug
687 options available. First, whenever you start <I>postmaster</I>,
688 make sure you send the standard output and error to a log file,
692 ./bin/postmaster >server.log 2>&1 &
695 <P>This will put a server.log file in the top-level PostgreSQL
696 directory. This file contains useful information about problems or
697 errors encountered by the server. <I>Postmaster</I> has a <I>-d</I>
698 option that allows even more detailed information to be reported.
699 The <I>-d</I> option takes a number that specifies the debug level.
700 Be warned that high debug level values generate large log
703 <P>If <I>postmaster</I> is not running, you can actually run the
704 <I>postgres</I> backend from the command line, and type your
705 <SMALL>SQL</SMALL> statement directly. This is recommended
706 <B>only</B> for debugging purposes. Note that a newline terminates
707 the query, not a semicolon. If you have compiled with debugging
708 symbols, you can use a debugger to see what is happening. Because
709 the backend was not started from <I>postmaster</I>, it is not
710 running in an identical environment and locking/backend interaction
711 problems may not be duplicated.</P>
713 <P>If <I>postmaster</I> is running, start <I>psql</I> in one
714 window, then find the <SMALL>PID</SMALL> of the <I>postgres</I>
715 process used by <I>psql</I>. Use a debugger to attach to the
716 <I>postgres</I> <SMALL>PID</SMALL>. You can set breakpoints in the
717 debugger and issue queries from <I>psql</I>. If you are debugging
718 <I>postgres</I> startup, you can set PGOPTIONS="-W n", then start
719 <I>psql</I>. This will cause startup to delay for <I>n</I> seconds
720 so you can attach to the process with the debugger, set any
721 breakpoints, and continue through the startup sequence.</P>
723 <P>The <I>postgres</I> program has <I>-s, -A</I>, and <I>-t</I>
724 options that can be very useful for debugging and performance
727 <P>You can also compile with profiling to see what functions are
728 taking execution time. The backend profile files will be deposited
729 in the <I>pgsql/data/base/dbname</I> directory. The client profile
730 file will be put in the client's current directory. Linux requires
731 a compile with <I>-DLINUX_PROFILE</I> for proper profiling.</P>
733 <H4><A name="3.8">3.8</A>) Why do I get <I>"Sorry, too many
734 clients"</I> when trying to connect?</H4>
736 <P>You need to increase <I>postmaster</I>'s limit on how many
737 concurrent backend processes it can start.</P>
739 <P>The default limit is 32 processes. You can increase it by
740 restarting <I>postmaster</I> with a suitable <I>-N</I> value or
741 modifying <I>postgresql.conf</I>.</P>
743 <P>Note that if you make <I>-N</I> larger than 32, you must also
744 increase <I>-B</I> beyond its default of 64; <I>-B</I> must be at
745 least twice <I>-N</I>, and probably should be more than that for
746 best performance. For large numbers of backend processes, you are
747 also likely to find that you need to increase various Unix kernel
748 configuration parameters. Things to check include the maximum size
749 of shared memory blocks, <SMALL>SHMMAX;</SMALL> the maximum number
750 of semaphores, <SMALL>SEMMNS</SMALL> and <SMALL>SEMMNI;</SMALL> the
751 maximum number of processes, <SMALL>NPROC;</SMALL> the maximum
752 number of processes per user, <SMALL>MAXUPRC;</SMALL> and the
753 maximum number of open files, <SMALL>NFILE</SMALL> and
754 <SMALL>NINODE</SMALL>. The reason that PostgreSQL has a limit on
755 the number of allowed backend processes is so your system won't run
756 out of resources.</P>
758 <H4><A name="3.9">3.9</A>) What is in the <I>pgsql_tmp</I> directory?</H4>
760 <P>This directory contains temporary files generated by the query
761 executor. For example, if a sort needs to be done to satisfy an
762 <SMALL>ORDER BY</SMALL> and the sort requires more space than the
763 backend's <I>-S</I> parameter allows, then temporary files are created
764 here to hold the extra data.</P>
766 <P>The temporary files are usually deleted automatically, but might
767 remain if a backend crashes during a sort. A stop and restart of the
768 <I>postmaster</I> will remove files from those directories.</P>
770 <H4><A name="3.10">3.10</A>) Why do I need to do a dump and restore
771 to upgrade between major PostgreSQL releases?</H4>
773 <P>The PostgreSQL team makes only small changes between minor releases,
774 so upgrading from 7.2 to 7.2.1 does not require a dump and restore.
775 However, major releases (e.g. from 7.2 to 7.3) often change the internal
776 format of system tables and data files. These changes are often complex,
777 so we don't maintain backward compatability for data files. A dump outputs
778 data in a generic format that can then be loaded in using the new internal
781 <P>In releases where the on-disk format does not change, the
782 <I>pg_upgrade</I> script can be used to upgrade without a dump/restore.
783 The release notes mention whether <I>pg_upgrade</I> is available for the
788 <H2 align="center">Operational Questions</H2>
790 <H4><A name="4.1">4.1</A>) What is the difference between binary
791 cursors and normal cursors?</H4>
793 <P>See the <SMALL>DECLARE</SMALL> manual page for a
796 <H4><A name="4.2">4.2</A>) How do I <SMALL>SELECT</SMALL> only the
797 first few rows of a query?</H4>
799 <P>See the <SMALL>FETCH</SMALL> manual page, or use
800 <SMALL>SELECT</SMALL> ... <SMALL>LIMIT</SMALL>....</P>
802 <P>The entire query may have to be evaluated, even if you only want
803 the first few rows. Consider using a query that has an <SMALL>ORDER
804 BY</SMALL>. If there is an index that matches the <SMALL>ORDER
805 BY</SMALL>, PostgreSQL may be able to evaluate only the first few
806 records requested, or the entire query may have to be evaluated
807 until the desired rows have been generated.</P>
809 <H4><A name="4.3">4.3</A>) How do I get a list of tables or other
810 things I can see in <I>psql</I>?</H4>
812 <P>You can read the source code for <I>psql</I> in file
813 <I>pgsql/src/bin/psql/describe.c</I>. It contains
814 <SMALL>SQL</SMALL> commands that generate the output for psql's
815 backslash commands. You can also start <I>psql</I> with the
816 <I>-E</I> option so it will print out the queries it uses to
817 execute the commands you give.</P>
819 <H4><A name="4.4">4.4</A>) How do you remove a column from a
822 <P>This functionality was added in release 7.3 with
823 <SMALL>ALTER TABLE DROP COLUMN</SMALL>. In earlier versions,
827 LOCK TABLE old_table;
828 SELECT ... -- select all columns but the one you want to remove
831 DROP TABLE old_table;
832 ALTER TABLE new_table RENAME TO old_table;
836 <H4><A name="4.5">4.5</A>) What is the maximum size for a row, a
837 table, and a database?</H4>
839 <P>These are the limits:</P>
841 Maximum size for a database? unlimited (4 TB databases exist)
842 Maximum size for a table? 16 TB
843 Maximum size for a row? 1.6TB
844 Maximum size for a field? 1 GB
845 Maximum number of rows in a table? unlimited
846 Maximum number of columns in a table? 250-1600 depending on column types
847 Maximum number of indexes on a table? unlimited
850 Of course, these are not actually unlimited, but limited to
851 available disk space and memory/swap space. Performance may suffer
852 when these values get unusually large.
854 <P>The maximum table size of 16 TB does not require large file
855 support from the operating system. Large tables are stored as
856 multiple 1 GB files so file system size limits are not
859 <P>The maximum table size and maximum number of columns can be
860 increased if the default block size is increased to 32k.</P>
862 <H4><A name="4.6">4.6</A>) How much database disk space is required
863 to store data from a typical text file?</H4>
865 <P>A PostgreSQL database may require up to five times the disk
866 space to store data from a text file.</P>
868 <P>As an example, consider a file of 100,000 lines with an integer
869 and text description on each line. Suppose the text string
870 avergages twenty bytes in length. The flat file would be 2.8 MB.
871 The size of the PostgreSQL database file containing this data can
872 be estimated as 6.4 MB:</P>
874 36 bytes: each row header (approximate)
875 24 bytes: one int field and one text field
876 + 4 bytes: pointer on page to tuple
877 ----------------------------------------
880 The data page size in PostgreSQL is 8192 bytes (8 KB), so:
883 ------------------- = 128 rows per database page (rounded down)
887 -------------------- = 782 database pages (rounded up)
890 782 database pages * 8192 bytes per page = 6,406,144 bytes (6.4 MB)
893 <P>Indexes do not require as much overhead, but do contain the data
894 that is being indexed, so they can be large also.</P>
896 <P><SMALL>NULL</SMALL>s are stored in bitmaps, so they
897 use very little space.</P>
899 <H4><A name="4.7">4.7</A>) How do I find out what tables, indexes,
900 databases, and users are defined?</H4>
902 <P><I>psql</I> has a variety of backslash commands to show such
903 information. Use \? to see them. There are also system tables
904 beginning with <I>pg_</I> that describe these too. Also, <I>psql
905 -l</I> will list all databases.</P>
907 <P>Also try the file <I>pgsql/src/tutorial/syscat.source</I>. It
908 illustrates many of the <SMALL>SELECT</SMALL>s needed to get
909 information from the database system tables.</P>
911 <H4><A name="4.8">4.8</A>) My queries are slow or don't make use of
912 the indexes. Why?</H4>
913 Indexes are not automatically used by every query. Indexes are only
914 used if the table is larger than a minimum size, and the query
915 selects only a small percentage of the rows in the table. This is
916 because the random disk access caused by an index scan can be
917 slower than a straight read through the table, or sequential scan.
919 <P>To determine if an index should be used, PostgreSQL must have
920 statistics about the table. These statistics are collected using
921 <SMALL>VACUUM ANALYZE</SMALL>, or simply <SMALL>ANALYZE</SMALL>.
922 Using statistics, the optimizer knows how many rows are in the
923 table, and can better determine if indexes should be used.
924 Statistics are also valuable in determining optimal join order and
925 join methods. Statistics collection should be performed
926 periodically as the contents of the table change.</P>
928 <P>Indexes are normally not used for <SMALL>ORDER BY</SMALL> or to
929 perform joins. A sequential scan followed by an explicit sort is
930 usually faster than an index scan of a large table.</P>
931 However, <SMALL>LIMIT</SMALL> combined with <SMALL>ORDER BY</SMALL>
932 often will use an index because only a small portion of the table
933 is returned. In fact, though MAX() and MIN() don't use indexes,
934 it is possible to retrieve such values using an index with ORDER BY
939 ORDER BY col [ DESC ]
943 <P>If you believe the optimizer is incorrect in choosing a
944 sequential scan, use <CODE>SET enable_seqscan TO 'off'</CODE> and
945 run tests to see if an index scan is indeed faster.</P>
947 <P>When using wild-card operators such as <SMALL>LIKE</SMALL> or
948 <I>~</I>, indexes can only be used in certain circumstances:</P>
950 <LI>The beginning of the search string must be anchored to the start
953 <LI><SMALL>LIKE</SMALL> patterns must not start with <I>%</I>.</LI>
954 <LI><I>~</I> (regular expression) patterns must start with
957 <LI>The search string can not start with a character class,
959 <LI>Case-insensitive searches such as <SMALL>ILIKE</SMALL> and
960 <I>~*</I> do not utilise indexes. Instead, use functional
961 indexes, which are described in section <a href="#4.12">4.12</a>.</LI>
962 <LI>The default <I>C</I> locale must be used during
967 <H4><A name="4.9">4.9</A>) How do I see how the query optimizer is
968 evaluating my query?</H4>
970 <P>See the <SMALL>EXPLAIN</SMALL> manual page.</P>
972 <H4><A name="4.10">4.10</A>) What is an R-tree index?</H4>
974 <P>An R-tree index is used for indexing spatial data. A hash index
975 can't handle range searches. A B-tree index only handles range
976 searches in a single dimension. R-trees can handle
977 multi-dimensional data. For example, if an R-tree index can be
978 built on an attribute of type <I>point</I>, the system can more
979 efficiently answer queries such as "select all points within a
980 bounding rectangle."</P>
982 <P>The canonical paper that describes the original R-tree design
985 <P>Guttman, A. "R-trees: A Dynamic Index Structure for Spatial
986 Searching." Proceedings of the 1984 ACM SIGMOD Int'l Conf on Mgmt
989 <P>You can also find this paper in Stonebraker's "Readings in
990 Database Systems".</P>
992 <P>Built-in R-trees can handle polygons and boxes. In theory,
993 R-trees can be extended to handle higher number of dimensions. In
994 practice, extending R-trees requires a bit of work and we don't
995 currently have any documentation on how to do it.</P>
997 <H4><A name="4.11">4.11</A>) What is the Genetic Query
1000 <P>The <SMALL>GEQO</SMALL> module speeds query optimization when
1001 joining many tables by means of a Genetic Algorithm (GA). It allows
1002 the handling of large join queries through nonexhaustive
1005 <H4><A name="4.12">4.12</A>) How do I perform regular expression
1006 searches and case-insensitive regular expression searches? How do I
1007 use an index for case-insensitive searches?</H4>
1009 <P>The <I>~</I> operator does regular expression matching, and
1010 <I>~*</I> does case-insensitive regular expression matching. The
1011 case-insensitive variant of <SMALL>LIKE</SMALL> is called
1012 <SMALL>ILIKE</SMALL>.</P>
1014 <P>Case-insensitive equality comparisons are normally expressed
1019 WHERE lower(col) = 'abc';
1022 This will not use an standard index. However, if you create a
1023 functional index, it will be used:
1025 CREATE INDEX tabindex ON tab (lower(col));
1028 <H4><A name="4.13">4.13</A>) In a query, how do I detect if a field
1029 is <SMALL>NULL</SMALL>?</H4>
1031 <P>You test the column with <SMALL>IS NULL</SMALL> and <SMALL>IS
1032 NOT NULL</SMALL>.</P>
1034 <H4><A name="4.14">4.14</A>) What is the difference between the
1035 various character types?</H4>
1037 Type Internal Name Notes
1038 --------------------------------------------------
1039 VARCHAR(n) varchar size specifies maximum length, no padding
1040 CHAR(n) bpchar blank padded to the specified fixed length
1041 TEXT text no specific upper limit on length
1042 BYTEA bytea variable-length byte array (null-byte safe)
1043 "char" char one character
1046 <P>You will see the internal name when examining system catalogs
1047 and in some error messages.</P>
1049 <P>The first four types above are "varlena" types (i.e., the first
1050 four bytes on disk are the length, followed by the data). Thus the
1051 actual space used is slightly greater than the declared size.
1052 However, these data types are also subject to compression or being
1053 stored out-of-line by <SMALL>TOAST</SMALL>, so the space on disk
1054 might also be less than expected.</P>
1056 <SMALL>VARCHAR(n)</SMALL> is best when storing variable-length
1057 strings and it limits how long a string can be. <SMALL>TEXT</SMALL>
1058 is for strings of unlimited length, with a maximum of one gigabyte.
1059 <P><SMALL>CHAR(n)</SMALL> is for storing strings that are all the
1060 same length. <SMALL>CHAR(n)</SMALL> pads with blanks to the specified
1061 length, while <SMALL>VARCHAR(n)</SMALL> only stores the characters
1062 supplied. <SMALL>BYTEA</SMALL> is for storing binary data,
1063 particularly values that include <SMALL>NULL</SMALL> bytes. All the
1064 types described here have similar performance characteristics.</P>
1066 <H4><A name="4.15.1">4.15.1</A>) How do I create a
1067 serial/auto-incrementing field?</H4>
1069 <P>PostgreSQL supports a <SMALL>SERIAL</SMALL> data type. It
1070 auto-creates a sequence and index on the column. For example,
1073 CREATE TABLE person (
1079 is automatically translated into this:
1081 CREATE SEQUENCE person_id_seq;
1082 CREATE TABLE person (
1083 id INT4 NOT NULL DEFAULT nextval('person_id_seq'),
1086 CREATE UNIQUE INDEX person_id_key ON person ( id );
1089 See the <I>create_sequence</I> manual page for more information
1090 about sequences. You can also use each row's <I>OID</I> field as a
1091 unique value. However, if you need to dump and reload the database,
1092 you need to use <I>pg_dump</I>'s <I>-o</I> option or <SMALL>COPY
1093 WITH OIDS</SMALL> option to preserve the <SMALL>OID</SMALL>s.
1095 <H4><A name="4.15.2">4.15.2</A>) How do I get the value of a
1096 <SMALL>SERIAL</SMALL> insert?</H4>
1098 <P>One approach is to retrieve the next <SMALL>SERIAL</SMALL> value
1099 from the sequence object with the <I>nextval()</I> function
1100 <I>before</I> inserting and then insert it explicitly. Using the
1101 example table in <A href="#4.15.1">4.15.1</A>, an example in a
1102 pseudo-language would look like this:</P>
1104 new_id = execute("SELECT nextval('person_id_seq')");
1105 execute("INSERT INTO person (id, name) VALUES (new_id, 'Blaise Pascal')");
1108 You would then also have the new value stored in
1109 <CODE>new_id</CODE> for use in other queries (e.g., as a foreign
1110 key to the <CODE>person</CODE> table). Note that the name of the
1111 automatically created <SMALL>SEQUENCE</SMALL> object will be named
1112 <<I>table</I>>_<<I>serialcolumn</I>>_<I>seq</I>, where
1113 <I>table</I> and <I>serialcolumn</I> are the names of your table
1114 and your <SMALL>SERIAL</SMALL> column, respectively.
1116 <P>Alternatively, you could retrieve the assigned
1117 <SMALL>SERIAL</SMALL> value with the <I>currval()</I> function
1118 <I>after</I> it was inserted by default, e.g.,</P>
1120 execute("INSERT INTO person (name) VALUES ('Blaise Pascal')");
1121 new_id = execute("SELECT currval('person_id_seq')");
1124 Finally, you could use the <A href="#4.16"><SMALL>OID</SMALL></A>
1125 returned from the <SMALL>INSERT</SMALL> statement to look up the
1126 default value, though this is probably the least portable approach.
1127 In Perl, using DBI with Edmund Mergl's DBD::Pg module, the oid
1128 value is made available via <I>$sth->{pg_oid_status}</I> after
1129 <I>$sth->execute()</I>.
1131 <H4><A name="4.15.3">4.15.3</A>) Don't <I>currval()</I> and
1132 <I>nextval()</I> lead to a race condition with other users?</H4>
1134 <P>No. <I>currval()</I> returns the current value assigned by your
1135 backend, not by all users.</P>
1137 <H4><A name="4.15.4">4.15.4</A>) Why aren't my sequence numbers
1138 reused on transaction abort? Why are there gaps in the numbering of
1139 my sequence/SERIAL column?</H4>
1141 <P>To improve concurrency, sequence values are given out to running
1142 transactions as needed and are not locked until the transaction
1143 completes. This causes gaps in numbering from aborted
1146 <H4><A name="4.16">4.16</A>) What is an <SMALL>OID</SMALL>? What is
1147 a <SMALL>TID</SMALL>?</H4>
1149 <P><SMALL>OID</SMALL>s are PostgreSQL's answer to unique row ids.
1150 Every row that is created in PostgreSQL gets a unique
1151 <SMALL>OID</SMALL>. All <SMALL>OID</SMALL>s generated during
1152 <I>initdb</I> are less than 16384 (from
1153 <I>include/access/transam.h</I>). All user-created
1154 <SMALL>OID</SMALL>s are equal to or greater than this. By default,
1155 all these <SMALL>OID</SMALL>s are unique not only within a table or
1156 database, but unique within the entire PostgreSQL installation.</P>
1158 <P>PostgreSQL uses <SMALL>OID</SMALL>s in its internal system
1159 tables to link rows between tables. These <SMALL>OID</SMALL>s can
1160 be used to identify specific user rows and used in joins. It is
1161 recommended you use column type <SMALL>OID</SMALL> to store
1162 <SMALL>OID</SMALL> values. You can create an index on the
1163 <SMALL>OID</SMALL> field for faster access.</P>
1165 <P>O<SMALL>ID</SMALL>s are assigned to all new rows from a central
1166 area that is used by all databases. If you want to change the
1167 <SMALL>OID</SMALL> to something else, or if you want to make a copy
1168 of the table, with the original <SMALL>OID</SMALL>s, there is no
1169 reason you can't do it:</P>
1171 CREATE TABLE new_table(old_oid oid, mycol int);
1172 SELECT old_oid, mycol INTO new FROM old;
1173 COPY new TO '/tmp/pgtable';
1175 COPY new WITH OIDS FROM '/tmp/pgtable';
1178 CREATE TABLE new_table (mycol int);
1179 INSERT INTO new_table (oid, mycol) SELECT oid, mycol FROM old_table;
1181 <P>O<SMALL>ID</SMALL>s are stored as 4-byte integers, and will
1182 overflow at 4 billion. No one has reported this ever happening, and
1183 we plan to have the limit removed before anyone does.</P>
1185 <P>T<SMALL>ID</SMALL>s are used to identify specific physical rows
1186 with block and offset values. T<SMALL>ID</SMALL>s change after rows
1187 are modified or reloaded. They are used by index entries to point
1188 to physical rows.</P>
1190 <H4><A name="4.17">4.17</A>) What is the meaning of some of the
1191 terms used in PostgreSQL?</H4>
1193 <P>Some of the source code and older documentation use terms that
1194 have more common usage. Here are some:</P>
1197 <LI>table, relation, class</LI>
1199 <LI>row, record, tuple</LI>
1201 <LI>column, field, attribute</LI>
1203 <LI>retrieve, select</LI>
1205 <LI>replace, update</LI>
1207 <LI>append, insert</LI>
1209 <LI><SMALL>OID</SMALL>, serial value</LI>
1211 <LI>portal, cursor</LI>
1213 <LI>range variable, table name, table alias</LI>
1216 <P>A list of general database terms can be found at: <A href=
1217 "http://hea-www.harvard.edu/MST/simul/software/docs/pkgs/pgsql/glossary/glossary.html">http://hea-www.harvard.edu/MST/simul/software/docs/pkgs/pgsql/glossary/glossary.html</A></P>
1219 <H4><A name="4.18">4.18</A>) Why do I get the error <I>"ERROR:
1220 Memory exhausted in AllocSetAlloc()"</I>?</H4>
1222 <P>You probably have run out of virtual memory on your system,
1223 or your kernel has a low limit for certain resources. Try this
1224 before starting <I>postmaster</I>:</P>
1230 Depending on your shell, only one of these may succeed, but it will
1231 set your process data segment limit much higher and perhaps allow
1232 the query to complete. This command applies to the current process,
1233 and all subprocesses created after the command is run. If you are
1234 having a problem with the <SMALL>SQL</SMALL> client because the
1235 backend is returning too much data, try it before starting the
1238 <H4><A name="4.19">4.19</A>) How do I tell what PostgreSQL version
1241 <P>From <I>psql</I>, type <CODE>SELECT version();</CODE></P>
1243 <H4><A name="4.20">4.20</A>) Why does my large-object operations
1244 get <I>"invalid large obj descriptor"</I>?</H4>
1246 <P>You need to put <CODE>BEGIN WORK</CODE> and <CODE>COMMIT</CODE>
1247 around any use of a large object handle, that is, surrounding
1248 <CODE>lo_open</CODE> ... <CODE>lo_close.</CODE></P>
1250 <P>Currently PostgreSQL enforces the rule by closing large object
1251 handles at transaction commit. So the first attempt to do anything
1252 with the handle will draw <I>invalid large obj descriptor</I>. So
1253 code that used to work (at least most of the time) will now
1254 generate that error message if you fail to use a transaction.</P>
1256 <P>If you are using a client interface like <SMALL>ODBC</SMALL> you
1257 may need to set <CODE>auto-commit off.</CODE></P>
1259 <H4><A name="4.21">4.21</A>) How do I create a column that will
1260 default to the current time?</H4>
1262 <P>Use <I>CURRENT_TIMESTAMP</I>:</P>
1264 <CODE>CREATE TABLE test (x int, modtime timestamp DEFAULT CURRENT_TIMESTAMP );
1268 <H4><A name="4.22">4.22</A>) Why are my subqueries using
1269 <CODE><SMALL>IN</SMALL></CODE> so slow?</H4>
1271 <P>Currently, we join subqueries to outer queries by sequentially
1272 scanning the result of the subquery for each row of the outer
1273 query. If the subquery returns only a few rows and the outer query
1274 returns many rows, <CODE><SMALL>IN</SMALL></CODE> is fastest. To
1275 speed up other queries, replace <CODE>IN</CODE> with
1276 <CODE>EXISTS</CODE>:</P>
1279 WHERE col IN (SELECT subcol FROM subtab);
1284 WHERE EXISTS (SELECT subcol FROM subtab WHERE subcol = col);
1287 For this to be fast, <CODE>subcol</CODE> should be an indexed column.
1288 This preformance problem will be fixed in 7.4.
1290 <H4><A name="4.23">4.23</A>) How do I perform an outer join?</H4>
1292 <P>PostgreSQL supports outer joins using the SQL standard syntax.
1293 Here are two examples:</P>
1296 FROM t1 LEFT OUTER JOIN t2 ON (t1.col = t2.col);
1301 FROM t1 LEFT OUTER JOIN t2 USING (col);
1304 <P>These identical queries join t1.col to t2.col, and also return
1305 any unjoined rows in t1 (those with no match in t2). A
1306 <SMALL>RIGHT</SMALL> join would add unjoined rows of t2. A
1307 <SMALL>FULL</SMALL> join would return the matched rows plus all
1308 unjoined rows from t1 and t2. The word <SMALL>OUTER</SMALL> is
1309 optional and is assumed in <SMALL>LEFT</SMALL>,
1310 <SMALL>RIGHT</SMALL>, and <SMALL>FULL</SMALL> joins. Ordinary joins
1311 are called <SMALL>INNER</SMALL> joins.</P>
1313 <P>In previous releases, outer joins can be simulated using
1314 <SMALL>UNION</SMALL> and <SMALL>NOT IN</SMALL>. For example, when
1315 joining <I>tab1</I> and <I>tab2</I>, the following query does an
1316 <I>outer</I> join of the two tables:<BR>
1320 SELECT tab1.col1, tab2.col2
1322 WHERE tab1.col1 = tab2.col1
1324 SELECT tab1.col1, NULL
1326 WHERE tab1.col1 NOT IN (SELECT tab2.col1 FROM tab2)
1330 <H4><A name="4.24">4.24</A>) How do I perform queries using
1331 multiple databases?</H4>
1333 <P>There is no way to query a database other than the current one.
1334 Because PostgreSQL loads database-specific system catalogs, it is
1335 uncertain how a cross-database query should even behave.</P>
1337 <P><I>contrib/dblink</I> allows cross-database queries using
1338 function calls. Of course, a client can make simultaneous
1339 connections to different databases and merge the results on the
1342 <H4><A name="4.25">4.25</A>) How do I return multiple rows or
1343 columns from a function?</H4>
1345 <P>You can return result sets from PL/pgSQL functions using
1346 <I>refcursors</I>. See <A href=
1347 "http://www.PostgreSQL.org/idocs/index.php?plpgsql-cursors.html">
1348 http://www.PostgreSQL.org/idocs/index.php?plpgsql-cursors.html</A>,
1349 section 23.7.3.3.</P>
1351 <H4><A name="4.26">4.26</A>) Why can't I reliably create/drop
1352 temporary tables in PL/PgSQL functions?</H4>
1353 <P>PL/PgSQL caches function contents, and an unfortunate side effect
1354 is that if a PL/PgSQL function accesses a temporary table, and that
1355 table is later dropped and recreated, and the function called
1356 again, the function will fail because the cached function contents
1357 still point to the old temporary table. The solution is to use
1358 <SMALL>EXECUTE</SMALL> for temporary table access in PL/PgSQL. This
1359 will cause the query to be reparsed every time.</P>
1361 <H4><A name="4.27">4.27</A>) What replication options are available?
1363 <P>There are several master/slave replication options available.
1364 These allow only the master to make database changes and the slave
1365 can only do database reads. The bottom of <a
1366 href="http://gborg.PostgreSQL.org/genpage?replication_research">
1367 http://gborg.PostgreSQL.org/genpage?replication_research</a> lists
1368 them. A multi-master replication solution is being worked on at <a
1369 href="http://gborg.PostgreSQL.org/project/pgreplication/projdisplay.php">http://gborg.PostgreSQL.org/project/pgreplication/projdisplay.php</a>.</P>
1371 <H4><A name="4.28">4.28</A>) What encryption options are available?
1374 <LI><I>contrib/pgcrypto</I> contains many encryption functions for
1375 use in <SMALL>SQL</SMALL> queries.</LI>
1376 <LI>The only way to encrypt transmission from the client to the
1377 server is by using <I>hostssl</I> in <I>pg_hba.conf</I>.</LI>
1378 <LI>Database user passwords are automatically encrypted when stored
1379 in version 7.3. In previous versions, you must enable the option
1380 <I>PASSWORD_ENCRYPTION</I> in <I>postgresql.conf</I>.</LI>
1381 <LI>The server can run using an encrypted file system.</LI>
1386 <H2 align="center">Extending PostgreSQL</H2>
1388 <H4><A name="5.1">5.1</A>) I wrote a user-defined function. When I
1389 run it in <I>psql</I>, why does it dump core?</H4>
1391 <P>The problem could be a number of things. Try testing your
1392 user-defined function in a stand-alone test program first.</P>
1394 <H4><A name="5.2">5.2</A>) How can I contribute some nifty new
1395 types and functions to PostgreSQL?</H4>
1397 <P>Send your extensions to the <I>pgsql-hackers</I> mailing list,
1398 and they will eventually end up in the <I>contrib/</I>
1401 <H4><A name="5.3">5.3</A>) How do I write a C function to return a
1404 <P>In versions of PostgreSQL beginning with 7.3, table-returning
1405 functions are fully supported in C, PL/PgSQL, and SQL. See the
1406 Programmer's Guide for more information. An example of a
1407 table-returning function defined in C can be found in
1408 <I>contrib/tablefunc</I>.</P>
1410 <H4><A name="5.4">5.4</A>) I have changed a source file. Why does
1411 the recompile not see the change?</H4>
1413 <P>The <I>Makefiles</I> do not have the proper dependencies for
1414 include files. You have to do a <I>make clean</I> and then another
1415 <I>make</I>. If you are using <SMALL>GCC</SMALL> you can use the
1416 <I>--enable-depend</I> option of <I>configure</I> to have the
1417 compiler compute the dependencies automatically.</P>
projects / postgresql / blob