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11 <H1>Frequently Asked Questions (FAQ) for PostgreSQL</H1>
13 <P>Last updated: Tue Sep 9 14:25:24 EDT 2003</P>
15 <P>Current maintainer: Bruce Momjian (<A href=
16 "mailto:pgman@candle.pha.pa.us">pgman@candle.pha.pa.us</A>)<BR>
19 <P>The most recent version of this document can be viewed at <A
21 "http://www.PostgreSQL.org/docs/faqs/FAQ.html">http://www.PostgreSQL.org/docs/faqs/FAQ.html</A>.</P>
23 <P>Platform-specific questions are answered at <A href=
24 "http://www.PostgreSQL.org/docs/index.html">http://www.PostgreSQL.org/docs/index.html</A>.</P>
27 <H2 align="center">General Questions</H2>
28 <A href="#1.1">1.1</A>) What is PostgreSQL? How is it pronounced?<BR>
29 <A href="#1.2">1.2</A>) What is the copyright on PostgreSQL?<BR>
30 <A href="#1.3">1.3</A>) What Unix platforms does PostgreSQL run
32 <A href="#1.4">1.4</A>) What non-Unix ports are available?<BR>
33 <A href="#1.5">1.5</A>) Where can I get PostgreSQL?<BR>
34 <A href="#1.6">1.6</A>) Where can I get support?<BR>
35 <A href="#1.7">1.7</A>) What is the latest release?<BR>
36 <A href="#1.8">1.8</A>) What documentation is available?<BR>
37 <A href="#1.9">1.9</A>) How do I find out about known bugs or
39 <A href="#1.10">1.10</A>) How can I learn <SMALL>SQL</SMALL>?<BR>
40 <A href="#1.11">1.11</A>) Is PostgreSQL Y2K compliant?<BR>
41 <A href="#1.12">1.12</A>) How do I join the development team?<BR>
42 <A href="#1.13">1.13</A>) How do I submit a bug report?<BR>
43 <A href="#1.14">1.14</A>) How does PostgreSQL compare to other
44 <SMALL>DBMS</SMALL>s?<BR>
45 <A href="#1.15">1.15</A>) How can I financially assist
49 <H2 align="center">User Client Questions</H2>
50 <A href="#2.1">2.1</A>) Are there <SMALL>ODBC</SMALL> drivers for
52 <A href="#2.2">2.2</A>) What tools are available for using
53 PostgreSQL with Web pages?<BR>
54 <A href="#2.3">2.3</A>) Does PostgreSQL have a graphical user
56 <A href="#2.4">2.4</A>) What languages are available to
57 communicate with PostgreSQL?<BR>
60 <H2 align="center">Administrative Questions</H2>
61 <A href="#3.1">3.1</A>) How do I install PostgreSQL somewhere other
62 than <I>/usr/local/pgsql</I>?<BR>
63 <A href="#3.2">3.2</A>) When I start <I>postmaster</I>, I get a
64 <I>Bad System Call</I> or core dumped message. Why?<BR>
65 <A href="#3.3">3.3</A>) When I try to start <I>postmaster</I>, I
66 get <I>IpcMemoryCreate</I> errors. Why?<BR>
67 <A href="#3.4">3.4</A>) When I try to start <I>postmaster</I>, I
68 get <I>IpcSemaphoreCreate</I> errors. Why?<BR>
69 <A href="#3.5">3.5</A>) How do I control connections from other
71 <A href="#3.6">3.6</A>) How do I tune the database engine for
72 better performance?<BR>
73 <A href="#3.7">3.7</A>) What debugging features are available?<BR>
74 <A href="#3.8">3.8</A>) Why do I get <I>"Sorry, too many
75 clients"</I> when trying to connect?<BR>
76 <A href="#3.9">3.9</A>) What is in the <I>pgsql_tmp</I>
78 <A href="#3.10">3.10</A>) Why do I need to do a dump and restore
79 to upgrade PostgreSQL releases?<BR>
82 <H2 align="center">Operational Questions</H2>
83 <A href="#4.1">4.1</A>) What is the difference between binary
84 cursors and normal cursors?<BR>
85 <A href="#4.2">4.2</A>) How do I <SMALL>SELECT</SMALL> only the
86 first few rows of a query? A random row?<BR>
87 <A href="#4.3">4.3</A>) How do I get a list of tables or other
88 things I can see in <I>psql</I>?<BR>
89 <A href="#4.4">4.4</A>) How do you remove a column from a
90 table, or change it's data type?<BR>
91 <A href="#4.5">4.5</A>) What is the maximum size for a row, a
92 table, and a database?<BR>
93 <A href="#4.6">4.6</A>) How much database disk space is required
94 to store data from a typical text file?<BR>
95 <A href="#4.7">4.7</A>) How do I find out what tables, indexes,
96 databases, and users are defined?<BR>
97 <A href="#4.8">4.8</A>) My queries are slow or don't make use of
99 <A href="#4.9">4.9</A>) How do I see how the query optimizer is
100 evaluating my query?<BR>
101 <A href="#4.10">4.10</A>) What is an R-tree index?<BR>
102 <A href="#4.11">4.11</A>) What is the Genetic Query Optimizer?<BR>
103 <A href="#4.12">4.12</A>) How do I perform regular expression
104 searches and case-insensitive regular expression searches? How do I
105 use an index for case-insensitive searches?<BR>
106 <A href="#4.13">4.13</A>) In a query, how do I detect if a field
107 is <SMALL>NULL</SMALL>?<BR>
108 <A href="#4.14">4.14</A>) What is the difference between the
109 various character types?<BR>
110 <A href="#4.15.1">4.15.1</A>) How do I create a
111 serial/auto-incrementing field?<BR>
112 <A href="#4.15.2">4.15.2</A>) How do I get the value of a
113 <SMALL>SERIAL</SMALL> insert?<BR>
114 <A href="#4.15.3">4.15.3</A>) Don't <I>currval()</I> and
115 <I>nextval()</I> lead to a race condition with other users?<BR>
116 <A href="#4.15.4">4.15.4</A>) Why aren't my sequence numbers
117 reused on transaction abort? Why are there gaps in the numbering of
118 my sequence/SERIAL column?<BR>
119 <A href="#4.16">4.16</A>) What is an <SMALL>OID</SMALL>? What is a
120 <SMALL>TID</SMALL>?<BR>
121 <A href="#4.17">4.17</A>) What is the meaning of some of the terms
122 used in PostgreSQL?<BR>
123 <A href="#4.18">4.18</A>) Why do I get the error <I>"ERROR: Memory
124 exhausted in AllocSetAlloc()"</I>?<BR>
125 <A href="#4.19">4.19</A>) How do I tell what PostgreSQL version I
127 <A href="#4.20">4.20</A>) Why does my large-object operations get
128 <I>"invalid large obj descriptor"</I>?<BR>
129 <A href="#4.21">4.21</A>) How do I create a column that will
130 default to the current time?<BR>
131 <A href="#4.22">4.22</A>) Why are my subqueries using
132 <CODE><SMALL>IN</SMALL></CODE> so slow?<BR>
133 <A href="#4.23">4.23</A>) How do I perform an outer join?<BR>
134 <A href="#4.24">4.24</A>) How do I perform queries using multiple
136 <A href="#4.25">4.25</A>) How do I return multiple rows or columns
138 <A href="#4.26">4.26</A>) Why can't I reliably create/drop
139 temporary tables in PL/PgSQL functions?<BR>
140 <A href="#4.27">4.27</A>) What replication options are available?<BR>
141 <A href="#4.28">4.28</A>) What encryption options are available?<BR>
144 <H2 align="center">Extending PostgreSQL</H2>
145 <A href="#5.1">5.1</A>) I wrote a user-defined function. When I run
146 it in <I>psql</I>, why does it dump core?<BR>
147 <A href="#5.2">5.2</A>) How can I contribute some nifty new types
148 and functions to PostgreSQL?<BR>
149 <A href="#5.3">5.3</A>) How do I write a C function to return a
151 <A href="#5.4">5.4</A>) I have changed a source file. Why does the
152 recompile not see the change?<BR>
156 <H2 align="center">General Questions</H2>
158 <H4><A name="1.1">1.1</A>) What is PostgreSQL? How is it pronounced?</H4>
160 <P>PostgreSQL is pronounced <I>Post-Gres-Q-L</I>.</P>
162 <P>PostgreSQL is an enhancement of the POSTGRES database management
163 system, a next-generation <SMALL>DBMS</SMALL> research prototype.
164 While PostgreSQL retains the powerful data model and rich data
165 types of POSTGRES, it replaces the PostQuel query language with an
166 extended subset of <SMALL>SQL</SMALL>. PostgreSQL is free and the
167 complete source is available.</P>
169 <P>PostgreSQL development is performed by a team of
170 developers who all subscribe to the PostgreSQL development mailing
171 list. The current coordinator is Marc G. Fournier (<A href=
172 "mailto:scrappy@PostgreSQL.org">scrappy@PostgreSQL.org</A>). (See
173 section <a href="#1.6">1.6</a> on how to join). This team is now
174 responsible for all development of PostgreSQL.</P>
176 <P>The authors of PostgreSQL 1.01 were Andrew Yu and Jolly Chen.
177 Many others have contributed to the porting, testing, debugging,
178 and enhancement of the code. The original Postgres code, from which
179 PostgreSQL is derived, was the effort of many graduate students,
180 undergraduate students, and staff programmers working under the
181 direction of Professor Michael Stonebraker at the University of
182 California, Berkeley.</P>
184 <P>The original name of the software at Berkeley was Postgres. When
185 <SMALL>SQL</SMALL> functionality was added in 1995, its name was
186 changed to Postgres95. The name was changed at the end of 1996 to
189 <H4><A name="1.2">1.2</A>) What is the copyright on
192 <P>PostgreSQL is subject to the following COPYRIGHT:</P>
194 <P>PostgreSQL Data Base Management System</P>
196 <P>Portions copyright (c) 1996-2002, PostgreSQL Global Development
197 Group Portions Copyright (c) 1994-6 Regents of the University of
200 <P>Permission to use, copy, modify, and distribute this software
201 and its documentation for any purpose, without fee, and without a
202 written agreement is hereby granted, provided that the above
203 copyright notice and this paragraph and the following two
204 paragraphs appear in all copies.</P>
206 <P>IN NO EVENT SHALL THE UNIVERSITY OF CALIFORNIA BE LIABLE TO ANY
207 PARTY FOR DIRECT, INDIRECT, SPECIAL, INCIDENTAL, OR CONSEQUENTIAL
208 DAMAGES, INCLUDING LOST PROFITS, ARISING OUT OF THE USE OF THIS
209 SOFTWARE AND ITS DOCUMENTATION, EVEN IF THE UNIVERSITY OF
210 CALIFORNIA HAS BEEN ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.</P>
212 <P>THE UNIVERSITY OF CALIFORNIA SPECIFICALLY DISCLAIMS ANY
213 WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
214 OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE. THE
215 SOFTWARE PROVIDED HEREUNDER IS ON AN "AS IS" BASIS, AND THE
216 UNIVERSITY OF CALIFORNIA HAS NO OBLIGATIONS TO PROVIDE MAINTENANCE,
217 SUPPORT, UPDATES, ENHANCEMENTS, OR MODIFICATIONS.</P>
219 <P>The above is the BSD license, the classic open-source license.
220 It has no restrictions on how the source code may be used. We like
221 it and have no intention of changing it.</P>
223 <H4><A name="1.3">1.3</A>) What Unix platforms does PostgreSQL run
226 <P>In general, a modern Unix-compatible platform should be able to
227 run PostgreSQL. The platforms that had received explicit testing at
228 the time of release are listed in the installation
231 <H4><A name="1.4">1.4</A>) What non-Unix ports are available?</H4>
233 <P><STRONG>Client</STRONG></P>
235 <P>It is possible to compile the <I>libpq</I> C library, psql, and
236 other interfaces and client applications to run on MS Windows platforms.
237 In this case, the client is running on MS Windows, and communicates
238 via TCP/IP to a server running on one of our supported Unix
239 platforms. A file <I>win32.mak</I> is included in the distribution
240 for making a Win32 <I>libpq</I> library and <I>psql</I>. PostgreSQL
241 also communicates with <SMALL>ODBC</SMALL> clients.</P>
243 <P><STRONG>Server</STRONG></P>
245 <P>The database server can run on Windows NT and Win2k using
246 Cygwin, the Cygnus Unix/NT porting library. See
247 <I>pgsql/doc/FAQ_MSWIN</I> in the distribution or the MS Windows FAQ
248 at <A href="http://www.PostgreSQL.org/docs/faqs/text/FAQ_MSWIN">
249 http://www.PostgreSQL.org/docs/faqs/text/FAQ_MSWIN</A>.</P>
251 <p>A native port to MS Win NT/2000/XP is currently being worked
252 on. For more details on the current status of PostgreSQL on Windows see
253 <a href="http://techdocs.postgresql.org/guides/Windows">
254 http://techdocs.postgresql.org/guides/Windows</a> and
255 <a href="http://momjian.postgresql.org/main/writings/pgsql/win32.html">
256 http://momjian.postgresql.org/main/writings/pgsql/win32.html</a>.</p>
258 <p>There is also a Novell Netware 6 port at
259 <a href="http://forge.novell.com">http://forge.novell.com</a>.</p>
261 <H4><A name="1.5">1.5</A>) Where can I get PostgreSQL?</H4>
263 <P>The primary anonymous ftp site for PostgreSQL is <A href=
264 "ftp://ftp.PostgreSQL.org/pub">ftp://ftp.PostgreSQL.org/pub</A>.
265 For mirror sites, see our main web site.</P>
267 <H4><A name="1.6">1.6</A>) Where can I get support?</H4>
269 <P>The main mailing list is: <A href=
270 "mailto:pgsql-general@PostgreSQL.org">pgsql-general@PostgreSQL.org</A>.
271 It is available for discussion of matters pertaining to PostgreSQL.
272 To subscribe, send mail with the following lines in the body (not
273 the subject line):</P>
280 "mailto:pgsql-general-request@PostgreSQL.org">pgsql-general-request@PostgreSQL.org</A>.</P>
282 <P>There is also a digest list available. To subscribe to this
283 list, send email to: <A href=
284 "mailto:pgsql-general-digest-request@PostgreSQL.org">pgsql-general-digest-request@PostgreSQL.org</A>
291 Digests are sent out to members of this list whenever the main list
292 has received around 30k of messages.
294 <P>The bugs mailing list is available. To subscribe to this list,
295 send email to <A href=
296 "mailto:pgsql-bugs-request@PostgreSQL.org">pgsql-bugs-request@PostgreSQL.org</A>
303 There is also a developers discussion mailing list available. To
304 subscribe to this list, send email to <A href=
305 "mailto:pgsql-hackers-request@PostgreSQL.org">pgsql-hackers-request@PostgreSQL.org</A>
312 <P>Additional mailing lists and information about PostgreSQL can be
313 found via the PostgreSQL WWW home page at:</P>
316 <A href="http://www.PostgreSQL.org">http://www.PostgreSQL.org</A>
319 <P>There is also an IRC channel on EFNet and OpenProjects,
320 channel <I>#PostgreSQL</I>. I use the Unix command <CODE>irc -c
321 '#PostgreSQL' "$USER" irc.phoenix.net.</CODE></P>
323 <P>A list of commercial support companies is available at <A href=
324 "http://techdocs.postgresql.org/companies.php">http://techdocs.postgresql.org/companies.php</A>.</P>
326 <H4><A name="1.7">1.7</A>) What is the latest release?</H4>
328 <P>The latest release of PostgreSQL is version 7.3.4.</P>
330 <P>We plan to have major releases every six to eight months.</P>
332 <H4><A name="1.8">1.8</A>) What documentation is available?</H4>
334 <P>Several manuals, manual pages, and some small test examples are
335 included in the distribution. See the <I>/doc</I> directory. You
336 can also browse the manuals online at <A href=
337 "http://www.PostgreSQL.org/docs">http://www.PostgreSQL.org/docs</A>.</P>
339 <P>There are two PostgreSQL books available online at <A href=
340 "http://www.PostgreSQL.org/docs/awbook.html">http://www.PostgreSQL.org/docs/awbook.html</A>
342 "http://www.commandprompt.com/ppbook/">http://www.commandprompt.com/ppbook/</A>.
343 There is a list of PostgreSQL books available for purchase at <A
345 "http://techdocs.postgresql.org/techdocs/bookreviews.php">http://techdocs.PostgreSQL.org/techdocs/bookreviews.php</A>.
346 There is also a collection of PostgreSQL technical articles at <A
348 "http://techdocs.PostgreSQL.org/">http://techdocs.PostgreSQL.org/</A>.</P>
350 <P><I>psql</I> has some nice \d commands to show information about
351 types, operators, functions, aggregates, etc.</P>
353 <P>Our web site contains even more documentation.</P>
355 <H4><A name="1.9">1.9</A>) How do I find out about known bugs or
356 missing features?</H4>
358 <P>PostgreSQL supports an extended subset of <SMALL>SQL</SMALL>-92.
359 See our <A href="http://developer.PostgreSQL.org/todo.php">TODO</A>
360 list for known bugs, missing features, and future plans.</P>
362 <H4><A name="1.10">1.10</A>) How can I learn
363 <SMALL>SQL</SMALL>?</H4>
365 <P>The PostgreSQL book at <A href=
366 "http://www.PostgreSQL.org/docs/awbook.html">http://www.PostgreSQL.org/docs/awbook.html</A>
367 teaches <SMALL>SQL</SMALL>. There is another PostgreSQL book at <A
369 "http://www.commandprompt.com/ppbook/">http://www.commandprompt.com/ppbook.</A>
370 There is a nice tutorial at <A href=
371 "http://www.intermedia.net/support/sql/sqltut.shtm">http://www.intermedia.net/support/sql/sqltut.shtm,</A>
373 "http://ourworld.compuserve.com/homepages/graeme_birchall/HTM_COOK.HTM">
374 http://ourworld.compuserve.com/homepages/graeme_birchall/HTM_COOK.HTM,</A>
376 "http://sqlcourse.com/">http://sqlcourse.com.</A></P>
378 <P>Another one is "Teach Yourself SQL in 21 Days, Second Edition"
380 "http://members.tripod.com/er4ebus/sql/index.htm">http://members.tripod.com/er4ebus/sql/index.htm</A></P>
382 <P>Many of our users like <I>The Practical SQL Handbook</I>,
383 Bowman, Judith S., et al., Addison-Wesley. Others like <I>The
384 Complete Reference SQL</I>, Groff et al., McGraw-Hill.</P>
386 <H4><A name="1.11">1.11</A>) Is PostgreSQL Y2K compliant?</H4>
388 <P>Yes, we easily handle dates past the year 2000 AD, and before
391 <H4><A name="1.12">1.12</A>) How do I join the development
394 <P>First, download the latest source and read the PostgreSQL
395 Developers documentation on our web site, or in the distribution.
396 Second, subscribe to the <I>pgsql-hackers</I> and
397 <I>pgsql-patches</I> mailing lists. Third, submit high quality
398 patches to pgsql-patches.</P>
400 <P>There are about a dozen people who have commit privileges to the
401 PostgreSQL <SMALL>CVS</SMALL> archive. They each have submitted so
402 many high-quality patches that it was impossible for the existing
403 committers to keep up, and we had confidence that patches they
404 committed were of high quality.</P>
406 <H4><A name="1.13">1.13</A>) How do I submit a bug report?</H4>
408 <P>Please visit the PostgreSQL BugTool page at <A href=
409 "http://www.PostgreSQL.org/bugs/bugs.php">http://www.PostgreSQL.org/bugs/bugs.php</A>,
410 which gives guidelines and directions on how to submit a
413 <P>Also check out our ftp site <A href=
414 "ftp://ftp.PostgreSQL.org/pub">ftp://ftp.PostgreSQL.org/pub</A> to
415 see if there is a more recent PostgreSQL version or patches.</P>
417 <H4><A name="1.14">1.14</A>) How does PostgreSQL compare to other
418 <SMALL>DBMS</SMALL>s?</H4>
420 <P>There are several ways of measuring software: features,
421 performance, reliability, support, and price.</P>
424 <DT><B>Features</B></DT>
426 <DD>PostgreSQL has most features present in large commercial
427 <SMALL>DBMS</SMALL>s, like transactions, subselects, triggers,
428 views, foreign key referential integrity, and sophisticated
429 locking. We have some features they do not have, like
430 user-defined types, inheritance, rules, and multi-version
431 concurrency control to reduce lock contention.<BR>
435 <DT><B>Performance</B></DT>
437 <DD>PostgreSQL has performance similar to other commercial and
438 open source databases. it is faster for some things, slower for
439 others. In comparison to MySQL or leaner database systems, we are
440 slower on inserts/updates because of transaction overhead. Of
441 course, MySQL does not have any of the features mentioned in the
442 <I>Features</I> section above. We are built for reliability and
443 features, though we continue to improve performance in every
444 release. There is an interesting Web page comparing PostgreSQL to
445 MySQL at <A href="http://openacs.org/philosophy/why-not-mysql.html">
446 http://openacs.org/philosophy/why-not-mysql.html</A><BR>
451 <DT><B>Reliability</B></DT>
453 <DD>We realize that a <SMALL>DBMS</SMALL> must be reliable, or it
454 is worthless. We strive to release well-tested, stable code that
455 has a minimum of bugs. Each release has at least one month of
456 beta testing, and our release history shows that we can provide
457 stable, solid releases that are ready for production use. We
458 believe we compare favorably to other database software in this
463 <DT><B>Support</B></DT>
465 <DD>Our mailing lists provide contact with a large group of developers
466 and users to help resolve any problems encountered. While we cannot
467 guarantee a fix, commercial <SMALL>DBMS</SMALL>s do not always
468 supply a fix either. Direct access to developers, the user
469 community, manuals, and the source code often make PostgreSQL
470 support superior to other <SMALL>DBMS</SMALL>s. There is
471 commercial per-incident support available for those who need it.
472 (See <A href="#1.6">FAQ section 1.6</A>.)<BR>
476 <DT><B>Price</B></DT>
478 <DD>We are free for all use, both commercial and non-commercial.
479 You can add our code to your product with no limitations, except
480 those outlined in our BSD-style license stated above.<BR>
485 <H4><A name="1.15">1.15</A>) How can I financially assist
488 <P>PostgreSQL has had a first-class infrastructure since we started
489 in 1996. This is all thanks to Marc Fournier, who has created
490 and managed this infrastructure over the years.</P>
492 <P>Quality infrastructure is very important to an open-source
493 project. It prevents disruptions that can greatly delay forward
494 movement of the project.</P>
496 <P>Of course, this infrastructure is not cheap. There are a variety
497 of monthly and one-time expenses that are required to keep it
498 going. If you or your company has money it can donate to help fund
499 this effort, please go to <A href="http://store.pgsql.com/shopping/">http://store.pgsql.com/shopping/</A>
500 and make a donation.</P>
502 <P>Although the web page mentions PostgreSQL, Inc, the
503 "contributions" item is solely to support the PostgreSQL project
504 and does not fund any specific company. If you prefer, you can also
505 send a check to the contact address.</P>
508 <P>Also, if you have a success story about PostgreSQL, please submit
509 it to our advocacy site at <a href="http://advocacy.postgresql.org">
510 http://advocacy.postgresql.org</a>.</P>
513 <H2 align="center">User Client Questions</H2>
515 <H4><A name="2.1">2.1</A>) Are there <SMALL>ODBC</SMALL> drivers
518 <P>There are two <SMALL>ODBC</SMALL> drivers available, PsqlODBC
519 and OpenLink <SMALL>ODBC</SMALL>.</P>
521 <P>You can download PsqlODBC from <A href=
522 "http://gborg.postgresql.org/project/psqlodbc/projdisplay.php">
523 http://gborg.postgresql.org/project/psqlodbc/projdisplay.php</A>.</P>
525 <P>OpenLink <SMALL>ODBC</SMALL> can be gotten from <A href=
526 "http://www.openlinksw.com/">http://www.openlinksw.com</A>. It
527 works with their standard <SMALL>ODBC</SMALL> client software so
528 you'll have PostgreSQL <SMALL>ODBC</SMALL> available on every
529 client platform they support (Win, Mac, Unix, VMS).</P>
531 <P>They will probably be selling this product to people who need
532 commercial-quality support, but a freeware version will always be
533 available. Please send questions to <A href=
534 "mailto:postgres95@openlink.co.uk">postgres95@openlink.co.uk</A>.</P>
536 <H4><A name="2.2">2.2</A>) What tools are available for using
537 PostgreSQL with Web pages?</H4>
539 <P>A nice introduction to Database-backed Web pages can be seen at:
540 <A href="http://www.webreview.com">http://www.webreview.com</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 or mod_perl.</P>
547 <H4><A name="2.3">2.3</A>) Does PostgreSQL have a graphical user
550 <P>Yes, there are several graphical interfaces to PostgreSQL available.
551 These include PgAccess <a href="http://www.pgaccess.org">
552 http://www.pgaccess.org</a>), PgAdmin II (<a
553 href="http://www.pgadmin.org">http://www.pgadmin.org</a>,
554 Win32-only), RHDB Admin (<a
555 href="http://sources.redhat.com/rhdb/">http://sources.redhat.com/rhdb/
556 </a>) and Rekall (<a href="http://www.thekompany.com/products/rekall/">
557 http://www.thekompany.com/products/rekall/</a>, proprietary). There is
558 also PHPPgAdmin (<a href="http://phppgadmin.sourceforge.net/">
559 http://phppgadmin.sourceforge.net/ </a>), a web-based interface to
562 <P>See <a href="http://techdocs.postgresql.org/guides/GUITools">http://techdocs.postgresql.org/guides/GUITools</a> for a more detailed list.</P>
564 <H4><A name="2.4">2.4</A>) What languages are able to communicate with
567 <P>Most popular programming languages contain an interface to
568 PostgreSQL. Check your programming language's list of extension
571 <P>The following interfaces are included in the PostgreSQL
577 <LI>Embedded C (ecpg)</LI>
581 <LI>Python (PyGreSQL)</LI>
583 <LI>TCL (libpgtcl)</LI>
586 <P>Additional interfaces are available at
587 <a href="http://gborg.postgresql.org">http://gborg.postgresql.org</A>
588 in the <I>Drivers/Interfaces</I> section.
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/docs/view.php?version=current&idoc=1&file=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 <H4><A name="3.9">3.9</A>) What is in the <I>pgsql_tmp</I> directory?</H4>
772 <P>This directory contains temporary files generated by the query
773 executor. For example, if a sort needs to be done to satisfy an
774 <SMALL>ORDER BY</SMALL> and the sort requires more space than the
775 backend's <I>-S</I> parameter allows, then temporary files are created
776 here to hold the extra data.</P>
778 <P>The temporary files are usually deleted automatically, but might
779 remain if a backend crashes during a sort. A stop and restart of the
780 <I>postmaster</I> will remove files from those directories.</P>
782 <H4><A name="3.10">3.10</A>) Why do I need to do a dump and restore
783 to upgrade between major PostgreSQL releases?</H4>
785 <P>The PostgreSQL team makes only small changes between minor releases,
786 so upgrading from 7.2 to 7.2.1 does not require a dump and restore.
787 However, major releases (e.g. from 7.2 to 7.3) often change the internal
788 format of system tables and data files. These changes are often complex,
789 so we don't maintain backward compatability for data files. A dump outputs
790 data in a generic format that can then be loaded in using the new internal
793 <P>In releases where the on-disk format does not change, the
794 <I>pg_upgrade</I> script can be used to upgrade without a dump/restore.
795 The release notes mention whether <I>pg_upgrade</I> is available for the
800 <H2 align="center">Operational Questions</H2>
802 <H4><A name="4.1">4.1</A>) What is the difference between binary
803 cursors and normal cursors?</H4>
805 <P>See the <SMALL>DECLARE</SMALL> manual page for a
808 <H4><A name="4.2">4.2</A>) How do I <SMALL>SELECT</SMALL> only the
809 first few rows of a query? A random row?</H4>
811 <P>See the <SMALL>FETCH</SMALL> manual page, or use
812 <SMALL>SELECT</SMALL> ... <SMALL>LIMIT</SMALL>....</P>
814 <P>The entire query may have to be evaluated, even if you only want
815 the first few rows. Consider using a query that has an <SMALL>ORDER
816 BY</SMALL>. If there is an index that matches the <SMALL>ORDER
817 BY</SMALL>, PostgreSQL may be able to evaluate only the first few
818 records requested, or the entire query may have to be evaluated
819 until the desired rows have been generated.</P>
821 <P>To <SMALL>SELECT</SMALL> a random row, use:
829 <H4><A name="4.3">4.3</A>) How do I get a list of tables or other
830 things I can see in <I>psql</I>?</H4>
832 <P>You can read the source code for <I>psql</I> in file
833 <I>pgsql/src/bin/psql/describe.c</I>. It contains
834 <SMALL>SQL</SMALL> commands that generate the output for psql's
835 backslash commands. You can also start <I>psql</I> with the
836 <I>-E</I> option so it will print out the queries it uses to
837 execute the commands you give.</P>
839 <H4><A name="4.4">4.4</A>) How do you remove a column from a
840 table, or change its data type?</H4>
842 <P><SMALL>DROP COLUMN</SMALL> functionality was added in release 7.3 with
843 <SMALL>ALTER TABLE DROP COLUMN</SMALL>. In earlier versions,
847 LOCK TABLE old_table;
848 SELECT ... -- select all columns but the one you want to remove
851 DROP TABLE old_table;
852 ALTER TABLE new_table RENAME TO old_table;
856 <P>To change the data type of a column, do this:</P>
859 ALTER TABLE tab ADD COLUMN new_col <i>new_data_type</i>;
860 UPDATE tab SET new_col = CAST(old_col AS <i>new_data_type</i>);
861 ALTER TABLE tab DROP COLUMN old_col;
864 <P>You might then want to do <I>VACUUM FULL tab</I> to reclaim the
865 disk space used by the expired rows.</P>
867 <H4><A name="4.5">4.5</A>) What is the maximum size for a row, a
868 table, and a database?</H4>
870 <P>These are the limits:</P>
872 Maximum size for a database? unlimited (4 TB databases exist)
873 Maximum size for a table? 32 TB
874 Maximum size for a row? 1.6TB
875 Maximum size for a field? 1 GB
876 Maximum number of rows in a table? unlimited
877 Maximum number of columns in a table? 250-1600 depending on column types
878 Maximum number of indexes on a table? unlimited
881 Of course, these are not actually unlimited, but limited to
882 available disk space and memory/swap space. Performance may suffer
883 when these values get unusually large.
885 <P>The maximum table size of 32 TB does not require large file
886 support from the operating system. Large tables are stored as
887 multiple 1 GB files so file system size limits are not
890 <P>The maximum table size and maximum number of columns can be
891 increased if the default block size is increased to 32k.</P>
893 <H4><A name="4.6">4.6</A>) How much database disk space is required
894 to store data from a typical text file?</H4>
896 <P>A PostgreSQL database may require up to five times the disk
897 space to store data from a text file.</P>
899 <P>As an example, consider a file of 100,000 lines with an integer
900 and text description on each line. Suppose the text string
901 avergages twenty bytes in length. The flat file would be 2.8 MB.
902 The size of the PostgreSQL database file containing this data can
903 be estimated as 6.4 MB:</P>
905 36 bytes: each row header (approximate)
906 24 bytes: one int field and one text field
907 + 4 bytes: pointer on page to tuple
908 ----------------------------------------
911 The data page size in PostgreSQL is 8192 bytes (8 KB), so:
914 ------------------- = 128 rows per database page (rounded down)
918 -------------------- = 782 database pages (rounded up)
921 782 database pages * 8192 bytes per page = 6,406,144 bytes (6.4 MB)
924 <P>Indexes do not require as much overhead, but do contain the data
925 that is being indexed, so they can be large also.</P>
927 <P><SMALL>NULL</SMALL>s are stored in bitmaps, so they
928 use very little space.</P>
930 <H4><A name="4.7">4.7</A>) How do I find out what tables, indexes,
931 databases, and users are defined?</H4>
933 <P><I>psql</I> has a variety of backslash commands to show such
934 information. Use \? to see them. There are also system tables
935 beginning with <I>pg_</I> that describe these too. Also, <I>psql
936 -l</I> will list all databases.</P>
938 <P>Also try the file <I>pgsql/src/tutorial/syscat.source</I>. It
939 illustrates many of the <SMALL>SELECT</SMALL>s needed to get
940 information from the database system tables.</P>
942 <H4><A name="4.8">4.8</A>) My queries are slow or don't make use of
943 the indexes. Why?</H4>
944 Indexes are not automatically used by every query. Indexes are only
945 used if the table is larger than a minimum size, and the query
946 selects only a small percentage of the rows in the table. This is
947 because the random disk access caused by an index scan can be
948 slower than a straight read through the table, or sequential scan.
950 <P>To determine if an index should be used, PostgreSQL must have
951 statistics about the table. These statistics are collected using
952 <SMALL>VACUUM ANALYZE</SMALL>, or simply <SMALL>ANALYZE</SMALL>.
953 Using statistics, the optimizer knows how many rows are in the
954 table, and can better determine if indexes should be used.
955 Statistics are also valuable in determining optimal join order and
956 join methods. Statistics collection should be performed
957 periodically as the contents of the table change.</P>
959 <P>Indexes are normally not used for <SMALL>ORDER BY</SMALL> or to
960 perform joins. A sequential scan followed by an explicit sort is
961 usually faster than an index scan of a large table.</P>
962 However, <SMALL>LIMIT</SMALL> combined with <SMALL>ORDER BY</SMALL>
963 often will use an index because only a small portion of the table
964 is returned. In fact, though MAX() and MIN() don't use indexes,
965 it is possible to retrieve such values using an index with ORDER BY
970 ORDER BY col [ DESC ]
974 <P>If you believe the optimizer is incorrect in choosing a
975 sequential scan, use <CODE>SET enable_seqscan TO 'off'</CODE> and
976 run tests to see if an index scan is indeed faster.</P>
978 <P>When using wild-card operators such as <SMALL>LIKE</SMALL> or
979 <I>~</I>, indexes can only be used in certain circumstances:</P>
981 <LI>The beginning of the search string must be anchored to the start
984 <LI><SMALL>LIKE</SMALL> patterns must not start with <I>%</I>.</LI>
985 <LI><I>~</I> (regular expression) patterns must start with
988 <LI>The search string can not start with a character class,
990 <LI>Case-insensitive searches such as <SMALL>ILIKE</SMALL> and
991 <I>~*</I> do not utilise indexes. Instead, use functional
992 indexes, which are described in section <a href="#4.12">4.12</a>.</LI>
993 <LI>The default <I>C</I> locale must be used during
998 <H4><A name="4.9">4.9</A>) How do I see how the query optimizer is
999 evaluating my query?</H4>
1001 <P>See the <SMALL>EXPLAIN</SMALL> manual page.</P>
1003 <H4><A name="4.10">4.10</A>) What is an R-tree index?</H4>
1005 <P>An R-tree index is used for indexing spatial data. A hash index
1006 can't handle range searches. A B-tree index only handles range
1007 searches in a single dimension. R-trees can handle
1008 multi-dimensional data. For example, if an R-tree index can be
1009 built on an attribute of type <I>point</I>, the system can more
1010 efficiently answer queries such as "select all points within a
1011 bounding rectangle."</P>
1013 <P>The canonical paper that describes the original R-tree design
1016 <P>Guttman, A. "R-trees: A Dynamic Index Structure for Spatial
1017 Searching." Proceedings of the 1984 ACM SIGMOD Int'l Conf on Mgmt
1020 <P>You can also find this paper in Stonebraker's "Readings in
1021 Database Systems".</P>
1023 <P>Built-in R-trees can handle polygons and boxes. In theory,
1024 R-trees can be extended to handle higher number of dimensions. In
1025 practice, extending R-trees requires a bit of work and we don't
1026 currently have any documentation on how to do it.</P>
1028 <H4><A name="4.11">4.11</A>) What is the Genetic Query
1031 <P>The <SMALL>GEQO</SMALL> module speeds query optimization when
1032 joining many tables by means of a Genetic Algorithm (GA). It allows
1033 the handling of large join queries through nonexhaustive
1036 <H4><A name="4.12">4.12</A>) How do I perform regular expression
1037 searches and case-insensitive regular expression searches? How do I
1038 use an index for case-insensitive searches?</H4>
1040 <P>The <I>~</I> operator does regular expression matching, and
1041 <I>~*</I> does case-insensitive regular expression matching. The
1042 case-insensitive variant of <SMALL>LIKE</SMALL> is called
1043 <SMALL>ILIKE</SMALL>.</P>
1045 <P>Case-insensitive equality comparisons are normally expressed
1050 WHERE lower(col) = 'abc';
1053 This will not use an standard index. However, if you create a
1054 functional index, it will be used:
1056 CREATE INDEX tabindex ON tab (lower(col));
1059 <H4><A name="4.13">4.13</A>) In a query, how do I detect if a field
1060 is <SMALL>NULL</SMALL>?</H4>
1062 <P>You test the column with <SMALL>IS NULL</SMALL> and <SMALL>IS
1063 NOT NULL</SMALL>.</P>
1065 <H4><A name="4.14">4.14</A>) What is the difference between the
1066 various character types?</H4>
1068 Type Internal Name Notes
1069 --------------------------------------------------
1070 VARCHAR(n) varchar size specifies maximum length, no padding
1071 CHAR(n) bpchar blank padded to the specified fixed length
1072 TEXT text no specific upper limit on length
1073 BYTEA bytea variable-length byte array (null-byte safe)
1074 "char" char one character
1077 <P>You will see the internal name when examining system catalogs
1078 and in some error messages.</P>
1080 <P>The first four types above are "varlena" types (i.e., the first
1081 four bytes on disk are the length, followed by the data). Thus the
1082 actual space used is slightly greater than the declared size.
1083 However, these data types are also subject to compression or being
1084 stored out-of-line by <SMALL>TOAST</SMALL>, so the space on disk
1085 might also be less than expected.</P>
1087 <SMALL>VARCHAR(n)</SMALL> is best when storing variable-length
1088 strings and it limits how long a string can be. <SMALL>TEXT</SMALL>
1089 is for strings of unlimited length, with a maximum of one gigabyte.
1090 <P><SMALL>CHAR(n)</SMALL> is for storing strings that are all the
1091 same length. <SMALL>CHAR(n)</SMALL> pads with blanks to the specified
1092 length, while <SMALL>VARCHAR(n)</SMALL> only stores the characters
1093 supplied. <SMALL>BYTEA</SMALL> is for storing binary data,
1094 particularly values that include <SMALL>NULL</SMALL> bytes. All the
1095 types described here have similar performance characteristics.</P>
1097 <H4><A name="4.15.1">4.15.1</A>) How do I create a
1098 serial/auto-incrementing field?</H4>
1100 <P>PostgreSQL supports a <SMALL>SERIAL</SMALL> data type. It
1101 auto-creates a sequence and index on the column. For example,
1104 CREATE TABLE person (
1110 is automatically translated into this:
1112 CREATE SEQUENCE person_id_seq;
1113 CREATE TABLE person (
1114 id INT4 NOT NULL DEFAULT nextval('person_id_seq'),
1117 CREATE UNIQUE INDEX person_id_key ON person ( id );
1120 See the <I>create_sequence</I> manual page for more information
1121 about sequences. You can also use each row's <I>OID</I> field as a
1122 unique value. However, if you need to dump and reload the database,
1123 you need to use <I>pg_dump</I>'s <I>-o</I> option or <SMALL>COPY
1124 WITH OIDS</SMALL> option to preserve the <SMALL>OID</SMALL>s.
1126 <H4><A name="4.15.2">4.15.2</A>) How do I get the value of a
1127 <SMALL>SERIAL</SMALL> insert?</H4>
1129 <P>One approach is to retrieve the next <SMALL>SERIAL</SMALL> value
1130 from the sequence object with the <I>nextval()</I> function
1131 <I>before</I> inserting and then insert it explicitly. Using the
1132 example table in <A href="#4.15.1">4.15.1</A>, an example in a
1133 pseudo-language would look like this:</P>
1135 new_id = execute("SELECT nextval('person_id_seq')");
1136 execute("INSERT INTO person (id, name) VALUES (new_id, 'Blaise Pascal')");
1139 You would then also have the new value stored in
1140 <CODE>new_id</CODE> for use in other queries (e.g., as a foreign
1141 key to the <CODE>person</CODE> table). Note that the name of the
1142 automatically created <SMALL>SEQUENCE</SMALL> object will be named
1143 <<I>table</I>>_<<I>serialcolumn</I>>_<I>seq</I>, where
1144 <I>table</I> and <I>serialcolumn</I> are the names of your table
1145 and your <SMALL>SERIAL</SMALL> column, respectively.
1147 <P>Alternatively, you could retrieve the assigned
1148 <SMALL>SERIAL</SMALL> value with the <I>currval()</I> function
1149 <I>after</I> it was inserted by default, e.g.,</P>
1151 execute("INSERT INTO person (name) VALUES ('Blaise Pascal')");
1152 new_id = execute("SELECT currval('person_id_seq')");
1155 Finally, you could use the <A href="#4.16"><SMALL>OID</SMALL></A>
1156 returned from the <SMALL>INSERT</SMALL> statement to look up the
1157 default value, though this is probably the least portable approach,
1158 and the oid value will wrap around when it reaches 4 billion.
1159 In Perl, using DBI with Edmund Mergl's DBD::Pg module, the oid
1160 value is made available via <I>$sth->{pg_oid_status}</I> after
1161 <I>$sth->execute()</I>.
1163 <H4><A name="4.15.3">4.15.3</A>) Don't <I>currval()</I> and
1164 <I>nextval()</I> lead to a race condition with other users?</H4>
1166 <P>No. <I>currval()</I> returns the current value assigned by your
1167 backend, not by all users.</P>
1169 <H4><A name="4.15.4">4.15.4</A>) Why aren't my sequence numbers
1170 reused on transaction abort? Why are there gaps in the numbering of
1171 my sequence/SERIAL column?</H4>
1173 <P>To improve concurrency, sequence values are given out to running
1174 transactions as needed and are not locked until the transaction
1175 completes. This causes gaps in numbering from aborted
1178 <H4><A name="4.16">4.16</A>) What is an <SMALL>OID</SMALL>? What is
1179 a <SMALL>TID</SMALL>?</H4>
1181 <P><SMALL>OID</SMALL>s are PostgreSQL's answer to unique row ids.
1182 Every row that is created in PostgreSQL gets a unique
1183 <SMALL>OID</SMALL>. All <SMALL>OID</SMALL>s generated during
1184 <I>initdb</I> are less than 16384 (from
1185 <I>include/access/transam.h</I>). All user-created
1186 <SMALL>OID</SMALL>s are equal to or greater than this. By default,
1187 all these <SMALL>OID</SMALL>s are unique not only within a table or
1188 database, but unique within the entire PostgreSQL installation.</P>
1190 <P>PostgreSQL uses <SMALL>OID</SMALL>s in its internal system
1191 tables to link rows between tables. These <SMALL>OID</SMALL>s can
1192 be used to identify specific user rows and used in joins. It is
1193 recommended you use column type <SMALL>OID</SMALL> to store
1194 <SMALL>OID</SMALL> values. You can create an index on the
1195 <SMALL>OID</SMALL> field for faster access.</P>
1197 <P>O<SMALL>ID</SMALL>s are assigned to all new rows from a central
1198 area that is used by all databases. If you want to change the
1199 <SMALL>OID</SMALL> to something else, or if you want to make a copy
1200 of the table, with the original <SMALL>OID</SMALL>s, there is no
1201 reason you can't do it:</P>
1203 CREATE TABLE new_table(old_oid oid, mycol int);
1204 SELECT old_oid, mycol INTO new FROM old;
1205 COPY new TO '/tmp/pgtable';
1207 COPY new WITH OIDS FROM '/tmp/pgtable';
1210 CREATE TABLE new_table (mycol int);
1211 INSERT INTO new_table (oid, mycol) SELECT oid, mycol FROM old_table;
1213 <P>O<SMALL>ID</SMALL>s are stored as 4-byte integers, and will
1214 overflow at 4 billion. No one has reported this ever happening, and
1215 we plan to have the limit removed before anyone does.</P>
1217 <P>T<SMALL>ID</SMALL>s are used to identify specific physical rows
1218 with block and offset values. T<SMALL>ID</SMALL>s change after rows
1219 are modified or reloaded. They are used by index entries to point
1220 to physical rows.</P>
1222 <H4><A name="4.17">4.17</A>) What is the meaning of some of the
1223 terms used in PostgreSQL?</H4>
1225 <P>Some of the source code and older documentation use terms that
1226 have more common usage. Here are some:</P>
1229 <LI>table, relation, class</LI>
1231 <LI>row, record, tuple</LI>
1233 <LI>column, field, attribute</LI>
1235 <LI>retrieve, select</LI>
1237 <LI>replace, update</LI>
1239 <LI>append, insert</LI>
1241 <LI><SMALL>OID</SMALL>, serial value</LI>
1243 <LI>portal, cursor</LI>
1245 <LI>range variable, table name, table alias</LI>
1248 <P>A list of general database terms can be found at: <A href=
1249 "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>
1251 <H4><A name="4.18">4.18</A>) Why do I get the error <I>"ERROR:
1252 Memory exhausted in AllocSetAlloc()"</I>?</H4>
1254 <P>You probably have run out of virtual memory on your system,
1255 or your kernel has a low limit for certain resources. Try this
1256 before starting <I>postmaster</I>:</P>
1262 Depending on your shell, only one of these may succeed, but it will
1263 set your process data segment limit much higher and perhaps allow
1264 the query to complete. This command applies to the current process,
1265 and all subprocesses created after the command is run. If you are
1266 having a problem with the <SMALL>SQL</SMALL> client because the
1267 backend is returning too much data, try it before starting the
1270 <H4><A name="4.19">4.19</A>) How do I tell what PostgreSQL version
1273 <P>From <I>psql</I>, type <CODE>SELECT version();</CODE></P>
1275 <H4><A name="4.20">4.20</A>) Why does my large-object operations
1276 get <I>"invalid large obj descriptor"</I>?</H4>
1278 <P>You need to put <CODE>BEGIN WORK</CODE> and <CODE>COMMIT</CODE>
1279 around any use of a large object handle, that is, surrounding
1280 <CODE>lo_open</CODE> ... <CODE>lo_close.</CODE></P>
1282 <P>Currently PostgreSQL enforces the rule by closing large object
1283 handles at transaction commit. So the first attempt to do anything
1284 with the handle will draw <I>invalid large obj descriptor</I>. So
1285 code that used to work (at least most of the time) will now
1286 generate that error message if you fail to use a transaction.</P>
1288 <P>If you are using a client interface like <SMALL>ODBC</SMALL> you
1289 may need to set <CODE>auto-commit off.</CODE></P>
1291 <H4><A name="4.21">4.21</A>) How do I create a column that will
1292 default to the current time?</H4>
1294 <P>Use <I>CURRENT_TIMESTAMP</I>:</P>
1296 <CODE>CREATE TABLE test (x int, modtime timestamp DEFAULT CURRENT_TIMESTAMP );
1300 <H4><A name="4.22">4.22</A>) Why are my subqueries using
1301 <CODE><SMALL>IN</SMALL></CODE> so slow?</H4>
1303 <P>Currently, we join subqueries to outer queries by sequentially
1304 scanning the result of the subquery for each row of the outer
1305 query. If the subquery returns only a few rows and the outer query
1306 returns many rows, <CODE><SMALL>IN</SMALL></CODE> is fastest. To
1307 speed up other queries, replace <CODE>IN</CODE> with
1308 <CODE>EXISTS</CODE>:</P>
1311 WHERE col IN (SELECT subcol FROM subtab);
1316 WHERE EXISTS (SELECT subcol FROM subtab WHERE subcol = col);
1319 For this to be fast, <CODE>subcol</CODE> should be an indexed column.
1320 This preformance problem will be fixed in 7.4.
1322 <H4><A name="4.23">4.23</A>) How do I perform an outer join?</H4>
1324 <P>PostgreSQL supports outer joins using the SQL standard syntax.
1325 Here are two examples:</P>
1328 FROM t1 LEFT OUTER JOIN t2 ON (t1.col = t2.col);
1333 FROM t1 LEFT OUTER JOIN t2 USING (col);
1336 <P>These identical queries join t1.col to t2.col, and also return
1337 any unjoined rows in t1 (those with no match in t2). A
1338 <SMALL>RIGHT</SMALL> join would add unjoined rows of t2. A
1339 <SMALL>FULL</SMALL> join would return the matched rows plus all
1340 unjoined rows from t1 and t2. The word <SMALL>OUTER</SMALL> is
1341 optional and is assumed in <SMALL>LEFT</SMALL>,
1342 <SMALL>RIGHT</SMALL>, and <SMALL>FULL</SMALL> joins. Ordinary joins
1343 are called <SMALL>INNER</SMALL> joins.</P>
1345 <P>In previous releases, outer joins can be simulated using
1346 <SMALL>UNION</SMALL> and <SMALL>NOT IN</SMALL>. For example, when
1347 joining <I>tab1</I> and <I>tab2</I>, the following query does an
1348 <I>outer</I> join of the two tables:<BR>
1352 SELECT tab1.col1, tab2.col2
1354 WHERE tab1.col1 = tab2.col1
1356 SELECT tab1.col1, NULL
1358 WHERE tab1.col1 NOT IN (SELECT tab2.col1 FROM tab2)
1362 <H4><A name="4.24">4.24</A>) How do I perform queries using
1363 multiple databases?</H4>
1365 <P>There is no way to query a database other than the current one.
1366 Because PostgreSQL loads database-specific system catalogs, it is
1367 uncertain how a cross-database query should even behave.</P>
1369 <P><I>contrib/dblink</I> allows cross-database queries using
1370 function calls. Of course, a client can make simultaneous
1371 connections to different databases and merge the results on the
1374 <H4><A name="4.25">4.25</A>) How do I return multiple rows or
1375 columns from a function?</H4>
1377 <P>In 7.3, you can easily return multiple rows or columns from a
1379 <a href="http://techdocs.postgresql.org/guides/SetReturningFunctions">
1380 http://techdocs.postgresql.org/guides/SetReturningFunctions</a>.
1382 <H4><A name="4.26">4.26</A>) Why can't I reliably create/drop
1383 temporary tables in PL/PgSQL functions?</H4>
1384 <P>PL/PgSQL caches function contents, and an unfortunate side effect
1385 is that if a PL/PgSQL function accesses a temporary table, and that
1386 table is later dropped and recreated, and the function called
1387 again, the function will fail because the cached function contents
1388 still point to the old temporary table. The solution is to use
1389 <SMALL>EXECUTE</SMALL> for temporary table access in PL/PgSQL. This
1390 will cause the query to be reparsed every time.</P>
1392 <H4><A name="4.27">4.27</A>) What replication options are available?
1394 <P>There are several master/slave replication options available.
1395 These allow only the master to make database changes and the slave
1396 can only do database reads. The bottom of <a
1397 href="http://gborg.PostgreSQL.org/genpage?replication_research">
1398 http://gborg.PostgreSQL.org/genpage?replication_research</a> lists
1399 them. A multi-master replication solution is being worked on at <a
1400 href="http://gborg.PostgreSQL.org/project/pgreplication/projdisplay.php">http://gborg.PostgreSQL.org/project/pgreplication/projdisplay.php</a>.</P>
1402 <H4><A name="4.28">4.28</A>) What encryption options are available?
1405 <LI><I>contrib/pgcrypto</I> contains many encryption functions for
1406 use in <SMALL>SQL</SMALL> queries.</LI>
1407 <LI>To encrypt transmission from the client to the server, the server
1408 must have the <I>ssl</I> option set to <I>true</I> in <I>postgresql.conf,
1409 </I> and an applicable <I>host</I> or <I>hostssl</I> record must exist in
1410 <I>pg_hba.conf</I>, and the client <I>sslmode</I> must not be
1411 <I>disable.</I> (Note that it is also possible to use a third-party
1412 encrypted transport, such as stunnel or ssh, rather than PostgreSQL's
1413 native SSL connections.)
1414 <LI>Database user passwords are automatically encrypted when stored in
1415 version 7.3. In previous versions, you must enable the option
1416 <I>PASSWORD_ENCRYPTION</I> in <I>postgresql.conf</I>.</LI>
1417 <LI>The server can run using an encrypted file system.</LI>
1422 <H2 align="center">Extending PostgreSQL</H2>
1424 <H4><A name="5.1">5.1</A>) I wrote a user-defined function. When I
1425 run it in <I>psql</I>, why does it dump core?</H4>
1427 <P>The problem could be a number of things. Try testing your
1428 user-defined function in a stand-alone test program first.</P>
1430 <H4><A name="5.2">5.2</A>) How can I contribute some nifty new
1431 types and functions to PostgreSQL?</H4>
1433 <P>Send your extensions to the <I>pgsql-hackers</I> mailing list,
1434 and they will eventually end up in the <I>contrib/</I>
1437 <H4><A name="5.3">5.3</A>) How do I write a C function to return a
1440 <P>In versions of PostgreSQL beginning with 7.3, table-returning
1441 functions are fully supported in C, PL/PgSQL, and SQL. See the
1442 Programmer's Guide for more information. An example of a
1443 table-returning function defined in C can be found in
1444 <I>contrib/tablefunc</I>.</P>
1446 <H4><A name="5.4">5.4</A>) I have changed a source file. Why does
1447 the recompile not see the change?</H4>
1449 <P>The <I>Makefiles</I> do not have the proper dependencies for
1450 include files. You have to do a <I>make clean</I> and then another
1451 <I>make</I>. If you are using <SMALL>GCC</SMALL> you can use the
1452 <I>--enable-depend</I> option of <I>configure</I> to have the
1453 compiler compute the dependencies automatically.</P>
projects / postgresql / blob