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11 <H1>Frequently Asked Questions (FAQ) for PostgreSQL</H1>
13 <P>Last updated: Sun Jul 20 17:52:41 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>.</p>
256 <p>There is also a Novell Netware 6 port at
257 <a href="http://forge.novell.com">http://forge.novell.com</a>.</p>
259 <H4><A name="1.5">1.5</A>) Where can I get PostgreSQL?</H4>
261 <P>The primary anonymous ftp site for PostgreSQL is <A href=
262 "ftp://ftp.PostgreSQL.org/pub">ftp://ftp.PostgreSQL.org/pub</A>.
263 For mirror sites, see our main web site.</P>
265 <H4><A name="1.6">1.6</A>) Where can I get support?</H4>
267 <P>The main mailing list is: <A href=
268 "mailto:pgsql-general@PostgreSQL.org">pgsql-general@PostgreSQL.org</A>.
269 It is available for discussion of matters pertaining to PostgreSQL.
270 To subscribe, send mail with the following lines in the body (not
271 the subject line):</P>
278 "mailto:pgsql-general-request@PostgreSQL.org">pgsql-general-request@PostgreSQL.org</A>.</P>
280 <P>There is also a digest list available. To subscribe to this
281 list, send email to: <A href=
282 "mailto:pgsql-general-digest-request@PostgreSQL.org">pgsql-general-digest-request@PostgreSQL.org</A>
289 Digests are sent out to members of this list whenever the main list
290 has received around 30k of messages.
292 <P>The bugs mailing list is available. To subscribe to this list,
293 send email to <A href=
294 "mailto:pgsql-bugs-request@PostgreSQL.org">pgsql-bugs-request@PostgreSQL.org</A>
301 There is also a developers discussion mailing list available. To
302 subscribe to this list, send email to <A href=
303 "mailto:pgsql-hackers-request@PostgreSQL.org">pgsql-hackers-request@PostgreSQL.org</A>
310 <P>Additional mailing lists and information about PostgreSQL can be
311 found via the PostgreSQL WWW home page at:</P>
314 <A href="http://www.PostgreSQL.org">http://www.PostgreSQL.org</A>
317 <P>There is also an IRC channel on EFNet and OpenProjects,
318 channel <I>#PostgreSQL</I>. I use the Unix command <CODE>irc -c
319 '#PostgreSQL' "$USER" irc.phoenix.net.</CODE></P>
321 <P>A list of commercial support companies is available at <A href=
322 "http://techdocs.postgresql.org/companies.php">http://techdocs.postgresql.org/companies.php</A>.</P>
324 <H4><A name="1.7">1.7</A>) What is the latest release?</H4>
326 <P>The latest release of PostgreSQL is version 7.3.3.</P>
328 <P>We plan to have major releases every four months.</P>
330 <H4><A name="1.8">1.8</A>) What documentation is available?</H4>
332 <P>Several manuals, manual pages, and some small test examples are
333 included in the distribution. See the <I>/doc</I> directory. You
334 can also browse the manuals online at <A href=
335 "http://www.PostgreSQL.org/docs">http://www.PostgreSQL.org/docs</A>.</P>
337 <P>There are two PostgreSQL books available online at <A href=
338 "http://www.PostgreSQL.org/docs/awbook.html">http://www.PostgreSQL.org/docs/awbook.html</A>
340 "http://www.commandprompt.com/ppbook/">http://www.commandprompt.com/ppbook/</A>.
341 There is a list of PostgreSQL books available for purchase at <A
343 "http://techdocs.postgresql.org/techdocs/bookreviews.php">http://techdocs.PostgreSQL.org/techdocs/bookreviews.php</A>.
344 There is also a collection of PostgreSQL technical articles at <A
346 "http://techdocs.PostgreSQL.org/">http://techdocs.PostgreSQL.org/</A>.</P>
348 <P><I>psql</I> has some nice \d commands to show information about
349 types, operators, functions, aggregates, etc.</P>
351 <P>Our web site contains even more documentation.</P>
353 <H4><A name="1.9">1.9</A>) How do I find out about known bugs or
354 missing features?</H4>
356 <P>PostgreSQL supports an extended subset of <SMALL>SQL</SMALL>-92.
357 See our <A href="http://developer.PostgreSQL.org/todo.php">TODO</A>
358 list for known bugs, missing features, and future plans.</P>
360 <H4><A name="1.10">1.10</A>) How can I learn
361 <SMALL>SQL</SMALL>?</H4>
363 <P>The PostgreSQL book at <A href=
364 "http://www.PostgreSQL.org/docs/awbook.html">http://www.PostgreSQL.org/docs/awbook.html</A>
365 teaches <SMALL>SQL</SMALL>. There is another PostgreSQL book at <A
367 "http://www.commandprompt.com/ppbook/">http://www.commandprompt.com/ppbook.</A>
368 There is a nice tutorial at <A href=
369 "http://www.intermedia.net/support/sql/sqltut.shtm">http://www.intermedia.net/support/sql/sqltut.shtm,</A>
371 "http://ourworld.compuserve.com/homepages/graeme_birchall/HTM_COOK.HTM">
372 http://ourworld.compuserve.com/homepages/graeme_birchall/HTM_COOK.HTM,</A>
374 "http://sqlcourse.com/">http://sqlcourse.com.</A></P>
376 <P>Another one is "Teach Yourself SQL in 21 Days, Second Edition"
378 "http://members.tripod.com/er4ebus/sql/index.htm">http://members.tripod.com/er4ebus/sql/index.htm</A></P>
380 <P>Many of our users like <I>The Practical SQL Handbook</I>,
381 Bowman, Judith S., et al., Addison-Wesley. Others like <I>The
382 Complete Reference SQL</I>, Groff et al., McGraw-Hill.</P>
384 <H4><A name="1.11">1.11</A>) Is PostgreSQL Y2K compliant?</H4>
386 <P>Yes, we easily handle dates past the year 2000 AD, and before
389 <H4><A name="1.12">1.12</A>) How do I join the development
392 <P>First, download the latest source and read the PostgreSQL
393 Developers documentation on our web site, or in the distribution.
394 Second, subscribe to the <I>pgsql-hackers</I> and
395 <I>pgsql-patches</I> mailing lists. Third, submit high quality
396 patches to pgsql-patches.</P>
398 <P>There are about a dozen people who have commit privileges to the
399 PostgreSQL <SMALL>CVS</SMALL> archive. They each have submitted so
400 many high-quality patches that it was impossible for the existing
401 committers to keep up, and we had confidence that patches they
402 committed were of high quality.</P>
404 <H4><A name="1.13">1.13</A>) How do I submit a bug report?</H4>
406 <P>Please visit the PostgreSQL BugTool page at <A href=
407 "http://www.PostgreSQL.org/bugs/bugs.php">http://www.PostgreSQL.org/bugs/bugs.php</A>,
408 which gives guidelines and directions on how to submit a
411 <P>Also check out our ftp site <A href=
412 "ftp://ftp.PostgreSQL.org/pub">ftp://ftp.PostgreSQL.org/pub</A> to
413 see if there is a more recent PostgreSQL version or patches.</P>
415 <H4><A name="1.14">1.14</A>) How does PostgreSQL compare to other
416 <SMALL>DBMS</SMALL>s?</H4>
418 <P>There are several ways of measuring software: features,
419 performance, reliability, support, and price.</P>
422 <DT><B>Features</B></DT>
424 <DD>PostgreSQL has most features present in large commercial
425 <SMALL>DBMS</SMALL>s, like transactions, subselects, triggers,
426 views, foreign key referential integrity, and sophisticated
427 locking. We have some features they do not have, like
428 user-defined types, inheritance, rules, and multi-version
429 concurrency control to reduce lock contention.<BR>
433 <DT><B>Performance</B></DT>
435 <DD>PostgreSQL has performance similar to other commercial and
436 open source databases. it is faster for some things, slower for
437 others. In comparison to MySQL or leaner database systems, we are
438 slower on inserts/updates because of transaction overhead. Of
439 course, MySQL does not have any of the features mentioned in the
440 <I>Features</I> section above. We are built for reliability and
441 features, though we continue to improve performance in every
442 release. There is an interesting Web page comparing PostgreSQL to
443 MySQL at <A href="http://openacs.org/philosophy/why-not-mysql.html">
444 http://openacs.org/philosophy/why-not-mysql.html</A><BR>
449 <DT><B>Reliability</B></DT>
451 <DD>We realize that a <SMALL>DBMS</SMALL> must be reliable, or it
452 is worthless. We strive to release well-tested, stable code that
453 has a minimum of bugs. Each release has at least one month of
454 beta testing, and our release history shows that we can provide
455 stable, solid releases that are ready for production use. We
456 believe we compare favorably to other database software in this
461 <DT><B>Support</B></DT>
463 <DD>Our mailing lists provide contact with a large group of developers
464 and users to help resolve any problems encountered. While we cannot
465 guarantee a fix, commercial <SMALL>DBMS</SMALL>s do not always
466 supply a fix either. Direct access to developers, the user
467 community, manuals, and the source code often make PostgreSQL
468 support superior to other <SMALL>DBMS</SMALL>s. There is
469 commercial per-incident support available for those who need it.
470 (See <A href="#1.6">FAQ section 1.6</A>.)<BR>
474 <DT><B>Price</B></DT>
476 <DD>We are free for all use, both commercial and non-commercial.
477 You can add our code to your product with no limitations, except
478 those outlined in our BSD-style license stated above.<BR>
483 <H4><A name="1.15">1.15</A>) How can I financially assist
486 <P>PostgreSQL has had a first-class infrastructure since we started
487 in 1996. This is all thanks to Marc Fournier, who has created
488 and managed this infrastructure over the years.</P>
490 <P>Quality infrastructure is very important to an open-source
491 project. It prevents disruptions that can greatly delay forward
492 movement of the project.</P>
494 <P>Of course, this infrastructure is not cheap. There are a variety
495 of monthly and one-time expenses that are required to keep it
496 going. If you or your company has money it can donate to help fund
497 this effort, please go to <A href="http://store.pgsql.com/shopping/">http://store.pgsql.com/shopping/</A>
498 and make a donation.</P>
500 <P>Although the web page mentions PostgreSQL, Inc, the
501 "contributions" item is solely to support the PostgreSQL project
502 and does not fund any specific company. If you prefer, you can also
503 send a check to the contact address.</P>
506 <P>Also, if you have a success story about PostgreSQL, please submit
507 it to our advocacy site at <a href="http://advocacy.postgresql.org">
508 http://advocacy.postgresql.org</a>.</P>
511 <H2 align="center">User Client Questions</H2>
513 <H4><A name="2.1">2.1</A>) Are there <SMALL>ODBC</SMALL> drivers
516 <P>There are two <SMALL>ODBC</SMALL> drivers available, PsqlODBC
517 and OpenLink <SMALL>ODBC</SMALL>.</P>
519 <P>You can download PsqlODBC from <A href=
520 "http://gborg.postgresql.org/project/psqlodbc/projdisplay.php">
521 http://gborg.postgresql.org/project/psqlodbc/projdisplay.php</A>.</P>
523 <P>OpenLink <SMALL>ODBC</SMALL> can be gotten from <A href=
524 "http://www.openlinksw.com/">http://www.openlinksw.com</A>. It
525 works with their standard <SMALL>ODBC</SMALL> client software so
526 you'll have PostgreSQL <SMALL>ODBC</SMALL> available on every
527 client platform they support (Win, Mac, Unix, VMS).</P>
529 <P>They will probably be selling this product to people who need
530 commercial-quality support, but a freeware version will always be
531 available. Please send questions to <A href=
532 "mailto:postgres95@openlink.co.uk">postgres95@openlink.co.uk</A>.</P>
534 <H4><A name="2.2">2.2</A>) What tools are available for using
535 PostgreSQL with Web pages?</H4>
537 <P>A nice introduction to Database-backed Web pages can be seen at:
538 <A href="http://www.webreview.com">http://www.webreview.com</A></P>
540 <P>For Web integration, PHP is an excellent interface. It is at <A
541 href="http://www.php.net">http://www.php.net</A>.</P>
543 <P>For complex cases, many use the Perl interface and CGI.pm or mod_perl.</P>
545 <H4><A name="2.3">2.3</A>) Does PostgreSQL have a graphical user
548 <P>Yes, there are several graphical interfaces to PostgreSQL available.
549 These include PgAccess <a href="http://www.pgaccess.org">
550 http://www.pgaccess.org</a>), PgAdmin II (<a
551 href="http://www.pgadmin.org">http://www.pgadmin.org</a>,
552 Win32-only), RHDB Admin (<a
553 href="http://sources.redhat.com/rhdb/">http://sources.redhat.com/rhdb/
554 </a>) and Rekall (<a href="http://www.thekompany.com/products/rekall/">
555 http://www.thekompany.com/products/rekall/</a>, proprietary). There is
556 also PHPPgAdmin (<a href="http://phppgadmin.sourceforge.net/">
557 http://phppgadmin.sourceforge.net/ </a>), a web-based interface to
560 <P>See <a href="http://techdocs.postgresql.org/guides/GUITools">http://techdocs.postgresql.org/guides/GUITools</a> for a more detailed list.</P>
562 <H4><A name="2.4">2.4</A>) What languages are able to communicate with
565 <P>Most popular programming languages contain an interface to
566 PostgreSQL. Check your programming language's list of extension
569 <P>The following interfaces are included in the PostgreSQL
575 <LI>Embedded C (ecpg)</LI>
579 <LI>Python (PyGreSQL)</LI>
581 <LI>TCL (libpgtcl)</LI>
584 <P>Additional interfaces are available at
585 <a href="http://gborg.postgresql.org">http://gborg.postgresql.org</A>
586 in the <I>Drivers/Interfaces</I> section.
590 <H2 align="center">Administrative Questions</H2>
592 <H4><A name="3.1">3.1</A>) How do I install PostgreSQL somewhere
593 other than <I>/usr/local/pgsql</I>?</H4>
595 <P>Specify the <I>--prefix</I> option when running
596 <I>configure</I>.</P>
598 <H4><A name="3.2">3.2</A>) When I start <I>postmaster</I>, I get a
599 <I>Bad System Call</I> or core dumped message. Why?</H4>
601 <P>It could be a variety of problems, but first check to see that
602 you have System V extensions installed in your kernel. PostgreSQL
603 requires kernel support for shared memory and semaphores.</P>
605 <H4><A name="3.3">3.3</A>) When I try to start <I>postmaster</I>, I
606 get <I>IpcMemoryCreate</I> errors. Why?</H4>
608 <P>You either do not have shared memory configured properly in your
609 kernel or you need to enlarge the shared memory available in the
610 kernel. The exact amount you need depends on your architecture and
611 how many buffers and backend processes you configure for
612 <I>postmaster</I>. For most systems, with default numbers of
613 buffers and processes, you need a minimum of ~1 MB. See the <A
615 "http://www.PostgreSQL.org/docs/view.php?version=current&idoc=1&file=kernel-resources.html">PostgreSQL
616 Administrator's Guide</A> for more detailed information about
617 shared memory and semaphores.</P>
619 <H4><A name="3.4">3.4</A>) When I try to start <I>postmaster</I>, I
620 get <I>IpcSemaphoreCreate</I> errors. Why?</H4>
622 <P>If the error message is <I>IpcSemaphoreCreate: semget failed (No
623 space left on device)</I> then your kernel is not configured with
624 enough semaphores. Postgres needs one semaphore per potential
625 backend process. A temporary solution is to start <I>postmaster</I>
626 with a smaller limit on the number of backend processes. Use
627 <I>-N</I> with a parameter less than the default of 32. A more
628 permanent solution is to increase your kernel's
629 <SMALL>SEMMNS</SMALL> and <SMALL>SEMMNI</SMALL> parameters.</P>
631 <P>Inoperative semaphores can also cause crashes during heavy
634 <P>If the error message is something else, you might not have
635 semaphore support configured in your kernel at all. See the
636 PostgreSQL Administrator's Guide for more detailed information
637 about shared memory and semaphores.</P>
639 <H4><A name="3.5">3.5</A>) How do I control connections from other
642 <P>By default, PostgreSQL only allows connections from the local
643 machine using Unix domain sockets. Other machines will not be able
644 to connect unless you add the <I>-i</I> flag to <I>postmaster</I>,
645 <B>and</B> enable host-based authentication by modifying the file
646 <I>$PGDATA/pg_hba.conf</I> accordingly. This will allow TCP/IP
649 <H4><A name="3.6">3.6</A>) How do I tune the database engine for
650 better performance?</H4>
652 <P>Certainly, indexes can speed up queries. The
653 <SMALL>EXPLAIN</SMALL> command allows you to see how PostgreSQL is
654 interpreting your query, and which indexes are being used.</P>
656 <P>If you are doing many <SMALL>INSERTs</SMALL>, consider doing
657 them in a large batch using the <SMALL>COPY</SMALL> command. This
658 is much faster than individual <SMALL>INSERTS</SMALL>. Second,
659 statements not in a <SMALL>BEGIN WORK/COMMIT</SMALL> transaction
660 block are considered to be in their own transaction. Consider
661 performing several statements in a single transaction block. This
662 reduces the transaction overhead. Also, consider dropping and
663 recreating indexes when making large data changes.</P>
665 <P>There are several tuning options. You can disable <I>fsync()</I>
666 by starting <I>postmaster</I> with a <I>-o -F</I> option. This will
667 prevent <I>fsync()</I>s from flushing to disk after every
670 <P>You can also use the <I>postmaster</I> <I>-B</I> option to
671 increase the number of shared memory buffers used by the backend
672 processes. If you make this parameter too high, the
673 <I>postmaster</I> may not start because you have exceeded your
674 kernel's limit on shared memory space. Each buffer is 8K and the
675 default is 64 buffers.</P>
677 <P>You can also use the backend <I>-S</I> option to increase the
678 maximum amount of memory used by the backend process for temporary
679 sorts. The <I>-S</I> value is measured in kilobytes, and the
680 default is 512 (i.e. 512K).</P>
682 <P>You can also use the <SMALL>CLUSTER</SMALL> command to group
683 data in tables to match an index. See the <SMALL>CLUSTER</SMALL>
684 manual page for more details.</P>
686 <H4><A name="3.7">3.7</A>) What debugging features are
689 <P>PostgreSQL has several features that report status information
690 that can be valuable for debugging purposes.</P>
692 <P>First, by running <I>configure</I> with the --enable-cassert
693 option, many <I>assert()</I>s monitor the progress of the backend
694 and halt the program when something unexpected occurs.</P>
696 <P>Both <I>postmaster</I> and <I>postgres</I> have several debug
697 options available. First, whenever you start <I>postmaster</I>,
698 make sure you send the standard output and error to a log file,
702 ./bin/postmaster >server.log 2>&1 &
705 <P>This will put a server.log file in the top-level PostgreSQL
706 directory. This file contains useful information about problems or
707 errors encountered by the server. <I>Postmaster</I> has a <I>-d</I>
708 option that allows even more detailed information to be reported.
709 The <I>-d</I> option takes a number that specifies the debug level.
710 Be warned that high debug level values generate large log
713 <P>If <I>postmaster</I> is not running, you can actually run the
714 <I>postgres</I> backend from the command line, and type your
715 <SMALL>SQL</SMALL> statement directly. This is recommended
716 <B>only</B> for debugging purposes. Note that a newline terminates
717 the query, not a semicolon. If you have compiled with debugging
718 symbols, you can use a debugger to see what is happening. Because
719 the backend was not started from <I>postmaster</I>, it is not
720 running in an identical environment and locking/backend interaction
721 problems may not be duplicated.</P>
723 <P>If <I>postmaster</I> is running, start <I>psql</I> in one
724 window, then find the <SMALL>PID</SMALL> of the <I>postgres</I>
725 process used by <I>psql</I>. Use a debugger to attach to the
726 <I>postgres</I> <SMALL>PID</SMALL>. You can set breakpoints in the
727 debugger and issue queries from <I>psql</I>. If you are debugging
728 <I>postgres</I> startup, you can set PGOPTIONS="-W n", then start
729 <I>psql</I>. This will cause startup to delay for <I>n</I> seconds
730 so you can attach to the process with the debugger, set any
731 breakpoints, and continue through the startup sequence.</P>
733 <P>The <I>postgres</I> program has <I>-s, -A</I>, and <I>-t</I>
734 options that can be very useful for debugging and performance
737 <P>You can also compile with profiling to see what functions are
738 taking execution time. The backend profile files will be deposited
739 in the <I>pgsql/data/base/dbname</I> directory. The client profile
740 file will be put in the client's current directory. Linux requires
741 a compile with <I>-DLINUX_PROFILE</I> for proper profiling.</P>
743 <H4><A name="3.8">3.8</A>) Why do I get <I>"Sorry, too many
744 clients"</I> when trying to connect?</H4>
746 <P>You need to increase <I>postmaster</I>'s limit on how many
747 concurrent backend processes it can start.</P>
749 <P>The default limit is 32 processes. You can increase it by
750 restarting <I>postmaster</I> with a suitable <I>-N</I> value or
751 modifying <I>postgresql.conf</I>.</P>
753 <P>Note that if you make <I>-N</I> larger than 32, you must also
754 increase <I>-B</I> beyond its default of 64; <I>-B</I> must be at
755 least twice <I>-N</I>, and probably should be more than that for
756 best performance. For large numbers of backend processes, you are
757 also likely to find that you need to increase various Unix kernel
758 configuration parameters. Things to check include the maximum size
759 of shared memory blocks, <SMALL>SHMMAX;</SMALL> the maximum number
760 of semaphores, <SMALL>SEMMNS</SMALL> and <SMALL>SEMMNI;</SMALL> the
761 maximum number of processes, <SMALL>NPROC;</SMALL> the maximum
762 number of processes per user, <SMALL>MAXUPRC;</SMALL> and the
763 maximum number of open files, <SMALL>NFILE</SMALL> and
764 <SMALL>NINODE</SMALL>. The reason that PostgreSQL has a limit on
765 the number of allowed backend processes is so your system won't run
766 out of resources.</P>
768 <H4><A name="3.9">3.9</A>) What is in the <I>pgsql_tmp</I> directory?</H4>
770 <P>This directory contains temporary files generated by the query
771 executor. For example, if a sort needs to be done to satisfy an
772 <SMALL>ORDER BY</SMALL> and the sort requires more space than the
773 backend's <I>-S</I> parameter allows, then temporary files are created
774 here to hold the extra data.</P>
776 <P>The temporary files are usually deleted automatically, but might
777 remain if a backend crashes during a sort. A stop and restart of the
778 <I>postmaster</I> will remove files from those directories.</P>
780 <H4><A name="3.10">3.10</A>) Why do I need to do a dump and restore
781 to upgrade between major PostgreSQL releases?</H4>
783 <P>The PostgreSQL team makes only small changes between minor releases,
784 so upgrading from 7.2 to 7.2.1 does not require a dump and restore.
785 However, major releases (e.g. from 7.2 to 7.3) often change the internal
786 format of system tables and data files. These changes are often complex,
787 so we don't maintain backward compatability for data files. A dump outputs
788 data in a generic format that can then be loaded in using the new internal
791 <P>In releases where the on-disk format does not change, the
792 <I>pg_upgrade</I> script can be used to upgrade without a dump/restore.
793 The release notes mention whether <I>pg_upgrade</I> is available for the
798 <H2 align="center">Operational Questions</H2>
800 <H4><A name="4.1">4.1</A>) What is the difference between binary
801 cursors and normal cursors?</H4>
803 <P>See the <SMALL>DECLARE</SMALL> manual page for a
806 <H4><A name="4.2">4.2</A>) How do I <SMALL>SELECT</SMALL> only the
807 first few rows of a query? A random row?</H4>
809 <P>See the <SMALL>FETCH</SMALL> manual page, or use
810 <SMALL>SELECT</SMALL> ... <SMALL>LIMIT</SMALL>....</P>
812 <P>The entire query may have to be evaluated, even if you only want
813 the first few rows. Consider using a query that has an <SMALL>ORDER
814 BY</SMALL>. If there is an index that matches the <SMALL>ORDER
815 BY</SMALL>, PostgreSQL may be able to evaluate only the first few
816 records requested, or the entire query may have to be evaluated
817 until the desired rows have been generated.</P>
819 <P>To <SMALL>SELECT</SMALL> a random row, use:
827 <H4><A name="4.3">4.3</A>) How do I get a list of tables or other
828 things I can see in <I>psql</I>?</H4>
830 <P>You can read the source code for <I>psql</I> in file
831 <I>pgsql/src/bin/psql/describe.c</I>. It contains
832 <SMALL>SQL</SMALL> commands that generate the output for psql's
833 backslash commands. You can also start <I>psql</I> with the
834 <I>-E</I> option so it will print out the queries it uses to
835 execute the commands you give.</P>
837 <H4><A name="4.4">4.4</A>) How do you remove a column from a
838 table, or change its data type?</H4>
840 <P><SMALL>DROP COLUMN</SMALL> functionality was added in release 7.3 with
841 <SMALL>ALTER TABLE DROP COLUMN</SMALL>. In earlier versions,
845 LOCK TABLE old_table;
846 SELECT ... -- select all columns but the one you want to remove
849 DROP TABLE old_table;
850 ALTER TABLE new_table RENAME TO old_table;
854 <P>To change the data type of a column, do this:</P>
857 ALTER TABLE tab ADD COLUMN new_col <i>new_data_type</i>;
858 UPDATE tab SET new_col = CAST(old_col AS <i>new_data_type</i>);
859 ALTER TABLE tab DROP COLUMN old_col;
862 <P>You might then want to do <I>VACUUM FULL tab</I> to reclaim the
863 disk space used by the expired rows.</P>
865 <H4><A name="4.5">4.5</A>) What is the maximum size for a row, a
866 table, and a database?</H4>
868 <P>These are the limits:</P>
870 Maximum size for a database? unlimited (4 TB databases exist)
871 Maximum size for a table? 16 TB
872 Maximum size for a row? 1.6TB
873 Maximum size for a field? 1 GB
874 Maximum number of rows in a table? unlimited
875 Maximum number of columns in a table? 250-1600 depending on column types
876 Maximum number of indexes on a table? unlimited
879 Of course, these are not actually unlimited, but limited to
880 available disk space and memory/swap space. Performance may suffer
881 when these values get unusually large.
883 <P>The maximum table size of 16 TB does not require large file
884 support from the operating system. Large tables are stored as
885 multiple 1 GB files so file system size limits are not
888 <P>The maximum table size and maximum number of columns can be
889 increased if the default block size is increased to 32k.</P>
891 <H4><A name="4.6">4.6</A>) How much database disk space is required
892 to store data from a typical text file?</H4>
894 <P>A PostgreSQL database may require up to five times the disk
895 space to store data from a text file.</P>
897 <P>As an example, consider a file of 100,000 lines with an integer
898 and text description on each line. Suppose the text string
899 avergages twenty bytes in length. The flat file would be 2.8 MB.
900 The size of the PostgreSQL database file containing this data can
901 be estimated as 6.4 MB:</P>
903 36 bytes: each row header (approximate)
904 24 bytes: one int field and one text field
905 + 4 bytes: pointer on page to tuple
906 ----------------------------------------
909 The data page size in PostgreSQL is 8192 bytes (8 KB), so:
912 ------------------- = 128 rows per database page (rounded down)
916 -------------------- = 782 database pages (rounded up)
919 782 database pages * 8192 bytes per page = 6,406,144 bytes (6.4 MB)
922 <P>Indexes do not require as much overhead, but do contain the data
923 that is being indexed, so they can be large also.</P>
925 <P><SMALL>NULL</SMALL>s are stored in bitmaps, so they
926 use very little space.</P>
928 <H4><A name="4.7">4.7</A>) How do I find out what tables, indexes,
929 databases, and users are defined?</H4>
931 <P><I>psql</I> has a variety of backslash commands to show such
932 information. Use \? to see them. There are also system tables
933 beginning with <I>pg_</I> that describe these too. Also, <I>psql
934 -l</I> will list all databases.</P>
936 <P>Also try the file <I>pgsql/src/tutorial/syscat.source</I>. It
937 illustrates many of the <SMALL>SELECT</SMALL>s needed to get
938 information from the database system tables.</P>
940 <H4><A name="4.8">4.8</A>) My queries are slow or don't make use of
941 the indexes. Why?</H4>
942 Indexes are not automatically used by every query. Indexes are only
943 used if the table is larger than a minimum size, and the query
944 selects only a small percentage of the rows in the table. This is
945 because the random disk access caused by an index scan can be
946 slower than a straight read through the table, or sequential scan.
948 <P>To determine if an index should be used, PostgreSQL must have
949 statistics about the table. These statistics are collected using
950 <SMALL>VACUUM ANALYZE</SMALL>, or simply <SMALL>ANALYZE</SMALL>.
951 Using statistics, the optimizer knows how many rows are in the
952 table, and can better determine if indexes should be used.
953 Statistics are also valuable in determining optimal join order and
954 join methods. Statistics collection should be performed
955 periodically as the contents of the table change.</P>
957 <P>Indexes are normally not used for <SMALL>ORDER BY</SMALL> or to
958 perform joins. A sequential scan followed by an explicit sort is
959 usually faster than an index scan of a large table.</P>
960 However, <SMALL>LIMIT</SMALL> combined with <SMALL>ORDER BY</SMALL>
961 often will use an index because only a small portion of the table
962 is returned. In fact, though MAX() and MIN() don't use indexes,
963 it is possible to retrieve such values using an index with ORDER BY
968 ORDER BY col [ DESC ]
972 <P>If you believe the optimizer is incorrect in choosing a
973 sequential scan, use <CODE>SET enable_seqscan TO 'off'</CODE> and
974 run tests to see if an index scan is indeed faster.</P>
976 <P>When using wild-card operators such as <SMALL>LIKE</SMALL> or
977 <I>~</I>, indexes can only be used in certain circumstances:</P>
979 <LI>The beginning of the search string must be anchored to the start
982 <LI><SMALL>LIKE</SMALL> patterns must not start with <I>%</I>.</LI>
983 <LI><I>~</I> (regular expression) patterns must start with
986 <LI>The search string can not start with a character class,
988 <LI>Case-insensitive searches such as <SMALL>ILIKE</SMALL> and
989 <I>~*</I> do not utilise indexes. Instead, use functional
990 indexes, which are described in section <a href="#4.12">4.12</a>.</LI>
991 <LI>The default <I>C</I> locale must be used during
996 <H4><A name="4.9">4.9</A>) How do I see how the query optimizer is
997 evaluating my query?</H4>
999 <P>See the <SMALL>EXPLAIN</SMALL> manual page.</P>
1001 <H4><A name="4.10">4.10</A>) What is an R-tree index?</H4>
1003 <P>An R-tree index is used for indexing spatial data. A hash index
1004 can't handle range searches. A B-tree index only handles range
1005 searches in a single dimension. R-trees can handle
1006 multi-dimensional data. For example, if an R-tree index can be
1007 built on an attribute of type <I>point</I>, the system can more
1008 efficiently answer queries such as "select all points within a
1009 bounding rectangle."</P>
1011 <P>The canonical paper that describes the original R-tree design
1014 <P>Guttman, A. "R-trees: A Dynamic Index Structure for Spatial
1015 Searching." Proceedings of the 1984 ACM SIGMOD Int'l Conf on Mgmt
1018 <P>You can also find this paper in Stonebraker's "Readings in
1019 Database Systems".</P>
1021 <P>Built-in R-trees can handle polygons and boxes. In theory,
1022 R-trees can be extended to handle higher number of dimensions. In
1023 practice, extending R-trees requires a bit of work and we don't
1024 currently have any documentation on how to do it.</P>
1026 <H4><A name="4.11">4.11</A>) What is the Genetic Query
1029 <P>The <SMALL>GEQO</SMALL> module speeds query optimization when
1030 joining many tables by means of a Genetic Algorithm (GA). It allows
1031 the handling of large join queries through nonexhaustive
1034 <H4><A name="4.12">4.12</A>) How do I perform regular expression
1035 searches and case-insensitive regular expression searches? How do I
1036 use an index for case-insensitive searches?</H4>
1038 <P>The <I>~</I> operator does regular expression matching, and
1039 <I>~*</I> does case-insensitive regular expression matching. The
1040 case-insensitive variant of <SMALL>LIKE</SMALL> is called
1041 <SMALL>ILIKE</SMALL>.</P>
1043 <P>Case-insensitive equality comparisons are normally expressed
1048 WHERE lower(col) = 'abc';
1051 This will not use an standard index. However, if you create a
1052 functional index, it will be used:
1054 CREATE INDEX tabindex ON tab (lower(col));
1057 <H4><A name="4.13">4.13</A>) In a query, how do I detect if a field
1058 is <SMALL>NULL</SMALL>?</H4>
1060 <P>You test the column with <SMALL>IS NULL</SMALL> and <SMALL>IS
1061 NOT NULL</SMALL>.</P>
1063 <H4><A name="4.14">4.14</A>) What is the difference between the
1064 various character types?</H4>
1066 Type Internal Name Notes
1067 --------------------------------------------------
1068 VARCHAR(n) varchar size specifies maximum length, no padding
1069 CHAR(n) bpchar blank padded to the specified fixed length
1070 TEXT text no specific upper limit on length
1071 BYTEA bytea variable-length byte array (null-byte safe)
1072 "char" char one character
1075 <P>You will see the internal name when examining system catalogs
1076 and in some error messages.</P>
1078 <P>The first four types above are "varlena" types (i.e., the first
1079 four bytes on disk are the length, followed by the data). Thus the
1080 actual space used is slightly greater than the declared size.
1081 However, these data types are also subject to compression or being
1082 stored out-of-line by <SMALL>TOAST</SMALL>, so the space on disk
1083 might also be less than expected.</P>
1085 <SMALL>VARCHAR(n)</SMALL> is best when storing variable-length
1086 strings and it limits how long a string can be. <SMALL>TEXT</SMALL>
1087 is for strings of unlimited length, with a maximum of one gigabyte.
1088 <P><SMALL>CHAR(n)</SMALL> is for storing strings that are all the
1089 same length. <SMALL>CHAR(n)</SMALL> pads with blanks to the specified
1090 length, while <SMALL>VARCHAR(n)</SMALL> only stores the characters
1091 supplied. <SMALL>BYTEA</SMALL> is for storing binary data,
1092 particularly values that include <SMALL>NULL</SMALL> bytes. All the
1093 types described here have similar performance characteristics.</P>
1095 <H4><A name="4.15.1">4.15.1</A>) How do I create a
1096 serial/auto-incrementing field?</H4>
1098 <P>PostgreSQL supports a <SMALL>SERIAL</SMALL> data type. It
1099 auto-creates a sequence and index on the column. For example,
1102 CREATE TABLE person (
1108 is automatically translated into this:
1110 CREATE SEQUENCE person_id_seq;
1111 CREATE TABLE person (
1112 id INT4 NOT NULL DEFAULT nextval('person_id_seq'),
1115 CREATE UNIQUE INDEX person_id_key ON person ( id );
1118 See the <I>create_sequence</I> manual page for more information
1119 about sequences. You can also use each row's <I>OID</I> field as a
1120 unique value. However, if you need to dump and reload the database,
1121 you need to use <I>pg_dump</I>'s <I>-o</I> option or <SMALL>COPY
1122 WITH OIDS</SMALL> option to preserve the <SMALL>OID</SMALL>s.
1124 <H4><A name="4.15.2">4.15.2</A>) How do I get the value of a
1125 <SMALL>SERIAL</SMALL> insert?</H4>
1127 <P>One approach is to retrieve the next <SMALL>SERIAL</SMALL> value
1128 from the sequence object with the <I>nextval()</I> function
1129 <I>before</I> inserting and then insert it explicitly. Using the
1130 example table in <A href="#4.15.1">4.15.1</A>, an example in a
1131 pseudo-language would look like this:</P>
1133 new_id = execute("SELECT nextval('person_id_seq')");
1134 execute("INSERT INTO person (id, name) VALUES (new_id, 'Blaise Pascal')");
1137 You would then also have the new value stored in
1138 <CODE>new_id</CODE> for use in other queries (e.g., as a foreign
1139 key to the <CODE>person</CODE> table). Note that the name of the
1140 automatically created <SMALL>SEQUENCE</SMALL> object will be named
1141 <<I>table</I>>_<<I>serialcolumn</I>>_<I>seq</I>, where
1142 <I>table</I> and <I>serialcolumn</I> are the names of your table
1143 and your <SMALL>SERIAL</SMALL> column, respectively.
1145 <P>Alternatively, you could retrieve the assigned
1146 <SMALL>SERIAL</SMALL> value with the <I>currval()</I> function
1147 <I>after</I> it was inserted by default, e.g.,</P>
1149 execute("INSERT INTO person (name) VALUES ('Blaise Pascal')");
1150 new_id = execute("SELECT currval('person_id_seq')");
1153 Finally, you could use the <A href="#4.16"><SMALL>OID</SMALL></A>
1154 returned from the <SMALL>INSERT</SMALL> statement to look up the
1155 default value, though this is probably the least portable approach.
1156 In Perl, using DBI with Edmund Mergl's DBD::Pg module, the oid
1157 value is made available via <I>$sth->{pg_oid_status}</I> after
1158 <I>$sth->execute()</I>.
1160 <H4><A name="4.15.3">4.15.3</A>) Don't <I>currval()</I> and
1161 <I>nextval()</I> lead to a race condition with other users?</H4>
1163 <P>No. <I>currval()</I> returns the current value assigned by your
1164 backend, not by all users.</P>
1166 <H4><A name="4.15.4">4.15.4</A>) Why aren't my sequence numbers
1167 reused on transaction abort? Why are there gaps in the numbering of
1168 my sequence/SERIAL column?</H4>
1170 <P>To improve concurrency, sequence values are given out to running
1171 transactions as needed and are not locked until the transaction
1172 completes. This causes gaps in numbering from aborted
1175 <H4><A name="4.16">4.16</A>) What is an <SMALL>OID</SMALL>? What is
1176 a <SMALL>TID</SMALL>?</H4>
1178 <P><SMALL>OID</SMALL>s are PostgreSQL's answer to unique row ids.
1179 Every row that is created in PostgreSQL gets a unique
1180 <SMALL>OID</SMALL>. All <SMALL>OID</SMALL>s generated during
1181 <I>initdb</I> are less than 16384 (from
1182 <I>include/access/transam.h</I>). All user-created
1183 <SMALL>OID</SMALL>s are equal to or greater than this. By default,
1184 all these <SMALL>OID</SMALL>s are unique not only within a table or
1185 database, but unique within the entire PostgreSQL installation.</P>
1187 <P>PostgreSQL uses <SMALL>OID</SMALL>s in its internal system
1188 tables to link rows between tables. These <SMALL>OID</SMALL>s can
1189 be used to identify specific user rows and used in joins. It is
1190 recommended you use column type <SMALL>OID</SMALL> to store
1191 <SMALL>OID</SMALL> values. You can create an index on the
1192 <SMALL>OID</SMALL> field for faster access.</P>
1194 <P>O<SMALL>ID</SMALL>s are assigned to all new rows from a central
1195 area that is used by all databases. If you want to change the
1196 <SMALL>OID</SMALL> to something else, or if you want to make a copy
1197 of the table, with the original <SMALL>OID</SMALL>s, there is no
1198 reason you can't do it:</P>
1200 CREATE TABLE new_table(old_oid oid, mycol int);
1201 SELECT old_oid, mycol INTO new FROM old;
1202 COPY new TO '/tmp/pgtable';
1204 COPY new WITH OIDS FROM '/tmp/pgtable';
1207 CREATE TABLE new_table (mycol int);
1208 INSERT INTO new_table (oid, mycol) SELECT oid, mycol FROM old_table;
1210 <P>O<SMALL>ID</SMALL>s are stored as 4-byte integers, and will
1211 overflow at 4 billion. No one has reported this ever happening, and
1212 we plan to have the limit removed before anyone does.</P>
1214 <P>T<SMALL>ID</SMALL>s are used to identify specific physical rows
1215 with block and offset values. T<SMALL>ID</SMALL>s change after rows
1216 are modified or reloaded. They are used by index entries to point
1217 to physical rows.</P>
1219 <H4><A name="4.17">4.17</A>) What is the meaning of some of the
1220 terms used in PostgreSQL?</H4>
1222 <P>Some of the source code and older documentation use terms that
1223 have more common usage. Here are some:</P>
1226 <LI>table, relation, class</LI>
1228 <LI>row, record, tuple</LI>
1230 <LI>column, field, attribute</LI>
1232 <LI>retrieve, select</LI>
1234 <LI>replace, update</LI>
1236 <LI>append, insert</LI>
1238 <LI><SMALL>OID</SMALL>, serial value</LI>
1240 <LI>portal, cursor</LI>
1242 <LI>range variable, table name, table alias</LI>
1245 <P>A list of general database terms can be found at: <A href=
1246 "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>
1248 <H4><A name="4.18">4.18</A>) Why do I get the error <I>"ERROR:
1249 Memory exhausted in AllocSetAlloc()"</I>?</H4>
1251 <P>You probably have run out of virtual memory on your system,
1252 or your kernel has a low limit for certain resources. Try this
1253 before starting <I>postmaster</I>:</P>
1259 Depending on your shell, only one of these may succeed, but it will
1260 set your process data segment limit much higher and perhaps allow
1261 the query to complete. This command applies to the current process,
1262 and all subprocesses created after the command is run. If you are
1263 having a problem with the <SMALL>SQL</SMALL> client because the
1264 backend is returning too much data, try it before starting the
1267 <H4><A name="4.19">4.19</A>) How do I tell what PostgreSQL version
1270 <P>From <I>psql</I>, type <CODE>SELECT version();</CODE></P>
1272 <H4><A name="4.20">4.20</A>) Why does my large-object operations
1273 get <I>"invalid large obj descriptor"</I>?</H4>
1275 <P>You need to put <CODE>BEGIN WORK</CODE> and <CODE>COMMIT</CODE>
1276 around any use of a large object handle, that is, surrounding
1277 <CODE>lo_open</CODE> ... <CODE>lo_close.</CODE></P>
1279 <P>Currently PostgreSQL enforces the rule by closing large object
1280 handles at transaction commit. So the first attempt to do anything
1281 with the handle will draw <I>invalid large obj descriptor</I>. So
1282 code that used to work (at least most of the time) will now
1283 generate that error message if you fail to use a transaction.</P>
1285 <P>If you are using a client interface like <SMALL>ODBC</SMALL> you
1286 may need to set <CODE>auto-commit off.</CODE></P>
1288 <H4><A name="4.21">4.21</A>) How do I create a column that will
1289 default to the current time?</H4>
1291 <P>Use <I>CURRENT_TIMESTAMP</I>:</P>
1293 <CODE>CREATE TABLE test (x int, modtime timestamp DEFAULT CURRENT_TIMESTAMP );
1297 <H4><A name="4.22">4.22</A>) Why are my subqueries using
1298 <CODE><SMALL>IN</SMALL></CODE> so slow?</H4>
1300 <P>Currently, we join subqueries to outer queries by sequentially
1301 scanning the result of the subquery for each row of the outer
1302 query. If the subquery returns only a few rows and the outer query
1303 returns many rows, <CODE><SMALL>IN</SMALL></CODE> is fastest. To
1304 speed up other queries, replace <CODE>IN</CODE> with
1305 <CODE>EXISTS</CODE>:</P>
1308 WHERE col IN (SELECT subcol FROM subtab);
1313 WHERE EXISTS (SELECT subcol FROM subtab WHERE subcol = col);
1316 For this to be fast, <CODE>subcol</CODE> should be an indexed column.
1317 This preformance problem will be fixed in 7.4.
1319 <H4><A name="4.23">4.23</A>) How do I perform an outer join?</H4>
1321 <P>PostgreSQL supports outer joins using the SQL standard syntax.
1322 Here are two examples:</P>
1325 FROM t1 LEFT OUTER JOIN t2 ON (t1.col = t2.col);
1330 FROM t1 LEFT OUTER JOIN t2 USING (col);
1333 <P>These identical queries join t1.col to t2.col, and also return
1334 any unjoined rows in t1 (those with no match in t2). A
1335 <SMALL>RIGHT</SMALL> join would add unjoined rows of t2. A
1336 <SMALL>FULL</SMALL> join would return the matched rows plus all
1337 unjoined rows from t1 and t2. The word <SMALL>OUTER</SMALL> is
1338 optional and is assumed in <SMALL>LEFT</SMALL>,
1339 <SMALL>RIGHT</SMALL>, and <SMALL>FULL</SMALL> joins. Ordinary joins
1340 are called <SMALL>INNER</SMALL> joins.</P>
1342 <P>In previous releases, outer joins can be simulated using
1343 <SMALL>UNION</SMALL> and <SMALL>NOT IN</SMALL>. For example, when
1344 joining <I>tab1</I> and <I>tab2</I>, the following query does an
1345 <I>outer</I> join of the two tables:<BR>
1349 SELECT tab1.col1, tab2.col2
1351 WHERE tab1.col1 = tab2.col1
1353 SELECT tab1.col1, NULL
1355 WHERE tab1.col1 NOT IN (SELECT tab2.col1 FROM tab2)
1359 <H4><A name="4.24">4.24</A>) How do I perform queries using
1360 multiple databases?</H4>
1362 <P>There is no way to query a database other than the current one.
1363 Because PostgreSQL loads database-specific system catalogs, it is
1364 uncertain how a cross-database query should even behave.</P>
1366 <P><I>contrib/dblink</I> allows cross-database queries using
1367 function calls. Of course, a client can make simultaneous
1368 connections to different databases and merge the results on the
1371 <H4><A name="4.25">4.25</A>) How do I return multiple rows or
1372 columns from a function?</H4>
1374 <P>In 7.3, you can easily return multiple rows or columns from a
1376 <a href="http://techdocs.postgresql.org/guides/SetReturningFunctions">
1377 http://techdocs.postgresql.org/guides/SetReturningFunctions</a>.
1379 <H4><A name="4.26">4.26</A>) Why can't I reliably create/drop
1380 temporary tables in PL/PgSQL functions?</H4>
1381 <P>PL/PgSQL caches function contents, and an unfortunate side effect
1382 is that if a PL/PgSQL function accesses a temporary table, and that
1383 table is later dropped and recreated, and the function called
1384 again, the function will fail because the cached function contents
1385 still point to the old temporary table. The solution is to use
1386 <SMALL>EXECUTE</SMALL> for temporary table access in PL/PgSQL. This
1387 will cause the query to be reparsed every time.</P>
1389 <H4><A name="4.27">4.27</A>) What replication options are available?
1391 <P>There are several master/slave replication options available.
1392 These allow only the master to make database changes and the slave
1393 can only do database reads. The bottom of <a
1394 href="http://gborg.PostgreSQL.org/genpage?replication_research">
1395 http://gborg.PostgreSQL.org/genpage?replication_research</a> lists
1396 them. A multi-master replication solution is being worked on at <a
1397 href="http://gborg.PostgreSQL.org/project/pgreplication/projdisplay.php">http://gborg.PostgreSQL.org/project/pgreplication/projdisplay.php</a>.</P>
1399 <H4><A name="4.28">4.28</A>) What encryption options are available?
1402 <LI><I>contrib/pgcrypto</I> contains many encryption functions for
1403 use in <SMALL>SQL</SMALL> queries.</LI>
1404 <LI>The only way to encrypt transmission from the client to the
1405 server is by using <I>hostssl</I> in <I>pg_hba.conf</I>.</LI>
1406 <LI>Database user passwords are automatically encrypted when stored
1407 in version 7.3. In previous versions, you must enable the option
1408 <I>PASSWORD_ENCRYPTION</I> in <I>postgresql.conf</I>.</LI>
1409 <LI>The server can run using an encrypted file system.</LI>
1414 <H2 align="center">Extending PostgreSQL</H2>
1416 <H4><A name="5.1">5.1</A>) I wrote a user-defined function. When I
1417 run it in <I>psql</I>, why does it dump core?</H4>
1419 <P>The problem could be a number of things. Try testing your
1420 user-defined function in a stand-alone test program first.</P>
1422 <H4><A name="5.2">5.2</A>) How can I contribute some nifty new
1423 types and functions to PostgreSQL?</H4>
1425 <P>Send your extensions to the <I>pgsql-hackers</I> mailing list,
1426 and they will eventually end up in the <I>contrib/</I>
1429 <H4><A name="5.3">5.3</A>) How do I write a C function to return a
1432 <P>In versions of PostgreSQL beginning with 7.3, table-returning
1433 functions are fully supported in C, PL/PgSQL, and SQL. See the
1434 Programmer's Guide for more information. An example of a
1435 table-returning function defined in C can be found in
1436 <I>contrib/tablefunc</I>.</P>
1438 <H4><A name="5.4">5.4</A>) I have changed a source file. Why does
1439 the recompile not see the change?</H4>
1441 <P>The <I>Makefiles</I> do not have the proper dependencies for
1442 include files. You have to do a <I>make clean</I> and then another
1443 <I>make</I>. If you are using <SMALL>GCC</SMALL> you can use the
1444 <I>--enable-depend</I> option of <I>configure</I> to have the
1445 compiler compute the dependencies automatically.</P>
projects / postgresql / blob