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15 <H1>Frequently Asked Questions (FAQ) for PostgreSQL</H1>
17 <P>Last updated: Sun Oct 13 22:49:56 EDT 2002</P>
19 <P>Current maintainer: Bruce Momjian (<A href=
20 "mailto:pgman@candle.pha.pa.us">pgman@candle.pha.pa.us</A>)<BR>
23 <P>The most recent version of this document can be viewed at <A
25 "http://www.PostgreSQL.org/docs/faq-english.html">http://www.PostgreSQL.org/docs/faq-english.html</A>.</P>
27 <P>Platform-specific questions are answered at <A href=
28 "http://www.PostgreSQL.org/users-lounge/docs/faq.html">http://www.PostgreSQL.org/users-lounge/docs/faq.html</A>.</P>
31 <H2 align="center">General Questions</H2>
32 <A href="#1.1">1.1</A>) What is PostgreSQL? How is it pronounced?<BR>
33 <A href="#1.2">1.2</A>) What is the copyright on PostgreSQL?<BR>
34 <A href="#1.3">1.3</A>) What Unix platforms does PostgreSQL run
36 <A href="#1.4">1.4</A>) What non-Unix ports are available?<BR>
37 <A href="#1.5">1.5</A>) Where can I get PostgreSQL?<BR>
38 <A href="#1.6">1.6</A>) Where can I get support?<BR>
39 <A href="#1.7">1.7</A>) What is the latest release?<BR>
40 <A href="#1.8">1.8</A>) What documentation is available?<BR>
41 <A href="#1.9">1.9</A>) How do I find out about known bugs or
43 <A href="#1.10">1.10</A>) How can I learn <SMALL>SQL</SMALL>?<BR>
44 <A href="#1.11">1.11</A>) Is PostgreSQL Y2K compliant?<BR>
45 <A href="#1.12">1.12</A>) How do I join the development team?<BR>
46 <A href="#1.13">1.13</A>) How do I submit a bug report?<BR>
47 <A href="#1.14">1.14</A>) How does PostgreSQL compare to other
48 <SMALL>DBMS</SMALL>s?<BR>
49 <A href="#1.15">1.15</A>) How can I financially assist
53 <H2 align="center">User Client Questions</H2>
54 <A href="#2.1">2.1</A>) Are there <SMALL>ODBC</SMALL> drivers for
56 <A href="#2.2">2.2</A>) What tools are available for using
57 PostgreSQL with Web pages?<BR>
58 <A href="#2.3">2.3</A>) Does PostgreSQL have a graphical user
59 interface? A report generator? An embedded query language
61 <A href="#2.4">2.4</A>) What languages are available to
62 communicate with PostgreSQL?<BR>
65 <H2 align="center">Administrative Questions</H2>
66 <A href="#3.1">3.1</A>) How do I install PostgreSQL somewhere other
67 than <I>/usr/local/pgsql</I>?<BR>
68 <A href="#3.2">3.2</A>) When I start <I>postmaster</I>, I get a
69 <I>Bad System Call</I> or core dumped message. Why?<BR>
70 <A href="#3.3">3.3</A>) When I try to start <I>postmaster</I>, I
71 get <I>IpcMemoryCreate</I> errors. Why?<BR>
72 <A href="#3.4">3.4</A>) When I try to start <I>postmaster</I>, I
73 get <I>IpcSemaphoreCreate</I> errors. Why?<BR>
74 <A href="#3.5">3.5</A>) How do I control connections from other
76 <A href="#3.6">3.6</A>) How do I tune the database engine for
77 better performance?<BR>
78 <A href="#3.7">3.7</A>) What debugging features are available?<BR>
79 <A href="#3.8">3.8</A>) Why do I get <I>"Sorry, too many
80 clients"</I> when trying to connect?<BR>
81 <A href="#3.9">3.9</A>) What are the <I>pg_sorttempNNN.NN</I>
82 files in my database directory?<BR>
83 <A href="#3.10">3.10</A>) Why do I need to do a dump and restore
84 to upgrade PostgreSQL releases?<BR>
87 <H2 align="center">Operational Questions</H2>
88 <A href="#4.1">4.1</A>) What is the difference between binary
89 cursors and normal cursors?<BR>
90 <A href="#4.2">4.2</A>) How do I <SMALL>SELECT</SMALL> only the
91 first few rows of a query?<BR>
92 <A href="#4.3">4.3</A>) How do I get a list of tables or other
93 things I can see in <I>psql</I>?<BR>
94 <A href="#4.4">4.4</A>) How do you remove a column from a
96 <A href="#4.5">4.5</A>) What is the maximum size for a row, a
97 table, and a database?<BR>
98 <A href="#4.6">4.6</A>) How much database disk space is required
99 to store data from a typical text file?<BR>
100 <A href="#4.7">4.7</A>) How do I find out what tables, indexes,
101 databases, and users are defined?<BR>
102 <A href="#4.8">4.8</A>) My queries are slow or don't make use of
103 the indexes. Why?<BR>
104 <A href="#4.9">4.9</A>) How do I see how the query optimizer is
105 evaluating my query?<BR>
106 <A href="#4.10">4.10</A>) What is an R-tree index?<BR>
107 <A href="#4.11">4.11</A>) What is the Genetic Query Optimizer?<BR>
108 <A href="#4.12">4.12</A>) How do I perform regular expression
109 searches and case-insensitive regular expression searches? How do I
110 use an index for case-insensitive searches?<BR>
111 <A href="#4.13">4.13</A>) In a query, how do I detect if a field
112 is <SMALL>NULL</SMALL>?<BR>
113 <A href="#4.14">4.14</A>) What is the difference between the
114 various character types?<BR>
115 <A href="#4.15.1">4.15.1</A>) How do I create a
116 serial/auto-incrementing field?<BR>
117 <A href="#4.15.2">4.15.2</A>) How do I get the value of a
118 <SMALL>SERIAL</SMALL> insert?<BR>
119 <A href="#4.15.3">4.15.3</A>) Don't <I>currval()</I> and
120 <I>nextval()</I> lead to a race condition with other users?<BR>
121 <A href="#4.15.4">4.15.4</A>) Why aren't my sequence numbers
122 reused on transaction abort? Why are there gaps in the numbering of
123 my sequence/SERIAL column?<BR>
124 <A href="#4.16">4.16</A>) What is an <SMALL>OID</SMALL>? What is a
125 <SMALL>TID</SMALL>?<BR>
126 <A href="#4.17">4.17</A>) What is the meaning of some of the terms
127 used in PostgreSQL?<BR>
128 <A href="#4.18">4.18</A>) Why do I get the error <I>"ERROR: Memory
129 exhausted in AllocSetAlloc()"</I>?<BR>
130 <A href="#4.19">4.19</A>) How do I tell what PostgreSQL version I
132 <A href="#4.20">4.20</A>) Why does my large-object operations get
133 <I>"invalid large obj descriptor"</I>?<BR>
134 <A href="#4.21">4.21</A>) How do I create a column that will
135 default to the current time?<BR>
136 <A href="#4.22">4.22</A>) Why are my subqueries using
137 <CODE><SMALL>IN</SMALL></CODE> so slow?<BR>
138 <A href="#4.23">4.23</A>) How do I perform an outer join?<BR>
139 <A href="#4.24">4.24</A>) How do I perform queries using multiple
141 <A href="#4.25">4.25</A>) How do I return multiple rows or columns
143 <A href="#4.26">4.26</A>) Why can't I reliably create/drop
144 temporary tables in PL/PgSQL functions?<BR>
145 <A href="#4.27">4.27</A>) What replication options are available?<BR>
146 <A href="#4.28">4.28</A>) What encryption options are available?<BR>
149 <H2 align="center">Extending PostgreSQL</H2>
150 <A href="#5.1">5.1</A>) I wrote a user-defined function. When I run
151 it in <I>psql</I>, why does it dump core?<BR>
152 <A href="#5.2">5.2</A>) How can I contribute some nifty new types
153 and functions to PostgreSQL?<BR>
154 <A href="#5.3">5.3</A>) How do I write a C function to return a
156 <A href="#5.4">5.4</A>) I have changed a source file. Why does the
157 recompile not see the change?<BR>
161 <H2 align="center">General Questions</H2>
163 <H4><A name="1.1">1.1</A>) What is PostgreSQL? How is it pronounced?</H4>
165 <P>PostgreSQL is pronounced <I>Post-Gres-Q-L</I>.</P>
167 <P>PostgreSQL is an enhancement of the POSTGRES database management
168 system, a next-generation <SMALL>DBMS</SMALL> research prototype.
169 While PostgreSQL retains the powerful data model and rich data
170 types of POSTGRES, it replaces the PostQuel query language with an
171 extended subset of <SMALL>SQL</SMALL>. PostgreSQL is free and the
172 complete source is available.</P>
174 <P>PostgreSQL development is performed by a team of
175 developers who all subscribe to the PostgreSQL development mailing
176 list. The current coordinator is Marc G. Fournier (<A href=
177 "mailto:scrappy@PostgreSQL.org">scrappy@PostgreSQL.org</A>). (See
178 section <a href="#1.6">1.6</a> on how to join). This team is now
179 responsible for all development of PostgreSQL.</P>
181 <P>The authors of PostgreSQL 1.01 were Andrew Yu and Jolly Chen.
182 Many others have contributed to the porting, testing, debugging,
183 and enhancement of the code. The original Postgres code, from which
184 PostgreSQL is derived, was the effort of many graduate students,
185 undergraduate students, and staff programmers working under the
186 direction of Professor Michael Stonebraker at the University of
187 California, Berkeley.</P>
189 <P>The original name of the software at Berkeley was Postgres. When
190 <SMALL>SQL</SMALL> functionality was added in 1995, its name was
191 changed to Postgres95. The name was changed at the end of 1996 to
194 <H4><A name="1.2">1.2</A>) What is the copyright on
197 <P>PostgreSQL is subject to the following COPYRIGHT:</P>
199 <P>PostgreSQL Data Base Management System</P>
201 <P>Portions copyright (c) 1996-2002, PostgreSQL Global Development
202 Group Portions Copyright (c) 1994-6 Regents of the University of
205 <P>Permission to use, copy, modify, and distribute this software
206 and its documentation for any purpose, without fee, and without a
207 written agreement is hereby granted, provided that the above
208 copyright notice and this paragraph and the following two
209 paragraphs appear in all copies.</P>
211 <P>IN NO EVENT SHALL THE UNIVERSITY OF CALIFORNIA BE LIABLE TO ANY
212 PARTY FOR DIRECT, INDIRECT, SPECIAL, INCIDENTAL, OR CONSEQUENTIAL
213 DAMAGES, INCLUDING LOST PROFITS, ARISING OUT OF THE USE OF THIS
214 SOFTWARE AND ITS DOCUMENTATION, EVEN IF THE UNIVERSITY OF
215 CALIFORNIA HAS BEEN ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.</P>
217 <P>THE UNIVERSITY OF CALIFORNIA SPECIFICALLY DISCLAIMS ANY
218 WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
219 OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE. THE
220 SOFTWARE PROVIDED HEREUNDER IS ON AN "AS IS" BASIS, AND THE
221 UNIVERSITY OF CALIFORNIA HAS NO OBLIGATIONS TO PROVIDE MAINTENANCE,
222 SUPPORT, UPDATES, ENHANCEMENTS, OR MODIFICATIONS.</P>
224 <P>The above is the BSD license, the classic open-source license.
225 It has no restrictions on how the source code may be used. We like
226 it and have no intention of changing it.</P>
228 <H4><A name="1.3">1.3</A>) What Unix platforms does PostgreSQL run
231 <P>In general, a modern Unix-compatible platform should be able to
232 run PostgreSQL. The platforms that had received explicit testing at
233 the time of release are listed in the installation
236 <H4><A name="1.4">1.4</A>) What non-Unix ports are available?</H4>
238 <P><STRONG>Client</STRONG></P>
240 <P>It is possible to compile the <I>libpq</I> C library, psql, and
241 other interfaces and client applications to run on MS Windows platforms.
242 In this case, the client is running on MS Windows, and communicates
243 via TCP/IP to a server running on one of our supported Unix
244 platforms. A file <I>win32.mak</I> is included in the distribution
245 for making a Win32 <I>libpq</I> library and <I>psql</I>. PostgreSQL
246 also communicates with <SMALL>ODBC</SMALL> clients.</P>
248 <P><STRONG>Server</STRONG></P>
250 <P>The database server can run on Windows NT and Win2k using
251 Cygwin, the Cygnus Unix/NT porting library. See
252 <I>pgsql/doc/FAQ_MSWIN</I> in the distribution or the MS Windows FAQ
253 at <A href="http://www.PostgreSQL.org/docs/faq-mswin.html">http://www.PostgreSQL.org/docs/faq-mswin.html</A>.</P>
255 <p>A native port to some Microsoft platforms is currently being worked
258 <H4><A name="1.5">1.5</A>) Where can I get PostgreSQL?</H4>
260 <P>The primary anonymous ftp site for PostgreSQL is <A href=
261 "ftp://ftp.PostgreSQL.org/pub">ftp://ftp.PostgreSQL.org/pub</A>.
262 For mirror sites, see our main web site.</P>
264 <H4><A name="1.6">1.6</A>) Where can I get support?</H4>
266 <P>The main mailing list is: <A href=
267 "mailto:pgsql-general@PostgreSQL.org">pgsql-general@PostgreSQL.org</A>.
268 It is available for discussion of matters pertaining to PostgreSQL.
269 To subscribe, send mail with the following lines in the body (not
270 the subject line):</P>
277 "mailto:pgsql-general-request@PostgreSQL.org">pgsql-general-request@PostgreSQL.org</A>.</P>
279 <P>There is also a digest list available. To subscribe to this
280 list, send email to: <A href=
281 "mailto:pgsql-general-digest-request@PostgreSQL.org">pgsql-general-digest-request@PostgreSQL.org</A>
288 Digests are sent out to members of this list whenever the main list
289 has received around 30k of messages.
291 <P>The bugs mailing list is available. To subscribe to this list,
292 send email to <A href=
293 "mailto:pgsql-bugs-request@PostgreSQL.org">pgsql-bugs-request@PostgreSQL.org</A>
300 There is also a developers discussion mailing list available. To
301 subscribe to this list, send email to <A href=
302 "mailto:pgsql-hackers-request@PostgreSQL.org">pgsql-hackers-request@PostgreSQL.org</A>
309 <P>Additional mailing lists and information about PostgreSQL can be
310 found via the PostgreSQL WWW home page at:</P>
313 <A href="http://www.PostgreSQL.org">http://www.PostgreSQL.org</A>
316 <P>There is also an IRC channel on EFNet, channel
317 <I>#PostgreSQL</I>. I use the Unix command <CODE>irc -c
318 '#PostgreSQL' "$USER" irc.phoenix.net.</CODE></P>
320 <P>A list of commercial support companies is available at <A href=
321 "http://www.PostgreSQL.org/users-lounge/commercial-support.html">http://www.PostgreSQL.org/users-lounge/commercial-support.html</A>.</P>
323 <H4><A name="1.7">1.7</A>) What is the latest release?</H4>
325 <P>The latest release of PostgreSQL is version 7.2.3.</P>
327 <P>We plan to have major releases every four months.</P>
329 <H4><A name="1.8">1.8</A>) What documentation is available?</H4>
331 <P>Several manuals, manual pages, and some small test examples are
332 included in the distribution. See the <I>/doc</I> directory. You
333 can also browse the manual online at <A href=
334 "http://www.PostgreSQL.org/users-lounge/docs/">http://www.PostgreSQL.org/users-lounge/docs/</A>.</P>
336 <P>There are two PostgreSQL books available online at <A href=
337 "http://www.PostgreSQL.org/docs/awbook.html">http://www.PostgreSQL.org/docs/awbook.html</A>
339 "http://www.commandprompt.com/ppbook/">http://www.commandprompt.com/ppbook/</A>.
340 There is a list of PostgreSQL books available for purchase at <A
342 "http://www.ca.PostgreSQL.org/books/">http://www.ca.PostgreSQL.org/books/</A>.
343 There is also a collection of PostgreSQL technical articles at <A
345 "http://techdocs.PostgreSQL.org/">http://techdocs.PostgreSQL.org/</A>.</P>
347 <P><I>psql</I> has some nice \d commands to show information about
348 types, operators, functions, aggregates, etc.</P>
350 <P>Our web site contains even more documentation.</P>
352 <H4><A name="1.9">1.9</A>) How do I find out about known bugs or
353 missing features?</H4>
355 <P>PostgreSQL supports an extended subset of <SMALL>SQL</SMALL>-92.
356 See our <A href="http://developer.PostgreSQL.org/todo.php">TODO</A>
357 list for known bugs, missing features, and future plans.</P>
359 <H4><A name="1.10">1.10</A>) How can I learn
360 <SMALL>SQL</SMALL>?</H4>
362 <P>The PostgreSQL book at <A href=
363 "http://www.PostgreSQL.org/docs/awbook.html">http://www.PostgreSQL.org/docs/awbook.html</A>
364 teaches <SMALL>SQL</SMALL>. There is another PostgreSQL book at <A
366 "http://www.commandprompt.com/ppbook/">http://www.commandprompt.com/ppbook.</A>
367 There is a nice tutorial at <A href=
368 "http://www.intermedia.net/support/sql/sqltut.shtm">http://www.intermedia.net/support/sql/sqltut.shtm,</A>
370 "http://ourworld.compuserve.com/homepages/graeme_birchall/HTM_COOK.HTM">
371 http://ourworld.compuserve.com/homepages/graeme_birchall/HTM_COOK.HTM,</A>
373 "http://sqlcourse.com/">http://sqlcourse.com.</A></P>
375 <P>Another one is "Teach Yourself SQL in 21 Days, Second Edition"
377 "http://members.tripod.com/er4ebus/sql/index.htm">http://members.tripod.com/er4ebus/sql/index.htm</A></P>
379 <P>Many of our users like <I>The Practical SQL Handbook</I>,
380 Bowman, Judith S., et al., Addison-Wesley. Others like <I>The
381 Complete Reference SQL</I>, Groff et al., McGraw-Hill.</P>
383 <H4><A name="1.11">1.11</A>) Is PostgreSQL Y2K compliant?</H4>
385 <P>Yes, we easily handle dates past the year 2000 AD, and before
388 <H4><A name="1.12">1.12</A>) How do I join the development
391 <P>First, download the latest source and read the PostgreSQL
392 Developers documentation on our web site, or in the distribution.
393 Second, subscribe to the <I>pgsql-hackers</I> and
394 <I>pgsql-patches</I> mailing lists. Third, submit high quality
395 patches to pgsql-patches.</P>
397 <P>There are about a dozen people who have commit privileges to the
398 PostgreSQL <SMALL>CVS</SMALL> archive. They each have submitted so
399 many high-quality patches that it was impossible for the existing
400 committers to keep up, and we had confidence that patches they
401 committed were of high quality.</P>
403 <H4><A name="1.13">1.13</A>) How do I submit a bug report?</H4>
405 <P>Please visit the PostgreSQL BugTool page at <A href=
406 "http://www.PostgreSQL.org/bugs/bugs.php">http://www.PostgreSQL.org/bugs/bugs.php</A>,
407 which gives guidelines and directions on how to submit a
410 <P>Also check out our ftp site <A href=
411 "ftp://ftp.PostgreSQL.org/pub">ftp://ftp.PostgreSQL.org/pub</A> to
412 see if there is a more recent PostgreSQL version or patches.</P>
414 <H4><A name="1.14">1.14</A>) How does PostgreSQL compare to other
415 <SMALL>DBMS</SMALL>s?</H4>
417 <P>There are several ways of measuring software: features,
418 performance, reliability, support, and price.</P>
421 <DT><B>Features</B></DT>
423 <DD>PostgreSQL has most features present in large commercial
424 <SMALL>DBMS</SMALL>s, like transactions, subselects, triggers,
425 views, foreign key referential integrity, and sophisticated
426 locking. We have some features they do not have, like
427 user-defined types, inheritance, rules, and multi-version
428 concurrency control to reduce lock contention.<BR>
432 <DT><B>Performance</B></DT>
434 <DD>PostgreSQL has performance similar to other commercial and
435 open source databases. it is faster for some things, slower for
436 others. In comparison to MySQL or leaner database systems, we are
437 slower on inserts/updates because of transaction overhead. Of
438 course, MySQL does not have any of the features mentioned in the
439 <I>Features</I> section above. We are built for reliability and
440 features, though we continue to improve performance in every
441 release. There is an interesting Web page comparing PostgreSQL to
442 MySQL at <A href= "http://openacs.org/why-not-mysql.html">
444 http://openacs.org/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 1994. 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 <H2 align="center">User Client Questions</H2>
508 <H4><A name="2.1">2.1</A>) Are there <SMALL>ODBC</SMALL> drivers
511 <P>There are two <SMALL>ODBC</SMALL> drivers available, PsqlODBC
512 and OpenLink <SMALL>ODBC</SMALL>.</P>
514 <P>PsqlODBC is included in the distribution. More information about
515 it can be gotten from <A href=
516 "ftp://ftp.PostgreSQL.org/pub/odbc/">ftp://ftp.PostgreSQL.org/pub/odbc/</A>.</P>
518 <P>OpenLink <SMALL>ODBC</SMALL> can be gotten from <A href=
519 "http://www.openlinksw.com/">http://www.openlinksw.com</A>. It
520 works with their standard <SMALL>ODBC</SMALL> client software so
521 you'll have PostgreSQL <SMALL>ODBC</SMALL> available on every
522 client platform they support (Win, Mac, Unix, VMS).</P>
524 <P>They will probably be selling this product to people who need
525 commercial-quality support, but a freeware version will always be
526 available. Please send questions to <A href=
527 "mailto:postgres95@openlink.co.uk">postgres95@openlink.co.uk</A>.</P>
529 <P>See also the <A href=
530 "http://www.postgresql.org/devel-corner/docs/programmer/odbc.html">ODBC
531 chapter of the Programmer's Guide</A>.</P>
533 <H4><A name="2.2">2.2</A>) What tools are available for using
534 PostgreSQL with Web pages?</H4>
536 <P>A nice introduction to Database-backed Web pages can be seen at:
537 <A href="http://www.webreview.com">http://www.webreview.com</A></P>
539 <P>For Web integration, PHP is an excellent interface. It is at <A
540 href="http://www.php.net">http://www.php.net</A>.</P>
542 <P>For complex cases, many use the Perl interface and CGI.pm or mod_perl.</P>
544 <H4><A name="2.3">2.3</A>) Does PostgreSQL have a graphical user
545 interface? A report generator? An embedded query language
548 <P>We have a nice graphical user interface called PgAccess, which is
549 shipped as part of the distribution. PgAccess also has a report
550 generator. The Web page is <A href="http://www.pgaccess.org/">http://www.pgaccess.org/</A>.</P>
552 <P>We also include <I>ecpg</I>, which is an embedded SQL query
553 language interface for C.</P>
555 <H4><A name="2.4">2.4</A>) What languages are able to communicate with PostgreSQL?</H4>
562 <LI>C++ (libpq++)</LI>
564 <LI>Embedded C (ecpg)</LI>
568 <LI>Perl (DBD::Pg)</LI>
572 <LI>Python (PyGreSQL)</LI>
574 <LI>TCL (libpgtcl)</LI>
576 <LI>C Easy API (libpgeasy)</LI>
578 <LI>PHP ('pg_' functions, Pear::DB)</LI>
580 <P>Additional interfaces are available at
581 <a href="http://www.PostgreSQL.org/interfaces.html">http://www.PostgreSQL.org/interfaces.html</A>.
585 <H2 align="center">Administrative Questions</H2>
587 <H4><A name="3.1">3.1</A>) How do I install PostgreSQL somewhere
588 other than <I>/usr/local/pgsql</I>?</H4>
590 <P>Specify the <I>--prefix</I> option when running
591 <I>configure</I>.</P>
593 <H4><A name="3.2">3.2</A>) When I start <I>postmaster</I>, I get a
594 <I>Bad System Call</I> or core dumped message. Why?</H4>
596 <P>It could be a variety of problems, but first check to see that
597 you have System V extensions installed in your kernel. PostgreSQL
598 requires kernel support for shared memory and semaphores.</P>
600 <H4><A name="3.3">3.3</A>) When I try to start <I>postmaster</I>, I
601 get <I>IpcMemoryCreate</I> errors. Why?</H4>
603 <P>You either do not have shared memory configured properly in your
604 kernel or you need to enlarge the shared memory available in the
605 kernel. The exact amount you need depends on your architecture and
606 how many buffers and backend processes you configure for
607 <I>postmaster</I>. For most systems, with default numbers of
608 buffers and processes, you need a minimum of ~1 MB. See the <A
610 "http://www.postgresql.org/idocs/index.php?kernel-resources.html">PostgreSQL
611 Administrator's Guide</A> for more detailed information about
612 shared memory and semaphores.</P>
614 <H4><A name="3.4">3.4</A>) When I try to start <I>postmaster</I>, I
615 get <I>IpcSemaphoreCreate</I> errors. Why?</H4>
617 <P>If the error message is <I>IpcSemaphoreCreate: semget failed (No
618 space left on device)</I> then your kernel is not configured with
619 enough semaphores. Postgres needs one semaphore per potential
620 backend process. A temporary solution is to start <I>postmaster</I>
621 with a smaller limit on the number of backend processes. Use
622 <I>-N</I> with a parameter less than the default of 32. A more
623 permanent solution is to increase your kernel's
624 <SMALL>SEMMNS</SMALL> and <SMALL>SEMMNI</SMALL> parameters.</P>
626 <P>Inoperative semaphores can also cause crashes during heavy
629 <P>If the error message is something else, you might not have
630 semaphore support configured in your kernel at all. See the
631 PostgreSQL Administrator's Guide for more detailed information
632 about shared memory and semaphores.</P>
634 <H4><A name="3.5">3.5</A>) How do I control connections from other
637 <P>By default, PostgreSQL only allows connections from the local
638 machine using Unix domain sockets. Other machines will not be able
639 to connect unless you add the <I>-i</I> flag to <I>postmaster</I>,
640 <B>and</B> enable host-based authentication by modifying the file
641 <I>$PGDATA/pg_hba.conf</I> accordingly. This will allow TCP/IP
644 <H4><A name="3.6">3.6</A>) How do I tune the database engine for
645 better performance?</H4>
647 <P>Certainly, indexes can speed up queries. The
648 <SMALL>EXPLAIN</SMALL> command allows you to see how PostgreSQL is
649 interpreting your query, and which indexes are being used.</P>
651 <P>If you are doing many <SMALL>INSERTs</SMALL>, consider doing
652 them in a large batch using the <SMALL>COPY</SMALL> command. This
653 is much faster than individual <SMALL>INSERTS.</SMALL> Second,
654 statements not in a <SMALL>BEGIN WORK/COMMIT</SMALL> transaction
655 block are considered to be in their own transaction. Consider
656 performing several statements in a single transaction block. This
657 reduces the transaction overhead. Also, consider dropping and
658 recreating indexes when making large data changes.</P>
660 <P>There are several tuning options. You can disable <I>fsync()</I>
661 by starting <I>postmaster</I> with a <I>-o -F</I> option. This will
662 prevent <I>fsync()</I>s from flushing to disk after every
665 <P>You can also use the <I>postmaster</I> <I>-B</I> option to
666 increase the number of shared memory buffers used by the backend
667 processes. If you make this parameter too high, the
668 <I>postmaster</I> may not start because you have exceeded your
669 kernel's limit on shared memory space. Each buffer is 8K and the
670 default is 64 buffers.</P>
672 <P>You can also use the backend <I>-S</I> option to increase the
673 maximum amount of memory used by the backend process for temporary
674 sorts. The <I>-S</I> value is measured in kilobytes, and the
675 default is 512 (i.e. 512K).</P>
677 <P>You can also use the <SMALL>CLUSTER</SMALL> command to group
678 data in tables to match an index. See the <SMALL>CLUSTER</SMALL>
679 manual page for more details.</P>
681 <H4><A name="3.7">3.7</A>) What debugging features are
684 <P>PostgreSQL has several features that report status information
685 that can be valuable for debugging purposes.</P>
687 <P>First, by running <I>configure</I> with the --enable-cassert
688 option, many <I>assert()</I>s monitor the progress of the backend
689 and halt the program when something unexpected occurs.</P>
691 <P>Both <I>postmaster</I> and <I>postgres</I> have several debug
692 options available. First, whenever you start <I>postmaster</I>,
693 make sure you send the standard output and error to a log file,
697 ./bin/postmaster >server.log 2>&1 &
700 <P>This will put a server.log file in the top-level PostgreSQL
701 directory. This file contains useful information about problems or
702 errors encountered by the server. <I>Postmaster</I> has a <I>-d</I>
703 option that allows even more detailed information to be reported.
704 The <I>-d</I> option takes a number that specifies the debug level.
705 Be warned that high debug level values generate large log
708 <P>If <I>postmaster</I> is not running, you can actually run the
709 <I>postgres</I> backend from the command line, and type your
710 <SMALL>SQL</SMALL> statement directly. This is recommended
711 <B>only</B> for debugging purposes. Note that a newline terminates
712 the query, not a semicolon. If you have compiled with debugging
713 symbols, you can use a debugger to see what is happening. Because
714 the backend was not started from <I>postmaster</I>, it is not
715 running in an identical environment and locking/backend interaction
716 problems may not be duplicated.</P>
718 <P>If <I>postmaster</I> is running, start <I>psql</I> in one
719 window, then find the <SMALL>PID</SMALL> of the <I>postgres</I>
720 process used by <I>psql</I>. Use a debugger to attach to the
721 <I>postgres</I> <SMALL>PID.</SMALL> You can set breakpoints in the
722 debugger and issue queries from <I>psql</I>. If you are debugging
723 <I>postgres</I> startup, you can set PGOPTIONS="-W n", then start
724 <I>psql</I>. This will cause startup to delay for <I>n</I> seconds
725 so you can attach to the process with the debugger, set any
726 breakpoints, and continue through the startup sequence.</P>
728 <P>The <I>postgres</I> program has <I>-s, -A</I>, and <I>-t</I>
729 options that can be very useful for debugging and performance
732 <P>You can also compile with profiling to see what functions are
733 taking execution time. The backend profile files will be deposited
734 in the <I>pgsql/data/base/dbname</I> directory. The client profile
735 file will be put in the client's current directory. Linux requires
736 a compile with <I>-DLINUX_PROFILE</I> for proper profiling.</P>
738 <H4><A name="3.8">3.8</A>) Why do I get <I>"Sorry, too many
739 clients"</I> when trying to connect?</H4>
741 <P>You need to increase <I>postmaster</I>'s limit on how many
742 concurrent backend processes it can start.</P>
744 <P>The default limit is 32 processes. You can increase it by
745 restarting <I>postmaster</I> with a suitable <I>-N</I> value or
746 modifying <I>postgresql.conf</I>.</P>
748 <P>Note that if you make <I>-N</I> larger than 32, you must also
749 increase <I>-B</I> beyond its default of 64; <I>-B</I> must be at
750 least twice <I>-N</I>, and probably should be more than that for
751 best performance. For large numbers of backend processes, you are
752 also likely to find that you need to increase various Unix kernel
753 configuration parameters. Things to check include the maximum size
754 of shared memory blocks, <SMALL>SHMMAX;</SMALL> the maximum number
755 of semaphores, <SMALL>SEMMNS</SMALL> and <SMALL>SEMMNI;</SMALL> the
756 maximum number of processes, <SMALL>NPROC;</SMALL> the maximum
757 number of processes per user, <SMALL>MAXUPRC;</SMALL> and the
758 maximum number of open files, <SMALL>NFILE</SMALL> and
759 <SMALL>NINODE.</SMALL> The reason that PostgreSQL has a limit on
760 the number of allowed backend processes is so your system won't run
761 out of resources.</P>
763 <P>In PostgreSQL versions prior to 6.5, the maximum number of
764 backends was 64, and changing it required a rebuild after altering
765 the MaxBackendId constant in
766 <I>include/storage/sinvaladt.h</I>.</P>
768 <H4><A name="3.9">3.9</A>) What are the <I>pg_tempNNN.NN</I>
769 files in my database directory?</H4>
771 <P>They are temporary files generated by the query executor. For
772 example, if a sort needs to be done to satisfy an <SMALL>ORDER
773 BY,</SMALL> and the sort requires more space than the backend's
774 <I>-S</I> parameter allows, then temporary files are created to
775 hold the extra data.</P>
777 <P>The temporary files should be deleted automatically, but might
778 not if a backend crashes during a sort. If you have no backends
779 running at the time, it is safe to delete the pg_tempNNN.NN
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?</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 <H4><A name="4.3">4.3</A>) How do I get a list of tables or other
822 things I can see in <I>psql</I>?</H4>
824 <P>You can read the source code for <I>psql</I> in file
825 <I>pgsql/src/bin/psql/describe.c</I>. It contains
826 <SMALL>SQL</SMALL> commands that generate the output for psql's
827 backslash commands. You can also start <I>psql</I> with the
828 <I>-E</I> option so it will print out the queries it uses to
829 execute the commands you give.</P>
831 <H4><A name="4.4">4.4</A>) How do you remove a column from a
834 <P>Prior to version 7.3, <SMALL>ALTER TABLE DROP COLUMN</SMALL> is not supported.
835 You can do this instead:</P>
838 LOCK TABLE old_table;
839 SELECT ... -- select all columns but the one you want to remove
842 DROP TABLE old_table;
843 ALTER TABLE new_table RENAME TO old_table;
847 <H4><A name="4.5">4.5</A>) What is the maximum size for a row, a
848 table, and a database?</H4>
850 <P>These are the limits:</P>
852 Maximum size for a database? unlimited (1 TB databases exist)
853 Maximum size for a table? 16 TB
854 Maximum size for a row? 1.6TB
855 Maximum size for a field? 1 GB
856 Maximum number of rows in a table? unlimited
857 Maximum number of columns in a table? 250-1600 depending on column types
858 Maximum number of indexes on a table? unlimited
861 Of course, these are not actually unlimited, but limited to
862 available disk space and memory/swap space. Performance may suffer
863 when these values get unusually large.
865 <P>The maximum table size of 16 TB does not require large file
866 support from the operating system. Large tables are stored as
867 multiple 1 GB files so file system size limits are not
870 <P>The maximum table size and maximum number of columns can be
871 increased if the default block size is increased to 32k.</P>
873 <H4><A name="4.6">4.6</A>) How much database disk space is required
874 to store data from a typical text file?</H4>
876 <P>A PostgreSQL database may require up to five times the disk
877 space to store data from a text file.</P>
879 <P>As an example, consider a file of 100,000 lines with an integer
880 and text description on each line. Suppose the text string
881 avergages twenty bytes in length. The flat file would be 2.8 MB.
882 The size of the PostgreSQL database file containing this data can
883 be estimated as 6.4 MB:</P>
885 36 bytes: each row header (approximate)
886 24 bytes: one int field and one text field
887 + 4 bytes: pointer on page to tuple
888 ----------------------------------------
891 The data page size in PostgreSQL is 8192 bytes (8 KB), so:
894 ------------------- = 128 rows per database page (rounded down)
898 -------------------- = 782 database pages (rounded up)
901 782 database pages * 8192 bytes per page = 6,406,144 bytes (6.4 MB)
904 <P>Indexes do not require as much overhead, but do contain the data
905 that is being indexed, so they can be large also.</P>
907 <P><SMALL>NULL</SMALL>s are stored in bitmaps, so they
908 use very little space.</P>
910 <H4><A name="4.7">4.7</A>) How do I find out what tables, indexes,
911 databases, and users are defined?</H4>
913 <P><I>psql</I> has a variety of backslash commands to show such
914 information. Use \? to see them. There are also system tables
915 beginning with <I>pg_</I> that describe these too. Also, <I>psql
916 -l</I> will list all databases.</P>
918 <P>Also try the file <I>pgsql/src/tutorial/syscat.source</I>. It
919 illustrates many of the <SMALL>SELECT</SMALL>s needed to get
920 information from the database system tables.</P>
922 <H4><A name="4.8">4.8</A>) My queries are slow or don't make use of
923 the indexes. Why?</H4>
924 Indexes are not automatically used by every query. Indexes are only
925 used if the table is larger than a minimum size, and the query
926 selects only a small percentage of the rows in the table. This is
927 because the random disk access caused by an index scan can be
928 slower than a straight read through the table, or sequential scan.
930 <P>To determine if an index should be used, PostgreSQL must have
931 statistics about the table. These statistics are collected using
932 <SMALL>VACUUM ANALYZE</SMALL>, or simply <SMALL>ANALYZE</SMALL>.
933 Using statistics, the optimizer knows how many rows are in the
934 table, and can better determine if indexes should be used.
935 Statistics are also valuable in determining optimal join order and
936 join methods. Statistics collection should be performed
937 periodically as the contents of the table change.</P>
939 <P>Indexes are normally not used for <SMALL>ORDER BY</SMALL> or to
940 perform joins. A sequential scan followed by an explicit sort is
941 usually faster than an index scan of a large table.</P>
942 However, <SMALL>LIMIT</SMALL> combined with <SMALL>ORDER BY</SMALL>
943 often will use an index because only a small portion of the table
944 is returned. In fact, though MAX() and MIN() don't use indexes,
945 it is possible to retrieve such values using an index with ORDER BY
950 ORDER BY col [ DESC ]
954 <P>When using wild-card operators such as <SMALL>LIKE</SMALL> or
955 <I>~</I>, indexes can only be used in certain circumstances:
957 <LI>The beginning of the search string must be anchored to the start
958 of the string, i.e.:</LI>
960 <LI><SMALL>LIKE</SMALL> patterns must not start with <I>%.</I></LI>
961 <LI><I>~</I> (regular expression) patterns must start with
964 <LI>The search string can not start with a character class,
966 <LI>Case-insensitive searches such as <SMALL>ILIKE</SMALL> and
967 <I>~*</I> do not utilise indexes. Instead, use functional
968 indexes, which are described in section <a href="#4.12">4.12</a>.</LI>
969 <LI>The default <I>C</I> locale must be used during
974 <H4><A name="4.9">4.9</A>) How do I see how the query optimizer is
975 evaluating my query?</H4>
977 <P>See the <SMALL>EXPLAIN</SMALL> manual page.</P>
979 <H4><A name="4.10">4.10</A>) What is an R-tree index?</H4>
981 <P>An R-tree index is used for indexing spatial data. A hash index
982 can't handle range searches. A B-tree index only handles range
983 searches in a single dimension. R-trees can handle
984 multi-dimensional data. For example, if an R-tree index can be
985 built on an attribute of type <I>point</I>, the system can more
986 efficiently answer queries such as "select all points within a
987 bounding rectangle."</P>
989 <P>The canonical paper that describes the original R-tree design
992 <P>Guttman, A. "R-trees: A Dynamic Index Structure for Spatial
993 Searching." Proceedings of the 1984 ACM SIGMOD Int'l Conf on Mgmt
996 <P>You can also find this paper in Stonebraker's "Readings in
997 Database Systems".</P>
999 <P>Built-in R-trees can handle polygons and boxes. In theory,
1000 R-trees can be extended to handle higher number of dimensions. In
1001 practice, extending R-trees requires a bit of work and we don't
1002 currently have any documentation on how to do it.</P>
1004 <H4><A name="4.11">4.11</A>) What is the Genetic Query
1007 <P>The <SMALL>GEQO</SMALL> module speeds query optimization when
1008 joining many tables by means of a Genetic Algorithm (GA). It allows
1009 the handling of large join queries through nonexhaustive
1012 <H4><A name="4.12">4.12</A>) How do I perform regular expression
1013 searches and case-insensitive regular expression searches? How do I
1014 use an index for case-insensitive searches?</H4>
1016 <P>The <I>~</I> operator does regular expression matching, and
1017 <I>~*</I> does case-insensitive regular expression matching. The
1018 case-insensitive variant of <SMALL>LIKE</SMALL> is called
1019 <SMALL>ILIKE</SMALL>.</P>
1021 <P>Case-insensitive equality comparisons are normally expressed
1026 WHERE LOWER(col) = 'abc';
1029 This will not use an standard index. However, if you create a
1030 functional index, it will be used:
1032 CREATE INDEX tabindex ON tab (LOWER(col));
1035 <H4><A name="4.13">4.13</A>) In a query, how do I detect if a field
1036 is <SMALL>NULL</SMALL>?</H4>
1038 <P>You test the column with <SMALL>IS NULL</SMALL> and <SMALL>IS
1039 NOT NULL</SMALL>.</P>
1041 <H4><A name="4.14">4.14</A>) What is the difference between the
1042 various character types?</H4>
1044 Type Internal Name Notes
1045 --------------------------------------------------
1046 "char" char 1 character
1047 CHAR(n) bpchar blank padded to the specified fixed length
1048 VARCHAR(n) varchar size specifies maximum length, no padding
1049 TEXT text no specific upper limit on length
1050 BYTEA bytea variable-length byte array (null-byte safe)
1053 <P>You will see the internal name when examining system catalogs
1054 and in some error messages.</P>
1056 <P>The last four types above are "varlena" types (i.e., the first
1057 four bytes on disk are the length, followed by the data). Thus the
1058 actual space used is slightly greater than the declared size.
1059 However, these data types are also subject to compression or being
1060 stored out-of-line by <SMALL>TOAST</SMALL>, so the space on disk
1061 might also be less than expected.</P>
1063 <P><SMALL>CHAR(n)</SMALL> is best when storing strings that are
1064 usually the same length. <SMALL>VARCHAR(n)</SMALL> is best when
1065 storing variable-length strings but it limits how long a string can
1066 be. <SMALL>TEXT</SMALL> is for strings of unlimited length, maximum
1067 1 gigabyte. <SMALL>BYTEA</SMALL> is for storing binary data,
1068 particularly values that include <SMALL>NULL</SMALL> bytes.</P>
1070 <H4><A name="4.15.1">4.15.1</A>) How do I create a
1071 serial/auto-incrementing field?</H4>
1073 <P>PostgreSQL supports a <SMALL>SERIAL</SMALL> data type. It
1074 auto-creates a sequence and index on the column. For example,
1077 CREATE TABLE person (
1083 is automatically translated into this:
1085 CREATE SEQUENCE person_id_seq;
1086 CREATE TABLE person (
1087 id INT4 NOT NULL DEFAULT nextval('person_id_seq'),
1090 CREATE UNIQUE INDEX person_id_key ON person ( id );
1093 See the <I>create_sequence</I> manual page for more information
1094 about sequences. You can also use each row's <I>OID</I> field as a
1095 unique value. However, if you need to dump and reload the database,
1096 you need to use <I>pg_dump</I>'s <I>-o</I> option or <SMALL>COPY
1097 WITH OIDS</SMALL> option to preserve the <SMALL>OID</SMALL>s.
1099 <H4><A name="4.15.2">4.15.2</A>) How do I get the value of a
1100 <SMALL>SERIAL</SMALL> insert?</H4>
1102 <P>One approach is to retrieve the next <SMALL>SERIAL</SMALL> value
1103 from the sequence object with the <I>nextval()</I> function
1104 <I>before</I> inserting and then insert it explicitly. Using the
1105 example table in <A href="#4.15.1">4.15.1</A>, an example in a
1106 pseudo-language would look like this:</P>
1108 new_id = execute("SELECT nextval('person_id_seq')");
1109 execute("INSERT INTO person (id, name) VALUES (new_id, 'Blaise Pascal')");
1112 You would then also have the new value stored in
1113 <CODE>new_id</CODE> for use in other queries (e.g., as a foreign
1114 key to the <CODE>person</CODE> table). Note that the name of the
1115 automatically created <SMALL>SEQUENCE</SMALL> object will be named
1116 <<I>table</I>>_<<I>serialcolumn</I>>_<I>seq</I>, where
1117 <I>table</I> and <I>serialcolumn</I> are the names of your table
1118 and your <SMALL>SERIAL</SMALL> column, respectively.
1120 <P>Alternatively, you could retrieve the assigned
1121 <SMALL>SERIAL</SMALL> value with the <I>currval</I>() function
1122 <I>after</I> it was inserted by default, e.g.,</P>
1124 execute("INSERT INTO person (name) VALUES ('Blaise Pascal')");
1125 new_id = execute("SELECT currval('person_id_seq')");
1128 Finally, you could use the <A href="#4.16"><SMALL>OID</SMALL></A>
1129 returned from the <SMALL>INSERT</SMALL> statement to look up the
1130 default value, though this is probably the least portable approach.
1131 In Perl, using DBI with Edmund Mergl's DBD::Pg module, the oid
1132 value is made available via <I>$sth->{pg_oid_status}</I> after
1133 <I>$sth->execute()</I>.
1135 <H4><A name="4.15.3">4.15.3</A>) Don't <I>currval()</I> and
1136 <I>nextval()</I> lead to a race condition with other users?</H4>
1138 <P>No. <i>currval</i>() returns the current value assigned by your
1139 backend, not by all users.</P>
1141 <H4><A name="4.15.4">4.15.4</A>) Why aren't my sequence numbers
1142 reused on transaction abort? Why are there gaps in the numbering of
1143 my sequence/SERIAL column?</H4>
1145 <P>To improve concurrency, sequence values are given out to running
1146 transactions as needed and are not locked until the transaction
1147 completes. This causes gaps in numbering from aborted
1150 <H4><A name="4.16">4.16</A>) What is an <SMALL>OID</SMALL>? What is
1151 a <SMALL>TID</SMALL>?</H4>
1153 <P><SMALL>OID</SMALL>s are PostgreSQL's answer to unique row ids.
1154 Every row that is created in PostgreSQL gets a unique
1155 <SMALL>OID</SMALL>. All <SMALL>OID</SMALL>s generated during
1156 <I>initdb</I> are less than 16384 (from
1157 <I>include/access/transam.h</I>). All user-created
1158 <SMALL>OID</SMALL>s are equal to or greater than this. By default,
1159 all these <SMALL>OID</SMALL>s are unique not only within a table or
1160 database, but unique within the entire PostgreSQL installation.</P>
1162 <P>PostgreSQL uses <SMALL>OID</SMALL>s in its internal system
1163 tables to link rows between tables. These <SMALL>OID</SMALL>s can
1164 be used to identify specific user rows and used in joins. It is
1165 recommended you use column type <SMALL>OID</SMALL> to store
1166 <SMALL>OID</SMALL> values. You can create an index on the
1167 <SMALL>OID</SMALL> field for faster access.</P>
1169 <P>O<SMALL>ID</SMALL>s are assigned to all new rows from a central
1170 area that is used by all databases. If you want to change the
1171 <SMALL>OID</SMALL> to something else, or if you want to make a copy
1172 of the table, with the original <SMALL>OID</SMALL>s, there is no
1173 reason you can't do it:</P>
1175 CREATE TABLE new_table(old_oid oid, mycol int);
1176 SELECT old_oid, mycol INTO new FROM old;
1177 COPY new TO '/tmp/pgtable';
1179 COPY new WITH OIDS FROM '/tmp/pgtable';
1182 CREATE TABLE new_table (mycol int);
1183 INSERT INTO new_table (oid, mycol) SELECT oid, mycol FROM old_table;
1185 <P>O<SMALL>ID</SMALL>s are stored as 4-byte integers, and will
1186 overflow at 4 billion. No one has reported this ever happening, and
1187 we plan to have the limit removed before anyone does.</P>
1189 <P>T<SMALL>ID</SMALL>s are used to identify specific physical rows
1190 with block and offset values. T<SMALL>ID</SMALL>s change after rows
1191 are modified or reloaded. They are used by index entries to point
1192 to physical rows.</P>
1194 <H4><A name="4.17">4.17</A>) What is the meaning of some of the
1195 terms used in PostgreSQL?</H4>
1197 <P>Some of the source code and older documentation use terms that
1198 have more common usage. Here are some:</P>
1201 <LI>table, relation, class</LI>
1203 <LI>row, record, tuple</LI>
1205 <LI>column, field, attribute</LI>
1207 <LI>retrieve, select</LI>
1209 <LI>replace, update</LI>
1211 <LI>append, insert</LI>
1213 <LI><SMALL>OID</SMALL>, serial value</LI>
1215 <LI>portal, cursor</LI>
1217 <LI>range variable, table name, table alias</LI>
1220 <P>A list of general database terms can be found at: <A href=
1221 "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>
1223 <H4><A name="4.18">4.18</A>) Why do I get the error <I>"ERROR:
1224 Memory exhausted in AllocSetAlloc()"</I>?</H4>
1226 <P>You probably have run out of virtual memory on your system,
1227 or your kernel has a low limit for certain resources. Try this
1228 before starting <I>postmaster</I>:</P>
1234 Depending on your shell, only one of these may succeed, but it will
1235 set your process data segment limit much higher and perhaps allow
1236 the query to complete. This command applies to the current process,
1237 and all subprocesses created after the command is run. If you are
1238 having a problem with the <SMALL>SQL</SMALL> client because the
1239 backend is returning too much data, try it before starting the
1242 <H4><A name="4.19">4.19</A>) How do I tell what PostgreSQL version
1245 <P>From <I>psql</I>, type <CODE>SELECT version();</CODE></P>
1247 <H4><A name="4.20">4.20</A>) Why does my large-object operations
1248 get <I>"invalid large obj descriptor"</I>?</H4>
1250 <P>You need to put <CODE>BEGIN WORK</CODE> and <CODE>COMMIT</CODE>
1251 around any use of a large object handle, that is, surrounding
1252 <CODE>lo_open</CODE> ... <CODE>lo_close.</CODE></P>
1254 <P>Currently PostgreSQL enforces the rule by closing large object
1255 handles at transaction commit. So the first attempt to do anything
1256 with the handle will draw <I>invalid large obj descriptor</I>. So
1257 code that used to work (at least most of the time) will now
1258 generate that error message if you fail to use a transaction.</P>
1260 <P>If you are using a client interface like <SMALL>ODBC</SMALL> you
1261 may need to set <CODE>auto-commit off.</CODE></P>
1263 <H4><A name="4.21">4.21</A>) How do I create a column that will
1264 default to the current time?</H4>
1266 <P>Use <I>CURRENT_TIMESTAMP</I>:</P>
1268 <CODE>CREATE TABLE test (x int, modtime timestamp DEFAULT CURRENT_TIMESTAMP );
1272 <H4><A name="4.22">4.22</A>) Why are my subqueries using
1273 <CODE><SMALL>IN</SMALL></CODE> so slow?</H4>
1275 <P>Currently, we join subqueries to outer queries by sequentially
1276 scanning the result of the subquery for each row of the outer
1277 query. If the subquery returns only a few rows and the outer query
1278 returns many rows, <CODE><SMALL>IN</SMALL></CODE> is fastest. To
1279 speed up other queries, replace <CODE>IN</CODE> with
1280 <CODE>EXISTS</CODE>:</P>
1283 WHERE col IN (SELECT subcol FROM subtab);
1288 WHERE EXISTS (SELECT subcol FROM subtab WHERE subcol = col);
1291 For this to be fast, <CODE>subcol</CODE> should be an indexed column.
1292 We hope to fix this limitation in a future release.
1294 <H4><A name="4.23">4.23</A>) How do I perform an outer join?</H4>
1296 <P>PostgreSQL supports outer joins using the SQL standard syntax.
1297 Here are two examples:</P>
1300 FROM t1 LEFT OUTER JOIN t2 ON (t1.col = t2.col);
1305 FROM t1 LEFT OUTER JOIN t2 USING (col);
1308 <P>These identical queries join t1.col to t2.col, and also return
1309 any unjoined rows in t1 (those with no match in t2). A
1310 <SMALL>RIGHT</SMALL> join would add unjoined rows of t2. A
1311 <SMALL>FULL</SMALL> join would return the matched rows plus all
1312 unjoined rows from t1 and t2. The word <SMALL>OUTER</SMALL> is
1313 optional and is assumed in <SMALL>LEFT</SMALL>,
1314 <SMALL>RIGHT</SMALL>, and <SMALL>FULL</SMALL> joins. Ordinary joins
1315 are called <SMALL>INNER</SMALL> joins.</P>
1317 <P>In previous releases, outer joins can be simulated using
1318 <SMALL>UNION</SMALL> and <SMALL>NOT IN</SMALL>. For example, when
1319 joining <I>tab1</I> and <I>tab2</I>, the following query does an
1320 <I>outer</I> join of the two tables:<BR>
1324 SELECT tab1.col1, tab2.col2
1326 WHERE tab1.col1 = tab2.col1
1328 SELECT tab1.col1, NULL
1330 WHERE tab1.col1 NOT IN (SELECT tab2.col1 FROM tab2)
1334 <H4><A name="4.24">4.24</A>) How do I perform queries using
1335 multiple databases?</H4>
1337 <P>There is no way to query a database other than the current one.
1338 Because PostgreSQL loads database-specific system catalogs, it is
1339 uncertain how a cross-database query should even behave.</P>
1341 <P><I>/contrib/dblink</I> allows cross-database queries using
1342 function calls. Of course, a client can make simultaneous
1343 connections to different databases and merge the results on the
1346 <H4><A name="4.25">4.25</A>) How do I return multiple rows or
1347 columns from a function?</H4>
1349 <P>You can return result sets from PL/pgSQL functions using
1350 <I>refcursors</I>. See <A href=
1351 "http://www.PostgreSQL.org/idocs/index.php?plpgsql-cursors.html">
1352 http://www.PostgreSQL.org/idocs/index.php?plpgsql-cursors.html</A>,
1353 section 23.7.3.3.</P>
1355 <H4><A name="4.26">4.26</A>) Why can't I reliably create/drop
1356 temporary tables in PL/PgSQL functions?</H4>
1357 <P>PL/PgSQL caches function contents, and an unfortunate side effect
1358 is that if a PL/PgSQL function accesses a temporary table, and that
1359 table is later dropped and recreated, and the function called
1360 again, the function will fail because the cached function contents
1361 still point to the old temporary table. The solution is to use
1362 <SMALL>EXECUTE</SMALL> for temporary table access in PL/PgSQL. This
1363 will cause the query to be reparsed every time.</P>
1365 <H4><A name="4.27">4.27</A>) What replication options are available?
1367 <P>There are several master/slave replication options available.
1368 These allow only the master to make database changes and the slave
1369 can only do database reads. The bottom of <a
1370 href="http://gborg.PostgreSQL.org/genpage?replication_research">
1371 http://gborg.PostgreSQL.org/genpage?replication_research</a> lists
1372 them. A multi-master replication solution is being worked on at <a
1373 href="http://gborg.PostgreSQL.org/project/pgreplication/projdisplay.php">http://gborg.PostgreSQL.org/project/pgreplication/projdisplay.php</a>.</P>
1375 <H4><A name="4.28">4.28</A>) What encryption options are available?
1378 <LI><I>/contrib/pgcrypto</I> contains many encryption functions for
1379 use in <SMALL>SQL</SMALL> queries.</LI>
1380 <LI>The only way to encrypt transmission from the client to the
1381 server is by using <I>hostssl</I> in <I>pg_hba.conf</I>.</LI>
1382 <LI>Database user passwords are automatically encrypted when stored
1383 in version 7.3. In previous versions, you must enable the option
1384 <i>PASSWORD_ENCRYPTION</i> in <i>postgresql.conf</i>.</LI>
1385 <LI>The server can run using an encrypted file system.</LI>
1390 <H2 align="center">Extending PostgreSQL</H2>
1392 <H4><A name="5.1">5.1</A>) I wrote a user-defined function. When I
1393 run it in <I>psql</I>, why does it dump core?</H4>
1395 <P>The problem could be a number of things. Try testing your
1396 user-defined function in a stand-alone test program first.</P>
1398 <H4><A name="5.2">5.2</A>) How can I contribute some nifty new
1399 types and functions to PostgreSQL?</H4>
1401 <P>Send your extensions to the <I>pgsql-hackers</I> mailing list,
1402 and they will eventually end up in the <I>contrib/</I>
1405 <H4><A name="5.3">5.3</A>) How do I write a C function to return a
1408 <P>This requires wizardry so extreme that the authors have never
1409 tried it, though in principle it can be done.</P>
1411 <H4><A name="5.4">5.4</A>) I have changed a source file. Why does
1412 the recompile not see the change?</H4>
1414 <P>The <I>Makefiles</I> do not have the proper dependencies for
1415 include files. You have to do a <I>make clean</I> and then another
1416 <I>make</I>. If you are using <SMALL>GCC</SMALL> you can use the
1417 <I>--enable-depend</I> option of <I>configure</I> to have the
1418 compiler compute the dependencies automatically.</P>
projects / postgresql / blob