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15 <H1>Frequently Asked Questions (FAQ) for PostgreSQL</H1>
17 <P>Last updated: Mon Jun 10 15:47:38 EDT 2002</P>
19 <P>Current maintainer: Bruce Momjian (<A href=
20 "mailto:pgman@candle.pha.pa.us">pgman@candle.pha.pa.us</A>)<BR>
23 <P>The most recent version of this document can be viewed at <A
25 "http://www.Postgresql.org/docs/faq-english.html">http://www.PostgreSQL.org/docs/faq-english.html</A>.</P>
27 <P>Platform-specific questions are answered at <A href=
28 "http://www.PostgreSQL.org/users-lounge/docs/faq.html">http://www.PostgreSQL.org/users-lounge/docs/faq.html</A>.</P>
31 <H2 align="center">General Questions</H2>
32 <A href="#1.1">1.1</A>) What is PostgreSQL? How is it
34 <A href="#1.2">1.2</A>) What is the copyright on PostgreSQL?<BR>
35 <A href="#1.3">1.3</A>) What Unix platforms does PostgreSQL run
37 <A href="#1.4">1.4</A>) What non-Unix ports are available?<BR>
38 <A href="#1.5">1.5</A>) Where can I get PostgreSQL?<BR>
39 <A href="#1.6">1.6</A>) Where can I get support?<BR>
40 <A href="#1.7">1.7</A>) What is the latest release?<BR>
41 <A href="#1.8">1.8</A>) What documentation is available?<BR>
42 <A href="#1.9">1.9</A>) How do I find out about known bugs or
44 <A href="#1.10">1.10</A>) How can I learn <SMALL>SQL</SMALL>?<BR>
45 <A href="#1.11">1.11</A>) Is PostgreSQL Y2K compliant?<BR>
46 <A href="#1.12">1.12</A>) How do I join the development team?<BR>
47 <A href="#1.13">1.13</A>) How do I submit a bug report?<BR>
48 <A href="#1.14">1.14</A>) How does PostgreSQL compare to other
49 <SMALL>DBMS</SMALL>s?<BR>
50 <A href="#1.15">1.15</A>) How can I financially assist
54 <H2 align="center">User Client Questions</H2>
55 <A href="#2.1">2.1</A>) Are there <SMALL>ODBC</SMALL> drivers for
57 <A href="#2.2">2.2</A>) What tools are available for using
58 PostgreSQL with Web pages?<BR>
59 <A href="#2.3">2.3</A>) Does PostgreSQL have a graphical user
60 interface? A report generator? An embedded query language
62 <A href="#2.4">2.4</A>) What languages are available to
63 communicate with PostgreSQL?<BR>
66 <H2 align="center">Administrative Questions</H2>
67 <A href="#3.1">3.1</A>) How do I install PostgreSQL somewhere other
68 than <I>/usr/local/pgsql</I>?<BR>
69 <A href="#3.2">3.2</A>) When I start <I>postmaster</I>, I get a
70 <I>Bad System Call</I> or core dumped message. Why?<BR>
71 <A href="#3.3">3.3</A>) When I try to start <I>postmaster</I>, I
72 get <I>IpcMemoryCreate</I> errors. Why?<BR>
73 <A href="#3.4">3.4</A>) When I try to start <I>postmaster</I>, I
74 get <I>IpcSemaphoreCreate</I> errors. Why?<BR>
75 <A href="#3.5">3.5</A>) How do I control connections from other
77 <A href="#3.6">3.6</A>) How do I tune the database engine for
78 better performance?<BR>
79 <A href="#3.7">3.7</A>) What debugging features are available?<BR>
80 <A href="#3.8">3.8</A>) Why do I get <I>"Sorry, too many
81 clients"</I> when trying to connect?<BR>
82 <A href="#3.9">3.9</A>) What are the <I>pg_sorttempNNN.NN</I>
83 files in my database directory?<BR>
86 <H2 align="center">Operational Questions</H2>
87 <A href="#4.1">4.1</A>) What is the difference between binary
88 cursors and normal cursors?<BR>
89 <A href="#4.2">4.2</A>) How do I <SMALL>SELECT</SMALL> only the
90 first few rows of a query?<BR>
91 <A href="#4.3">4.3</A>) How do I get a list of tables or other
92 things I can see in <I>psql</I>?<BR>
93 <A href="#4.4">4.4</A>) How do you remove a column from a
95 <A href="#4.5">4.5</A>) What is the maximum size for a row, a
96 table, and a database?<BR>
97 <A href="#4.6">4.6</A>) How much database disk space is required
98 to store data from a typical text file?<BR>
99 <A href="#4.7">4.7</A>) How do I find out what tables, indexes,
100 databases, and users are defined?<BR>
101 <A href="#4.8">4.8</A>) My queries are slow or don't make use of
102 the indexes. Why?<BR>
103 <A href="#4.9">4.9</A>) How do I see how the query optimizer is
104 evaluating my query?<BR>
105 <A href="#4.10">4.10</A>) What is an R-tree index?<BR>
106 <A href="#4.11">4.11</A>) What is the Genetic Query Optimizer?<BR>
107 <A href="#4.12">4.12</A>) How do I perform regular expression
108 searches and case-insensitive regular expression searches? How do I
109 use an index for case-insensitive searches?<BR>
110 <A href="#4.13">4.13</A>) In a query, how do I detect if a field
111 is <SMALL>NULL</SMALL>?<BR>
112 <A href="#4.14">4.14</A>) What is the difference between the
113 various character types?<BR>
114 <A href="#4.15.1">4.15.1</A>) How do I create a
115 serial/auto-incrementing field?<BR>
116 <A href="#4.15.2">4.15.2</A>) How do I get the value of a
117 <SMALL>SERIAL</SMALL> insert?<BR>
118 <A href="#4.15.3">4.15.3</A>) Don't <I>currval()</I> and
119 <I>nextval()</I> lead to a race condition with other users?<BR>
120 <A href="#4.15.4">4.15.4</A>) Why aren't my sequence numbers
121 reused on transaction abort? Why are there gaps in the numbering of
122 my sequence/SERIAL column?<BR>
123 <A href="#4.16">4.16</A>) What is an <SMALL>OID</SMALL>? What is a
124 <SMALL>TID</SMALL>?<BR>
125 <A href="#4.17">4.17</A>) What is the meaning of some of the terms
126 used in PostgreSQL?<BR>
127 <A href="#4.18">4.18</A>) Why do I get the error <I>"ERROR: Memory
128 exhausted in AllocSetAlloc()"</I>?<BR>
129 <A href="#4.19">4.19</A>) How do I tell what PostgreSQL version I
131 <A href="#4.20">4.20</A>) Why does my large-object operations get
132 <I>"invalid large obj descriptor"</I>?<BR>
133 <A href="#4.21">4.21</A>) How do I create a column that will
134 default to the current time?<BR>
135 <A href="#4.22">4.22</A>) Why are my subqueries using
136 <CODE><SMALL>IN</SMALL></CODE> so slow?<BR>
137 <A href="#4.23">4.23</A>) How do I perform an outer join?<BR>
138 <A href="#4.24">4.24</A>) How do I perform queries using multiple
140 <A href="#4.25">4.25</A>) How do I return multiple rows or columns
142 <A href="#4.26">4.26</A>) Why can't I reliably create/drop
143 temporary tables in PL/PgSQL functions?<BR>
146 <H2 align="center">Extending PostgreSQL</H2>
147 <A href="#5.1">5.1</A>) I wrote a user-defined function. When I run
148 it in <I>psql</I>, why does it dump core?<BR>
149 <A href="#5.2">5.2</A>) How can I contribute some nifty new types
150 and functions to PostgreSQL?<BR>
151 <A href="#5.3">5.3</A>) How do I write a C function to return a
153 <A href="#5.4">5.4</A>) I have changed a source file. Why does the
154 recompile not see the change?<BR>
158 <H2 align="center">General Questions</H2>
160 <H4><A name="1.1">1.1</A>) What is PostgreSQL?</H4>
162 <P>PostgreSQL is pronounced <I>Post-Gres-Q-L</I>.</P>
164 <P>PostgreSQL is an enhancement of the POSTGRES database management
165 system, a next-generation <SMALL>DBMS</SMALL> research prototype.
166 While PostgreSQL retains the powerful data model and rich data
167 types of POSTGRES, it replaces the PostQuel query language with an
168 extended subset of <SMALL>SQL</SMALL>. PostgreSQL is free and the
169 complete source is available.</P>
171 <P>PostgreSQL development is performed by a team of Internet
172 developers who all subscribe to the PostgreSQL development mailing
173 list. The current coordinator is Marc G. Fournier (<A href=
174 "mailto:scrappy@PostgreSQL.org">scrappy@PostgreSQL.org</A>). (See
175 below on how to join). This team is now responsible for all
176 development of PostgreSQL.</P>
178 <P>The authors of PostgreSQL 1.01 were Andrew Yu and Jolly Chen.
179 Many others have contributed to the porting, testing, debugging,
180 and enhancement of the code. The original Postgres code, from which
181 PostgreSQL is derived, was the effort of many graduate students,
182 undergraduate students, and staff programmers working under the
183 direction of Professor Michael Stonebraker at the University of
184 California, Berkeley.</P>
186 <P>The original name of the software at Berkeley was Postgres. When
187 <SMALL>SQL</SMALL> functionality was added in 1995, its name was
188 changed to Postgres95. The name was changed at the end of 1996 to
191 <H4><A name="1.2">1.2</A>) What is the copyright on
194 <P>PostgreSQL is subject to the following COPYRIGHT:</P>
196 <P>PostgreSQL Data Base Management System</P>
198 <P>Portions copyright (c) 1996-2002, PostgreSQL Global Development
199 Group Portions Copyright (c) 1994-6 Regents of the University of
202 <P>Permission to use, copy, modify, and distribute this software
203 and its documentation for any purpose, without fee, and without a
204 written agreement is hereby granted, provided that the above
205 copyright notice and this paragraph and the following two
206 paragraphs appear in all copies.</P>
208 <P>IN NO EVENT SHALL THE UNIVERSITY OF CALIFORNIA BE LIABLE TO ANY
209 PARTY FOR DIRECT, INDIRECT, SPECIAL, INCIDENTAL, OR CONSEQUENTIAL
210 DAMAGES, INCLUDING LOST PROFITS, ARISING OUT OF THE USE OF THIS
211 SOFTWARE AND ITS DOCUMENTATION, EVEN IF THE UNIVERSITY OF
212 CALIFORNIA HAS BEEN ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.</P>
214 <P>THE UNIVERSITY OF CALIFORNIA SPECIFICALLY DISCLAIMS ANY
215 WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
216 OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE. THE
217 SOFTWARE PROVIDED HEREUNDER IS ON AN "AS IS" BASIS, AND THE
218 UNIVERSITY OF CALIFORNIA HAS NO OBLIGATIONS TO PROVIDE MAINTENANCE,
219 SUPPORT, UPDATES, ENHANCEMENTS, OR MODIFICATIONS.</P>
221 <P>The above is the BSD license, the classic open-source license.
222 It has no restrictions on how the source code may be used. We like
223 it and have no intention of changing it.</P>
225 <H4><A name="1.3">1.3</A>) What Unix platforms does PostgreSQL run
228 <P>In general, a modern Unix-compatible platform should be able to
229 run PostgreSQL. The platforms that had received explicit testing at
230 the time of release are listed in the installation
233 <H4><A name="1.4">1.4</A>) What non-Unix ports are available?</H4>
235 <P><STRONG>Client</STRONG></P>
237 <P>It is possible to compile the <I>libpq</I> C library, psql, and
238 other interfaces and binaries to run on MS Windows platforms. In
239 this case, the client is running on MS Windows, and communicates
240 via TCP/IP to a server running on one of our supported Unix
241 platforms. A file <I>win31.mak</I> is included in the distribution
242 for making a Win32 <I>libpq</I> library and <I>psql</I>. PostgreSQL
243 also communicates with <SMALL>ODBC</SMALL> clients.</P>
245 <P><STRONG>Server</STRONG></P>
247 <P>The database server can run on Windows NT and Win2k using
248 Cygwin, the Cygnus Unix/NT porting library. See
249 <I>pgsql/doc/FAQ_MSWIN</I> in the distribution or the <A href=
250 "http://www.postgresql.org/docs/faq-mswin.html">MS Windows FAQ</A>
251 on our web site. We have no plan to do a native port to any
252 Microsoft platform.</P>
254 <H4><A name="1.5">1.5</A>) Where can I get PostgreSQL?</H4>
256 <P>The primary anonymous ftp site for PostgreSQL is <A href=
257 "ftp://ftp.PostgreSQL.org/pub">ftp://ftp.PostgreSQL.org/pub</A>.
258 For mirror sites, see our main web site.</P>
260 <H4><A name="1.6">1.6</A>) Where can I get support?</H4>
262 <P>The main mailing list is: <A href=
263 "mailto:pgsql-general@PostgreSQL.org">pgsql-general@PostgreSQL.org</A>.
264 It is available for discussion of matters pertaining to PostgreSQL.
265 To subscribe, send mail with the following lines in the body (not
266 the subject line):</P>
273 "mailto:pgsql-general-request@PostgreSQL.org">pgsql-general-request@PostgreSQL.org</A>.</P>
275 <P>There is also a digest list available. To subscribe to this
276 list, send email to: <A href=
277 "mailto:pgsql-general-digest-request@PostgreSQL.org">pgsql-general-digest-request@PostgreSQL.org</A>
283 Digests are sent out to members of this list whenever the main list
284 has received around 30k of messages.
286 <P>The bugs mailing list is available. To subscribe to this list,
287 send email to <A href=
288 "mailto:pgsql-bugs-request@PostgreSQL.org">pgsql-bugs-request@PostgreSQL.org</A>
294 There is also a developers discussion mailing list available. To
295 subscribe to this list, send email to <A href=
296 "mailto:pgsql-hackers-request@PostgreSQL.org">pgsql-hackers-request@PostgreSQL.org</A>
303 <P>Additional mailing lists and information about PostgreSQL can be
304 found via the PostgreSQL WWW home page at:</P>
307 <A href="http://www.PostgreSQL.org">http://www.PostgreSQL.org</A>
310 <P>There is also an IRC channel on EFNet, channel
311 <I>#PostgreSQL</I>. I use the Unix command <CODE>irc -c
312 '#PostgreSQL' "$USER" irc.phoenix.net.</CODE></P>
314 <P>A list of commercial support companies is available at <A href=
315 "http://www.postgresql.org/users-lounge/commercial-support.html">http://www.postgresql.org/users-lounge/commercial-support.html</A>.</P>
317 <H4><A name="1.7">1.7</A>) What is the latest release?</H4>
319 <P>The latest release of PostgreSQL is version 7.2.1.</P>
321 <P>We plan to have major releases every four months.</P>
323 <H4><A name="1.8">1.8</A>) What documentation is available?</H4>
325 <P>Several manuals, manual pages, and some small test examples are
326 included in the distribution. See the <I>/doc</I> directory. You
327 can also browse the manual online at <A href=
328 "http://www.PostgreSQL.org/users-lounge/docs/">http://www.PostgreSQL.org/users-lounge/docs/</A>.</P>
330 <P>There are two PostgreSQL books available online at <A href=
331 "http://www.PostgreSQL.org/docs/awbook.html">http://www.PostgreSQL.org/docs/awbook.html</A>
333 "http://www.commandprompt.com/ppbook/">http://www.commandprompt.com/ppbook/</A>.
334 There is a list of PostgreSQL books available for purchase at <A
336 "http://www.postgresql.org/books/">http://www.postgresql.org/books/</A>.
337 There is also a collection of PostgreSQL technical articles at <A
339 "http://techdocs.postgresql.org/">http://techdocs.postgresql.org/</A>.</P>
341 <P><I>psql</I> has some nice \d commands to show information about
342 types, operators, functions, aggregates, etc.</P>
344 <P>Our web site contains even more documentation.</P>
346 <H4><A name="1.9">1.9</A>) How do I find out about known bugs or
347 missing features?</H4>
349 <P>PostgreSQL supports an extended subset of <SMALL>SQL</SMALL>-92.
350 See our <A href="http://developer.PostgreSQL.org/todo.php">TODO</A>
351 list for known bugs, missing features, and future plans.</P>
353 <H4><A name="1.10">1.10</A>) How can I learn
354 <SMALL>SQL</SMALL>?</H4>
356 <P>The PostgreSQL book at <A href=
357 "http://www.PostgreSQL.org/docs/awbook.html">http://www.PostgreSQL.org/docs/awbook.html</A>
358 teaches <SMALL>SQL</SMALL>. There is another PostgreSQL book at <A
360 "http://www.commandprompt.com/ppbook/">http://www.commandprompt.com/ppbook.</A>
361 There is a nice tutorial at <A href=
362 "http://www.intermedia.net/support/sql/sqltut.shtm">http://www.intermedia.net/support/sql/sqltut.shtm,</A>
364 "http://ourworld.compuserve.com/homepages/graeme_birchall/HTM_COOK.HTM">
365 http://ourworld.compuserve.com/homepages/graeme_birchall/HTM_COOK.HTM,</A>
367 "http://sqlcourse.com/">http://sqlcourse.com.</A></P>
369 <P>Another one is "Teach Yourself SQL in 21 Days, Second Edition"
371 "http://members.tripod.com/er4ebus/sql/index.htm">http://members.tripod.com/er4ebus/sql/index.htm</A></P>
373 <P>Many of our users like <I>The Practical SQL Handbook</I>,
374 Bowman, Judith S., et al., Addison-Wesley. Others like <I>The
375 Complete Reference SQL</I>, Groff et al., McGraw-Hill.</P>
377 <H4><A name="1.11">1.11</A>) Is PostgreSQL Y2K compliant?</H4>
379 <P>Yes, we easily handle dates past the year 2000 AD, and before
382 <H4><A name="1.12">1.12</A>) How do I join the development
385 <P>First, download the latest source and read the PostgreSQL
386 Developers documentation on our web site, or in the distribution.
387 Second, subscribe to the <I>pgsql-hackers</I> and
388 <I>pgsql-patches</I> mailing lists. Third, submit high quality
389 patches to pgsql-patches.</P>
391 <P>There are about a dozen people who have commit privileges to the
392 PostgreSQL <SMALL>CVS</SMALL> archive. They each have submitted so
393 many high-quality patches that it was impossible for the existing
394 committers to keep up, and we had confidence that patches they
395 committed were of high quality.</P>
397 <H4><A name="1.13">1.13</A>) How do I submit a bug report?</H4>
399 <P>Please visit the <A href=
400 "http://www.postgresql.org/bugs/bugs.php">PostgreSQL BugTool</A>
401 page, which gives guidelines and directions on how to submit a
404 <P>Also check out our ftp site <A href=
405 "ftp://ftp.PostgreSQL.org/pub">ftp://ftp.PostgreSQL.org/pub</A> to
406 see if there is a more recent PostgreSQL version or patches.</P>
408 <H4><A name="1.14">1.14</A>) How does PostgreSQL compare to other
409 <SMALL>DBMS</SMALL>s?</H4>
411 <P>There are several ways of measuring software: features,
412 performance, reliability, support, and price.</P>
415 <DT><B>Features</B></DT>
417 <DD>PostgreSQL has most features present in large commercial
418 <SMALL>DBMS</SMALL>s, like transactions, subselects, triggers,
419 views, foreign key referential integrity, and sophisticated
420 locking. We have some features they do not have, like
421 user-defined types, inheritance, rules, and multi-version
422 concurrency control to reduce lock contention.<BR>
426 <DT><B>Performance</B></DT>
428 <DD>PostgreSQL runs in two modes. Normal <I>fsync</I> mode
429 flushes every completed transaction to disk, guaranteeing that if
430 the OS crashes or loses power in the next few seconds, all your
431 data is safely stored on disk. In this mode, we are slower than
432 most commercial databases, partly because few of them do such
433 conservative flushing to disk in their default modes. In
434 <I>no-fsync</I> mode, we are usually faster than commercial
435 databases, though in this mode, an OS crash could cause data
436 corruption. We are working to provide an intermediate mode that
437 suffers less performance overhead than full fsync mode, and will
438 allow data integrity within 30 seconds of an OS crash.<BR>
440 In comparison to MySQL or leaner database systems, we are slower
441 on inserts/updates because we have transaction overhead. Of
442 course, MySQL does not have any of the features mentioned in the
443 <I>Features</I> section above. We are built for flexibility and
444 features, though we continue to improve performance through
445 profiling and source code analysis. There is an interesting Web
446 page comparing PostgreSQL to MySQL at <A href=
447 "http://openacs.org/why-not-mysql.html">http://openacs.org/why-not-mysql.html</A><BR>
450 We handle each user connection by creating a Unix process.
451 Backend processes share data buffers and locking information.
452 With multiple CPUs, multiple backends can easily run on different
457 <DT><B>Reliability</B></DT>
459 <DD>We realize that a <SMALL>DBMS</SMALL> must be reliable, or it
460 is worthless. We strive to release well-tested, stable code that
461 has a minimum of bugs. Each release has at least one month of
462 beta testing, and our release history shows that we can provide
463 stable, solid releases that are ready for production use. We
464 believe we compare favorably to other database software in this
469 <DT><B>Support</B></DT>
471 <DD>Our mailing list provides a large group of developers and
472 users to help resolve any problems encountered. While we can not
473 guarantee a fix, commercial <SMALL>DBMS</SMALL>s do not always
474 supply a fix either. Direct access to developers, the user
475 community, manuals, and the source code often make PostgreSQL
476 support superior to other <SMALL>DBMS</SMALL>s. There is
477 commercial per-incident support available for those who need it.
478 (See <A href="#1.6">support FAQ item</A>.)<BR>
482 <DT><B>Price</B></DT>
484 <DD>We are free for all use, both commercial and non-commercial.
485 You can add our code to your product with no limitations, except
486 those outlined in our BSD-style license stated above.<BR>
491 <H4><A name="1.15">1.15</A>) How can I financially assist
494 <P>PostgreSQL has had a first-class infrastructure since we started
495 six years ago. This is all thanks to Marc Fournier, who has created
496 and managed this infrastructure over the years.</P>
498 <P>Quality infrastructure is very important to an open-source
499 project. It prevents disruptions that can greatly delay forward
500 movement of the project.</P>
502 <P>Of course, this infrastructure is not cheap. There are a variety
503 of monthly and one-time expenses that are required to keep it
504 going. If you or your company has money it can donate to help fund
505 this effort, please go to <A href=
506 "http://www.pgsql.com/pg_goodies">http://www.pgsql.com/pg_goodies</A>
507 and make a donation.</P>
509 <P>Although the web page mentions PostgreSQL, Inc, the
510 "contributions" item is solely to support the PostgreSQL project
511 and does not fund any specific company. If you prefer, you can also
512 send a check to the contact address.</P>
515 <H2 align="center">User Client Questions</H2>
517 <H4><A name="2.1">2.1</A>) Are there <SMALL>ODBC</SMALL> drivers
520 <P>There are two <SMALL>ODBC</SMALL> drivers available, PsqlODBC
521 and OpenLink <SMALL>ODBC</SMALL>.</P>
523 <P>PsqlODBC is included in the distribution. More information about
524 it can be gotten from <A href=
525 "ftp://ftp.PostgreSQL.org/pub/odbc/">ftp://ftp.PostgreSQL.org/pub/odbc/</A>.</P>
527 <P>OpenLink <SMALL>ODBC</SMALL> can be gotten from <A href=
528 "http://www.openlinksw.com/">http://www.openlinksw.com</A>. It
529 works with their standard <SMALL>ODBC</SMALL> client software so
530 you'll have PostgreSQL <SMALL>ODBC</SMALL> available on every
531 client platform they support (Win, Mac, Unix, VMS).</P>
533 <P>They will probably be selling this product to people who need
534 commercial-quality support, but a freeware version will always be
535 available. Please send questions to <A href=
536 "mailto:postgres95@openlink.co.uk">postgres95@openlink.co.uk</A>.</P>
538 <P>See also the <A href=
539 "http://www.postgresql.org/devel-corner/docs/programmer/odbc.html">ODBC
540 chapter of the Programmer's Guide</A>.</P>
542 <H4><A name="2.2">2.2</A>) What tools are available for using
543 PostgreSQL with Web pages?</H4>
545 <P>A nice introduction to Database-backed Web pages can be seen at:
546 <A href="http://www.webreview.com">http://www.webreview.com</A></P>
548 <P>There is also one at <A href=
549 "http://www.phone.net/home/mwm/hotlist/">http://www.phone.net/home/mwm/hotlist/.</A></P>
551 <P>For Web integration, PHP is an excellent interface. It is at <A
552 href="http://www.php.net">http://www.php.net</A>.</P>
554 <P>For complex cases, many use the Perl interface and CGI.pm.</P>
556 <H4><A name="2.3">2.3</A>) Does PostgreSQL have a graphical user
557 interface? A report generator? An embedded query language
560 <P>We have a nice graphical user interface called <I>pgaccess</I>,
561 which is shipped as part of the distribution. <I>pgaccess</I> also
562 has a report generator. The Web page is <A href=
563 "http://www.flex.ro/pgaccess">http://www.flex.ro/pgaccess</A></P>
565 <P>We also include <I>ecpg</I>, which is an embedded SQL query
566 language interface for C.</P>
568 <H4><A name="2.4">2.4</A>) What languages are available to
569 communicate with PostgreSQL?</H4>
576 <LI>C++ (libpq++)</LI>
578 <LI>Embedded C (ecpg)</LI>
582 <LI>Perl (perl5)</LI>
586 <LI>Python (PyGreSQL)</LI>
588 <LI>TCL (libpgtcl)</LI>
590 <LI>C Easy API (libpgeasy)</LI>
592 <LI>Embedded <SMALL>HTML</SMALL> (<A href=
593 "http://www.php.net">PHP from http://www.php.net</A>)</LI>
597 <H2 align="center">Administrative Questions</H2>
599 <H4><A name="3.1">3.1</A>) How do I install PostgreSQL somewhere
600 other than <I>/usr/local/pgsql</I>?</H4>
602 <P>Specify the <I>--prefix</I> option when running
603 <I>configure</I>.</P>
605 <H4><A name="3.2">3.2</A>) When I start <I>postmaster</I>, I get a
606 <I>Bad System Call</I> or core dumped message. Why?</H4>
608 <P>It could be a variety of problems, but first check to see that
609 you have System V extensions installed in your kernel. PostgreSQL
610 requires kernel support for shared memory and semaphores.</P>
612 <H4><A name="3.3">3.3</A>) When I try to start <I>postmaster</I>, I
613 get <I>IpcMemoryCreate</I> errors. Why?</H4>
615 <P>You either do not have shared memory configured properly in your
616 kernel or you need to enlarge the shared memory available in the
617 kernel. The exact amount you need depends on your architecture and
618 how many buffers and backend processes you configure for
619 <I>postmaster</I>. For most systems, with default numbers of
620 buffers and processes, you need a minimum of ~1 MB. See the <A
622 "http://www.postgresql.org/idocs/index.php?kernel-resources.html">PostgreSQL
623 Administrator's Guide</A> for more detailed information about
624 shared memory and semaphores.</P>
626 <H4><A name="3.4">3.4</A>) When I try to start <I>postmaster</I>, I
627 get <I>IpcSemaphoreCreate</I> errors. Why?</H4>
629 <P>If the error message is <I>IpcSemaphoreCreate: semget failed (No
630 space left on device)</I> then your kernel is not configured with
631 enough semaphores. Postgres needs one semaphore per potential
632 backend process. A temporary solution is to start <I>postmaster</I>
633 with a smaller limit on the number of backend processes. Use
634 <I>-N</I> with a parameter less than the default of 32. A more
635 permanent solution is to increase your kernel's
636 <SMALL>SEMMNS</SMALL> and <SMALL>SEMMNI</SMALL> parameters.</P>
638 <P>Inoperative semaphores can also cause crashes during heavy
641 <P>If the error message is something else, you might not have
642 semaphore support configured in your kernel at all. See the
643 PostgreSQL Administrator's Guide for more detailed information
644 about shared memory and semaphores.</P>
646 <H4><A name="3.5">3.5</A>) How do I control connections from other
649 <P>By default, PostgreSQL only allows connections from the local
650 machine using Unix domain sockets. Other machines will not be able
651 to connect unless you add the <I>-i</I> flag to <I>postmaster</I>,
652 <B>and</B> enable host-based authentication by modifying the file
653 <I>$PGDATA/pg_hba.conf</I> accordingly. This will allow TCP/IP
656 <H4><A name="3.6">3.6</A>) How do I tune the database engine for
657 better performance?</H4>
659 <P>Certainly, indexes can speed up queries. The
660 <SMALL>EXPLAIN</SMALL> command allows you to see how PostgreSQL is
661 interpreting your query, and which indexes are being used.</P>
663 <P>If you are doing many <SMALL>INSERTs</SMALL>, consider doing
664 them in a large batch using the <SMALL>COPY</SMALL> command. This
665 is much faster than individual <SMALL>INSERTS.</SMALL> Second,
666 statements not in a <SMALL>BEGIN WORK/COMMIT</SMALL> transaction
667 block are considered to be in their own transaction. Consider
668 performing several statements in a single transaction block. This
669 reduces the transaction overhead. Also, consider dropping and
670 recreating indexes when making large data changes.</P>
672 <P>There are several tuning options. You can disable <I>fsync()</I>
673 by starting <I>postmaster</I> with a <I>-o -F</I> option. This will
674 prevent <I>fsync()</I>s from flushing to disk after every
677 <P>You can also use the <I>postmaster</I> <I>-B</I> option to
678 increase the number of shared memory buffers used by the backend
679 processes. If you make this parameter too high, the
680 <I>postmaster</I> may not start because you have exceeded your
681 kernel's limit on shared memory space. Each buffer is 8K and the
682 default is 64 buffers.</P>
684 <P>You can also use the backend <I>-S</I> option to increase the
685 maximum amount of memory used by the backend process for temporary
686 sorts. The <I>-S</I> value is measured in kilobytes, and the
687 default is 512 (i.e. 512K).</P>
689 <P>You can also use the <SMALL>CLUSTER</SMALL> command to group
690 data in tables to match an index. See the <SMALL>CLUSTER</SMALL>
691 manual page for more details.</P>
693 <H4><A name="3.7">3.7</A>) What debugging features are
696 <P>PostgreSQL has several features that report status information
697 that can be valuable for debugging purposes.</P>
699 <P>First, by running <I>configure</I> with the --enable-cassert
700 option, many <I>assert()</I>s monitor the progress of the backend
701 and halt the program when something unexpected occurs.</P>
703 <P>Both <I>postmaster</I> and <I>postgres</I> have several debug
704 options available. First, whenever you start <I>postmaster</I>,
705 make sure you send the standard output and error to a log file,
709 ./bin/postmaster >server.log 2>&1 &
712 <P>This will put a server.log file in the top-level PostgreSQL
713 directory. This file contains useful information about problems or
714 errors encountered by the server. <I>Postmaster</I> has a <I>-d</I>
715 option that allows even more detailed information to be reported.
716 The <I>-d</I> option takes a number that specifies the debug level.
717 Be warned that high debug level values generate large log
720 <P>If <I>postmaster</I> is not running, you can actually run the
721 <I>postgres</I> backend from the command line, and type your
722 <SMALL>SQL</SMALL> statement directly. This is recommended
723 <B>only</B> for debugging purposes. Note that a newline terminates
724 the query, not a semicolon. If you have compiled with debugging
725 symbols, you can use a debugger to see what is happening. Because
726 the backend was not started from <I>postmaster</I>, it is not
727 running in an identical environment and locking/backend interaction
728 problems may not be duplicated.</P>
730 <P>If <I>postmaster</I> is running, start <I>psql</I> in one
731 window, then find the <SMALL>PID</SMALL> of the <I>postgres</I>
732 process used by <I>psql</I>. Use a debugger to attach to the
733 <I>postgres</I> <SMALL>PID.</SMALL> You can set breakpoints in the
734 debugger and issue queries from <I>psql</I>. If you are debugging
735 <I>postgres</I> startup, you can set PGOPTIONS="-W n", then start
736 <I>psql</I>. This will cause startup to delay for <I>n</I> seconds
737 so you can attach to the process with the debugger, set any
738 breakpoints, and continue through the startup sequence.</P>
740 <P>The <I>postgres</I> program has <I>-s, -A</I>, and <I>-t</I>
741 options that can be very useful for debugging and performance
744 <P>You can also compile with profiling to see what functions are
745 taking execution time. The backend profile files will be deposited
746 in the <I>pgsql/data/base/dbname</I> directory. The client profile
747 file will be put in the client's current directory. Linux requires
748 a compile with <I>-DLINUX_PROFILE</I> for proper profiling.</P>
750 <H4><A name="3.8">3.8</A>) Why do I get <I>"Sorry, too many
751 clients"</I> when trying to connect?</H4>
753 <P>You need to increase <I>postmaster</I>'s limit on how many
754 concurrent backend processes it can start.</P>
756 <P>The default limit is 32 processes. You can increase it by
757 restarting <I>postmaster</I> with a suitable <I>-N</I> value or
758 modifying <I>postgresql.conf</I>.</P>
760 <P>Note that if you make <I>-N</I> larger than 32, you must also
761 increase <I>-B</I> beyond its default of 64; <I>-B</I> must be at
762 least twice <I>-N</I>, and probably should be more than that for
763 best performance. For large numbers of backend processes, you are
764 also likely to find that you need to increase various Unix kernel
765 configuration parameters. Things to check include the maximum size
766 of shared memory blocks, <SMALL>SHMMAX;</SMALL> the maximum number
767 of semaphores, <SMALL>SEMMNS</SMALL> and <SMALL>SEMMNI;</SMALL> the
768 maximum number of processes, <SMALL>NPROC;</SMALL> the maximum
769 number of processes per user, <SMALL>MAXUPRC;</SMALL> and the
770 maximum number of open files, <SMALL>NFILE</SMALL> and
771 <SMALL>NINODE.</SMALL> The reason that PostgreSQL has a limit on
772 the number of allowed backend processes is so your system won't run
773 out of resources.</P>
775 <P>In PostgreSQL versions prior to 6.5, the maximum number of
776 backends was 64, and changing it required a rebuild after altering
777 the MaxBackendId constant in
778 <I>include/storage/sinvaladt.h</I>.</P>
780 <H4><A name="3.9">3.9</A>) What are the <I>pg_sorttempNNN.NN</I>
781 files in my database directory?</H4>
783 <P>They are temporary files generated by the query executor. For
784 example, if a sort needs to be done to satisfy an <SMALL>ORDER
785 BY,</SMALL> and the sort requires more space than the backend's
786 <I>-S</I> parameter allows, then temporary files are created to
787 hold the extra data.</P>
789 <P>The temporary files should be deleted automatically, but might
790 not if a backend crashes during a sort. If you have no backends
791 running at the time, it is safe to delete the pg_tempNNN.NN
795 <H2 align="center">Operational Questions</H2>
797 <H4><A name="4.1">4.1</A>) What is the difference between binary
798 cursors and normal cursors?</H4>
800 <P>See the <SMALL>DECLARE</SMALL> manual page for a
803 <H4><A name="4.2">4.2</A>) How do I <SMALL>SELECT</SMALL> only the
804 first few rows of a query?</H4>
806 <P>See the <SMALL>FETCH</SMALL> manual page, or use
807 <SMALL>SELECT</SMALL> ... <SMALL>LIMIT</SMALL>....</P>
809 <P>The entire query may have to be evaluated, even if you only want
810 the first few rows. Consider a query that has an <SMALL>ORDER
811 BY.</SMALL> If there is an index that matches the <SMALL>ORDER
812 BY</SMALL>, PostgreSQL may be able to evaluate only the first few
813 records requested, or the entire query may have to be evaluated
814 until the desired rows have been generated.</P>
816 <H4><A name="4.3">4.3</A>) How do I get a list of tables or other
817 things I can see in <I>psql</I>?</H4>
819 <P>You can read the source code for <I>psql</I> in file
820 <I>pgsql/src/bin/psql/describe.c</I>. It contains
821 <SMALL>SQL</SMALL> commands that generate the output for psql's
822 backslash commands. You can also start <I>psql</I> with the
823 <I>-E</I> option so it will print out the queries it uses to
824 execute the commands you give.</P>
826 <H4><A name="4.4">4.4</A>) How do you remove a column from a
829 <P>We do not support <SMALL>ALTER TABLE DROP COLUMN,</SMALL> but do
833 LOCK TABLE old_table;
834 SELECT ... -- select all columns but the one you want to remove
837 DROP TABLE old_table;
838 ALTER TABLE new_table RENAME TO old_table;
842 <H4><A name="4.5">4.5</A>) What is the maximum size for a row, a
843 table, and a database?</H4>
845 <P>These are the limits:</P>
847 Maximum size for a database? unlimited (500 GB databases exist)
848 Maximum size for a table? 16 TB
849 Maximum size for a row? unlimited in 7.1 and later
850 Maximum size for a field? 1 GB in 7.1 and later
851 Maximum number of rows in a table? unlimited
852 Maximum number of columns in a table? 250-1600 depending on column types
853 Maximum number of indexes on a table? unlimited
855 Of course, these are not actually unlimited, but limited to
856 available disk space and memory/swap space. Performance may suffer
857 when these values get unusually large.
859 <P>The maximum table size of 16 TB does not require large file
860 support from the operating system. Large tables are stored as
861 multiple 1 GB files so file system size limits are not
864 <P>The maximum table size and maximum number of columns can be
865 increased if the default block size is increased to 32k.</P>
867 <H4><A name="4.6">4.6</A>) How much database disk space is required
868 to store data from a typical text file?</H4>
870 <P>A PostgreSQL database may require up to five times the disk
871 space to store data from a text file.</P>
873 <P>As an example, consider a file of 100,000 lines with an integer
874 and text description on each line. Suppose the text string
875 avergages twenty bytes in length. The flat file would be 2.8 MB.
876 The size of the PostgreSQL database file containing this data can
877 be estimated as 6.4 MB:</P>
879 36 bytes: each row header (approximate)
880 24 bytes: one int field and one text filed
881 + 4 bytes: pointer on page to tuple
882 ----------------------------------------
885 The data page size in PostgreSQL is 8192 bytes (8 KB), so:
888 ------------------- = 128 rows per database page (rounded down)
892 -------------------- = 782 database pages (rounded up)
895 782 database pages * 8192 bytes per page = 6,406,144 bytes (6.4 MB)
898 <P>Indexes do not require as much overhead, but do contain the data
899 that is being indexed, so they can be large also.</P>
901 <H4><A name="4.7">4.7</A>) How do I find out what tables, indexes,
902 databases, and users are defined?</H4>
904 <P><I>psql</I> has a variety of backslash commands to show such
905 information. Use \? to see them. There are also system tables
906 beginning with <I>pg_</I> that describe these too. Also, <I>psql
907 -l</I> will list all databases.</P>
909 <P>Also try the file <I>pgsql/src/tutorial/syscat.source</I>. It
910 illustrates many of the <SMALL>SELECT</SMALL>s needed to get
911 information from the database system tables.</P>
913 <H4><A name="4.8">4.8</A>) My queries are slow or don't make use of
914 the indexes. Why?</H4>
915 Indexes are not automatically used by every query. Indexes are only
916 used if the table is larger than a minimum size, and the query
917 selects only a small percentage of the rows in the table. This is
918 because the random disk access caused by an index scan is sometimes
919 slower than a straight read through the table, or sequential scan.
921 <P>To determine if an index should be used, PostgreSQL must have
922 statistics about the table. These statistics are collected using
923 <SMALL>VACUUM ANALYZE</SMALL>, or simply <SMALL>ANALYZE</SMALL>.
924 Using statistics, the optimizer knows how many rows are in the
925 table, and can better determine if indexes should be used.
926 Statistics are also valuable in determining optimal join order and
927 join methods. Statistics collection should be performed
928 periodically as the contents of the table change.</P>
930 <P>Indexes are normally not used for <SMALL>ORDER BY</SMALL> or to
931 perform joins. A sequential scan followed by an explicit sort is
932 usually faster than an index scan of a large table.</P>
933 However, <SMALL>LIMIT</SMALL> combined with <SMALL>ORDER BY</SMALL>
934 often will use an index because only a small portion of the table
937 <P>When using wild-card operators such as <SMALL>LIKE</SMALL> or
938 <I>~</I>, indexes can only be used if the beginning of the search
939 is anchored to the start of the string. Therefore, to use indexes,
940 <SMALL>LIKE</SMALL> patterns must not start with <I>%</I>, and
941 <I>~</I>(regular expression) patterns must start with <I>^</I>.</P>
943 <H4><A name="4.9">4.9</A>) How do I see how the query optimizer is
944 evaluating my query?</H4>
946 <P>See the <SMALL>EXPLAIN</SMALL> manual page.</P>
948 <H4><A name="4.10">4.10</A>) What is an R-tree index?</H4>
950 <P>An R-tree index is used for indexing spatial data. A hash index
951 can't handle range searches. A B-tree index only handles range
952 searches in a single dimension. R-trees can handle
953 multi-dimensional data. For example, if an R-tree index can be
954 built on an attribute of type <I>point</I>, the system can more
955 efficiently answer queries such as "select all points within a
956 bounding rectangle."</P>
958 <P>The canonical paper that describes the original R-tree design
961 <P>Guttman, A. "R-trees: A Dynamic Index Structure for Spatial
962 Searching." Proceedings of the 1984 ACM SIGMOD Int'l Conf on Mgmt
965 <P>You can also find this paper in Stonebraker's "Readings in
966 Database Systems".</P>
968 <P>Built-in R-trees can handle polygons and boxes. In theory,
969 R-trees can be extended to handle higher number of dimensions. In
970 practice, extending R-trees requires a bit of work and we don't
971 currently have any documentation on how to do it.</P>
973 <H4><A name="4.11">4.11</A>) What is the Genetic Query
976 <P>The <SMALL>GEQO</SMALL> module speeds query optimization when
977 joining many tables by means of a Genetic Algorithm (GA). It allows
978 the handling of large join queries through nonexhaustive
981 <H4><A name="4.12">4.12</A>) How do I perform regular expression
982 searches and case-insensitive regular expression searches? How do I
983 use an index for case-insensitive searches?</H4>
985 <P>The <I>~</I> operator does regular expression matching, and
986 <I>~*</I> does case-insensitive regular expression matching. The
987 case-insensitive variant of <SMALL>LIKE</SMALL> is called
988 <SMALL>ILIKE</SMALL> in PostgreSQL 7.1 and later.</P>
990 <P>Case-insensitive equality comparisons are normally expressed
995 WHERE lower(col) = 'abc'
998 This will not use an standard index. However, if you create a
999 functional index, it will be used:
1001 CREATE INDEX tabindex on tab (lower(col));
1005 <H4><A name="4.13">4.13</A>) In a query, how do I detect if a field
1006 is <SMALL>NULL</SMALL>?</H4>
1008 <P>You test the column with <SMALL>IS NULL</SMALL> and <SMALL>IS
1009 NOT NULL</SMALL>.</P>
1011 <H4><A name="4.14">4.14</A>) What is the difference between the
1012 various character types?</H4>
1014 Type Internal Name Notes
1015 --------------------------------------------------
1016 "char" char 1 character
1017 CHAR(#) bpchar blank padded to the specified fixed length
1018 VARCHAR(#) varchar size specifies maximum length, no padding
1019 TEXT text no specific upper limit on length
1020 BYTEA bytea variable-length byte array (null-byte safe)
1023 <P>You will see the internal name when examining system catalogs
1024 and in some error messages.</P>
1026 <P>The last four types above are "varlena" types (i.e., the first
1027 four bytes on disk are the length, followed by the data). Thus the
1028 actual space used is slightly greater than the declared size.
1029 However, these data types are also subject to compression or being
1030 stored out-of-line by <SMALL>TOAST</SMALL>, so the space on disk
1031 might also be less than expected.</P>
1033 <P><SMALL>CHAR()</SMALL> is best when storing strings that are
1034 usually the same length. <SMALL>VARCHAR()</SMALL> is best when
1035 storing variable-length strings but it limits how long a string can
1036 be. <SMALL>TEXT</SMALL> is for strings of unlimited length, maximum
1037 1 gigabyte. <SMALL>BYTEA</SMALL> is for storing binary data,
1038 particularly values that include <SMALL>NULL</SMALL> bytes.</P>
1040 <H4><A name="4.15.1">4.15.1</A>) How do I create a
1041 serial/auto-incrementing field?</H4>
1043 <P>PostgreSQL supports a <SMALL>SERIAL</SMALL> data type. It
1044 auto-creates a sequence and index on the column. For example,
1047 CREATE TABLE person (
1052 is automatically translated into this:
1054 CREATE SEQUENCE person_id_seq;
1055 CREATE TABLE person (
1056 id INT4 NOT NULL DEFAULT nextval('person_id_seq'),
1059 CREATE UNIQUE INDEX person_id_key ON person ( id );
1061 See the <I>create_sequence</I> manual page for more information
1062 about sequences. You can also use each row's <I>OID</I> field as a
1063 unique value. However, if you need to dump and reload the database,
1064 you need to use <I>pg_dump</I>'s <I>-o</I> option or <SMALL>COPY
1065 WITH OIDS</SMALL> option to preserve the <SMALL>OID</SMALL>s.
1067 <H4><A name="4.15.2">4.15.2</A>) How do I get the value of a
1068 <SMALL>SERIAL</SMALL> insert?</H4>
1070 <P>One approach is to retrieve the next <SMALL>SERIAL</SMALL> value
1071 from the sequence object with the <I>nextval()</I> function
1072 <I>before</I> inserting and then insert it explicitly. Using the
1073 example table in <A href="#4.15.1">4.15.1</A>, that might look like
1076 new_id = output of "SELECT nextval('person_id_seq')"
1077 INSERT INTO person (id, name) VALUES (new_id, 'Blaise Pascal');
1079 You would then also have the new value stored in
1080 <CODE>new_id</CODE> for use in other queries (e.g., as a foreign
1081 key to the <CODE>person</CODE> table). Note that the name of the
1082 automatically created <SMALL>SEQUENCE</SMALL> object will be named
1083 <<I>table</I>>_<<I>serialcolumn</I>>_<I>seq</I>, where
1084 <I>table</I> and <I>serialcolumn</I> are the names of your table
1085 and your <SMALL>SERIAL</SMALL> column, respectively.
1087 <P>Alternatively, you could retrieve the assigned
1088 <SMALL>SERIAL</SMALL> value with the <I>currval</I>() function
1089 <I>after</I> it was inserted by default, e.g.,</P>
1091 INSERT INTO person (name) VALUES ('Blaise Pascal');
1092 new_id = output of "SELECT currval('person_id_seq')";
1094 Finally, you could use the <A href="#4.16"><SMALL>OID</SMALL></A>
1095 returned from the <SMALL>INSERT</SMALL> statement to look up the
1096 default value, though this is probably the least portable approach.
1097 In Perl, using DBI with Edmund Mergl's DBD::Pg module, the oid
1098 value is made available via <I>$sth->{pg_oid_status}</I> after
1099 <I>$sth->execute()</I>.
1101 <H4><A name="4.15.3">4.15.3</A>) Don't <I>currval()</I> and
1102 <I>nextval()</I> lead to a race condition with other users?</H4>
1104 <P>No. Currval() returns the current value assigned by your
1105 backend, not by all users.</P>
1107 <H4><A name="4.15.4">4.15.4</A>) Why aren't my sequence numbers
1108 reused on transaction abort? Why are there gaps in the numbering of
1109 my sequence/SERIAL column?</H4>
1111 <P>To improve concurrency, sequence values are given out to running
1112 transactions as needed and are not locked until the transaction
1113 completes. This causes gaps in numbering from aborted
1116 <H4><A name="4.16">4.16</A>) What is an <SMALL>OID</SMALL>? What is
1117 a <SMALL>TID</SMALL>?</H4>
1119 <P><SMALL>OID</SMALL>s are PostgreSQL's answer to unique row ids.
1120 Every row that is created in PostgreSQL gets a unique
1121 <SMALL>OID</SMALL>. All <SMALL>OID</SMALL>s generated during
1122 <I>initdb</I> are less than 16384 (from
1123 <I>backend/access/transam.h</I>). All user-created
1124 <SMALL>OID</SMALL>s are equal to or greater than this. By default,
1125 all these <SMALL>OID</SMALL>s are unique not only within a table or
1126 database, but unique within the entire PostgreSQL installation.</P>
1128 <P>PostgreSQL uses <SMALL>OID</SMALL>s in its internal system
1129 tables to link rows between tables. These <SMALL>OID</SMALL>s can
1130 be used to identify specific user rows and used in joins. It is
1131 recommended you use column type <SMALL>OID</SMALL> to store
1132 <SMALL>OID</SMALL> values. You can create an index on the
1133 <SMALL>OID</SMALL> field for faster access.</P>
1135 <P>O<SMALL>ID</SMALL>s are assigned to all new rows from a central
1136 area that is used by all databases. If you want to change the
1137 <SMALL>OID</SMALL> to something else, or if you want to make a copy
1138 of the table, with the original <SMALL>OID</SMALL>s, there is no
1139 reason you can't do it:</P>
1141 CREATE TABLE new_table(old_oid oid, mycol int);
1142 SELECT old_oid, mycol INTO new FROM old;
1143 COPY new TO '/tmp/pgtable';
1145 COPY new WITH OIDS FROM '/tmp/pgtable';
1147 CREATE TABLE new_table (mycol int);
1148 INSERT INTO new_table (oid, mycol) SELECT oid, mycol FROM old_table;
1152 <P>O<SMALL>ID</SMALL>s are stored as 4-byte integers, and will
1153 overflow at 4 billion. No one has reported this ever happening, and
1154 we plan to have the limit removed before anyone does.</P>
1156 <P>T<SMALL>ID</SMALL>s are used to identify specific physical rows
1157 with block and offset values. T<SMALL>ID</SMALL>s change after rows
1158 are modified or reloaded. They are used by index entries to point
1159 to physical rows.</P>
1161 <H4><A name="4.17">4.17</A>) What is the meaning of some of the
1162 terms used in PostgreSQL?</H4>
1164 <P>Some of the source code and older documentation use terms that
1165 have more common usage. Here are some:</P>
1168 <LI>table, relation, class</LI>
1170 <LI>row, record, tuple</LI>
1172 <LI>column, field, attribute</LI>
1174 <LI>retrieve, select</LI>
1176 <LI>replace, update</LI>
1178 <LI>append, insert</LI>
1180 <LI><SMALL>OID</SMALL>, serial value</LI>
1182 <LI>portal, cursor</LI>
1184 <LI>range variable, table name, table alias</LI>
1187 <P>A list of general database terms can be found at: <A href=
1188 "http://www.comptechnews.com/~reaster/dbdesign.html">http://www.comptechnews.com/~reaster/dbdesign.html</A></P>
1190 <H4><A name="4.18">4.18</A>) Why do I get the error <I>"ERROR:
1191 Memory exhausted in AllocSetAlloc()"</I>?</H4>
1193 <P>If you are running a version older than 7.1, an upgrade may fix
1194 the problem. Also it is possible you have run out of virtual memory
1195 on your system, or your kernel has a low limit for certain
1196 resources. Try this before starting <I>postmaster</I>:</P>
1201 Depending on your shell, only one of these may succeed, but it will
1202 set your process data segment limit much higher and perhaps allow
1203 the query to complete. This command applies to the current process,
1204 and all subprocesses created after the command is run. If you are
1205 having a problem with the <SMALL>SQL</SMALL> client because the
1206 backend is returning too much data, try it before starting the
1209 <H4><A name="4.19">4.19</A>) How do I tell what PostgreSQL version
1212 <P>From <I>psql</I>, type <CODE>select version();</CODE></P>
1214 <H4><A name="4.20">4.20</A>) Why does my large-object operations
1215 get <I>"invalid large obj descriptor"</I>?</H4>
1217 <P>You need to put <CODE>BEGIN WORK</CODE> and <CODE>COMMIT</CODE>
1218 around any use of a large object handle, that is, surrounding
1219 <CODE>lo_open</CODE> ... <CODE>lo_close.</CODE></P>
1221 <P>Currently PostgreSQL enforces the rule by closing large object
1222 handles at transaction commit. So the first attempt to do anything
1223 with the handle will draw <I>invalid large obj descriptor</I>. So
1224 code that used to work (at least most of the time) will now
1225 generate that error message if you fail to use a transaction.</P>
1227 <P>If you are using a client interface like <SMALL>ODBC</SMALL> you
1228 may need to set <CODE>auto-commit off.</CODE></P>
1230 <H4><A name="4.21">4.21</A>) How do I create a column that will
1231 default to the current time?</H4>
1233 <P>Use <I>CURRENT_TIMESTAMP</I>:</P>
1235 <CODE>CREATE TABLE test (x int, modtime timestamp DEFAULT CURRENT_TIMESTAMP );
1239 <H4><A name="4.22">4.22</A>) Why are my subqueries using
1240 <CODE><SMALL>IN</SMALL></CODE> so slow?</H4>
1242 <P>Currently, we join subqueries to outer queries by sequentially
1243 scanning the result of the subquery for each row of the outer
1244 query. A workaround is to replace <CODE>IN</CODE> with
1245 <CODE>EXISTS</CODE>:</P>
1249 WHERE col1 IN (SELECT col2 FROM TAB2)
1256 WHERE EXISTS (SELECT col2 FROM TAB2 WHERE col1 = col2)
1259 We hope to fix this limitation in a future release.
1261 <H4><A name="4.23">4.23</A>) How do I perform an outer join?</H4>
1263 <P>PostgreSQL 7.1 and later supports outer joins using the SQL
1264 standard syntax. Here are two examples:</P>
1267 FROM t1 LEFT OUTER JOIN t2 ON (t1.col = t2.col);
1272 FROM t1 LEFT OUTER JOIN t2 USING (col);
1275 <P>These identical queries join t1.col to t2.col, and also return
1276 any unjoined rows in t1 (those with no match in t2). A
1277 <SMALL>RIGHT</SMALL> join would add unjoined rows of t2. A
1278 <SMALL>FULL</SMALL> join would return the matched rows plus all
1279 unjoined rows from t1 and t2. The word <SMALL>OUTER</SMALL> is
1280 optional and is assumed in <SMALL>LEFT</SMALL>,
1281 <SMALL>RIGHT</SMALL>, and <SMALL>FULL</SMALL> joins. Ordinary joins
1282 are called <SMALL>INNER</SMALL> joins.</P>
1284 <P>In previous releases, outer joins can be simulated using
1285 <SMALL>UNION</SMALL> and <SMALL>NOT IN</SMALL>. For example, when
1286 joining <I>tab1</I> and <I>tab2</I>, the following query does an
1287 <I>outer</I> join of the two tables:<BR>
1291 SELECT tab1.col1, tab2.col2
1293 WHERE tab1.col1 = tab2.col1
1295 SELECT tab1.col1, NULL
1297 WHERE tab1.col1 NOT IN (SELECT tab2.col1 FROM tab2)
1301 <H4><A name="4.24">4.24</A>) How do I perform queries using
1302 multiple databases?</H4>
1304 <P>There is no way to query any database except the current one.
1305 Because PostgreSQL loads database-specific system catalogs, it is
1306 uncertain how a cross-database query should even behave.</P>
1308 <P>Of course, a client can make simultaneous connections to
1309 different databases and merge the information that way.</P>
1312 <H4><A name="4.25">4.25</A>) How do I return multiple rows or
1313 columns from a function?</H4>
1315 <P>You can return result sets from PL/pgSQL functions using
1316 <I>refcursors</I>. See <A href=
1317 "http://developer.postgresql.org/docs/postgres/plpgsql-cursors.html">
1318 http://developer.postgresql.org/docs/postgres/plpgsql-cursors.html,</A>
1319 section 23.7.3.3.</P>
1321 <H4><A href="#4.26">4.26</A>) Why can't I reliably create/drop
1322 temporary tables in PL/PgSQL functions?</H4>
1323 PL/PgSQL caches function contents, and an unfortunate side effect
1324 is that if a PL/PgSQL function accesses a temporary table, and that
1325 table is later dropped and recreated, and the function called
1326 again, the function will fail because the cached function contents
1327 still point to the old temporary table. The solution is to use
1328 <SMALL>EXECUTE</SMALL> for temporary table access in PL/PgSQL. This
1329 will cause the query to be reparsed every time.
1331 <H2 align="center">Extending PostgreSQL</H2>
1333 <H4><A name="5.1">5.1</A>) I wrote a user-defined function. When I
1334 run it in <I>psql</I>, why does it dump core?</H4>
1336 <P>The problem could be a number of things. Try testing your
1337 user-defined function in a stand-alone test program first.</P>
1339 <H4><A name="5.2">5.2</A>) How can I contribute some nifty new
1340 types and functions to PostgreSQL?</H4>
1342 <P>Send your extensions to the <I>pgsql-hackers</I> mailing list,
1343 and they will eventually end up in the <I>contrib/</I>
1346 <H4><A name="5.3">5.3</A>) How do I write a C function to return a
1349 <P>This requires wizardry so extreme that the authors have never
1350 tried it, though in principle it can be done.</P>
1352 <H4><A name="5.4">5.4</A>) I have changed a source file. Why does
1353 the recompile not see the change?</H4>
1355 <P>The <I>Makefiles</I> do not have the proper dependencies for
1356 include files. You have to do a <I>make clean</I> and then another
1357 <I>make</I>. If you are using <SMALL>GCC</SMALL> you can use the
1358 <I>--enable-depend</I> option of <I>configure</I> to have the
1359 compiler compute the dependencies automatically.</P>
projects / postgresql / blob