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15 <H1>Frequently Asked Questions (FAQ) for PostgreSQL</H1>
17 <P>Last updated: Wed Apr 17 22:48:04 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 or indexes
100 are defined in the database?<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
144 <H2 align="center">Extending PostgreSQL</H2>
145 <A href="#5.1">5.1</A>) I wrote a user-defined function. When I run
146 it in <I>psql</I>, why does it dump core?<BR>
147 <A href="#5.2">5.2</A>) How can I contribute some nifty new types
148 and functions to PostgreSQL?<BR>
149 <A href="#5.3">5.3</A>) How do I write a C function to return a
151 <A href="#5.4">5.4</A>) I have changed a source file. Why does the
152 recompile not see the change?<BR>
156 <H2 align="center">General Questions</H2>
158 <H4><A name="1.1">1.1</A>) What is PostgreSQL?</H4>
160 <P>PostgreSQL is pronounced <I>Post-Gres-Q-L</I>.</P>
162 <P>PostgreSQL is an enhancement of the POSTGRES database management
163 system, a next-generation <SMALL>DBMS</SMALL> research prototype.
164 While PostgreSQL retains the powerful data model and rich data
165 types of POSTGRES, it replaces the PostQuel query language with an
166 extended subset of <SMALL>SQL</SMALL>. PostgreSQL is free and the
167 complete source is available.</P>
169 <P>PostgreSQL development is performed by a team of Internet
170 developers who all subscribe to the PostgreSQL development mailing
171 list. The current coordinator is Marc G. Fournier (<A href=
172 "mailto:scrappy@PostgreSQL.org">scrappy@PostgreSQL.org</A>). (See
173 below on how to join). This team is now responsible for all
174 development of PostgreSQL.</P>
176 <P>The authors of PostgreSQL 1.01 were Andrew Yu and Jolly Chen.
177 Many others have contributed to the porting, testing, debugging,
178 and enhancement of the code. The original Postgres code, from which
179 PostgreSQL is derived, was the effort of many graduate students,
180 undergraduate students, and staff programmers working under the
181 direction of Professor Michael Stonebraker at the University of
182 California, Berkeley.</P>
184 <P>The original name of the software at Berkeley was Postgres. When
185 <SMALL>SQL</SMALL> functionality was added in 1995, its name was
186 changed to Postgres95. The name was changed at the end of 1996 to
189 <H4><A name="1.2">1.2</A>) What is the copyright on
192 <P>PostgreSQL is subject to the following COPYRIGHT:</P>
194 <P>PostgreSQL Data Base Management System</P>
196 <P>Portions copyright (c) 1996-2002, PostgreSQL Global Development
197 Group Portions Copyright (c) 1994-6 Regents of the University of
200 <P>Permission to use, copy, modify, and distribute this software
201 and its documentation for any purpose, without fee, and without a
202 written agreement is hereby granted, provided that the above
203 copyright notice and this paragraph and the following two
204 paragraphs appear in all copies.</P>
206 <P>IN NO EVENT SHALL THE UNIVERSITY OF CALIFORNIA BE LIABLE TO ANY
207 PARTY FOR DIRECT, INDIRECT, SPECIAL, INCIDENTAL, OR CONSEQUENTIAL
208 DAMAGES, INCLUDING LOST PROFITS, ARISING OUT OF THE USE OF THIS
209 SOFTWARE AND ITS DOCUMENTATION, EVEN IF THE UNIVERSITY OF
210 CALIFORNIA HAS BEEN ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.</P>
212 <P>THE UNIVERSITY OF CALIFORNIA SPECIFICALLY DISCLAIMS ANY
213 WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
214 OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE. THE
215 SOFTWARE PROVIDED HEREUNDER IS ON AN "AS IS" BASIS, AND THE
216 UNIVERSITY OF CALIFORNIA HAS NO OBLIGATIONS TO PROVIDE MAINTENANCE,
217 SUPPORT, UPDATES, ENHANCEMENTS, OR MODIFICATIONS.</P>
219 <P>The above is the BSD license, the classic open-source license.
220 It has no restrictions on how the source code may be used. We like
221 it and have no intention of changing it.</P>
223 <H4><A name="1.3">1.3</A>) What Unix platforms does PostgreSQL run
226 <P>In general, a modern Unix-compatible platform should be able to
227 run PostgreSQL. The platforms that had received explicit testing at
228 the time of release are listed in the installation
231 <H4><A name="1.4">1.4</A>) What non-Unix ports are available?</H4>
233 <P><STRONG>Client</STRONG></P>
235 <P>It is possible to compile the <I>libpq</I> C library, psql, and
236 other interfaces and binaries to run on MS Windows platforms. In
237 this case, the client is running on MS Windows, and communicates
238 via TCP/IP to a server running on one of our supported Unix
239 platforms. A file <I>win31.mak</I> is included in the distribution
240 for making a Win32 <I>libpq</I> library and <I>psql</I>. PostgreSQL
241 also communicates with <SMALL>ODBC</SMALL> clients.</P>
243 <P><STRONG>Server</STRONG></P>
245 <P>The database server can run on Windows NT and Win2k using
246 Cygwin, the Cygnus Unix/NT porting library. See
247 <I>pgsql/doc/FAQ_MSWIN</I> in the distribution or the <A href=
248 "http://www.postgresql.org/docs/faq-mswin.html">MS Windows FAQ</A>
249 on our web site. We have no plan to do a native port to any
250 Microsoft platform.</P>
252 <H4><A name="1.5">1.5</A>) Where can I get PostgreSQL?</H4>
254 <P>The primary anonymous ftp site for PostgreSQL is <A href=
255 "ftp://ftp.PostgreSQL.org/pub">ftp://ftp.PostgreSQL.org/pub</A>.
256 For mirror sites, see our main web site.</P>
258 <H4><A name="1.6">1.6</A>) Where can I get support?</H4>
260 <P>The main mailing list is: <A href=
261 "mailto:pgsql-general@PostgreSQL.org">pgsql-general@PostgreSQL.org</A>.
262 It is available for discussion of matters pertaining to PostgreSQL.
263 To subscribe, send mail with the following lines in the body (not
264 the subject line):</P>
271 "mailto:pgsql-general-request@PostgreSQL.org">pgsql-general-request@PostgreSQL.org</A>.</P>
273 <P>There is also a digest list available. To subscribe to this
274 list, send email to: <A href=
275 "mailto:pgsql-general-digest-request@PostgreSQL.org">pgsql-general-digest-request@PostgreSQL.org</A>
281 Digests are sent out to members of this list whenever the main list
282 has received around 30k of messages.
284 <P>The bugs mailing list is available. To subscribe to this list,
285 send email to <A href=
286 "mailto:pgsql-bugs-request@PostgreSQL.org">pgsql-bugs-request@PostgreSQL.org</A>
292 There is also a developers discussion mailing list available. To
293 subscribe to this list, send email to <A href=
294 "mailto:pgsql-hackers-request@PostgreSQL.org">pgsql-hackers-request@PostgreSQL.org</A>
301 <P>Additional mailing lists and information about PostgreSQL can be
302 found via the PostgreSQL WWW home page at:</P>
305 <A href="http://www.PostgreSQL.org">http://www.PostgreSQL.org</A>
308 <P>There is also an IRC channel on EFNet, channel
309 <I>#PostgreSQL</I>. I use the Unix command <CODE>irc -c
310 '#PostgreSQL' "$USER" irc.phoenix.net.</CODE></P>
312 <P>A list of commercial support companies is available at <A href=
313 "http://www.postgresql.org/users-lounge/commercial-support.html">http://www.postgresql.org/users-lounge/commercial-support.html</A>.</P>
315 <H4><A name="1.7">1.7</A>) What is the latest release?</H4>
317 <P>The latest release of PostgreSQL is version 7.2.1.</P>
319 <P>We plan to have major releases every four months.</P>
321 <H4><A name="1.8">1.8</A>) What documentation is available?</H4>
323 <P>Several manuals, manual pages, and some small test examples are
324 included in the distribution. See the <I>/doc</I> directory. You
325 can also browse the manual online at <A href=
326 "http://www.PostgreSQL.org/users-lounge/docs/">http://www.PostgreSQL.org/users-lounge/docs/</A>.</P>
328 <P>There are two PostgreSQL books available online at <A href=
329 "http://www.PostgreSQL.org/docs/awbook.html">http://www.PostgreSQL.org/docs/awbook.html</A>
331 "http://www.commandprompt.com/ppbook/">http://www.commandprompt.com/ppbook/</A>.
332 There is a list of PostgreSQL books available for purchase at <A
334 "http://www.postgresql.org/books/">http://www.postgresql.org/books/</A>.
335 There is also a collection of PostgreSQL technical articles at <A
337 "http://techdocs.postgresql.org/">http://techdocs.postgresql.org/</A>.</P>
339 <P><I>psql</I> has some nice \d commands to show information about
340 types, operators, functions, aggregates, etc.</P>
342 <P>Our web site contains even more documentation.</P>
344 <H4><A name="1.9">1.9</A>) How do I find out about known bugs or
345 missing features?</H4>
347 <P>PostgreSQL supports an extended subset of <SMALL>SQL</SMALL>-92.
348 See our <A href="http://developer.PostgreSQL.org/todo.php">TODO</A>
349 list for known bugs, missing features, and future plans.</P>
351 <H4><A name="1.10">1.10</A>) How can I learn
352 <SMALL>SQL</SMALL>?</H4>
354 <P>The PostgreSQL book at <A href=
355 "http://www.PostgreSQL.org/docs/awbook.html">http://www.PostgreSQL.org/docs/awbook.html</A>
356 teaches <SMALL>SQL</SMALL>. There is another PostgreSQL book at <A
358 "http://www.commandprompt.com/ppbook/">http://www.commandprompt.com/ppbook.</A>
359 There is a nice tutorial at <A href=
360 "http://www.intermedia.net/support/sql/sqltut.shtm">http://www.intermedia.net/support/sql/sqltut.shtm,</A>
362 "http://ourworld.compuserve.com/homepages/graeme_birchall/HTM_COOK.HTM">
363 http://ourworld.compuserve.com/homepages/graeme_birchall/HTM_COOK.HTM,</A>
365 "http://sqlcourse.com/">http://sqlcourse.com.</A></P>
367 <P>Another one is "Teach Yourself SQL in 21 Days, Second Edition"
369 "http://members.tripod.com/er4ebus/sql/index.htm">http://members.tripod.com/er4ebus/sql/index.htm</A></P>
371 <P>Many of our users like <I>The Practical SQL Handbook</I>,
372 Bowman, Judith S., et al., Addison-Wesley. Others like <I>The
373 Complete Reference SQL</I>, Groff et al., McGraw-Hill.</P>
375 <H4><A name="1.11">1.11</A>) Is PostgreSQL Y2K compliant?</H4>
377 <P>Yes, we easily handle dates past the year 2000 AD, and before
380 <H4><A name="1.12">1.12</A>) How do I join the development
383 <P>First, download the latest source and read the PostgreSQL
384 Developers documentation on our web site, or in the distribution.
385 Second, subscribe to the <I>pgsql-hackers</I> and
386 <I>pgsql-patches</I> mailing lists. Third, submit high quality
387 patches to pgsql-patches.</P>
389 <P>There are about a dozen people who have commit privileges to the
390 PostgreSQL <SMALL>CVS</SMALL> archive. They each have submitted so
391 many high-quality patches that it was impossible for the existing
392 committers to keep up, and we had confidence that patches they
393 committed were of high quality.</P>
395 <H4><A name="1.13">1.13</A>) How do I submit a bug report?</H4>
397 <P>Please visit the <A href=
398 "http://www.postgresql.org/bugs/bugs.php">PostgreSQL BugTool</A>
399 page, which gives guidelines and directions on how to submit a
402 <P>Also check out our ftp site <A href=
403 "ftp://ftp.PostgreSQL.org/pub">ftp://ftp.PostgreSQL.org/pub</A> to
404 see if there is a more recent PostgreSQL version or patches.</P>
406 <H4><A name="1.14">1.14</A>) How does PostgreSQL compare to other
407 <SMALL>DBMS</SMALL>s?</H4>
409 <P>There are several ways of measuring software: features,
410 performance, reliability, support, and price.</P>
413 <DT><B>Features</B></DT>
415 <DD>PostgreSQL has most features present in large commercial
416 <SMALL>DBMS</SMALL>s, like transactions, subselects, triggers,
417 views, foreign key referential integrity, and sophisticated
418 locking. We have some features they do not have, like
419 user-defined types, inheritance, rules, and multi-version
420 concurrency control to reduce lock contention.<BR>
424 <DT><B>Performance</B></DT>
426 <DD>PostgreSQL runs in two modes. Normal <I>fsync</I> mode
427 flushes every completed transaction to disk, guaranteeing that if
428 the OS crashes or loses power in the next few seconds, all your
429 data is safely stored on disk. In this mode, we are slower than
430 most commercial databases, partly because few of them do such
431 conservative flushing to disk in their default modes. In
432 <I>no-fsync</I> mode, we are usually faster than commercial
433 databases, though in this mode, an OS crash could cause data
434 corruption. We are working to provide an intermediate mode that
435 suffers less performance overhead than full fsync mode, and will
436 allow data integrity within 30 seconds of an OS crash.<BR>
438 In comparison to MySQL or leaner database systems, we are slower
439 on inserts/updates because we have transaction overhead. Of
440 course, MySQL does not have any of the features mentioned in the
441 <I>Features</I> section above. We are built for flexibility and
442 features, though we continue to improve performance through
443 profiling and source code analysis. There is an interesting Web
444 page comparing PostgreSQL to MySQL at <A href=
445 "http://openacs.org/why-not-mysql.html">http://openacs.org/why-not-mysql.html</A><BR>
448 We handle each user connection by creating a Unix process.
449 Backend processes share data buffers and locking information.
450 With multiple CPUs, multiple backends can easily run on different
455 <DT><B>Reliability</B></DT>
457 <DD>We realize that a <SMALL>DBMS</SMALL> must be reliable, or it
458 is worthless. We strive to release well-tested, stable code that
459 has a minimum of bugs. Each release has at least one month of
460 beta testing, and our release history shows that we can provide
461 stable, solid releases that are ready for production use. We
462 believe we compare favorably to other database software in this
467 <DT><B>Support</B></DT>
469 <DD>Our mailing list provides a large group of developers and
470 users to help resolve any problems encountered. While we can not
471 guarantee a fix, commercial <SMALL>DBMS</SMALL>s do not always
472 supply a fix either. Direct access to developers, the user
473 community, manuals, and the source code often make PostgreSQL
474 support superior to other <SMALL>DBMS</SMALL>s. There is
475 commercial per-incident support available for those who need it.
476 (See <A href="#1.6">support FAQ item</A>.)<BR>
480 <DT><B>Price</B></DT>
482 <DD>We are free for all use, both commercial and non-commercial.
483 You can add our code to your product with no limitations, except
484 those outlined in our BSD-style license stated above.<BR>
489 <H4><A name="1.15">1.15</A>) How can I financially assist
492 <P>PostgreSQL has had a first-class infrastructure since we started
493 six years ago. This is all thanks to Marc Fournier, who has created
494 and managed this infrastructure over the years.</P>
496 <P>Quality infrastructure is very important to an open-source
497 project. It prevents disruptions that can greatly delay forward
498 movement of the project.</P>
500 <P>Of course, this infrastructure is not cheap. There are a variety
501 of monthly and one-time expenses that are required to keep it
502 going. If you or your company has money it can donate to help fund
503 this effort, please go to <A href=
504 "http://www.pgsql.com/pg_goodies">http://www.pgsql.com/pg_goodies</A>
505 and make a donation.</P>
507 <P>Although the web page mentions PostgreSQL, Inc, the
508 "contributions" item is solely to support the PostgreSQL project
509 and does not fund any specific company. If you prefer, you can also
510 send a check to the contact address.</P>
513 <H2 align="center">User Client Questions</H2>
515 <H4><A name="2.1">2.1</A>) Are there <SMALL>ODBC</SMALL> drivers
518 <P>There are two <SMALL>ODBC</SMALL> drivers available, PsqlODBC
519 and OpenLink <SMALL>ODBC</SMALL>.</P>
521 <P>PsqlODBC is included in the distribution. More information about
522 it can be gotten from <A href=
523 "ftp://ftp.PostgreSQL.org/pub/odbc/">ftp://ftp.PostgreSQL.org/pub/odbc/</A>.</P>
525 <P>OpenLink <SMALL>ODBC</SMALL> can be gotten from <A href=
526 "http://www.openlinksw.com/">http://www.openlinksw.com</A>. It
527 works with their standard <SMALL>ODBC</SMALL> client software so
528 you'll have PostgreSQL <SMALL>ODBC</SMALL> available on every
529 client platform they support (Win, Mac, Unix, VMS).</P>
531 <P>They will probably be selling this product to people who need
532 commercial-quality support, but a freeware version will always be
533 available. Please send questions to <A href=
534 "mailto:postgres95@openlink.co.uk">postgres95@openlink.co.uk</A>.</P>
536 <P>See also the <A href=
537 "http://www.postgresql.org/devel-corner/docs/programmer/odbc.html">ODBC
538 chapter of the Programmer's Guide</A>.</P>
540 <H4><A name="2.2">2.2</A>) What tools are available for using
541 PostgreSQL with Web pages?</H4>
543 <P>A nice introduction to Database-backed Web pages can be seen at:
544 <A href="http://www.webreview.com">http://www.webreview.com</A></P>
546 <P>There is also one at <A href=
547 "http://www.phone.net/home/mwm/hotlist/">http://www.phone.net/home/mwm/hotlist/.</A></P>
549 <P>For Web integration, PHP is an excellent interface. It is at <A
550 href="http://www.php.net">http://www.php.net</A>.</P>
552 <P>For complex cases, many use the Perl interface and CGI.pm.</P>
554 <H4><A name="2.3">2.3</A>) Does PostgreSQL have a graphical user
555 interface? A report generator? An embedded query language
558 <P>We have a nice graphical user interface called <I>pgaccess</I>,
559 which is shipped as part of the distribution. <I>pgaccess</I> also
560 has a report generator. The Web page is <A href=
561 "http://www.flex.ro/pgaccess">http://www.flex.ro/pgaccess</A></P>
563 <P>We also include <I>ecpg</I>, which is an embedded SQL query
564 language interface for C.</P>
566 <H4><A name="2.4">2.4</A>) What languages are available to
567 communicate with PostgreSQL?</H4>
574 <LI>C++ (libpq++)</LI>
576 <LI>Embedded C (ecpg)</LI>
580 <LI>Perl (perl5)</LI>
584 <LI>Python (PyGreSQL)</LI>
586 <LI>TCL (libpgtcl)</LI>
588 <LI>C Easy API (libpgeasy)</LI>
590 <LI>Embedded <SMALL>HTML</SMALL> (<A href=
591 "http://www.php.net">PHP from http://www.php.net</A>)</LI>
595 <H2 align="center">Administrative Questions</H2>
597 <H4><A name="3.1">3.1</A>) How do I install PostgreSQL somewhere
598 other than <I>/usr/local/pgsql</I>?</H4>
600 <P>Specify the <I>--prefix</I> option when running
601 <I>configure</I>.</P>
603 <H4><A name="3.2">3.2</A>) When I start <I>postmaster</I>, I get a
604 <I>Bad System Call</I> or core dumped message. Why?</H4>
606 <P>It could be a variety of problems, but first check to see that
607 you have System V extensions installed in your kernel. PostgreSQL
608 requires kernel support for shared memory and semaphores.</P>
610 <H4><A name="3.3">3.3</A>) When I try to start <I>postmaster</I>, I
611 get <I>IpcMemoryCreate</I> errors. Why?</H4>
613 <P>You either do not have shared memory configured properly in your
614 kernel or you need to enlarge the shared memory available in the
615 kernel. The exact amount you need depends on your architecture and
616 how many buffers and backend processes you configure for
617 <I>postmaster</I>. For most systems, with default numbers of
618 buffers and processes, you need a minimum of ~1 MB. See the <A
620 "http://www.postgresql.org/idocs/index.php?kernel-resources.html">PostgreSQL
621 Administrator's Guide</A> for more detailed information about
622 shared memory and semaphores.</P>
624 <H4><A name="3.4">3.4</A>) When I try to start <I>postmaster</I>, I
625 get <I>IpcSemaphoreCreate</I> errors. Why?</H4>
627 <P>If the error message is <I>IpcSemaphoreCreate: semget failed (No
628 space left on device)</I> then your kernel is not configured with
629 enough semaphores. Postgres needs one semaphore per potential
630 backend process. A temporary solution is to start <I>postmaster</I>
631 with a smaller limit on the number of backend processes. Use
632 <I>-N</I> with a parameter less than the default of 32. A more
633 permanent solution is to increase your kernel's
634 <SMALL>SEMMNS</SMALL> and <SMALL>SEMMNI</SMALL> parameters.</P>
636 <P>Inoperative semaphores can also cause crashes during heavy
639 <P>If the error message is something else, you might not have
640 semaphore support configured in your kernel at all. See the
641 PostgreSQL Administrator's Guide for more detailed information
642 about shared memory and semaphores.</P>
644 <H4><A name="3.5">3.5</A>) How do I control connections from other
647 <P>By default, PostgreSQL only allows connections from the local
648 machine using Unix domain sockets. Other machines will not be able
649 to connect unless you add the <I>-i</I> flag to <I>postmaster</I>,
650 <B>and</B> enable host-based authentication by modifying the file
651 <I>$PGDATA/pg_hba.conf</I> accordingly. This will allow TCP/IP
654 <H4><A name="3.6">3.6</A>) How do I tune the database engine for
655 better performance?</H4>
657 <P>Certainly, indexes can speed up queries. The
658 <SMALL>EXPLAIN</SMALL> command allows you to see how PostgreSQL is
659 interpreting your query, and which indexes are being used.</P>
661 <P>If you are doing many <SMALL>INSERTs</SMALL>, consider doing
662 them in a large batch using the <SMALL>COPY</SMALL> command. This
663 is much faster than individual <SMALL>INSERTS.</SMALL> Second,
664 statements not in a <SMALL>BEGIN WORK/COMMIT</SMALL> transaction
665 block are considered to be in their own transaction. Consider
666 performing several statements in a single transaction block. This
667 reduces the transaction overhead. Also, consider dropping and
668 recreating indexes when making large data changes.</P>
670 <P>There are several tuning options. You can disable <I>fsync()</I>
671 by starting <I>postmaster</I> with a <I>-o -F</I> option. This will
672 prevent <I>fsync()</I>s from flushing to disk after every
675 <P>You can also use the <I>postmaster</I> <I>-B</I> option to
676 increase the number of shared memory buffers used by the backend
677 processes. If you make this parameter too high, the
678 <I>postmaster</I> may not start because you have exceeded your
679 kernel's limit on shared memory space. Each buffer is 8K and the
680 default is 64 buffers.</P>
682 <P>You can also use the backend <I>-S</I> option to increase the
683 maximum amount of memory used by the backend process for temporary
684 sorts. The <I>-S</I> value is measured in kilobytes, and the
685 default is 512 (i.e. 512K).</P>
687 <P>You can also use the <SMALL>CLUSTER</SMALL> command to group
688 data in tables to match an index. See the <SMALL>CLUSTER</SMALL>
689 manual page for more details.</P>
691 <H4><A name="3.7">3.7</A>) What debugging features are
694 <P>PostgreSQL has several features that report status information
695 that can be valuable for debugging purposes.</P>
697 <P>First, by running <I>configure</I> with the --enable-cassert
698 option, many <I>assert()</I>s monitor the progress of the backend
699 and halt the program when something unexpected occurs.</P>
701 <P>Both <I>postmaster</I> and <I>postgres</I> have several debug
702 options available. First, whenever you start <I>postmaster</I>,
703 make sure you send the standard output and error to a log file,
707 ./bin/postmaster >server.log 2>&1 &
710 <P>This will put a server.log file in the top-level PostgreSQL
711 directory. This file contains useful information about problems or
712 errors encountered by the server. <I>Postmaster</I> has a <I>-d</I>
713 option that allows even more detailed information to be reported.
714 The <I>-d</I> option takes a number that specifies the debug level.
715 Be warned that high debug level values generate large log
718 <P>If <I>postmaster</I> is not running, you can actually run the
719 <I>postgres</I> backend from the command line, and type your
720 <SMALL>SQL</SMALL> statement directly. This is recommended
721 <B>only</B> for debugging purposes. Note that a newline terminates
722 the query, not a semicolon. If you have compiled with debugging
723 symbols, you can use a debugger to see what is happening. Because
724 the backend was not started from <I>postmaster</I>, it is not
725 running in an identical environment and locking/backend interaction
726 problems may not be duplicated.</P>
728 <P>If <I>postmaster</I> is running, start <I>psql</I> in one
729 window, then find the <SMALL>PID</SMALL> of the <I>postgres</I>
730 process used by <I>psql</I>. Use a debugger to attach to the
731 <I>postgres</I> <SMALL>PID.</SMALL> You can set breakpoints in the
732 debugger and issue queries from <I>psql</I>. If you are debugging
733 <I>postgres</I> startup, you can set PGOPTIONS="-W n", then start
734 <I>psql</I>. This will cause startup to delay for <I>n</I> seconds
735 so you can attach to the process with the debugger, set any
736 breakpoints, and continue through the startup sequence.</P>
738 <P>The <I>postgres</I> program has <I>-s, -A</I>, and <I>-t</I>
739 options that can be very useful for debugging and performance
742 <P>You can also compile with profiling to see what functions are
743 taking execution time. The backend profile files will be deposited
744 in the <I>pgsql/data/base/dbname</I> directory. The client profile
745 file will be put in the client's current directory. Linux requires
746 a compile with <I>-DLINUX_PROFILE</I> for proper profiling.</P>
748 <H4><A name="3.8">3.8</A>) Why do I get <I>"Sorry, too many
749 clients"</I> when trying to connect?</H4>
751 <P>You need to increase <I>postmaster</I>'s limit on how many
752 concurrent backend processes it can start.</P>
754 <P>The default limit is 32 processes. You can increase it by
755 restarting <I>postmaster</I> with a suitable <I>-N</I> value or
756 modifying <I>postgresql.conf</I>.</P>
758 <P>Note that if you make <I>-N</I> larger than 32, you must also
759 increase <I>-B</I> beyond its default of 64; <I>-B</I> must be at
760 least twice <I>-N</I>, and probably should be more than that for
761 best performance. For large numbers of backend processes, you are
762 also likely to find that you need to increase various Unix kernel
763 configuration parameters. Things to check include the maximum size
764 of shared memory blocks, <SMALL>SHMMAX;</SMALL> the maximum number
765 of semaphores, <SMALL>SEMMNS</SMALL> and <SMALL>SEMMNI;</SMALL> the
766 maximum number of processes, <SMALL>NPROC;</SMALL> the maximum
767 number of processes per user, <SMALL>MAXUPRC;</SMALL> and the
768 maximum number of open files, <SMALL>NFILE</SMALL> and
769 <SMALL>NINODE.</SMALL> The reason that PostgreSQL has a limit on
770 the number of allowed backend processes is so your system won't run
771 out of resources.</P>
773 <P>In PostgreSQL versions prior to 6.5, the maximum number of
774 backends was 64, and changing it required a rebuild after altering
775 the MaxBackendId constant in
776 <I>include/storage/sinvaladt.h</I>.</P>
778 <H4><A name="3.9">3.9</A>) What are the <I>pg_sorttempNNN.NN</I>
779 files in my database directory?</H4>
781 <P>They are temporary files generated by the query executor. For
782 example, if a sort needs to be done to satisfy an <SMALL>ORDER
783 BY,</SMALL> and the sort requires more space than the backend's
784 <I>-S</I> parameter allows, then temporary files are created to
785 hold the extra data.</P>
787 <P>The temporary files should be deleted automatically, but might
788 not if a backend crashes during a sort. If you have no backends
789 running at the time, it is safe to delete the pg_tempNNN.NN
793 <H2 align="center">Operational Questions</H2>
795 <H4><A name="4.1">4.1</A>) What is the difference between binary
796 cursors and normal cursors?</H4>
798 <P>See the <SMALL>DECLARE</SMALL> manual page for a
801 <H4><A name="4.2">4.2</A>) How do I <SMALL>SELECT</SMALL> only the
802 first few rows of a query?</H4>
804 <P>See the <SMALL>FETCH</SMALL> manual page, or use
805 <SMALL>SELECT</SMALL> ... <SMALL>LIMIT</SMALL>....</P>
807 <P>The entire query may have to be evaluated, even if you only want
808 the first few rows. Consider a query that has an <SMALL>ORDER
809 BY.</SMALL> If there is an index that matches the <SMALL>ORDER
810 BY</SMALL>, PostgreSQL may be able to evaluate only the first few
811 records requested, or the entire query may have to be evaluated
812 until the desired rows have been generated.</P>
814 <H4><A name="4.3">4.3</A>) How do I get a list of tables or other
815 things I can see in <I>psql</I>?</H4>
817 <P>You can read the source code for <I>psql</I> in file
818 <I>pgsql/src/bin/psql/describe.c</I>. It contains
819 <SMALL>SQL</SMALL> commands that generate the output for psql's
820 backslash commands. You can also start <I>psql</I> with the
821 <I>-E</I> option so it will print out the queries it uses to
822 execute the commands you give.</P>
824 <H4><A name="4.4">4.4</A>) How do you remove a column from a
827 <P>We do not support <SMALL>ALTER TABLE DROP COLUMN,</SMALL> but do
830 SELECT ... -- select all columns but the one you want to remove
833 DROP TABLE old_table;
834 ALTER TABLE new_table RENAME TO old_table;
837 <H4><A name="4.5">4.5</A>) What is the maximum size for a row, a
838 table, and a database?</H4>
840 <P>These are the limits:</P>
842 Maximum size for a database? unlimited (500 GB databases exist)
843 Maximum size for a table? 16 TB
844 Maximum size for a row? unlimited in 7.1 and later
845 Maximum size for a field? 1 GB in 7.1 and later
846 Maximum number of rows in a table? unlimited
847 Maximum number of columns in a table? 250-1600 depending on column types
848 Maximum number of indexes on a table? unlimited
850 Of course, these are not actually unlimited, but limited to
851 available disk space and memory/swap space. Performance may suffer
852 when these values get unusually large.
854 <P>The maximum table size of 16 TB does not require large file
855 support from the operating system. Large tables are stored as
856 multiple 1 GB files so file system size limits are not
859 <P>The maximum table size and maximum number of columns can be
860 increased if the default block size is increased to 32k.</P>
862 <H4><A name="4.6">4.6</A>) How much database disk space is required
863 to store data from a typical text file?</H4>
865 <P>A PostgreSQL database may require up to five times the disk
866 space to store data from a text file.</P>
868 <P>As an example, consider a file of 100,000 lines with an integer
869 and text description on each line. Suppose the text string
870 avergages twenty bytes in length. The flat file would be 2.8 MB.
871 The size of the PostgreSQL database file containing this data can
872 be estimated as 6.4 MB:</P>
874 36 bytes: each row header (approximate)
875 24 bytes: one int field and one text filed
876 + 4 bytes: pointer on page to tuple
877 ----------------------------------------
880 The data page size in PostgreSQL is 8192 bytes (8 KB), so:
883 ------------------- = 128 rows per database page (rounded down)
887 -------------------- = 782 database pages (rounded up)
890 782 database pages * 8192 bytes per page = 6,406,144 bytes (6.4 MB)
893 <P>Indexes do not require as much overhead, but do contain the data
894 that is being indexed, so they can be large also.</P>
896 <H4><A name="4.7">4.7</A>) How do I find out what tables or indexes
897 are defined in the database?</H4>
899 <P><I>psql</I> has a variety of backslash commands to show such
900 information. Use \? to see them.</P>
902 <P>Also try the file <I>pgsql/src/tutorial/syscat.source</I>. It
903 illustrates many of the <SMALL>SELECT</SMALL>s needed to get
904 information from the database system tables.</P>
906 <H4><A name="4.8">4.8</A>) My queries are slow or don't make use of
907 the indexes. Why?</H4>
908 Indexes are not automatically used by every query. Indexes are only
909 used if the table is larger than a minimum size, and the query
910 selects only a small percentage of the rows in the table. This is
911 because the random disk access caused by an index scan is sometimes
912 slower than a straight read through the table, or sequential scan.
914 <P>To determine if an index should be used, PostgreSQL must have
915 statistics about the table. These statistics are collected using
916 <SMALL>VACUUM ANALYZE</SMALL>, or simply <SMALL>ANALYZE</SMALL>.
917 Using statistics, the optimizer knows how many rows are in the
918 table, and can better determine if indexes should be used.
919 Statistics are also valuable in determining optimal join order and
920 join methods. Statistics collection should be performed
921 periodically as the contents of the table change.</P>
923 <P>Indexes are normally not used for <SMALL>ORDER BY</SMALL> or to
924 perform joins. A sequential scan followed by an explicit sort is
925 usually faster than an index scan of a large table.</P>
926 However, <SMALL>LIMIT</SMALL> combined with <SMALL>ORDER BY</SMALL>
927 often will use an index because only a small portion of the table
930 <P>When using wild-card operators such as <SMALL>LIKE</SMALL> or
931 <I>~</I>, indexes can only be used if the beginning of the search
932 is anchored to the start of the string. Therefore, to use indexes,
933 <SMALL>LIKE</SMALL> patterns must not start with <I>%</I>, and
934 <I>~</I>(regular expression) patterns must start with <I>^</I>.</P>
936 <H4><A name="4.9">4.9</A>) How do I see how the query optimizer is
937 evaluating my query?</H4>
939 <P>See the <SMALL>EXPLAIN</SMALL> manual page.</P>
941 <H4><A name="4.10">4.10</A>) What is an R-tree index?</H4>
943 <P>An R-tree index is used for indexing spatial data. A hash index
944 can't handle range searches. A B-tree index only handles range
945 searches in a single dimension. R-trees can handle
946 multi-dimensional data. For example, if an R-tree index can be
947 built on an attribute of type <I>point</I>, the system can more
948 efficiently answer queries such as "select all points within a
949 bounding rectangle."</P>
951 <P>The canonical paper that describes the original R-tree design
954 <P>Guttman, A. "R-trees: A Dynamic Index Structure for Spatial
955 Searching." Proceedings of the 1984 ACM SIGMOD Int'l Conf on Mgmt
958 <P>You can also find this paper in Stonebraker's "Readings in
959 Database Systems".</P>
961 <P>Built-in R-trees can handle polygons and boxes. In theory,
962 R-trees can be extended to handle higher number of dimensions. In
963 practice, extending R-trees requires a bit of work and we don't
964 currently have any documentation on how to do it.</P>
966 <H4><A name="4.11">4.11</A>) What is the Genetic Query
969 <P>The <SMALL>GEQO</SMALL> module speeds query optimization when
970 joining many tables by means of a Genetic Algorithm (GA). It allows
971 the handling of large join queries through nonexhaustive
974 <H4><A name="4.12">4.12</A>) How do I perform regular expression
975 searches and case-insensitive regular expression searches? How do I
976 use an index for case-insensitive searches?</H4>
978 <P>The <I>~</I> operator does regular expression matching, and
979 <I>~*</I> does case-insensitive regular expression matching. The
980 case-insensitive variant of <SMALL>LIKE</SMALL> is called
981 <SMALL>ILIKE</SMALL> in PostgreSQL 7.1 and later.</P>
983 <P>Case-insensitive equality comparisons are normally expressed
988 WHERE lower(col) = 'abc'
991 This will not use an standard index. However, if you create a
992 functional index, it will be used:
994 CREATE INDEX tabindex on tab (lower(col));
998 <H4><A name="4.13">4.13</A>) In a query, how do I detect if a field
999 is <SMALL>NULL</SMALL>?</H4>
1001 <P>You test the column with <SMALL>IS NULL</SMALL> and <SMALL>IS
1002 NOT NULL</SMALL>.</P>
1004 <H4><A name="4.14">4.14</A>) What is the difference between the
1005 various character types?</H4>
1007 Type Internal Name Notes
1008 --------------------------------------------------
1009 "char" char 1 character
1010 CHAR(#) bpchar blank padded to the specified fixed length
1011 VARCHAR(#) varchar size specifies maximum length, no padding
1012 TEXT text no specific upper limit on length
1013 BYTEA bytea variable-length byte array (null-byte safe)
1016 <P>You will see the internal name when examining system catalogs
1017 and in some error messages.</P>
1019 <P>The last four types above are "varlena" types (i.e., the first
1020 four bytes on disk are the length, followed by the data). Thus the
1021 actual space used is slightly greater than the declared size.
1022 However, these data types are also subject to compression or being
1023 stored out-of-line by <SMALL>TOAST</SMALL>, so the space on disk
1024 might also be less than expected.</P>
1026 <P><SMALL>CHAR()</SMALL> is best when storing strings that are
1027 usually the same length. <SMALL>VARCHAR()</SMALL> is best when
1028 storing variable-length strings but it limits how long a string can
1029 be. <SMALL>TEXT</SMALL> is for strings of unlimited length, maximum
1030 1 gigabyte. <SMALL>BYTEA</SMALL> is for storing binary data,
1031 particularly values that include <SMALL>NULL</SMALL> bytes.</P>
1033 <H4><A name="4.15.1">4.15.1</A>) How do I create a
1034 serial/auto-incrementing field?</H4>
1036 <P>PostgreSQL supports a <SMALL>SERIAL</SMALL> data type. It
1037 auto-creates a sequence and index on the column. For example,
1040 CREATE TABLE person (
1045 is automatically translated into this:
1047 CREATE SEQUENCE person_id_seq;
1048 CREATE TABLE person (
1049 id INT4 NOT NULL DEFAULT nextval('person_id_seq'),
1052 CREATE UNIQUE INDEX person_id_key ON person ( id );
1054 See the <I>create_sequence</I> manual page for more information
1055 about sequences. You can also use each row's <I>OID</I> field as a
1056 unique value. However, if you need to dump and reload the database,
1057 you need to use <I>pg_dump</I>'s <I>-o</I> option or <SMALL>COPY
1058 WITH OIDS</SMALL> option to preserve the <SMALL>OID</SMALL>s.
1060 <H4><A name="4.15.2">4.15.2</A>) How do I get the value of a
1061 <SMALL>SERIAL</SMALL> insert?</H4>
1063 <P>One approach is to retrieve the next <SMALL>SERIAL</SMALL> value
1064 from the sequence object with the <I>nextval()</I> function
1065 <I>before</I> inserting and then insert it explicitly. Using the
1066 example table in <A href="#4.15.1">4.15.1</A>, that might look like
1069 new_id = output of "SELECT nextval('person_id_seq')"
1070 INSERT INTO person (id, name) VALUES (new_id, 'Blaise Pascal');
1072 You would then also have the new value stored in
1073 <CODE>new_id</CODE> for use in other queries (e.g., as a foreign
1074 key to the <CODE>person</CODE> table). Note that the name of the
1075 automatically created <SMALL>SEQUENCE</SMALL> object will be named
1076 <<I>table</I>>_<<I>serialcolumn</I>>_<I>seq</I>, where
1077 <I>table</I> and <I>serialcolumn</I> are the names of your table
1078 and your <SMALL>SERIAL</SMALL> column, respectively.
1080 <P>Alternatively, you could retrieve the assigned
1081 <SMALL>SERIAL</SMALL> value with the <I>currval</I>() function
1082 <I>after</I> it was inserted by default, e.g.,</P>
1084 INSERT INTO person (name) VALUES ('Blaise Pascal');
1085 new_id = output of "SELECT currval('person_id_seq')";
1087 Finally, you could use the <A href="#4.16"><SMALL>OID</SMALL></A>
1088 returned from the <SMALL>INSERT</SMALL> statement to look up the
1089 default value, though this is probably the least portable approach.
1090 In Perl, using DBI with Edmund Mergl's DBD::Pg module, the oid
1091 value is made available via <I>$sth->{pg_oid_status}</I> after
1092 <I>$sth->execute()</I>.
1094 <H4><A name="4.15.3">4.15.3</A>) Don't <I>currval()</I> and
1095 <I>nextval()</I> lead to a race condition with other users?</H4>
1097 <P>No. Currval() returns the current value assigned by your
1098 backend, not by all users.</P>
1100 <H4><A name="4.15.4">4.15.4</A>) Why aren't my sequence numbers
1101 reused on transaction abort? Why are there gaps in the numbering of
1102 my sequence/SERIAL column?</H4>
1104 <P>To improve concurrency, sequence values are given out to running
1105 transactions as needed and are not locked until the transaction
1106 completes. This causes gaps in numbering from aborted
1109 <H4><A name="4.16">4.16</A>) What is an <SMALL>OID</SMALL>? What is
1110 a <SMALL>TID</SMALL>?</H4>
1112 <P><SMALL>OID</SMALL>s are PostgreSQL's answer to unique row ids.
1113 Every row that is created in PostgreSQL gets a unique
1114 <SMALL>OID</SMALL>. All <SMALL>OID</SMALL>s generated during
1115 <I>initdb</I> are less than 16384 (from
1116 <I>backend/access/transam.h</I>). All user-created
1117 <SMALL>OID</SMALL>s are equal to or greater than this. By default,
1118 all these <SMALL>OID</SMALL>s are unique not only within a table or
1119 database, but unique within the entire PostgreSQL installation.</P>
1121 <P>PostgreSQL uses <SMALL>OID</SMALL>s in its internal system
1122 tables to link rows between tables. These <SMALL>OID</SMALL>s can
1123 be used to identify specific user rows and used in joins. It is
1124 recommended you use column type <SMALL>OID</SMALL> to store
1125 <SMALL>OID</SMALL> values. You can create an index on the
1126 <SMALL>OID</SMALL> field for faster access.</P>
1128 <P>O<SMALL>ID</SMALL>s are assigned to all new rows from a central
1129 area that is used by all databases. If you want to change the
1130 <SMALL>OID</SMALL> to something else, or if you want to make a copy
1131 of the table, with the original <SMALL>OID</SMALL>s, there is no
1132 reason you can't do it:</P>
1134 CREATE TABLE new_table(old_oid oid, mycol int);
1135 SELECT old_oid, mycol INTO new FROM old;
1136 COPY new TO '/tmp/pgtable';
1138 COPY new WITH OIDS FROM '/tmp/pgtable';
1140 CREATE TABLE new_table (mycol int);
1141 INSERT INTO new_table (oid, mycol) SELECT oid, mycol FROM old_table;
1145 <P>O<SMALL>ID</SMALL>s are stored as 4-byte integers, and will
1146 overflow at 4 billion. No one has reported this ever happening, and
1147 we plan to have the limit removed before anyone does.</P>
1149 <P>T<SMALL>ID</SMALL>s are used to identify specific physical rows
1150 with block and offset values. T<SMALL>ID</SMALL>s change after rows
1151 are modified or reloaded. They are used by index entries to point
1152 to physical rows.</P>
1154 <H4><A name="4.17">4.17</A>) What is the meaning of some of the
1155 terms used in PostgreSQL?</H4>
1157 <P>Some of the source code and older documentation use terms that
1158 have more common usage. Here are some:</P>
1161 <LI>table, relation, class</LI>
1163 <LI>row, record, tuple</LI>
1165 <LI>column, field, attribute</LI>
1167 <LI>retrieve, select</LI>
1169 <LI>replace, update</LI>
1171 <LI>append, insert</LI>
1173 <LI><SMALL>OID</SMALL>, serial value</LI>
1175 <LI>portal, cursor</LI>
1177 <LI>range variable, table name, table alias</LI>
1180 <P>A list of general database terms can be found at: <A href=
1181 "http://www.comptechnews.com/~reaster/dbdesign.html">http://www.comptechnews.com/~reaster/dbdesign.html</A></P>
1183 <H4><A name="4.18">4.18</A>) Why do I get the error <I>"ERROR:
1184 Memory exhausted in AllocSetAlloc()"</I>?</H4>
1186 <P>If you are running a version older than 7.1, an upgrade may fix
1187 the problem. Also it is possible you have run out of virtual memory
1188 on your system, or your kernel has a low limit for certain
1189 resources. Try this before starting <I>postmaster</I>:</P>
1194 Depending on your shell, only one of these may succeed, but it will
1195 set your process data segment limit much higher and perhaps allow
1196 the query to complete. This command applies to the current process,
1197 and all subprocesses created after the command is run. If you are
1198 having a problem with the <SMALL>SQL</SMALL> client because the
1199 backend is returning too much data, try it before starting the
1202 <H4><A name="4.19">4.19</A>) How do I tell what PostgreSQL version
1205 <P>From <I>psql</I>, type <CODE>select version();</CODE></P>
1207 <H4><A name="4.20">4.20</A>) Why does my large-object operations
1208 get <I>"invalid large obj descriptor"</I>?</H4>
1210 <P>You need to put <CODE>BEGIN WORK</CODE> and <CODE>COMMIT</CODE>
1211 around any use of a large object handle, that is, surrounding
1212 <CODE>lo_open</CODE> ... <CODE>lo_close.</CODE></P>
1214 <P>Currently PostgreSQL enforces the rule by closing large object
1215 handles at transaction commit. So the first attempt to do anything
1216 with the handle will draw <I>invalid large obj descriptor</I>. So
1217 code that used to work (at least most of the time) will now
1218 generate that error message if you fail to use a transaction.</P>
1220 <P>If you are using a client interface like <SMALL>ODBC</SMALL> you
1221 may need to set <CODE>auto-commit off.</CODE></P>
1223 <H4><A name="4.21">4.21</A>) How do I create a column that will
1224 default to the current time?</H4>
1226 <P>Use <I>CURRENT_TIMESTAMP</I>:</P>
1228 <CODE>CREATE TABLE test (x int, modtime timestamp DEFAULT CURRENT_TIMESTAMP );
1232 <H4><A name="4.22">4.22</A>) Why are my subqueries using
1233 <CODE><SMALL>IN</SMALL></CODE> so slow?</H4>
1235 <P>Currently, we join subqueries to outer queries by sequentially
1236 scanning the result of the subquery for each row of the outer
1237 query. A workaround is to replace <CODE>IN</CODE> with
1238 <CODE>EXISTS</CODE>:</P>
1242 WHERE col1 IN (SELECT col2 FROM TAB2)
1249 WHERE EXISTS (SELECT col2 FROM TAB2 WHERE col1 = col2)
1252 We hope to fix this limitation in a future release.
1254 <H4><A name="4.23">4.23</A>) How do I perform an outer join?</H4>
1256 <P>PostgreSQL 7.1 and later supports outer joins using the SQL
1257 standard syntax. Here are two examples:</P>
1260 FROM t1 LEFT OUTER JOIN t2 ON (t1.col = t2.col);
1265 FROM t1 LEFT OUTER JOIN t2 USING (col);
1268 <P>These identical queries join t1.col to t2.col, and also return
1269 any unjoined rows in t1 (those with no match in t2). A
1270 <SMALL>RIGHT</SMALL> join would add unjoined rows of t2. A
1271 <SMALL>FULL</SMALL> join would return the matched rows plus all
1272 unjoined rows from t1 and t2. The word <SMALL>OUTER</SMALL> is
1273 optional and is assumed in <SMALL>LEFT</SMALL>,
1274 <SMALL>RIGHT</SMALL>, and <SMALL>FULL</SMALL> joins. Ordinary joins
1275 are called <SMALL>INNER</SMALL> joins.</P>
1277 <P>In previous releases, outer joins can be simulated using
1278 <SMALL>UNION</SMALL> and <SMALL>NOT IN</SMALL>. For example, when
1279 joining <I>tab1</I> and <I>tab2</I>, the following query does an
1280 <I>outer</I> join of the two tables:<BR>
1284 SELECT tab1.col1, tab2.col2
1286 WHERE tab1.col1 = tab2.col1
1288 SELECT tab1.col1, NULL
1290 WHERE tab1.col1 NOT IN (SELECT tab2.col1 FROM tab2)
1294 <H4><A name="4.24">4.24</A>) How do I perform queries using
1295 multiple databases?</H4>
1297 <P>There is no way to query any database except the current one.
1298 Because PostgreSQL loads database-specific system catalogs, it is
1299 uncertain how a cross-database query should even behave.</P>
1301 <P>Of course, a client can make simultaneous connections to
1302 different databases and merge the information that way.</P>
1305 <H4><A name="4.25">4.25</A>) How do I return multiple rows or columns
1306 from a function?</H4>
1308 <P>You can return result sets from PL/pgSQL functions using
1309 <i>refcursors</i>. See <a
1310 href="http://developer.postgresql.org/docs/postgres/plpgsql-cursors.html">
1311 http://developer.postgresql.org/docs/postgres/plpgsql-cursors.html,</a>
1312 section 23.7.3.3.</P>
1315 <H2 align="center">Extending PostgreSQL</H2>
1317 <H4><A name="5.1">5.1</A>) I wrote a user-defined function. When I
1318 run it in <I>psql</I>, why does it dump core?</H4>
1320 <P>The problem could be a number of things. Try testing your
1321 user-defined function in a stand-alone test program first.</P>
1323 <H4><A name="5.2">5.2</A>) How can I contribute some nifty new
1324 types and functions to PostgreSQL?</H4>
1326 <P>Send your extensions to the <I>pgsql-hackers</I> mailing list,
1327 and they will eventually end up in the <I>contrib/</I>
1330 <H4><A name="5.3">5.3</A>) How do I write a C function to return a
1333 <P>This requires wizardry so extreme that the authors have never
1334 tried it, though in principle it can be done.</P>
1336 <H4><A name="5.4">5.4</A>) I have changed a source file. Why does
1337 the recompile not see the change?</H4>
1339 <P>The <I>Makefiles</I> do not have the proper dependencies for
1340 include files. You have to do a <I>make clean</I> and then another
1341 <I>make</I>. If you are using <SMALL>GCC</SMALL> you can use the
1342 <I>--enable-depend</I> option of <I>configure</I> to have the
1343 compiler compute the dependencies automatically.</P>
projects / postgresql / blob