2 Frequently Asked Questions (FAQ) for PostgreSQL
4 Last updated: Fri Apr 25 10:34:17 EDT 2008
6 Current maintainer: Bruce Momjian (bruce@momjian.us)
8 The most recent version of this document can be viewed at
9 http://www.postgresql.org/files/documentation/faqs/FAQ.html.
11 Platform-specific questions are answered at
12 http://www.postgresql.org/docs/faq/.
13 _________________________________________________________________
17 1.1) What is PostgreSQL? How is it pronounced? What is Postgres?
18 1.2) Who controls PostgreSQL?
19 1.3) What is the copyright of PostgreSQL?
20 1.4) What platforms does PostgreSQL support?
21 1.5) Where can I get PostgreSQL?
22 1.6) What is the most recent release?
23 1.7) Where can I get support?
24 1.8) How do I submit a bug report?
25 1.9) How do I find out about known bugs or missing features?
26 1.10) What documentation is available?
27 1.11) How can I learn SQL?
28 1.12) How do I submit a patch or join the development team?
29 1.13) How does PostgreSQL compare to other DBMSs? Can PostgreSQL be
31 1.14) Will PostgreSQL handle recent daylight saving time changes in
33 1.15) How do I unsubscribe from the PostgreSQL email lists? How do I
34 avoid receiving duplicate emails?
38 2.1) What interfaces are available for PostgreSQL?
39 2.2) What tools are available for using PostgreSQL with Web pages?
40 2.3) Does PostgreSQL have a graphical user interface?
42 Administrative Questions
44 3.1) How do I install PostgreSQL somewhere other than
46 3.2) How do I control connections from other hosts?
47 3.3) How do I tune the database engine for better performance?
48 3.4) What debugging features are available?
49 3.5) Why do I get "Sorry, too many clients" when trying to connect?
50 3.6 What is the upgrade process for PostgreSQL?
51 3.7) What computer hardware should I use?
55 4.1) How do I SELECT only the first few rows of a query? A random row?
56 4.2) How do I find out what tables, indexes, databases, and users are
57 defined? How do I see the queries used by psql to display them?
58 4.3) How do you change a column's data type?
59 4.4) What is the maximum size for a row, a table, and a database?
60 4.5) How much database disk space is required to store data from a
62 4.6) Why are my queries slow? Why don't they use my indexes?
63 4.7) How do I see how the query optimizer is evaluating my query?
64 4.8) How do I perform regular expression searches and case-insensitive
65 regular expression searches? How do I use an index for
66 case-insensitive searches?
67 4.9) In a query, how do I detect if a field is NULL? How do I
68 concatenate possible NULLs? How can I sort on whether a field is NULL
70 4.10) What is the difference between the various character types?
71 4.11.1) How do I create a serial/auto-incrementing field?
72 4.11.2) How do I get the value of a SERIAL insert?
73 4.11.3) Doesn't currval() lead to a race condition with other users?
74 4.11.4) Why aren't my sequence numbers reused on transaction abort?
75 Why are there gaps in the numbering of my sequence/SERIAL column?
76 4.12) What is an OID? What is a CTID?
77 4.13) Why do I get the error "ERROR: Memory exhausted in
79 4.14) How do I tell what PostgreSQL version I am running?
80 4.15) How do I create a column that will default to the current time?
81 4.16) How do I perform an outer join?
82 4.17) How do I perform queries using multiple databases?
83 4.18) How do I return multiple rows or columns from a function?
84 4.19) Why do I get "relation with OID ##### does not exist" errors
85 when accessing temporary tables in PL/PgSQL functions?
86 4.20) What replication solutions are available?
87 4.21) Why are my table and column names not recognized in my query?
88 Why is capitalization not preserved?
89 _________________________________________________________________
93 1.1) What is PostgreSQL? How is it pronounced? What is Postgres?
95 PostgreSQL is pronounced Post-Gres-Q-L. (For those curious about how
96 to say "PostgreSQL", an audio file is available.)
98 PostgreSQL is an object-relational database system that has the
99 features of traditional commercial database systems with enhancements
100 to be found in next-generation DBMS systems. PostgreSQL is free and
101 the complete source code is available.
103 PostgreSQL development is performed by a team of mostly volunteer
104 developers spread throughout the world and communicating via the
105 Internet. It is a community project and is not controlled by any
106 company. To get involved, see the developer's FAQ at
107 http://www.postgresql.org/docs/faqs.FAQ_DEV.html
109 Postgres is a widely-used nickname for PostgreSQL. It was the original
110 name of the project at Berkeley and is strongly preferred over other
111 nicknames. If you find 'PostgreSQL' hard to pronounce, call it
114 1.2) Who controls PostgreSQL?
116 If you are looking for a PostgreSQL gatekeeper, central committee, or
117 controlling company, give up --- there isn't one. We do have a core
118 committee and CVS committers, but these groups are more for
119 administrative purposes than control. The project is directed by the
120 community of developers and users, which anyone can join. All you need
121 to do is subscribe to the mailing lists and participate in the
122 discussions. (See the Developer's FAQ for information on how to get
123 involved in PostgreSQL development.)
125 1.3) What is the copyright of PostgreSQL?
127 PostgreSQL is distributed under the classic BSD license. Basically, it
128 allows users to do anything they want with the code, including
129 reselling binaries without the source code. The only restriction is
130 that you not hold us legally liable for problems with the software.
131 There is also the requirement that this copyright appear in all copies
132 of the software. Here is the actual BSD license we use:
134 PostgreSQL Data Base Management System
136 Portions Copyright (c) 1996-2008, PostgreSQL Global Development Group
137 Portions Copyright (c) 1994-1996 Regents of the University of
140 Permission to use, copy, modify, and distribute this software and its
141 documentation for any purpose, without fee, and without a written
142 agreement is hereby granted, provided that the above copyright notice
143 and this paragraph and the following two paragraphs appear in all
146 IN NO EVENT SHALL THE UNIVERSITY OF CALIFORNIA BE LIABLE TO ANY PARTY
147 FOR DIRECT, INDIRECT, SPECIAL, INCIDENTAL, OR CONSEQUENTIAL DAMAGES,
148 INCLUDING LOST PROFITS, ARISING OUT OF THE USE OF THIS SOFTWARE AND
149 ITS DOCUMENTATION, EVEN IF THE UNIVERSITY OF CALIFORNIA HAS BEEN
150 ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
152 THE UNIVERSITY OF CALIFORNIA SPECIFICALLY DISCLAIMS ANY WARRANTIES,
153 INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF
154 MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE. THE SOFTWARE
155 PROVIDED HEREUNDER IS ON AN "AS IS" BASIS, AND THE UNIVERSITY OF
156 CALIFORNIA HAS NO OBLIGATIONS TO PROVIDE MAINTENANCE, SUPPORT,
157 UPDATES, ENHANCEMENTS, OR MODIFICATIONS.
159 1.4) What platforms does PostgreSQL support?
161 In general, any modern Unix-compatible platform should be able to run
162 PostgreSQL. The platforms that had received explicit testing at the
163 time of release are listed in the installation instructions.
165 PostgreSQL also runs natively on Microsoft Windows NT-based operating
166 systems like Win2000 SP4, WinXP, and Win2003. A prepackaged installer
167 is available at http://pgfoundry.org/projects/pginstaller. MSDOS-based
168 versions of Windows (Win95, Win98, WinMe) can run PostgreSQL using
171 There is also a Novell Netware 6 port at http://forge.novell.com, and
172 an OS/2 (eComStation) version at
173 http://hobbes.nmsu.edu/cgi-bin/h-search?sh=1&button=Search&key=postgre
174 SQL&stype=all&sort=type&dir=%2F.
176 1.5) Where can I get PostgreSQL?
178 Via web browser, use http://www.postgresql.org/ftp/, and via ftp, use
179 ftp://ftp.postgresql.org/pub/.
181 1.6) What is the most recent release?
183 The latest release of PostgreSQL is version 8.3.1.
185 We plan to have a major release every year, with minor releases every
188 1.7) Where can I get support?
190 The PostgreSQL community provides assistance to many of its users via
191 email. The main web site to subscribe to the email lists is
192 http://www.postgresql.org/community/lists/. The general or bugs lists
193 are a good place to start.
195 The major IRC channel is #postgresql on Freenode (irc.freenode.net).
196 To connect you can use the Unix program irc -c '#postgresql' "$USER"
197 irc.freenode.net or use any other IRC clients. A Spanish one also
198 exists on the same network, (#postgresql-es), a French one,
199 (#postgresqlfr), and a Brazilian one, (#postgresql-br). There is also
200 a PostgreSQL channel on EFNet.
202 A list of commercial support companies is available at
203 http://www.postgresql.org/support/professional_support.
205 1.8) How do I submit a bug report?
207 Visit the PostgreSQL bug form at
208 http://www.postgresql.org/support/submitbug. Also check out our ftp
209 site ftp://ftp.postgresql.org/pub/ to see if there is a more recent
212 Bugs submitted using the bug form or posted to any PostgreSQL mailing
213 list typically generates one of the following replies:
214 * It is not a bug, and why
215 * It is a known bug and is already on the TODO list
216 * The bug has been fixed in the current release
217 * The bug has been fixed but is not packaged yet in an official
219 * A request is made for more detailed information:
222 + Reproducible test case
223 + Debugging information
224 + Debugger backtrace output
225 * The bug is new. The following might happen:
226 + A patch is created and will be included in the next major or
228 + The bug cannot be fixed immediately and is added to the TODO
231 1.9) How do I find out about known bugs or missing features?
233 PostgreSQL supports an extended subset of SQL:2003. See our TODO list
234 for known bugs, missing features, and future plans.
236 A feature request usually results in one of the following replies:
237 * The feature is already on the TODO list
238 * The feature is not desired because:
239 + It duplicates existing functionality that already follows the
241 + The feature would increase code complexity but add little
243 + The feature would be insecure or unreliable
244 * The new feature is added to the TODO list
246 PostgreSQL does not use a bug tracking system because we find it more
247 efficient to respond directly to email and keep the TODO list
248 up-to-date. In practice, bugs don't last very long in the software,
249 and bugs that affect a large number of users are fixed rapidly. The
250 only place to find all changes, improvements, and fixes in a
251 PostgreSQL release is to read the CVS log messages. Even the release
252 notes do not list every change made to the software.
254 1.10) What documentation is available?
256 PostgreSQL includes extensive documentation, including a large manual,
257 manual pages, and some test examples. See the /doc directory. You can
258 also browse the manuals online at http://www.postgresql.org/docs.
260 There are two PostgreSQL books available online at
261 http://www.postgresql.org/docs/books/awbook.html and
262 http://www.commandprompt.com/ppbook/. There are a number of PostgreSQL
263 books available for purchase. One of the most popular ones is by Korry
264 Douglas. A list of book reviews can be found at
265 http://www.postgresql.org/docs/books/. There is also a collection of
266 PostgreSQL technical articles at
267 http://www.postgresql.org/docs/techdocs.
269 The command line client program psql has some \d commands to show
270 information about types, operators, functions, aggregates, etc. - use
271 \? to display the available commands.
273 Our web site contains even more documentation.
275 1.11) How can I learn SQL?
277 First, consider the PostgreSQL-specific books mentioned above. Many of
278 our users also like The Practical SQL Handbook, Bowman, Judith S., et
279 al., Addison-Wesley. Others like The Complete Reference SQL, Groff et
282 There are also many nice tutorials available online:
283 * http://www.intermedia.net/support/sql/sqltut.shtm
284 * http://sqlcourse.com
285 * http://www.w3schools.com/sql/default.asp
286 * http://mysite.verizon.net/Graeme_Birchall/id1.html
288 1.12) How do I submit a patch or join the development team?
290 See the Developer's FAQ.
292 1.13) How does PostgreSQL compare to other DBMSs? Can PostgreSQL be embedded?
294 There are several ways of measuring software: features, performance,
295 reliability, support, and price.
298 PostgreSQL has most features present in large commercial DBMSs,
299 like transactions, subselects, triggers, views, foreign key
300 referential integrity, and sophisticated locking. We have some
301 features they do not have, like user-defined types,
302 inheritance, rules, and multi-version concurrency control to
303 reduce lock contention.
306 PostgreSQL's performance is comparable to other commercial and
307 open source databases. It is faster for some things, slower for
308 others. Our performance is usually +/-10% compared to other
312 We realize that a DBMS must be reliable, or it is worthless. We
313 strive to release well-tested, stable code that has a minimum
314 of bugs. Each release has at least one month of beta testing,
315 and our release history shows that we can provide stable, solid
316 releases that are ready for production use. We believe we
317 compare favorably to other database software in this area.
320 Our mailing lists provide contact with a large group of
321 developers and users to help resolve any problems encountered.
322 While we cannot guarantee a fix, commercial DBMSs do not always
323 supply a fix either. Direct access to developers, the user
324 community, manuals, and the source code often make PostgreSQL
325 support superior to other DBMSs. There is commercial
326 per-incident support available for those who need it. (See FAQ
330 We are free for all use, both commercial and non-commercial.
331 You can add our code to your product with no limitations,
332 except those outlined in our BSD-style license stated above.
334 PostgreSQL is designed as a client/server architecture, which requires
335 separate processes for each client and server, and various helper
336 processes. Many embedded architectures can support such requirements.
337 However, if your embedded architecture requires the database server to
338 run inside the application process, you cannot use Postgres and should
339 select a lighter-weight database solution.
341 1.14) Will PostgreSQL handle recent daylight saving time changes in various
344 USA daylight saving time changes are included in PostgreSQL release
345 8.0.[4+], and all later major releases, e.g. 8.1. Canada and Western
346 Australia changes are included in 8.0.[10+], 8.1.[6+], and all later
347 major releases. PostgreSQL releases prior to 8.0 use the operating
348 system's timezone database for daylight saving information.
350 1.15) How do I unsubscribe from the PostgreSQL email lists? How do I avoid
351 receiving duplicate emails?
353 The PostgreSQL Majordomo page allows subscribing or unsubscribing from
354 any of the PostgreSQL email lists. (You might need to have your
355 Majordomo password emailed to you to log in.)
357 All PostgreSQL email lists are configured so a group reply goes to the
358 email list and the original email author. This is done so users
359 receive the quickest possible email replies. If you would prefer not
360 to receive duplicate email from the list in cases where you already
361 receive an email directly, check eliminatecc from the Majordomo Change
362 Settings page. You can also prevent yourself from receiving copies of
363 emails you post to the lists by unchecking selfcopy.
364 _________________________________________________________________
366 User Client Questions
368 2.1) What interfaces are available for PostgreSQL?
370 The PostgreSQL install includes only the C and embedded C interfaces.
371 All other interfaces are independent projects that are downloaded
372 separately; being separate allows them to have their own release
373 schedule and development teams.
375 Some programming languages like PHP include an interface to
376 PostgreSQL. Interfaces for languages like Perl, TCL, Python, and many
377 others are available at http://pgfoundry.org.
379 2.2) What tools are available for using PostgreSQL with Web pages?
381 A nice introduction to Database-backed Web pages can be seen at:
382 http://www.webreview.com
384 For Web integration, PHP (http://www.php.net) is an excellent
387 For complex cases, many use the Perl and DBD::Pg with CGI.pm or
390 2.3) Does PostgreSQL have a graphical user interface?
392 There are a large number of GUI Tools that are available for
393 PostgreSQL from both commercial and open source developers. A detailed
394 list can be found in the PostgreSQL Community Documentation
395 _________________________________________________________________
397 Administrative Questions
399 3.1) How do I install PostgreSQL somewhere other than /usr/local/pgsql?
401 Specify the --prefix option when running configure.
403 3.2) How do I control connections from other hosts?
405 By default, PostgreSQL only allows connections from the local machine
406 using Unix domain sockets or TCP/IP connections. Other machines will
407 not be able to connect unless you modify listen_addresses in the
408 postgresql.conf file, enable host-based authentication by modifying
409 the $PGDATA/pg_hba.conf file, and restart the database server.
411 3.3) How do I tune the database engine for better performance?
413 There are three major areas for potential performance improvement:
416 This involves modifying queries to obtain better performance:
418 + Creation of indexes, including expression and partial indexes
419 + Use of COPY instead of multiple INSERTs
420 + Grouping of multiple statements into a single transaction to
421 reduce commit overhead
422 + Use of CLUSTER when retrieving many rows from an index
423 + Use of LIMIT for returning a subset of a query's output
424 + Use of Prepared queries
425 + Use of ANALYZE to maintain accurate optimizer statistics
426 + Regular use of VACUUM or pg_autovacuum
427 + Dropping of indexes during large data changes
430 A number of postgresql.conf settings affect performance. For
431 more details, see Administration Guide/Server Run-time
432 Environment/Run-time Configuration for a full listing, and for
434 http://www.varlena.com/varlena/GeneralBits/Tidbits/annotated_co
436 http://www.varlena.com/varlena/GeneralBits/Tidbits/perf.html.
439 The effect of hardware on performance is detailed in
440 http://www.powerpostgresql.com/PerfList/ and
441 http://momjian.us/main/writings/pgsql/hw_performance/index.html
444 3.4) What debugging features are available?
446 There are many log_* server configuration variables that enable
447 printing of query and process statistics which can be very useful for
448 debugging and performance measurements.
450 3.5) Why do I get "Sorry, too many clients" when trying to connect?
452 You have reached the default limit of 100 database sessions. You need
453 to increase the server's limit on how many concurrent backend
454 processes it can start by changing the max_connections value in
455 postgresql.conf and restarting the server.
457 3.6) What is the upgrade process for PostgreSQL?
459 See http://www.postgresql.org/support/versioning for a general
460 discussion about upgrading, and
461 http://www.postgresql.org/docs/current/static/install-upgrading.html
462 for specific instructions.
464 3.7) What computer hardware should I use?
466 Because PC hardware is mostly compatible, people tend to believe that
467 all PC hardware is of equal quality. It is not. ECC RAM, SCSI, and
468 quality motherboards are more reliable and have better performance
469 than less expensive hardware. PostgreSQL will run on almost any
470 hardware, but if reliability and performance are important it is wise
471 to research your hardware options thoroughly. Our email lists can be
472 used to discuss hardware options and tradeoffs.
473 _________________________________________________________________
475 Operational Questions
477 4.1) How do I SELECT only the first few rows of a query? A random row?
479 To retrieve only a few rows, if you know at the number of rows needed
480 at the time of the SELECT use LIMIT . If an index matches the ORDER BY
481 it is possible the entire query does not have to be executed. If you
482 don't know the number of rows at SELECT time, use a cursor and FETCH.
484 To SELECT a random row, use:
490 4.2) How do I find out what tables, indexes, databases, and users are
491 defined? How do I see the queries used by psql to display them?
493 Use the \dt command to see tables in psql. For a complete list of
494 commands inside psql you can use \?. Alternatively you can read the
495 source code for psql in file pgsql/src/bin/psql/describe.c, it
496 contains SQL commands that generate the output for psql's backslash
497 commands. You can also start psql with the -E option so it will print
498 out the queries it uses to execute the commands you give. PostgreSQL
499 also provides an SQL compliant INFORMATION SCHEMA interface you can
500 query to get information about the database.
502 There are also system tables beginning with pg_ that describe these
505 Use psql -l will list all databases.
507 Also try the file pgsql/src/tutorial/syscat.source. It illustrates
508 many of the SELECTs needed to get information from the database system
511 4.3) How do you change a column's data type?
513 Changing the data type of a column can be done easily in 8.0 and later
514 with ALTER TABLE ALTER COLUMN TYPE.
516 In earlier releases, do this:
518 ALTER TABLE tab ADD COLUMN new_col new_data_type;
519 UPDATE tab SET new_col = CAST(old_col AS new_data_type);
520 ALTER TABLE tab DROP COLUMN old_col;
523 You might then want to do VACUUM FULL tab to reclaim the disk space
524 used by the expired rows.
526 4.4) What is the maximum size for a row, a table, and a database?
528 These are the limits:
530 Maximum size for a database? unlimited (32 TB databases exist)
531 Maximum size for a table? 32 TB
532 Maximum size for a row? 400 GB
533 Maximum size for a field? 1 GB
534 Maximum number of rows in a table? unlimited
535 Maximum number of columns in a table? 250-1600 depending on column
537 Maximum number of indexes on a table? unlimited
539 Of course, these are not actually unlimited, but limited to available
540 disk space and memory/swap space. Performance may suffer when these
541 values get unusually large.
543 The maximum table size of 32 TB does not require large file support
544 from the operating system. Large tables are stored as multiple 1 GB
545 files so file system size limits are not important.
547 The maximum table size, row size, and maximum number of columns can be
548 quadrupled by increasing the default block size to 32k. The maximum
549 table size can also be increased using table partitioning.
551 One limitation is that indexes can not be created on columns longer
552 than about 2,000 characters. Fortunately, such indexes are rarely
553 needed. Uniqueness is best guaranteed by a function index of an MD5
554 hash of the long column, and full text indexing allows for searching
555 of words within the column.
557 4.5) How much database disk space is required to store data from a typical
560 A PostgreSQL database may require up to five times the disk space to
561 store data from a text file.
563 As an example, consider a file of 100,000 lines with an integer and
564 text description on each line. Suppose the text string avergages
565 twenty bytes in length. The flat file would be 2.8 MB. The size of the
566 PostgreSQL database file containing this data can be estimated as 5.2
568 24 bytes: each row header (approximate)
569 24 bytes: one int field and one text field
570 + 4 bytes: pointer on page to tuple
571 ----------------------------------------
574 The data page size in PostgreSQL is 8192 bytes (8 KB), so:
577 ------------------- = 158 rows per database page (rounded down)
581 -------------------- = 633 database pages (rounded up)
584 633 database pages * 8192 bytes per page = 5,185,536 bytes (5.2 MB)
586 Indexes do not require as much overhead, but do contain the data that
587 is being indexed, so they can be large also.
589 NULLs are stored as bitmaps, so they use very little space.
591 4.6) Why are my queries slow? Why don't they use my indexes?
593 Indexes are not used by every query. Indexes are used only if the
594 table is larger than a minimum size, and the query selects only a
595 small percentage of the rows in the table. This is because the random
596 disk access caused by an index scan can be slower than a straight read
597 through the table, or sequential scan.
599 To determine if an index should be used, PostgreSQL must have
600 statistics about the table. These statistics are collected using
601 VACUUM ANALYZE, or simply ANALYZE. Using statistics, the optimizer
602 knows how many rows are in the table, and can better determine if
603 indexes should be used. Statistics are also valuable in determining
604 optimal join order and join methods. Statistics collection should be
605 performed periodically as the contents of the table change.
607 Indexes are normally not used for ORDER BY or to perform joins. A
608 sequential scan followed by an explicit sort is usually faster than an
609 index scan of a large table. However, LIMIT combined with ORDER BY
610 often will use an index because only a small portion of the table is
613 If you believe the optimizer is incorrect in choosing a sequential
614 scan, use SET enable_seqscan TO 'off' and run query again to see if an
615 index scan is indeed faster.
617 When using wild-card operators such as LIKE or ~, indexes can only be
618 used in certain circumstances:
619 * The beginning of the search string must be anchored to the start
621 + LIKE patterns must not start with %.
622 + ~ (regular expression) patterns must start with ^.
623 * The search string can not start with a character class, e.g.
625 * Case-insensitive searches such as ILIKE and ~* do not utilize
626 indexes. Instead, use expression indexes, which are described in
628 * The default C locale must be used during initdb because it is not
629 possible to know the next-greatest character in a non-C locale.
630 You can create a special text_pattern_ops index for such cases
631 that work only for LIKE indexing. It is also possible to use full
632 text indexing for word searches.
634 4.7) How do I see how the query optimizer is evaluating my query?
636 See the EXPLAIN manual page.
638 4.8) How do I perform regular expression searches and case-insensitive
639 regular expression searches? How do I use an index for case-insensitive
642 The ~ operator does regular expression matching, and ~* does
643 case-insensitive regular expression matching. The case-insensitive
644 variant of LIKE is called ILIKE.
646 Case-insensitive equality comparisons are normally expressed as:
649 WHERE lower(col) = 'abc';
651 This will not use an standard index. However, if you create an
652 expression index, it will be used:
653 CREATE INDEX tabindex ON tab (lower(col));
655 If the above index is created as UNIQUE, though the column can store
656 upper and lowercase characters, it can not have identical values that
657 differ only in case. To force a particular case to be stored in the
658 column, use a CHECK constraint or a trigger.
660 4.9) In a query, how do I detect if a field is NULL? How do I concatenate
661 possible NULLs? How can I sort on whether a field is NULL or not?
663 You test the column with IS NULL and IS NOT NULL, like this:
668 To concatentate with possible NULLs, use COALESCE(), like this:
669 SELECT COALESCE(col1, '') || COALESCE(col2, '')
672 To sort by the NULL status, use the IS NULL and IS NOT NULL modifiers
673 in your ORDER BY clause. Things that are true will sort higher than
674 things that are false, so the following will put NULL entries at the
675 top of the resulting list:
678 ORDER BY (col IS NOT NULL)
680 4.10) What is the difference between the various character types?
682 Type Internal Name Notes
683 VARCHAR(n) varchar size specifies maximum length, no padding
684 CHAR(n) bpchar blank padded to the specified fixed length
685 TEXT text no specific upper limit on length
686 BYTEA bytea variable-length byte array (null-byte safe)
687 "char" char one character
689 You will see the internal name when examining system catalogs and in
692 The first four types above are "varlena" types (i.e., the first four
693 bytes on disk are the length, followed by the data). Thus the actual
694 space used is slightly greater than the declared size. However, long
695 values are also subject to compression, so the space on disk might
696 also be less than expected.
697 VARCHAR(n) is best when storing variable-length strings and it limits
698 how long a string can be. TEXT is for strings of unlimited length,
699 with a maximum of one gigabyte.
701 CHAR(n) is for storing strings that are all the same length. CHAR(n)
702 pads with blanks to the specified length, while VARCHAR(n) only stores
703 the characters supplied. BYTEA is for storing binary data,
704 particularly values that include NULL bytes. All the types described
705 here have similar performance characteristics.
707 4.11.1) How do I create a serial/auto-incrementing field?
709 PostgreSQL supports a SERIAL data type. It auto-creates a sequence.
711 CREATE TABLE person (
716 is automatically translated into this:
717 CREATE SEQUENCE person_id_seq;
718 CREATE TABLE person (
719 id INT4 NOT NULL DEFAULT nextval('person_id_seq'),
723 Automatically created sequence are named <table>_<serialcolumn>_seq,
724 where table and serialcolumn are the names of the table and SERIAL
725 column, respectively. See the create_sequence manual page for more
726 information about sequences.
728 4.11.2) How do I get the value of a SERIAL insert?
730 The simplest way is to retrieve the assigned SERIAL value with
731 RETURNING. Using the example table in 4.11.1, it would look like this:
732 INSERT INTO person (name) VALUES ('Blaise Pascal') RETURNING id;
734 You can also call nextval() and use that value in the INSERT, or call
735 currval() after the INSERT.
737 4.11.3) Doesn't currval() lead to a race condition with other users?
739 No. currval() returns the current value assigned by your session, not
742 4.11.4) Why aren't my sequence numbers reused on transaction abort? Why are
743 there gaps in the numbering of my sequence/SERIAL column?
745 To improve concurrency, sequence values are given out to running
746 transactions as needed and are not locked until the transaction
747 completes. This causes gaps in numbering from aborted transactions.
749 4.12) What is an OID? What is a CTID?
751 If a table is created WITH OIDS, each row gets a unique a OID. OIDs
752 are automatically assigned unique 4-byte integers that are unique
753 across the entire installation. However, they overflow at 4 billion,
754 and then the OIDs start being duplicated. PostgreSQL uses OIDs to link
755 its internal system tables together.
757 To uniquely number rows in user tables, it is best to use SERIAL
758 rather than OIDs because SERIAL sequences are unique only within a
759 single table. and are therefore less likely to overflow. SERIAL8 is
760 available for storing eight-byte sequence values.
762 CTIDs are used to identify specific physical rows with block and
763 offset values. CTIDs change after rows are modified or reloaded. They
764 are used by index entries to point to physical rows.
766 4.13) Why do I get the error "ERROR: Memory exhausted in AllocSetAlloc()"?
768 You probably have run out of virtual memory on your system, or your
769 kernel has a low limit for certain resources. Try this before starting
774 Depending on your shell, only one of these may succeed, but it will
775 set your process data segment limit much higher and perhaps allow the
776 query to complete. This command applies to the current process, and
777 all subprocesses created after the command is run. If you are having a
778 problem with the SQL client because the backend is returning too much
779 data, try it before starting the client.
781 4.14) How do I tell what PostgreSQL version I am running?
783 From psql, type SELECT version();
785 4.15) How do I create a column that will default to the current time?
787 Use CURRENT_TIMESTAMP:
788 CREATE TABLE test (x int, modtime TIMESTAMP DEFAULT CURRENT_TIMESTAMP );
790 4.16) How do I perform an outer join?
792 PostgreSQL supports outer joins using the SQL standard syntax. Here
795 FROM t1 LEFT OUTER JOIN t2 ON (t1.col = t2.col);
799 FROM t1 LEFT OUTER JOIN t2 USING (col);
801 These identical queries join t1.col to t2.col, and also return any
802 unjoined rows in t1 (those with no match in t2). A RIGHT join would
803 add unjoined rows of t2. A FULL join would return the matched rows
804 plus all unjoined rows from t1 and t2. The word OUTER is optional and
805 is assumed in LEFT, RIGHT, and FULL joins. Ordinary joins are called
808 4.17) How do I perform queries using multiple databases?
810 There is no way to query a database other than the current one.
811 Because PostgreSQL loads database-specific system catalogs, it is
812 uncertain how a cross-database query should even behave.
814 contrib/dblink allows cross-database queries using function calls. Of
815 course, a client can also make simultaneous connections to different
816 databases and merge the results on the client side.
818 4.18) How do I return multiple rows or columns from a function?
820 It is easy using set-returning functions,
821 http://www.postgresql.org/docs/techdocs.17.
823 4.19) Why do I get "relation with OID ##### does not exist" errors when
824 accessing temporary tables in PL/PgSQL functions?
826 In PostgreSQL versions < 8.3, PL/PgSQL caches function scripts, and an
827 unfortunate side effect is that if a PL/PgSQL function accesses a
828 temporary table, and that table is later dropped and recreated, and
829 the function called again, the function will fail because the cached
830 function contents still point to the old temporary table. The solution
831 is to use EXECUTE for temporary table access in PL/PgSQL. This will
832 cause the query to be reparsed every time.
834 This problem does not occur in PostgreSQL 8.3 and later.
836 4.20) What replication solutions are available?
838 Though "replication" is a single term, there are several technologies
839 for doing replication, with advantages and disadvantages for each.
841 Master/slave replication allows a single master to receive read/write
842 queries, while slaves can only accept read/SELECT queries. The most
843 popular freely available master-slave PostgreSQL replication solution
846 Multi-master replication allows read/write queries to be sent to
847 multiple replicated computers. This capability also has a severe
848 impact on performance due to the need to synchronize changes between
849 servers. PGCluster is the most popular such solution freely available
852 There are also commercial and hardware-based replication solutions
853 available supporting a variety of replication models.
855 4.21) Why are my table and column names not recognized in my query? Why is
856 capitalization not preserved?
858 The most common cause of unrecognized names is the use of
859 double-quotes around table or column names during table creation. When
860 double-quotes are used, table and column names (called identifiers)
861 are stored case-sensitive, meaning you must use double-quotes when
862 referencing the names in a query. Some interfaces, like pgAdmin,
863 automatically double-quote identifiers during table creation. So, for
864 identifiers to be recognized, you must either:
865 * Avoid double-quoting identifiers when creating tables
866 * Use only lowercase characters in identifiers
867 * Double-quote identifiers when referencing them in queries