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853 lines
37 KiB
Plaintext
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Frequently Asked Questions (FAQ) for PostgreSQL
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Last updated: Tue Sep 11 13:34:02 EDT 2007
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Current maintainer: Bruce Momjian (bruce@momjian.us)
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The most recent version of this document can be viewed at
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http://www.postgresql.org/files/documentation/faqs/FAQ.html.
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Platform-specific questions are answered at
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http://www.postgresql.org/docs/faq/.
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_________________________________________________________________
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General Questions
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1.1) What is PostgreSQL? How is it pronounced?
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1.2) Who controls PostgreSQL?
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1.3) What is the copyright of PostgreSQL?
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1.4) What platforms does PostgreSQL support?
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1.5) Where can I get PostgreSQL?
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1.6) What is the most recent release?
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1.7) Where can I get support?
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1.8) How do I submit a bug report?
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1.9) How do I find out about known bugs or missing features?
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1.10) What documentation is available?
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1.11) How can I learn SQL?
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1.12) How do I submit a patch or join the development team?
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1.13) How does PostgreSQL compare to other DBMSs?
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1.14) Will PostgreSQL handle recent daylight saving time changes in
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various countries?
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User Client Questions
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2.1) What interfaces are available for PostgreSQL?
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2.2) What tools are available for using PostgreSQL with Web pages?
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2.3) Does PostgreSQL have a graphical user interface?
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Administrative Questions
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3.1) How do I install PostgreSQL somewhere other than
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/usr/local/pgsql?
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3.2) How do I control connections from other hosts?
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3.3) How do I tune the database engine for better performance?
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3.4) What debugging features are available?
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3.5) Why do I get "Sorry, too many clients" when trying to connect?
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3.6 What is the upgrade process for PostgreSQL?
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3.7) What computer hardware should I use?
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Operational Questions
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4.1) How do I SELECT only the first few rows of a query? A random row?
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4.2) How do I find out what tables, indexes, databases, and users are
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defined? How do I see the queries used by psql to display them?
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4.3) How do you change a column's data type?
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4.4) What is the maximum size for a row, a table, and a database?
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4.5) How much database disk space is required to store data from a
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typical text file?
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4.6) Why are my queries slow? Why don't they use my indexes?
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4.7) How do I see how the query optimizer is evaluating my query?
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4.8) How do I perform regular expression searches and case-insensitive
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regular expression searches? How do I use an index for
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case-insensitive searches?
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4.9) In a query, how do I detect if a field is NULL? How do I
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concatenate possible NULLs? How can I sort on whether a field is NULL
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or not?
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4.10) What is the difference between the various character types?
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4.11.1) How do I create a serial/auto-incrementing field?
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4.11.2) How do I get the value of a SERIAL insert?
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4.11.3) Doesn't currval() lead to a race condition with other users?
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4.11.4) Why aren't my sequence numbers reused on transaction abort?
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Why are there gaps in the numbering of my sequence/SERIAL column?
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4.12) What is an OID? What is a CTID?
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4.13) Why do I get the error "ERROR: Memory exhausted in
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AllocSetAlloc()"?
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4.14) How do I tell what PostgreSQL version I am running?
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4.15) How do I create a column that will default to the current time?
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4.16) How do I perform an outer join?
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4.17) How do I perform queries using multiple databases?
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4.18) How do I return multiple rows or columns from a function?
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4.19) Why do I get "relation with OID ##### does not exist" errors
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when accessing temporary tables in PL/PgSQL functions?
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4.20) What replication solutions are available?
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4.21) Why are my table and column names not recognized in my query?
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Why is capitalization not preserved?
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_________________________________________________________________
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General Questions
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1.1) What is PostgreSQL? How is it pronounced?
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PostgreSQL is pronounced Post-Gres-Q-L, and is also sometimes referred
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to as just Postgres. An audio file is available in MP3 format for
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those would like to hear the pronunciation.
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PostgreSQL is an object-relational database system that has the
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features of traditional commercial database systems with enhancements
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to be found in next-generation DBMS systems. PostgreSQL is free and
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the complete source code is available.
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PostgreSQL development is performed by a team of mostly volunteer
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developers spread throughout the world and communicating via the
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Internet. It is a community project and is not controlled by any
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company. To get involved, see the developer's FAQ at
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http://www.postgresql.org/files/documentation/faqs/FAQ_DEV.html
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1.2) Who controls PostgreSQL?
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If you are looking for a PostgreSQL gatekeeper, central committee, or
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controlling company, give up --- there isn't one. We do have a core
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committee and CVS committers, but these groups are more for
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administrative purposes than control. The project is directed by the
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community of developers and users, which anyone can join. All you need
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to do is subscribe to the mailing lists and participate in the
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discussions. (See the Developer's FAQ for information on how to get
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involved in PostgreSQL development.)
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1.3) What is the copyright of PostgreSQL?
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PostgreSQL is distributed under the classic BSD license. Basically, it
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allows users to do anything they want with the code, including
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reselling binaries without the source code. The only restriction is
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that you not hold us legally liable for problems with the software.
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There is also the requirement that this copyright appear in all copies
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of the software. Here is the actual BSD license we use:
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PostgreSQL Data Base Management System
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Portions Copyright (c) 1996-2007, PostgreSQL Global Development Group
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Portions Copyright (c) 1994-1996 Regents of the University of
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California
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Permission to use, copy, modify, and distribute this software and its
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documentation for any purpose, without fee, and without a written
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agreement is hereby granted, provided that the above copyright notice
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and this paragraph and the following two paragraphs appear in all
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copies.
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IN NO EVENT SHALL THE UNIVERSITY OF CALIFORNIA BE LIABLE TO ANY PARTY
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FOR DIRECT, INDIRECT, SPECIAL, INCIDENTAL, OR CONSEQUENTIAL DAMAGES,
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INCLUDING LOST PROFITS, ARISING OUT OF THE USE OF THIS SOFTWARE AND
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ITS DOCUMENTATION, EVEN IF THE UNIVERSITY OF CALIFORNIA HAS BEEN
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ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
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THE UNIVERSITY OF CALIFORNIA SPECIFICALLY DISCLAIMS ANY WARRANTIES,
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INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF
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MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE. THE SOFTWARE
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PROVIDED HEREUNDER IS ON AN "AS IS" BASIS, AND THE UNIVERSITY OF
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CALIFORNIA HAS NO OBLIGATIONS TO PROVIDE MAINTENANCE, SUPPORT,
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UPDATES, ENHANCEMENTS, OR MODIFICATIONS.
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1.4) What platforms does PostgreSQL support?
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In general, any modern Unix-compatible platform should be able to run
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PostgreSQL. The platforms that had received explicit testing at the
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time of release are listed in the installation instructions.
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PostgreSQL also runs natively on Microsoft Windows NT-based operating
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systems like Win2000 SP4, WinXP, and Win2003. A prepackaged installer
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is available at http://pgfoundry.org/projects/pginstaller. MSDOS-based
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versions of Windows (Win95, Win98, WinMe) can run PostgreSQL using
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Cygwin.
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There is also a Novell Netware 6 port at http://forge.novell.com, and
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an OS/2 (eComStation) version at
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http://hobbes.nmsu.edu/cgi-bin/h-search?sh=1&button=Search&key=postgre
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SQL&stype=all&sort=type&dir=%2F.
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1.5) Where can I get PostgreSQL?
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Via web browser, use http://www.postgresql.org/ftp/, and via ftp, use
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ftp://ftp.PostgreSQL.org/pub/.
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1.6) What is the most recent release?
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The latest release of PostgreSQL is version 8.2.5.
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We plan to have a major release every year, with minor releases every
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few months.
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1.7) Where can I get support?
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The PostgreSQL community provides assistance to many of its users via
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email. The main web site to subscribe to the email lists is
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http://www.postgresql.org/community/lists/. The general or bugs lists
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are a good place to start.
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The major IRC channel is #postgresql on Freenode (irc.freenode.net).
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To connect you can use the Unix program irc -c '#postgresql' "$USER"
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irc.freenode.net or use any other IRC clients. A Spanish one also
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exists on the same network, (#postgresql-es), a French one,
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(#postgresqlfr), and a Brazilian one, (#postgresql-br). There is also
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a PostgreSQL channel on EFNet.
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A list of commercial support companies is available at
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http://www.postgresql.org/support/professional_support.
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1.8) How do I submit a bug report?
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Visit the PostgreSQL bug form at
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http://www.postgresql.org/support/submitbug. Also check out our ftp
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site ftp://ftp.PostgreSQL.org/pub/ to see if there is a more recent
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PostgreSQL version.
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Bugs submitted using the bug form or posted to any PostgreSQL mailing
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list typically generates one of the following replies:
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* It is not a bug, and why
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* It is a known bug and is already on the TODO list
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* The bug has been fixed in the current release
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* The bug has been fixed but is not packaged yet in an official
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release
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* A request is made for more detailed information:
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+ Operating system
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+ PostgreSQL version
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+ Reproducible test case
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+ Debugging information
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+ Debugger backtrace output
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* The bug is new. The following might happen:
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+ A patch is created and will be included in the next major or
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minor release
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+ The bug cannot be fixed immediately and is added to the TODO
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list
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1.9) How do I find out about known bugs or missing features?
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PostgreSQL supports an extended subset of SQL:2003. See our TODO list
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for known bugs, missing features, and future plans.
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A feature request usually results in one of the following replies:
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* The feature is already on the TODO list
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* The feature is not desired because:
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+ It duplicates existing functionality that already follows the
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SQL standard
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+ The feature would increase code complexity but add little
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benefit
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+ The feature would be insecure or unreliable
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* The new feature is added to the TODO list
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PostgreSQL does not use a bug tracking system because we find it more
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efficient to respond directly to email and keep the TODO list
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up-to-date. In practice, bugs don't last very long in the software,
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and bugs that affect a large number of users are fixed rapidly. The
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only place to find all changes, improvements, and fixes in a
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PostgreSQL release is to read the CVS log messages. Even the release
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notes do not list every change made to the software.
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1.10) What documentation is available?
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PostgreSQL includes extensive documentation, including a large manual,
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manual pages, and some test examples. See the /doc directory. You can
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also browse the manuals online at http://www.PostgreSQL.org/docs.
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There are two PostgreSQL books available online at
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http://www.postgresql.org/docs/books/awbook.html and
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http://www.commandprompt.com/ppbook/. There are a number of PostgreSQL
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books available for purchase. One of the most popular ones is by Korry
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Douglas. A list of book reviews can be found at
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http://techdocs.PostgreSQL.org/techdocs/bookreviews.php. There is also
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a collection of PostgreSQL technical articles at
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http://techdocs.PostgreSQL.org/.
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The command line client program psql has some \d commands to show
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information about types, operators, functions, aggregates, etc. - use
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\? to display the available commands.
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Our web site contains even more documentation.
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1.11) How can I learn SQL?
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First, consider the PostgreSQL-specific books mentioned above. Another
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one is "Teach Yourself SQL in 21 Days, Second Edition" at
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http://members.tripod.com/er4ebus/sql/index.htm. Many of our users
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like The Practical SQL Handbook, Bowman, Judith S., et al.,
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Addison-Wesley. Others like The Complete Reference SQL, Groff et al.,
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McGraw-Hill.
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There are also many nice tutorials available online:
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* http://www.intermedia.net/support/sql/sqltut.shtm
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* http://sqlcourse.com
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* http://www.w3schools.com/sql/default.asp
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* http://mysite.verizon.net/Graeme_Birchall/id1.html
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1.12) How do I submit a patch or join the development team?
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See the Developer's FAQ.
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1.13) How does PostgreSQL compare to other DBMSs?
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There are several ways of measuring software: features, performance,
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reliability, support, and price.
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Features
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PostgreSQL has most features present in large commercial DBMSs,
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like transactions, subselects, triggers, views, foreign key
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referential integrity, and sophisticated locking. We have some
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features they do not have, like user-defined types,
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inheritance, rules, and multi-version concurrency control to
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reduce lock contention.
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Performance
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PostgreSQL's performance is comparable to other commercial and
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open source databases. It is faster for some things, slower for
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others. Our performance is usually +/-10% compared to other
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databases.
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Reliability
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We realize that a DBMS must be reliable, or it is worthless. We
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strive to release well-tested, stable code that has a minimum
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of bugs. Each release has at least one month of beta testing,
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and our release history shows that we can provide stable, solid
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releases that are ready for production use. We believe we
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compare favorably to other database software in this area.
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Support
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Our mailing lists provide contact with a large group of
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developers and users to help resolve any problems encountered.
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While we cannot guarantee a fix, commercial DBMSs do not always
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supply a fix either. Direct access to developers, the user
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community, manuals, and the source code often make PostgreSQL
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support superior to other DBMSs. There is commercial
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per-incident support available for those who need it. (See FAQ
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section 1.7.)
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Price
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We are free for all use, both commercial and non-commercial.
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You can add our code to your product with no limitations,
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except those outlined in our BSD-style license stated above.
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1.14) Will PostgreSQL handle recent daylight saving time changes in various
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countries?
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USA daylight saving time changes are included in PostgreSQL release
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8.0.[4+], and all later major releases, e.g. 8.1. Canada and Western
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Australia changes are included in 8.0.[10+], 8.1.[6+], and all later
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major releases. PostgreSQL releases prior to 8.0 use the operating
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system's timezone database for daylight saving information.
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_________________________________________________________________
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User Client Questions
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2.1) What interfaces are available for PostgreSQL?
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The PostgreSQL install includes only the C and embedded C interfaces.
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All other interfaces are independent projects that are downloaded
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separately; being separate allows them to have their own release
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schedule and development teams.
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Some programming languages like PHP include an interface to
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PostgreSQL. Interfaces for languages like Perl, TCL, Python, and many
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others are available at http://gborg.postgresql.org in the
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Drivers/Interfaces section and via Internet search.
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2.2) What tools are available for using PostgreSQL with Web pages?
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A nice introduction to Database-backed Web pages can be seen at:
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http://www.webreview.com
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For Web integration, PHP (http://www.php.net) is an excellent
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interface.
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For complex cases, many use the Perl and DBD::Pg with CGI.pm or
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mod_perl.
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2.3) Does PostgreSQL have a graphical user interface?
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There are a large number of GUI Tools that are available for
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PostgreSQL from both commercial and open source developers. A detailed
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list can be found in the PostgreSQL Community Documentation
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_________________________________________________________________
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Administrative Questions
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3.1) How do I install PostgreSQL somewhere other than /usr/local/pgsql?
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Specify the --prefix option when running configure.
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3.2) How do I control connections from other hosts?
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By default, PostgreSQL only allows connections from the local machine
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using Unix domain sockets or TCP/IP connections. Other machines will
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not be able to connect unless you modify listen_addresses in the
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postgresql.conf file, enable host-based authentication by modifying
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the $PGDATA/pg_hba.conf file, and restart the server.
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3.3) How do I tune the database engine for better performance?
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There are three major areas for potential performance improvement:
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Query Changes
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This involves modifying queries to obtain better performance:
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+ Creation of indexes, including expression and partial indexes
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+ Use of COPY instead of multiple INSERTs
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+ Grouping of multiple statements into a single transaction to
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reduce commit overhead
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+ Use of CLUSTER when retrieving many rows from an index
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+ Use of LIMIT for returning a subset of a query's output
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+ Use of Prepared queries
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+ Use of ANALYZE to maintain accurate optimizer statistics
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+ Regular use of VACUUM or pg_autovacuum
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+ Dropping of indexes during large data changes
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Server Configuration
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A number of postgresql.conf settings affect performance. For
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more details, see Administration Guide/Server Run-time
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Environment/Run-time Configuration for a full listing, and for
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commentary see
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http://www.varlena.com/varlena/GeneralBits/Tidbits/annotated_co
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nf_e.html and
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http://www.varlena.com/varlena/GeneralBits/Tidbits/perf.html.
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Hardware Selection
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The effect of hardware on performance is detailed in
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http://www.powerpostgresql.com/PerfList/ and
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http://momjian.us/main/writings/pgsql/hw_performance/index.html
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.
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3.4) What debugging features are available?
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There are many log_* server configuration variables that enable
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printing of query and process statistics which can be very useful for
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debugging and performance measurements.
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3.5) Why do I get "Sorry, too many clients" when trying to connect?
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You have reached the default limit is 100 database sessions. You need
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to increase the postmaster's limit on how many concurrent backend
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processes it can start by changing the max_connections value in
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postgresql.conf and restarting the postmaster.
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3.6) What is the upgrade process for PostgreSQL?
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See http://www.postgresql.org/support/versioning for a general
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discussion about upgrading, and
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http://www.postgresql.org/docs/current/static/install-upgrading.html
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for specific instructions.
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3.7) What computer hardware should I use?
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Because PC hardware is mostly compatible, people tend to believe that
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all PC hardware is of equal quality. It is not. ECC RAM, SCSI, and
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quality motherboards are more reliable and have better performance
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than less expensive hardware. PostgreSQL will run on almost any
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hardware, but if reliability and performance are important it is wise
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to research your hardware options thoroughly. Our email lists can be
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used to discuss hardware options and tradeoffs.
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_________________________________________________________________
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Operational Questions
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4.1) How do I SELECT only the first few rows of a query? A random row?
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To retrieve only a few rows, if you know at the number of rows needed
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at the time of the SELECT use LIMIT . If an index matches the ORDER BY
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it is possible the entire query does not have to be executed. If you
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don't know the number of rows at SELECT time, use a cursor and FETCH.
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To SELECT a random row, use:
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SELECT col
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FROM tab
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ORDER BY random()
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LIMIT 1;
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4.2) How do I find out what tables, indexes, databases, and users are
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defined? How do I see the queries used by psql to display them?
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Use the \dt command to see tables in psql. For a complete list of
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commands inside psql you can use \?. Alternatively you can read the
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source code for psql in file pgsql/src/bin/psql/describe.c, it
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contains SQL commands that generate the output for psql's backslash
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|
commands. You can also start psql with the -E option so it will print
|
|
out the queries it uses to execute the commands you give. PostgreSQL
|
|
also provides an SQL compliant INFORMATION SCHEMA interface you can
|
|
query to get information about the database.
|
|
|
|
There are also system tables beginning with pg_ that describe these
|
|
too.
|
|
|
|
Use psql -l will list all databases.
|
|
|
|
Also try the file pgsql/src/tutorial/syscat.source. It illustrates
|
|
many of the SELECTs needed to get information from the database system
|
|
tables.
|
|
|
|
4.3) How do you change a column's data type?
|
|
|
|
Changing the data type of a column can be done easily in 8.0 and later
|
|
with ALTER TABLE ALTER COLUMN TYPE.
|
|
|
|
In earlier releases, do this:
|
|
BEGIN;
|
|
ALTER TABLE tab ADD COLUMN new_col new_data_type;
|
|
UPDATE tab SET new_col = CAST(old_col AS new_data_type);
|
|
ALTER TABLE tab DROP COLUMN old_col;
|
|
COMMIT;
|
|
|
|
You might then want to do VACUUM FULL tab to reclaim the disk space
|
|
used by the expired rows.
|
|
|
|
4.4) What is the maximum size for a row, a table, and a database?
|
|
|
|
These are the limits:
|
|
|
|
Maximum size for a database? unlimited (32 TB databases exist)
|
|
Maximum size for a table? 32 TB
|
|
Maximum size for a row? 400 GB
|
|
Maximum size for a field? 1 GB
|
|
Maximum number of rows in a table? unlimited
|
|
Maximum number of columns in a table? 250-1600 depending on column
|
|
types
|
|
Maximum number of indexes on a table? unlimited
|
|
|
|
Of course, these are not actually unlimited, but limited to available
|
|
disk space and memory/swap space. Performance may suffer when these
|
|
values get unusually large.
|
|
|
|
The maximum table size of 32 TB does not require large file support
|
|
from the operating system. Large tables are stored as multiple 1 GB
|
|
files so file system size limits are not important.
|
|
|
|
The maximum table size, row size, and maximum number of columns can be
|
|
quadrupled by increasing the default block size to 32k. The maximum
|
|
table size can also be increased using table partitioning.
|
|
|
|
One limitation is that indexes can not be created on columns longer
|
|
than about 2,000 characters. Fortunately, such indexes are rarely
|
|
needed. Uniqueness is best guaranteed by a function index of an MD5
|
|
hash of the long column, and full text indexing allows for searching
|
|
of words within the column.
|
|
|
|
4.5) How much database disk space is required to store data from a typical
|
|
text file?
|
|
|
|
A PostgreSQL database may require up to five times the disk space to
|
|
store data from a text file.
|
|
|
|
As an example, consider a file of 100,000 lines with an integer and
|
|
text description on each line. Suppose the text string avergages
|
|
twenty bytes in length. The flat file would be 2.8 MB. The size of the
|
|
PostgreSQL database file containing this data can be estimated as 5.2
|
|
MB:
|
|
24 bytes: each row header (approximate)
|
|
24 bytes: one int field and one text field
|
|
+ 4 bytes: pointer on page to tuple
|
|
----------------------------------------
|
|
52 bytes per row
|
|
|
|
The data page size in PostgreSQL is 8192 bytes (8 KB), so:
|
|
|
|
8192 bytes per page
|
|
------------------- = 158 rows per database page (rounded down)
|
|
52 bytes per row
|
|
|
|
100000 data rows
|
|
-------------------- = 633 database pages (rounded up)
|
|
158 rows per page
|
|
|
|
633 database pages * 8192 bytes per page = 5,185,536 bytes (5.2 MB)
|
|
|
|
Indexes do not require as much overhead, but do contain the data that
|
|
is being indexed, so they can be large also.
|
|
|
|
NULLs are stored as bitmaps, so they use very little space.
|
|
|
|
4.6) Why are my queries slow? Why don't they use my indexes?
|
|
|
|
Indexes are not used by every query. Indexes are used only if the
|
|
table is larger than a minimum size, and the query selects only a
|
|
small percentage of the rows in the table. This is because the random
|
|
disk access caused by an index scan can be slower than a straight read
|
|
through the table, or sequential scan.
|
|
|
|
To determine if an index should be used, PostgreSQL must have
|
|
statistics about the table. These statistics are collected using
|
|
VACUUM ANALYZE, or simply ANALYZE. Using statistics, the optimizer
|
|
knows how many rows are in the table, and can better determine if
|
|
indexes should be used. Statistics are also valuable in determining
|
|
optimal join order and join methods. Statistics collection should be
|
|
performed periodically as the contents of the table change.
|
|
|
|
Indexes are normally not used for ORDER BY or to perform joins. A
|
|
sequential scan followed by an explicit sort is usually faster than an
|
|
index scan of a large table. However, LIMIT combined with ORDER BY
|
|
often will use an index because only a small portion of the table is
|
|
returned.
|
|
|
|
If you believe the optimizer is incorrect in choosing a sequential
|
|
scan, use SET enable_seqscan TO 'off' and run query again to see if an
|
|
index scan is indeed faster.
|
|
|
|
When using wild-card operators such as LIKE or ~, indexes can only be
|
|
used in certain circumstances:
|
|
* The beginning of the search string must be anchored to the start
|
|
of the string, i.e.
|
|
+ LIKE patterns must not start with %.
|
|
+ ~ (regular expression) patterns must start with ^.
|
|
* The search string can not start with a character class, e.g.
|
|
[a-e].
|
|
* Case-insensitive searches such as ILIKE and ~* do not utilize
|
|
indexes. Instead, use expression indexes, which are described in
|
|
section 4.8.
|
|
* The default C locale must be used during initdb because it is not
|
|
possible to know the next-greatest character in a non-C locale.
|
|
You can create a special text_pattern_ops index for such cases
|
|
that work only for LIKE indexing.
|
|
|
|
In pre-8.0 releases, indexes often can not be used unless the data
|
|
types exactly match the index's column types. This was particularly
|
|
true of int2, int8, and numeric column indexes.
|
|
|
|
4.7) How do I see how the query optimizer is evaluating my query?
|
|
|
|
See the EXPLAIN manual page.
|
|
|
|
4.8) How do I perform regular expression searches and case-insensitive
|
|
regular expression searches? How do I use an index for case-insensitive
|
|
searches?
|
|
|
|
The ~ operator does regular expression matching, and ~* does
|
|
case-insensitive regular expression matching. The case-insensitive
|
|
variant of LIKE is called ILIKE.
|
|
|
|
Case-insensitive equality comparisons are normally expressed as:
|
|
SELECT *
|
|
FROM tab
|
|
WHERE lower(col) = 'abc';
|
|
|
|
This will not use an standard index. However, if you create an
|
|
expression index, it will be used:
|
|
CREATE INDEX tabindex ON tab (lower(col));
|
|
|
|
If the above index is created as UNIQUE, though the column can store
|
|
upper and lowercase characters, it can not have identical values that
|
|
differ only in case. To force a particular case to be stored in the
|
|
column, use a CHECK constraint or a trigger.
|
|
|
|
4.9) In a query, how do I detect if a field is NULL? How do I concatenate
|
|
possible NULLs? How can I sort on whether a field is NULL or not?
|
|
|
|
You test the column with IS NULL and IS NOT NULL, like this:
|
|
SELECT *
|
|
FROM tab
|
|
WHERE col IS NULL;
|
|
|
|
To concatentate with possible NULLs, use COALESCE(), like this:
|
|
SELECT COALESCE(col1, '') || COALESCE(col2, '')
|
|
FROM tab
|
|
|
|
To sort by the NULL status, use the IS NULL and IS NOT NULL modifiers
|
|
in your ORDER BY clause. Things that are true will sort higher than
|
|
things that are false, so the following will put NULL entries at the
|
|
top of the resulting list:
|
|
SELECT *
|
|
FROM tab
|
|
ORDER BY (col IS NOT NULL)
|
|
|
|
4.10) What is the difference between the various character types?
|
|
|
|
Type Internal Name Notes
|
|
VARCHAR(n) varchar size specifies maximum length, no padding
|
|
CHAR(n) bpchar blank padded to the specified fixed length
|
|
TEXT text no specific upper limit on length
|
|
BYTEA bytea variable-length byte array (null-byte safe)
|
|
"char" char one character
|
|
|
|
You will see the internal name when examining system catalogs and in
|
|
some error messages.
|
|
|
|
The first four types above are "varlena" types (i.e., the first four
|
|
bytes on disk are the length, followed by the data). Thus the actual
|
|
space used is slightly greater than the declared size. However, long
|
|
values are also subject to compression, so the space on disk might
|
|
also be less than expected.
|
|
VARCHAR(n) is best when storing variable-length strings and it limits
|
|
how long a string can be. TEXT is for strings of unlimited length,
|
|
with a maximum of one gigabyte.
|
|
|
|
CHAR(n) is for storing strings that are all the same length. CHAR(n)
|
|
pads with blanks to the specified length, while VARCHAR(n) only stores
|
|
the characters supplied. BYTEA is for storing binary data,
|
|
particularly values that include NULL bytes. All the types described
|
|
here have similar performance characteristics.
|
|
|
|
4.11.1) How do I create a serial/auto-incrementing field?
|
|
|
|
PostgreSQL supports a SERIAL data type. It auto-creates a sequence.
|
|
For example, this:
|
|
CREATE TABLE person (
|
|
id SERIAL,
|
|
name TEXT
|
|
);
|
|
|
|
is automatically translated into this:
|
|
CREATE SEQUENCE person_id_seq;
|
|
CREATE TABLE person (
|
|
id INT4 NOT NULL DEFAULT nextval('person_id_seq'),
|
|
name TEXT
|
|
);
|
|
|
|
See the create_sequence manual page for more information about
|
|
sequences.
|
|
|
|
4.11.2) How do I get the value of a SERIAL insert?
|
|
|
|
One approach is to retrieve the next SERIAL value from the sequence
|
|
object with the nextval() function before inserting and then insert it
|
|
explicitly. Using the example table in 4.11.1, an example in a
|
|
pseudo-language would look like this:
|
|
new_id = execute("SELECT nextval('person_id_seq')");
|
|
execute("INSERT INTO person (id, name) VALUES (new_id, 'Blaise Pascal')");
|
|
|
|
You would then also have the new value stored in new_id for use in
|
|
other queries (e.g., as a foreign key to the person table). Note that
|
|
the name of the automatically created SEQUENCE object will be named
|
|
<table>_< serialcolumn>_seq, where table and serialcolumn are the
|
|
names of your table and your SERIAL column, respectively.
|
|
|
|
Alternatively, you could retrieve the assigned SERIAL value with the
|
|
currval() function after it was inserted by default, e.g.,
|
|
execute("INSERT INTO person (name) VALUES ('Blaise Pascal')");
|
|
new_id = execute("SELECT currval('person_id_seq')");
|
|
|
|
4.11.3) Doesn't currval() lead to a race condition with other users?
|
|
|
|
No. currval() returns the current value assigned by your session, not
|
|
by all sessions.
|
|
|
|
4.11.4) Why aren't my sequence numbers reused on transaction abort? Why are
|
|
there gaps in the numbering of my sequence/SERIAL column?
|
|
|
|
To improve concurrency, sequence values are given out to running
|
|
transactions as needed and are not locked until the transaction
|
|
completes. This causes gaps in numbering from aborted transactions.
|
|
|
|
4.12) What is an OID? What is a CTID?
|
|
|
|
If a table is created WITH OIDS, each row gets a unique a OID. OIDs
|
|
are automatically assigned unique 4-byte integers that are unique
|
|
across the entire installation. However, they overflow at 4 billion,
|
|
and then the OIDs start being duplicated. PostgreSQL uses OIDs to link
|
|
its internal system tables together.
|
|
|
|
To uniquely number rows in user tables, it is best to use SERIAL
|
|
rather than OIDs because SERIAL sequences are unique only within a
|
|
single table. and are therefore less likely to overflow. SERIAL8 is
|
|
available for storing eight-byte sequence values.
|
|
|
|
CTIDs are used to identify specific physical rows with block and
|
|
offset values. CTIDs change after rows are modified or reloaded. They
|
|
are used by index entries to point to physical rows.
|
|
|
|
4.13) Why do I get the error "ERROR: Memory exhausted in AllocSetAlloc()"?
|
|
|
|
You probably have run out of virtual memory on your system, or your
|
|
kernel has a low limit for certain resources. Try this before starting
|
|
postmaster:
|
|
ulimit -d 262144
|
|
limit datasize 256m
|
|
|
|
Depending on your shell, only one of these may succeed, but it will
|
|
set your process data segment limit much higher and perhaps allow the
|
|
query to complete. This command applies to the current process, and
|
|
all subprocesses created after the command is run. If you are having a
|
|
problem with the SQL client because the backend is returning too much
|
|
data, try it before starting the client.
|
|
|
|
4.14) How do I tell what PostgreSQL version I am running?
|
|
|
|
From psql, type SELECT version();
|
|
|
|
4.15) How do I create a column that will default to the current time?
|
|
|
|
Use CURRENT_TIMESTAMP:
|
|
CREATE TABLE test (x int, modtime TIMESTAMP DEFAULT CURRENT_TIMESTAMP );
|
|
|
|
4.16) How do I perform an outer join?
|
|
|
|
PostgreSQL supports outer joins using the SQL standard syntax. Here
|
|
are two examples:
|
|
SELECT *
|
|
FROM t1 LEFT OUTER JOIN t2 ON (t1.col = t2.col);
|
|
|
|
or
|
|
SELECT *
|
|
FROM t1 LEFT OUTER JOIN t2 USING (col);
|
|
|
|
These identical queries join t1.col to t2.col, and also return any
|
|
unjoined rows in t1 (those with no match in t2). A RIGHT join would
|
|
add unjoined rows of t2. A FULL join would return the matched rows
|
|
plus all unjoined rows from t1 and t2. The word OUTER is optional and
|
|
is assumed in LEFT, RIGHT, and FULL joins. Ordinary joins are called
|
|
INNER joins.
|
|
|
|
4.17) How do I perform queries using multiple databases?
|
|
|
|
There is no way to query a database other than the current one.
|
|
Because PostgreSQL loads database-specific system catalogs, it is
|
|
uncertain how a cross-database query should even behave.
|
|
|
|
contrib/dblink allows cross-database queries using function calls. Of
|
|
course, a client can also make simultaneous connections to different
|
|
databases and merge the results on the client side.
|
|
|
|
4.18) How do I return multiple rows or columns from a function?
|
|
|
|
It is easy using set-returning functions,
|
|
http://www.postgresql.org/docs/techdocs.17
|
|
.
|
|
|
|
4.19) Why do I get "relation with OID ##### does not exist" errors when
|
|
accessing temporary tables in PL/PgSQL functions?
|
|
|
|
PL/PgSQL caches function scripts, and an unfortunate side effect is
|
|
that if a PL/PgSQL function accesses a temporary table, and that table
|
|
is later dropped and recreated, and the function called again, the
|
|
function will fail because the cached function contents still point to
|
|
the old temporary table. The solution is to use EXECUTE for temporary
|
|
table access in PL/PgSQL. This will cause the query to be reparsed
|
|
every time.
|
|
|
|
4.20) What replication solutions are available?
|
|
|
|
Though "replication" is a single term, there are several technologies
|
|
for doing replication, with advantages and disadvantages for each.
|
|
|
|
Master/slave replication allows a single master to receive read/write
|
|
queries, while slaves can only accept read/SELECT queries. The most
|
|
popular freely available master-slave PostgreSQL replication solution
|
|
is Slony-I.
|
|
|
|
Multi-master replication allows read/write queries to be sent to
|
|
multiple replicated computers. This capability also has a severe
|
|
impact on performance due to the need to synchronize changes between
|
|
servers. PGCluster is the most popular such solution freely available
|
|
for PostgreSQL.
|
|
|
|
There are also commercial and hardware-based replication solutions
|
|
available supporting a variety of replication models.
|
|
|
|
4.21) Why are my table and column names not recognized in my query? Why is
|
|
capitalization not preserved?
|
|
|
|
The most common cause of recognized names is the use of double-quotes
|
|
around table or column names during table creation. When double-quotes
|
|
are used, table and column names (called identifiers) are stored
|
|
case-sensitive, meaning you must use double-quotes when referencing
|
|
the names in a query. Some interfaces, like pgAdmin, automatically
|
|
double-quote identifiers during table creation. So, for identifiers to
|
|
be recognized, you must either:
|
|
* Avoid double-quoting identifiers when creating tables
|
|
* Use only lowercase characters in identifiers
|
|
* Double-quote identifiers when referencing them in queries
|