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doc/FAQ
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doc/FAQ
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Frequently Asked Questions (FAQ) for PostgreSQL
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Last updated: Sat Apr 23 14:56:41 EDT 2005
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Last updated: Sat Apr 23 14:59:01 EDT 2005
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Current maintainer: Bruce Momjian (pgman@candle.pha.pa.us)
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@ -94,8 +94,8 @@
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General Questions
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1.1) What is PostgreSQL? How is it pronounced?
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1.1) What is PostgreSQL? How is it pronounced?
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PostgreSQL is pronounced Post-Gres-Q-L, also called just Postgres.
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PostgreSQL is an object-relational database system that has the
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@ -109,8 +109,8 @@
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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) What is the copyright of PostgreSQL?
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1.2) What is the copyright of PostgreSQL?
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PostgreSQL is distributed under the classic BSD license. It has no
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restrictions on how the source code can be used. We like it and have
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no intention of changing it.
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@ -142,8 +142,8 @@
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CALIFORNIA HAS NO OBLIGATIONS TO PROVIDE MAINTENANCE, SUPPORT,
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UPDATES, ENHANCEMENTS, OR MODIFICATIONS.
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1.3) What platforms does PostgreSQL support?
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1.3) 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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@ -159,13 +159,13 @@
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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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1.4) Where can I get PostgreSQL?
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1.4) 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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1.5) Where can I get support?
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1.5) 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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@ -180,23 +180,23 @@
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A list of commercial support companies is available at
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http://techdocs.postgresql.org/companies.php.
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1.6) How do I submit a bug report?
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1.6) 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.
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Also check out our ftp site ftp://ftp.PostgreSQL.org/pub/ to see if
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there is a more recent PostgreSQL version.
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1.7) What is the latest release?
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1.7) What is the latest release?
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The latest release of PostgreSQL is version 8.0.2.
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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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1.8) What documentation is available?
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1.8) 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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@ -215,13 +215,13 @@
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Our web site contains even more documentation.
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1.9) How do I find out about known bugs or missing features?
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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-92. See our TODO list
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for known bugs, missing features, and future plans.
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1.10) How can I learn SQL?
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1.10) 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
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@ -232,12 +232,12 @@
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http://ourworld.compuserve.com/homepages/graeme_birchall/HTM_COOK.HTM,
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and at http://sqlcourse.com.
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1.11) How do I join the development team?
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1.11) How do I join the development team?
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See the Developer's FAQ.
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1.12) How does PostgreSQL compare to other DBMSs?
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1.12) 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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@ -281,8 +281,8 @@
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User Client Questions
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2.1) What interfaces are available for PostgreSQL?
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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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@ -293,8 +293,8 @@
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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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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 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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2.3) Does PostgreSQL have a graphical user interface?
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Yes, see http://techdocs.postgresql.org/guides/GUITools for a detailed
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list.
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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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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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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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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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http://candle.pha.pa.us/main/writings/pgsql/hw_performance/inde
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x.html and http://www.powerpostgresql.com/PerfList/.
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3.4) What debugging features are available?
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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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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) Why do I need to do a dump and restore to upgrade between major
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PostgreSQL releases?
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3.6) Why do I need to do a dump and restore to upgrade between major
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PostgreSQL releases?
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The PostgreSQL team makes only small changes between minor releases,
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so upgrading from 7.4.0 to 7.4.1 does not require a dump and restore.
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However, major releases (e.g. from 7.3 to 7.4) often change the
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@ -380,8 +380,8 @@
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files. A dump outputs data in a generic format that can then be loaded
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in using the new internal format.
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3.7) What computer hardware should I use?
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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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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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4.1) 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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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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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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many of the SELECTs needed to get information from the database system
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tables.
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4.3) How do you change a column's data type?
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4.3) How do you change a column's data type?
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Changing the data type of a column can be done easily in 8.0 and later
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with ALTER TABLE ALTER COLUMN TYPE.
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You might then want to do VACUUM FULL tab to reclaim the disk space
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used by the expired rows.
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4.4) What is the maximum size for a row, a table, and a database?
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4.4) What is the maximum size for a row, a table, and a database?
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These are the limits:
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Maximum size for a database? unlimited (32 TB databases exist)
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The maximum table size and maximum number of columns can be quadrupled
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by increasing the default block size to 32k.
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4.5) How much database disk space is required to store data from a typical
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text file?
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4.5) How much database disk space is required to store data from a typical
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text file?
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A PostgreSQL database may require up to five times the disk space to
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store data from a text file.
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NULLs are stored as bitmaps, so they use very little space.
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4.6) Why are my queries slow? Why don't they use my indexes?
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4.6) Why are my queries slow? Why don't they use my indexes?
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Indexes are not used by every query. Indexes are used only if the
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table is larger than a minimum size, and the query selects only a
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small percentage of the rows in the table. This is because the random
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types exactly match the index's column types. This was particularly
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true of int2, int8, and numeric column indexes.
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4.7) How do I see how the query optimizer is evaluating my query?
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4.7) How do I see how the query optimizer is evaluating my query?
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See the EXPLAIN manual page.
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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 case-insensitive
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searches?
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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 case-insensitive
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searches?
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The ~ operator does regular expression matching, and ~* does
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case-insensitive regular expression matching. The case-insensitive
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variant of LIKE is called ILIKE.
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expresssion index, it will be used:
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CREATE INDEX tabindex ON tab (lower(col));
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4.9) In a query, how do I detect if a field is NULL? How can I sort on
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whether a field is NULL or not?
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4.9) In a query, how do I detect if a field is NULL? How can I sort on
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whether a field is NULL or not?
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You test the column with IS NULL and IS NOT NULL.
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SELECT *
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FROM tab
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FROM tab
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ORDER BY (col IS NOT NULL)
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4.10) What is the difference between the various character types?
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4.10) What is the difference between the various character types?
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Type Internal Name Notes
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VARCHAR(n) varchar size specifies maximum length, no padding
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CHAR(n) bpchar blank padded to the specified fixed length
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particularly values that include NULL bytes. All the types described
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here have similar performance characteristics.
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4.11.1) How do I create a serial/auto-incrementing field?
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4.11.1) How do I create a serial/auto-incrementing field?
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PostgreSQL supports a SERIAL data type. It auto-creates a sequence.
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For example, this:
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CREATE TABLE person (
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See the create_sequence manual page for more information about
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sequences.
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4.11.2) How do I get the value of a SERIAL insert?
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4.11.2) How do I get the value of a SERIAL insert?
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One approach is to retrieve the next SERIAL value from the sequence
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object with the nextval() function before inserting and then insert it
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explicitly. Using the example table in 4.11.1, an example in a
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execute("INSERT INTO person (name) VALUES ('Blaise Pascal')");
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new_id = execute("SELECT currval('person_id_seq')");
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4.11.3) Doesn't currval() lead to a race condition with other users?
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4.11.3) Doesn't currval() lead to a race condition with other users?
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No. currval() returns the current value assigned by your session, not
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by all sessions.
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4.11.4) Why aren't my sequence numbers reused on transaction abort? Why are
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there gaps in the numbering of my sequence/SERIAL column?
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4.11.4) Why aren't my sequence numbers reused on transaction abort? Why are
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there gaps in the numbering of my sequence/SERIAL column?
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To improve concurrency, sequence values are given out to running
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transactions as needed and are not locked until the transaction
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completes. This causes gaps in numbering from aborted transactions.
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4.12) What is an OID? What is a TID?
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4.12) What is an OID? What is a TID?
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Every row that is created in PostgreSQL gets a unique OID unless
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created WITHOUT OIDS. OIDs are autotomatically assigned unique 4-byte
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integers that are unique across the entire installation. However, they
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values. TIDs change after rows are modified or reloaded. They are used
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by index entries to point to physical rows.
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4.13) Why do I get the error "ERROR: Memory exhausted in AllocSetAlloc()"?
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4.13) Why do I get the error "ERROR: Memory exhausted in AllocSetAlloc()"?
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You probably have run out of virtual memory on your system, or your
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kernel has a low limit for certain resources. Try this before starting
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postmaster:
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problem with the SQL client because the backend is returning too much
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data, try it before starting the client.
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4.14) How do I tell what PostgreSQL version I am running?
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4.14) How do I tell what PostgreSQL version I am running?
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From psql, type SELECT version();
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4.15) Why does my large-object operations get "invalid large obj
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descriptor"?
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4.15) Why does my large-object operations get "invalid large obj descriptor"?
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You need to put BEGIN WORK and COMMIT around any use of a large object
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handle, that is, surrounding lo_open ... lo_close.
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If you are using a client interface like ODBC you may need to set
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auto-commit off.
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4.16) How do I create a column that will default to the current time?
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4.16) How do I create a column that will default to the current time?
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Use CURRENT_TIMESTAMP:
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CREATE TABLE test (x int, modtime TIMESTAMP DEFAULT CURRENT_TIMESTAMP );
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4.17) How do I perform an outer join?
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4.17) How do I perform an outer join?
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PostgreSQL supports outer joins using the SQL standard syntax. Here
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are two examples:
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SELECT *
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is assumed in LEFT, RIGHT, and FULL joins. Ordinary joins are called
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INNER joins.
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4.18) How do I perform queries using multiple databases?
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4.18) How do I perform queries using multiple databases?
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There is no way to query a database other than the current one.
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Because PostgreSQL loads database-specific system catalogs, it is
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uncertain how a cross-database query should even behave.
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@ -751,15 +750,15 @@
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course, a client can also make simultaneous connections to different
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databases and merge the results on the client side.
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4.19) How do I return multiple rows or columns from a function?
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4.19) How do I return multiple rows or columns from a function?
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It is easy using set-returning functions,
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http://techdocs.postgresql.org/guides/SetReturningFunctions
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.
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4.20) Why do I get "relation with OID ##### does not exist" errors when
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accessing temporary tables in PL/PgSQL functions?
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4.20) Why do I get "relation with OID ##### does not exist" errors when
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accessing temporary tables in PL/PgSQL functions?
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PL/PgSQL caches function scripts, and an unfortunate side effect is
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that if a PL/PgSQL function accesses a temporary table, and that table
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is later dropped and recreated, and the function called again, the
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table access in PL/PgSQL. This will cause the query to be reparsed
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every time.
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4.21) What encryption options are available?
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4.21) What encryption options are available?
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|
||||
* contrib/pgcrypto contains many encryption functions for use in SQL
|
||||
queries.
|
||||
* To encrypt transmission from the client to the server, the server
|
||||
@ -785,28 +784,27 @@
|
||||
|
||||
Extending PostgreSQL
|
||||
|
||||
5.1) I wrote a user-defined function. When I run it in psql, why does it
|
||||
dump core?
|
||||
|
||||
5.1) I wrote a user-defined function. When I run it in psql, why does it dump
|
||||
core?
|
||||
|
||||
The problem could be a number of things. Try testing your user-defined
|
||||
function in a stand-alone test program first.
|
||||
|
||||
5.2) How can I contribute some nifty new types and functions to PostgreSQL?
|
||||
|
||||
5.2) How can I contribute some nifty new types and functions to PostgreSQL?
|
||||
|
||||
Send your extensions to the pgsql-hackers mailing list, and they will
|
||||
eventually end up in the contrib/ subdirectory.
|
||||
|
||||
5.3) How do I write a C function to return a tuple?
|
||||
|
||||
5.3) How do I write a C function to return a tuple?
|
||||
|
||||
In versions of PostgreSQL beginning with 7.3, table-returning
|
||||
functions are fully supported in C, PL/PgSQL, and SQL. See the
|
||||
Programmer's Guide for more information. An example of a
|
||||
table-returning function defined in C can be found in
|
||||
contrib/tablefunc.
|
||||
|
||||
5.4) I have changed a source file. Why does the recompile not see the
|
||||
change?
|
||||
|
||||
5.4) I have changed a source file. Why does the recompile not see the change?
|
||||
|
||||
The Makefiles do not have the proper dependencies for include files.
|
||||
You have to do a make clean and then another make. If you are using
|
||||
GCC you can use the --enable-depend option of configure to have the
|
||||
|
||||
@ -10,7 +10,7 @@
|
||||
alink="#0000ff">
|
||||
<H1>Frequently Asked Questions (FAQ) for PostgreSQL</H1>
|
||||
|
||||
<P>Last updated: Sat Apr 23 14:56:41 EDT 2005</P>
|
||||
<P>Last updated: Sat Apr 23 14:59:01 EDT 2005</P>
|
||||
|
||||
<P>Current maintainer: Bruce Momjian (<A href=
|
||||
"mailto:pgman@candle.pha.pa.us">pgman@candle.pha.pa.us</A>)
|
||||
@ -133,7 +133,7 @@
|
||||
|
||||
<H2 align="center">General Questions</H2>
|
||||
|
||||
<H4><A name="1.1">1.1</A>) What is PostgreSQL? How is it pronounced?</H4>
|
||||
<H3><A name="1.1">1.1</A>) What is PostgreSQL? How is it pronounced?</H3>
|
||||
|
||||
<P>PostgreSQL is pronounced <I>Post-Gres-Q-L</I>, also called just
|
||||
<I>Postgres</I>.</P>
|
||||
@ -152,8 +152,8 @@
|
||||
http://www.postgresql.org/files/documentation/faqs/FAQ_DEV.html</A>
|
||||
</P>
|
||||
|
||||
<H4><A name="1.2">1.2</A>) What is the copyright of
|
||||
PostgreSQL?</H4>
|
||||
<H3><A name="1.2">1.2</A>) What is the copyright of
|
||||
PostgreSQL?</H3>
|
||||
|
||||
<P>PostgreSQL is distributed under the classic BSD license. It has
|
||||
no restrictions on how the source code can be used. We like it and
|
||||
@ -185,7 +185,7 @@
|
||||
UNIVERSITY OF CALIFORNIA HAS NO OBLIGATIONS TO PROVIDE MAINTENANCE,
|
||||
SUPPORT, UPDATES, ENHANCEMENTS, OR MODIFICATIONS.</P>
|
||||
|
||||
<H4><A name="1.3">1.3</A>) What platforms does PostgreSQL support?</H4>
|
||||
<H3><A name="1.3">1.3</A>) What platforms does PostgreSQL support?</H3>
|
||||
|
||||
<P>In general, any modern Unix-compatible platform should be able to
|
||||
run PostgreSQL. The platforms that had received explicit testing at
|
||||
@ -204,14 +204,14 @@
|
||||
"http://hobbes.nmsu.edu/cgi-bin/h-search?sh=1&button=Search&key=postgreSQL&stype=all&sort=type&dir=%2F">
|
||||
http://hobbes.nmsu.edu/cgi-bin/h-search?sh=1&button=Search&key=postgreSQL&stype=all&sort=type&dir=%2F</a>.</p>
|
||||
|
||||
<H4><A name="1.4">1.4</A>) Where can I get PostgreSQL?</H4>
|
||||
<H3><A name="1.4">1.4</A>) Where can I get PostgreSQL?</H3>
|
||||
|
||||
<P>Via web browser, use <a href="http://www.postgresql.org/ftp/">
|
||||
http://www.postgresql.org/ftp/</a>, and via ftp, use
|
||||
<A href="ftp://ftp.PostgreSQL.org/pub/">
|
||||
ftp://ftp.PostgreSQL.org/pub/</A>.</P>
|
||||
|
||||
<H4><A name="1.5">1.5</A>) Where can I get support?</H4>
|
||||
<H3><A name="1.5">1.5</A>) Where can I get support?</H3>
|
||||
|
||||
<P>The PostgreSQL community provides assistance to many of its users
|
||||
via email. The main web site to subscribe to the email lists is
|
||||
@ -230,7 +230,7 @@
|
||||
"http://techdocs.postgresql.org/companies.php">
|
||||
http://techdocs.postgresql.org/companies.php</A>.</P>
|
||||
|
||||
<H4><A name="1.6">1.6</A>) How do I submit a bug report?</H4>
|
||||
<H3><A name="1.6">1.6</A>) How do I submit a bug report?</H3>
|
||||
|
||||
<P>Visit the PostgreSQL bug form at <A href=
|
||||
"http://www.postgresql.org/support/submitbug">
|
||||
@ -240,14 +240,14 @@
|
||||
"ftp://ftp.PostgreSQL.org/pub/">ftp://ftp.PostgreSQL.org/pub/</A> to
|
||||
see if there is a more recent PostgreSQL version.</P>
|
||||
|
||||
<H4><A name="1.7">1.7</A>) What is the latest release?</H4>
|
||||
<H3><A name="1.7">1.7</A>) What is the latest release?</H3>
|
||||
|
||||
<P>The latest release of PostgreSQL is version 8.0.2.</P>
|
||||
|
||||
<P>We plan to have a major release every year, with minor releases
|
||||
every few months.</P>
|
||||
|
||||
<H4><A name="1.8">1.8</A>) What documentation is available?</H4>
|
||||
<H3><A name="1.8">1.8</A>) What documentation is available?</H3>
|
||||
|
||||
<P>PostgreSQL includes extensive documentation, including a large
|
||||
manual, manual pages, and some test examples. See the <I>/doc</I>
|
||||
@ -272,15 +272,15 @@
|
||||
|
||||
<P>Our web site contains even more documentation.</P>
|
||||
|
||||
<H4><A name="1.9">1.9</A>) How do I find out about known bugs or
|
||||
missing features?</H4>
|
||||
<H3><A name="1.9">1.9</A>) How do I find out about known bugs or
|
||||
missing features?</H3>
|
||||
|
||||
<P>PostgreSQL supports an extended subset of <SMALL>SQL</SMALL>-92.
|
||||
See our <A href="http://www.postgresql.org/docs/faqs.TODO.html">TODO</A>
|
||||
list for known bugs, missing features, and future plans.</P>
|
||||
|
||||
<H4><A name="1.10">1.10</A>) How can I learn
|
||||
<SMALL>SQL</SMALL>?</H4>
|
||||
<H3><A name="1.10">1.10</A>) How can I learn
|
||||
<SMALL>SQL</SMALL>?</H3>
|
||||
|
||||
<P>First, consider the PostgreSQL-specific books mentioned above.
|
||||
Another one is "Teach Yourself SQL in 21 Days, Second Edition"
|
||||
@ -298,14 +298,14 @@
|
||||
and at <A href=
|
||||
"http://sqlcourse.com/">http://sqlcourse.com.</A></P>
|
||||
|
||||
<H4><A name="1.11">1.11</A>) How do I join the development
|
||||
team?</H4>
|
||||
<H3><A name="1.11">1.11</A>) How do I join the development
|
||||
team?</H3>
|
||||
|
||||
<P>See the <a href="http://www.postgresql.org/docs/faqs.FAQ_DEV.html">
|
||||
Developer's FAQ</A>.
|
||||
|
||||
<H4><A name="1.12">1.12</A>) How does PostgreSQL compare to other
|
||||
<SMALL>DBMS</SMALL>s?</H4>
|
||||
<H3><A name="1.12">1.12</A>) How does PostgreSQL compare to other
|
||||
<SMALL>DBMS</SMALL>s?</H3>
|
||||
|
||||
<P>There are several ways of measuring software: features,
|
||||
performance, reliability, support, and price.</P>
|
||||
@ -368,8 +368,8 @@
|
||||
|
||||
<H2 align="center">User Client Questions</H2>
|
||||
|
||||
<H4><A name="2.1">2.1</A>) What interfaces are available for
|
||||
PostgreSQL?</H4>
|
||||
<H3><A name="2.1">2.1</A>) What interfaces are available for
|
||||
PostgreSQL?</H3>
|
||||
|
||||
<P>The PostgreSQL install includes only the <SMALL>C</SMALL> and embedded
|
||||
<SMALL>C</SMALL> interfaces. All other interfaces are independent projects
|
||||
@ -383,8 +383,8 @@
|
||||
in the <I>Drivers/Interfaces</I> section and via Internet search.
|
||||
</P>
|
||||
|
||||
<H4><A name="2.2">2.2</A>) What tools are available for using
|
||||
PostgreSQL with Web pages?</H4>
|
||||
<H3><A name="2.2">2.2</A>) What tools are available for using
|
||||
PostgreSQL with Web pages?</H3>
|
||||
|
||||
<P>A nice introduction to Database-backed Web pages can be seen at:
|
||||
<A href="http://www.webreview.com">http://www.webreview.com</A></P>
|
||||
@ -396,8 +396,8 @@
|
||||
<P>For complex cases, many use the Perl and DBD::Pg with CGI.pm or
|
||||
mod_perl.</P>
|
||||
|
||||
<H4><A name="2.3">2.3</A>) Does PostgreSQL have a graphical user
|
||||
interface?</H4>
|
||||
<H3><A name="2.3">2.3</A>) Does PostgreSQL have a graphical user
|
||||
interface?</H3>
|
||||
|
||||
<P>Yes, see <a href="http://techdocs.postgresql.org/guides/GUITools">
|
||||
http://techdocs.postgresql.org/guides/GUITools</a> for a
|
||||
@ -408,14 +408,14 @@
|
||||
|
||||
<H2 align="center">Administrative Questions</H2>
|
||||
|
||||
<H4><A name="3.1">3.1</A>) How do I install PostgreSQL somewhere
|
||||
other than <I>/usr/local/pgsql</I>?</H4>
|
||||
<H3><A name="3.1">3.1</A>) How do I install PostgreSQL somewhere
|
||||
other than <I>/usr/local/pgsql</I>?</H3>
|
||||
|
||||
<P>Specify the <I>--prefix</I> option when running
|
||||
<I>configure</I>.</P>
|
||||
|
||||
<H4><A name="3.2">3.2</A>) How do I control connections from other
|
||||
hosts?</H4>
|
||||
<H3><A name="3.2">3.2</A>) How do I control connections from other
|
||||
hosts?</H3>
|
||||
|
||||
<P>By default, PostgreSQL only allows connections from the local
|
||||
machine using Unix domain sockets or TCP/IP connections. Other
|
||||
@ -424,8 +424,8 @@
|
||||
host-based authentication by modifying the
|
||||
<I>$PGDATA/pg_hba.conf</I> file, and restart the server.</P>
|
||||
|
||||
<H4><A name="3.3">3.3</A>) How do I tune the database engine for
|
||||
better performance?</H4>
|
||||
<H3><A name="3.3">3.3</A>) How do I tune the database engine for
|
||||
better performance?</H3>
|
||||
|
||||
<P>There are three major areas for potential performance
|
||||
improvement:</P>
|
||||
@ -481,15 +481,15 @@
|
||||
</DD>
|
||||
</DL>
|
||||
|
||||
<H4><A name="3.4">3.4</A>) What debugging features are
|
||||
available?</H4>
|
||||
<H3><A name="3.4">3.4</A>) What debugging features are
|
||||
available?</H3>
|
||||
|
||||
<P>There are many <CODE>log_*</CODE> server configuration variables
|
||||
that enable printing of query and process statistics which can be
|
||||
very useful for debugging and performance measurements.</P>
|
||||
|
||||
<H4><A name="3.5">3.5</A>) Why do I get <I>"Sorry, too many
|
||||
clients"</I> when trying to connect?</H4>
|
||||
<H3><A name="3.5">3.5</A>) Why do I get <I>"Sorry, too many
|
||||
clients"</I> when trying to connect?</H3>
|
||||
|
||||
<P>You have reached the default limit is 100 database sessions. You
|
||||
need to increase the <I>postmaster</I>'s limit on how many
|
||||
@ -497,8 +497,8 @@
|
||||
<I>max_connections</I> value in <I>postgresql.conf</I> and
|
||||
restarting the <I>postmaster</I>.</P>
|
||||
|
||||
<H4><A name="3.6">3.6</A>) Why do I need to do a dump and restore
|
||||
to upgrade between major PostgreSQL releases?</H4>
|
||||
<H3><A name="3.6">3.6</A>) Why do I need to do a dump and restore
|
||||
to upgrade between major PostgreSQL releases?</H3>
|
||||
|
||||
<P>The PostgreSQL team makes only small changes between minor releases,
|
||||
so upgrading from 7.4.0 to 7.4.1 does not require a dump and restore.
|
||||
@ -508,7 +508,7 @@
|
||||
data in a generic format that can then be loaded in using the new internal
|
||||
format.</P>
|
||||
|
||||
<H4><A name="3.7">3.7</A>) What computer hardware should I use?</H4>
|
||||
<H3><A name="3.7">3.7</A>) What computer hardware should I use?</H3>
|
||||
|
||||
<P>Because PC hardware is mostly compatible, people tend to believe that
|
||||
all PC hardware is of equal quality. It is not. ECC RAM, SCSI, and
|
||||
@ -522,8 +522,8 @@
|
||||
|
||||
<H2 align="center">Operational Questions</H2>
|
||||
|
||||
<H4><A name="4.1">4.1</A>) How do I <SMALL>SELECT</SMALL> only the
|
||||
first few rows of a query? A random row?</H4>
|
||||
<H3><A name="4.1">4.1</A>) How do I <SMALL>SELECT</SMALL> only the
|
||||
first few rows of a query? A random row?</H3>
|
||||
|
||||
<P>To retrieve only a few rows, if you know at the number of rows
|
||||
needed at the time of the <SMALL>SELECT</SMALL> use
|
||||
@ -541,9 +541,9 @@
|
||||
LIMIT 1;
|
||||
</PRE>
|
||||
|
||||
<H4><A name="4.2">4.2</A>) How do I find out what tables, indexes,
|
||||
<H3><A name="4.2">4.2</A>) How do I find out what tables, indexes,
|
||||
databases, and users are defined? How do I see the queries used
|
||||
by <I>psql</I> to display them?</H4>
|
||||
by <I>psql</I> to display them?</H3>
|
||||
|
||||
<P>Use the \dt command to see tables in <I>psql</I>. For a complete list of
|
||||
commands inside psql you can use \?. Alternatively you can read the source
|
||||
@ -564,7 +564,7 @@
|
||||
illustrates many of the <SMALL>SELECT</SMALL>s needed to get
|
||||
information from the database system tables.</P>
|
||||
|
||||
<H4><A name="4.3">4.3</A>) How do you change a column's data type?</H4>
|
||||
<H3><A name="4.3">4.3</A>) How do you change a column's data type?</H3>
|
||||
|
||||
<P>Changing the data type of a column can be done easily in 8.0
|
||||
and later with <SMALL>ALTER TABLE ALTER COLUMN TYPE</SMALL>.</P>
|
||||
@ -580,8 +580,8 @@
|
||||
<P>You might then want to do <I>VACUUM FULL tab</I> to reclaim the
|
||||
disk space used by the expired rows.</P>
|
||||
|
||||
<H4><A name="4.4">4.4</A>) What is the maximum size for a row, a
|
||||
table, and a database?</H4>
|
||||
<H3><A name="4.4">4.4</A>) What is the maximum size for a row, a
|
||||
table, and a database?</H3>
|
||||
|
||||
<P>These are the limits:</P>
|
||||
<BLOCKQUOTE>
|
||||
@ -611,8 +611,8 @@ table?</TD><TD>unlimited</TD></TR>
|
||||
<P>The maximum table size and maximum number of columns can be
|
||||
quadrupled by increasing the default block size to 32k.</P>
|
||||
|
||||
<H4><A name="4.5">4.5</A>) How much database disk space is required
|
||||
to store data from a typical text file?</H4>
|
||||
<H3><A name="4.5">4.5</A>) How much database disk space is required
|
||||
to store data from a typical text file?</H3>
|
||||
|
||||
<P>A PostgreSQL database may require up to five times the disk
|
||||
space to store data from a text file.</P>
|
||||
@ -648,8 +648,8 @@ table?</TD><TD>unlimited</TD></TR>
|
||||
<P><SMALL>NULL</SMALL>s are stored as bitmaps, so they
|
||||
use very little space.</P>
|
||||
|
||||
<H4><A name="4.6">4.6</A>) Why are my queries slow? Why don't they
|
||||
use my indexes?</H4>
|
||||
<H3><A name="4.6">4.6</A>) Why are my queries slow? Why don't they
|
||||
use my indexes?</H3>
|
||||
|
||||
<P>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
|
||||
@ -712,14 +712,14 @@ table?</TD><TD>unlimited</TD></TR>
|
||||
types exactly match the index's column types. This was particularly
|
||||
true of int2, int8, and numeric column indexes.</P>
|
||||
|
||||
<H4><A name="4.7">4.7</A>) How do I see how the query optimizer is
|
||||
evaluating my query?</H4>
|
||||
<H3><A name="4.7">4.7</A>) How do I see how the query optimizer is
|
||||
evaluating my query?</H3>
|
||||
|
||||
<P>See the <SMALL>EXPLAIN</SMALL> manual page.</P>
|
||||
|
||||
<H4><A name="4.8">4.8</A>) How do I perform regular expression
|
||||
<H3><A name="4.8">4.8</A>) How do I perform regular expression
|
||||
searches and case-insensitive regular expression searches? How do I
|
||||
use an index for case-insensitive searches?</H4>
|
||||
use an index for case-insensitive searches?</H3>
|
||||
|
||||
<P>The <I>~</I> operator does regular expression matching, and
|
||||
<I>~*</I> does case-insensitive regular expression matching. The
|
||||
@ -740,9 +740,9 @@ table?</TD><TD>unlimited</TD></TR>
|
||||
CREATE INDEX tabindex ON tab (lower(col));
|
||||
</PRE>
|
||||
|
||||
<H4><A name="4.9">4.9</A>) In a query, how do I detect if a field
|
||||
<H3><A name="4.9">4.9</A>) In a query, how do I detect if a field
|
||||
is <SMALL>NULL</SMALL>? How can I sort on whether a field is <SMALL>
|
||||
NULL</SMALL> or not?</H4>
|
||||
NULL</SMALL> or not?</H3>
|
||||
|
||||
<P>You test the column with <SMALL>IS NULL</SMALL> and <SMALL>IS
|
||||
NOT NULL</SMALL>.</P>
|
||||
@ -764,8 +764,8 @@ table?</TD><TD>unlimited</TD></TR>
|
||||
ORDER BY (col IS NOT NULL)
|
||||
</PRE>
|
||||
|
||||
<H4><A name="4.10">4.10</A>) What is the difference between the
|
||||
various character types?</H4>
|
||||
<H3><A name="4.10">4.10</A>) What is the difference between the
|
||||
various character types?</H3>
|
||||
<BLOCKQUOTE>
|
||||
<TABLE >
|
||||
<TR><TH>Type</TH><TH>Internal Name</TH><TH>Notes</TH></TR>
|
||||
@ -801,8 +801,8 @@ length</TD></TR>
|
||||
particularly values that include <SMALL>NULL</SMALL> bytes. All the
|
||||
types described here have similar performance characteristics.</P>
|
||||
|
||||
<H4><A name="4.11.1">4.11.1</A>) How do I create a
|
||||
serial/auto-incrementing field?</H4>
|
||||
<H3><A name="4.11.1">4.11.1</A>) How do I create a
|
||||
serial/auto-incrementing field?</H3>
|
||||
|
||||
<P>PostgreSQL supports a <SMALL>SERIAL</SMALL> data type. It
|
||||
auto-creates a sequence. For example,
|
||||
@ -826,8 +826,8 @@ length</TD></TR>
|
||||
See the <I>create_sequence</I> manual page for more information
|
||||
about sequences.
|
||||
|
||||
<H4><A name="4.11.2">4.11.2</A>) How do I get the value of a
|
||||
<SMALL>SERIAL</SMALL> insert?</H4>
|
||||
<H3><A name="4.11.2">4.11.2</A>) How do I get the value of a
|
||||
<SMALL>SERIAL</SMALL> insert?</H3>
|
||||
|
||||
<P>One approach is to retrieve the next <SMALL>SERIAL</SMALL> value
|
||||
from the sequence object with the <I>nextval()</I> function
|
||||
@ -855,23 +855,23 @@ length</TD></TR>
|
||||
new_id = execute("SELECT currval('person_id_seq')");
|
||||
</PRE>
|
||||
|
||||
<H4><A name="4.11.3">4.11.3</A>) Doesn't <I>currval()</I>
|
||||
lead to a race condition with other users?</H4>
|
||||
<H3><A name="4.11.3">4.11.3</A>) Doesn't <I>currval()</I>
|
||||
lead to a race condition with other users?</H3>
|
||||
|
||||
<P>No. <I>currval()</I> returns the current value assigned by your
|
||||
session, not by all sessions.</P>
|
||||
|
||||
<H4><A name="4.11.4">4.11.4</A>) Why aren't my sequence numbers
|
||||
<H3><A name="4.11.4">4.11.4</A>) Why aren't my sequence numbers
|
||||
reused on transaction abort? Why are there gaps in the numbering of
|
||||
my sequence/SERIAL column?</H4>
|
||||
my sequence/SERIAL column?</H3>
|
||||
|
||||
<P>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.</P>
|
||||
|
||||
<H4><A name="4.12">4.12</A>) What is an <SMALL>OID</SMALL>? What is
|
||||
a <SMALL>TID</SMALL>?</H4>
|
||||
<H3><A name="4.12">4.12</A>) What is an <SMALL>OID</SMALL>? What is
|
||||
a <SMALL>TID</SMALL>?</H3>
|
||||
|
||||
<P>Every row that is created in PostgreSQL gets a unique
|
||||
<SMALL>OID</SMALL> unless created <SMALL>WITHOUT OIDS</SMALL>.
|
||||
@ -893,8 +893,8 @@ length</TD></TR>
|
||||
are modified or reloaded. They are used by index entries to point
|
||||
to physical rows.</P>
|
||||
|
||||
<H4><A name="4.13">4.13</A>) Why do I get the error <I>"ERROR:
|
||||
Memory exhausted in AllocSetAlloc()"</I>?</H4>
|
||||
<H3><A name="4.13">4.13</A>) Why do I get the error <I>"ERROR:
|
||||
Memory exhausted in AllocSetAlloc()"</I>?</H3>
|
||||
|
||||
<P>You probably have run out of virtual memory on your system,
|
||||
or your kernel has a low limit for certain resources. Try this
|
||||
@ -912,13 +912,13 @@ length</TD></TR>
|
||||
backend is returning too much data, try it before starting the
|
||||
client.
|
||||
|
||||
<H4><A name="4.14">4.14</A>) How do I tell what PostgreSQL version
|
||||
I am running?</H4>
|
||||
<H3><A name="4.14">4.14</A>) How do I tell what PostgreSQL version
|
||||
I am running?</H3>
|
||||
|
||||
<P>From <I>psql</I>, type <CODE>SELECT version();</CODE></P>
|
||||
|
||||
<H4><A name="4.15">4.15</A>) Why does my large-object operations
|
||||
get <I>"invalid large obj descriptor"</I>?</H4>
|
||||
<H3><A name="4.15">4.15</A>) Why does my large-object operations
|
||||
get <I>"invalid large obj descriptor"</I>?</H3>
|
||||
|
||||
<P>You need to put <CODE>BEGIN WORK</CODE> and <CODE>COMMIT</CODE>
|
||||
around any use of a large object handle, that is, surrounding
|
||||
@ -933,15 +933,15 @@ length</TD></TR>
|
||||
<P>If you are using a client interface like <SMALL>ODBC</SMALL> you
|
||||
may need to set <CODE>auto-commit off.</CODE></P>
|
||||
|
||||
<H4><A name="4.16">4.16</A>) How do I create a column that will
|
||||
default to the current time?</H4>
|
||||
<H3><A name="4.16">4.16</A>) How do I create a column that will
|
||||
default to the current time?</H3>
|
||||
|
||||
<P>Use <I>CURRENT_TIMESTAMP</I>:</P>
|
||||
<PRE>
|
||||
CREATE TABLE test (x int, modtime TIMESTAMP DEFAULT CURRENT_TIMESTAMP );
|
||||
</PRE>
|
||||
|
||||
<H4><A name="4.17">4.17</A>) How do I perform an outer join?</H4>
|
||||
<H3><A name="4.17">4.17</A>) How do I perform an outer join?</H3>
|
||||
|
||||
<P>PostgreSQL supports outer joins using the SQL standard syntax.
|
||||
Here are two examples:</P>
|
||||
@ -964,8 +964,8 @@ length</TD></TR>
|
||||
<SMALL>RIGHT</SMALL>, and <SMALL>FULL</SMALL> joins. Ordinary joins
|
||||
are called <SMALL>INNER</SMALL> joins.</P>
|
||||
|
||||
<H4><A name="4.18">4.18</A>) How do I perform queries using
|
||||
multiple databases?</H4>
|
||||
<H3><A name="4.18">4.18</A>) How do I perform queries using
|
||||
multiple databases?</H3>
|
||||
|
||||
<P>There is no way to query a database other than the current one.
|
||||
Because PostgreSQL loads database-specific system catalogs, it is
|
||||
@ -976,16 +976,16 @@ length</TD></TR>
|
||||
connections to different databases and merge the results on the
|
||||
client side.</P>
|
||||
|
||||
<H4><A name="4.19">4.19</A>) How do I return multiple rows or
|
||||
columns from a function?</H4>
|
||||
<H3><A name="4.19">4.19</A>) How do I return multiple rows or
|
||||
columns from a function?</H3>
|
||||
|
||||
<P>It is easy using set-returning functions,
|
||||
<a href="http://techdocs.postgresql.org/guides/SetReturningFunctions">
|
||||
http://techdocs.postgresql.org/guides/SetReturningFunctions</a></P>.
|
||||
|
||||
<H4><A name="4.20">4.20</A>) Why do I get "relation with OID #####
|
||||
<H3><A name="4.20">4.20</A>) Why do I get "relation with OID #####
|
||||
does not exist" errors when accessing temporary tables in PL/PgSQL
|
||||
functions?</H4>
|
||||
functions?</H3>
|
||||
|
||||
<P>PL/PgSQL caches function scripts, and an unfortunate side effect
|
||||
is that if a PL/PgSQL function accesses a temporary table, and that
|
||||
@ -995,8 +995,8 @@ length</TD></TR>
|
||||
<SMALL>EXECUTE</SMALL> for temporary table access in PL/PgSQL. This
|
||||
will cause the query to be reparsed every time.</P>
|
||||
|
||||
<H4><A name="4.21">4.21</A>) What encryption options are available?
|
||||
</H4>
|
||||
<H3><A name="4.21">4.21</A>) What encryption options are available?
|
||||
</H3>
|
||||
<UL>
|
||||
<LI><I>contrib/pgcrypto</I> contains many encryption functions for
|
||||
use in <SMALL>SQL</SMALL> queries.</LI>
|
||||
@ -1016,21 +1016,21 @@ length</TD></TR>
|
||||
|
||||
<H2 align="center">Extending PostgreSQL</H2>
|
||||
|
||||
<H4><A name="5.1">5.1</A>) I wrote a user-defined function. When I
|
||||
run it in <I>psql</I>, why does it dump core?</H4>
|
||||
<H3><A name="5.1">5.1</A>) I wrote a user-defined function. When I
|
||||
run it in <I>psql</I>, why does it dump core?</H3>
|
||||
|
||||
<P>The problem could be a number of things. Try testing your
|
||||
user-defined function in a stand-alone test program first.</P>
|
||||
|
||||
<H4><A name="5.2">5.2</A>) How can I contribute some nifty new
|
||||
types and functions to PostgreSQL?</H4>
|
||||
<H3><A name="5.2">5.2</A>) How can I contribute some nifty new
|
||||
types and functions to PostgreSQL?</H3>
|
||||
|
||||
<P>Send your extensions to the <I>pgsql-hackers</I> mailing list,
|
||||
and they will eventually end up in the <I>contrib/</I>
|
||||
subdirectory.</P>
|
||||
|
||||
<H4><A name="5.3">5.3</A>) How do I write a C function to return a
|
||||
tuple?</H4>
|
||||
<H3><A name="5.3">5.3</A>) How do I write a C function to return a
|
||||
tuple?</H3>
|
||||
|
||||
<P>In versions of PostgreSQL beginning with 7.3, table-returning
|
||||
functions are fully supported in C, PL/PgSQL, and SQL. See the
|
||||
@ -1038,8 +1038,8 @@ length</TD></TR>
|
||||
table-returning function defined in C can be found in
|
||||
<I>contrib/tablefunc</I>.</P>
|
||||
|
||||
<H4><A name="5.4">5.4</A>) I have changed a source file. Why does
|
||||
the recompile not see the change?</H4>
|
||||
<H3><A name="5.4">5.4</A>) I have changed a source file. Why does
|
||||
the recompile not see the change?</H3>
|
||||
|
||||
<P>The <I>Makefiles</I> do not have the proper dependencies for
|
||||
include files. You have to do a <I>make clean</I> and then another
|
||||
|
||||
Loading…
Reference in New Issue
Block a user