+++ /dev/null
-
- Frequently Asked Questions (FAQ) for PostgreSQL
-
- Last updated: Tue Sep 23 16:19:49 EDT 2008
-
- Current maintainer: Bruce Momjian (bruce@momjian.us)
-
- The most recent version of this document can be viewed at
- http://www.postgresql.org/files/documentation/faqs/FAQ.html.
-
- Platform-specific questions are answered at
- http://www.postgresql.org/docs/faq/.
- _________________________________________________________________
-
- General Questions
-
- 1.1) What is PostgreSQL? How is it pronounced? What is Postgres?
- 1.2) Who controls PostgreSQL?
- 1.3) What is the copyright of PostgreSQL?
- 1.4) What platforms does PostgreSQL support?
- 1.5) Where can I get PostgreSQL?
- 1.6) What is the most recent release?
- 1.7) Where can I get support?
- 1.8) How do I submit a bug report?
- 1.9) How do I find out about known bugs or missing features?
- 1.10) What documentation is available?
- 1.11) How can I learn SQL?
- 1.12) How do I submit a patch or join the development team?
- 1.13) How does PostgreSQL compare to other DBMSs? Can PostgreSQL be
- embedded?
- 1.14) Will PostgreSQL handle recent daylight saving time changes in
- various countries?
- 1.15) How do I unsubscribe from the PostgreSQL email lists? How do I
- avoid receiving duplicate emails?
-
- User Client Questions
-
- 2.1) What interfaces are available for PostgreSQL?
- 2.2) What tools are available for using PostgreSQL with Web pages?
- 2.3) Does PostgreSQL have a graphical user interface?
-
- Administrative Questions
-
- 3.1) How do I install PostgreSQL somewhere other than
- /usr/local/pgsql?
- 3.2) How do I control connections from other hosts?
- 3.3) How do I tune the database engine for better performance?
- 3.4) What debugging features are available?
- 3.5) Why do I get "Sorry, too many clients" when trying to connect?
- 3.6 What is the upgrade process for PostgreSQL?
- 3.7) What computer hardware should I use?
-
- Operational Questions
-
- 4.1) How do I SELECT only the first few rows of a query? A random row?
- 4.2) How do I find out what tables, indexes, databases, and users are
- defined? How do I see the queries used by psql to display them?
- 4.3) How do you change a column's data type?
- 4.4) What is the maximum size for a row, a table, and a database?
- 4.5) How much database disk space is required to store data from a
- typical text file?
- 4.6) Why are my queries slow? Why don't they use my indexes?
- 4.7) How do I see how the query optimizer is evaluating my query?
- 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?
- 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?
- 4.10) What is the difference between the various character types?
- 4.11.1) How do I create a serial/auto-incrementing field?
- 4.11.2) How do I get the value of a SERIAL insert?
- 4.11.3) Doesn't currval() lead to a race condition with other users?
- 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?
- 4.12) What is an OID? What is a CTID?
- 4.13) Why do I get the error "ERROR: Memory exhausted in
- AllocSetAlloc()"?
- 4.14) How do I tell what PostgreSQL version I am running?
- 4.15) How do I create a column that will default to the current time?
- 4.16) How do I perform an outer join?
- 4.17) How do I perform queries using multiple databases?
- 4.18) How do I return multiple rows or columns from a function?
- 4.19) Why do I get "relation with OID ##### does not exist" errors
- when accessing temporary tables in PL/PgSQL functions?
- 4.20) What replication solutions are available?
- 4.21) Why are my table and column names not recognized in my query?
- Why is capitalization not preserved?
- _________________________________________________________________
-
- General Questions
-
- 1.1) What is PostgreSQL? How is it pronounced? What is Postgres?
-
- PostgreSQL is pronounced Post-Gres-Q-L. (For those curious about how
- to say "PostgreSQL", an audio file is available.)
-
- PostgreSQL is an object-relational database system that has the
- features of traditional commercial database systems with enhancements
- to be found in next-generation DBMS systems. PostgreSQL is free and
- the complete source code is available.
-
- PostgreSQL development is performed by a team of mostly volunteer
- developers spread throughout the world and communicating via the
- Internet. It is a community project and is not controlled by any
- company. To get involved, see the developer's FAQ at
- http://www.postgresql.org/docs/faqs.FAQ_DEV.html
-
- Postgres is a widely-used nickname for PostgreSQL. It was the original
- name of the project at Berkeley and is strongly preferred over other
- nicknames. If you find 'PostgreSQL' hard to pronounce, call it
- 'Postgres' instead.
-
- 1.2) Who controls PostgreSQL?
-
- If you are looking for a PostgreSQL gatekeeper, central committee, or
- controlling company, give up --- there isn't one. We do have a core
- committee and CVS committers, but these groups are more for
- administrative purposes than control. The project is directed by the
- community of developers and users, which anyone can join. All you need
- to do is subscribe to the mailing lists and participate in the
- discussions. (See the Developer's FAQ for information on how to get
- involved in PostgreSQL development.)
-
- 1.3) What is the copyright of PostgreSQL?
-
- PostgreSQL is distributed under the classic BSD license. Basically, it
- allows users to do anything they want with the code, including
- reselling binaries without the source code. The only restriction is
- that you not hold us legally liable for problems with the software.
- There is also the requirement that this copyright appear in all copies
- of the software. Here is the actual BSD license we use:
-
- PostgreSQL Data Base Management System
-
- Portions Copyright (c) 1996-2009, PostgreSQL Global Development Group
- Portions Copyright (c) 1994-1996 Regents of the University of
- California
-
- Permission to use, copy, modify, and distribute this software and its
- documentation for any purpose, without fee, and without a written
- agreement is hereby granted, provided that the above copyright notice
- and this paragraph and the following two paragraphs appear in all
- copies.
-
- IN NO EVENT SHALL THE UNIVERSITY OF CALIFORNIA BE LIABLE TO ANY PARTY
- FOR DIRECT, INDIRECT, SPECIAL, INCIDENTAL, OR CONSEQUENTIAL DAMAGES,
- INCLUDING LOST PROFITS, ARISING OUT OF THE USE OF THIS SOFTWARE AND
- ITS DOCUMENTATION, EVEN IF THE UNIVERSITY OF CALIFORNIA HAS BEEN
- ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
-
- THE UNIVERSITY OF CALIFORNIA SPECIFICALLY DISCLAIMS ANY WARRANTIES,
- INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF
- MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE. THE SOFTWARE
- PROVIDED HEREUNDER IS ON AN "AS IS" BASIS, AND THE UNIVERSITY OF
- CALIFORNIA HAS NO OBLIGATIONS TO PROVIDE MAINTENANCE, SUPPORT,
- UPDATES, ENHANCEMENTS, OR MODIFICATIONS.
-
- 1.4) What platforms does PostgreSQL support?
-
- In general, any modern Unix-compatible platform should be able to run
- PostgreSQL. The platforms that had received explicit testing at the
- time of release are listed in the installation instructions.
-
- PostgreSQL also runs natively on Microsoft Windows NT-based operating
- systems like Win2000 SP4, WinXP, and Win2003. A prepackaged installer
- is available at http://www.postgresql.org/download/windows.
- MSDOS-based versions of Windows (Win95, Win98, WinMe) can run
- PostgreSQL using Cygwin.
-
- There is also a Novell Netware 6 port at
- http://developer.novell.com/wiki/index.php/Postgresql, and an OS/2
- (eComStation) version at
- http://hobbes.nmsu.edu/cgi-bin/h-search?sh=1&button=Search&key=postgre
- SQL&stype=all&sort=type&dir=%2F.
-
- 1.5) Where can I get PostgreSQL?
-
- Via web browser, use http://www.postgresql.org/ftp/, and via ftp, use
- ftp://ftp.postgresql.org/pub/.
-
- 1.6) What is the most recent release?
-
- The latest release of PostgreSQL is version 8.3.3.
-
- We plan to have a major release every year, with minor releases every
- few months.
-
- 1.7) Where can I get support?
-
- The PostgreSQL community provides assistance to many of its users via
- email. The main web site to subscribe to the email lists is
- http://www.postgresql.org/community/lists/. The general or bugs lists
- are a good place to start.
-
- The major IRC channel is #postgresql on Freenode (irc.freenode.net).
- To connect you can use the Unix program irc -c '#postgresql' "$USER"
- irc.freenode.net or use any other IRC clients. A Spanish one also
- exists on the same network, (#postgresql-es), a French one,
- (#postgresqlfr), and a Brazilian one, (#postgresql-br). There is also
- a PostgreSQL channel on EFNet.
-
- A list of commercial support companies is available at
- http://www.postgresql.org/support/professional_support.
-
- 1.8) How do I submit a bug report?
-
- Visit the PostgreSQL bug form at
- http://www.postgresql.org/support/submitbug. Also check out our ftp
- site ftp://ftp.postgresql.org/pub/ to see if there is a more recent
- PostgreSQL version.
-
- Bugs submitted using the bug form or posted to any PostgreSQL mailing
- list typically generates one of the following replies:
- * It is not a bug, and why
- * It is a known bug and is already on the TODO list
- * The bug has been fixed in the current release
- * The bug has been fixed but is not packaged yet in an official
- release
- * A request is made for more detailed information:
- + Operating system
- + PostgreSQL version
- + Reproducible test case
- + Debugging information
- + Debugger backtrace output
- * The bug is new. The following might happen:
- + A patch is created and will be included in the next major or
- minor release
- + The bug cannot be fixed immediately and is added to the TODO
- list
-
- 1.9) How do I find out about known bugs or missing features?
-
- PostgreSQL supports an extended subset of SQL:2003. See our TODO list
- for known bugs, missing features, and future plans.
-
- A feature request usually results in one of the following replies:
- * The feature is already on the TODO list
- * The feature is not desired because:
- + It duplicates existing functionality that already follows the
- SQL standard
- + The feature would increase code complexity but add little
- benefit
- + The feature would be insecure or unreliable
- * The new feature is added to the TODO list
-
- PostgreSQL does not use a bug tracking system because we find it more
- efficient to respond directly to email and keep the TODO list
- up-to-date. In practice, bugs don't last very long in the software,
- and bugs that affect a large number of users are fixed rapidly. The
- only place to find all changes, improvements, and fixes in a
- PostgreSQL release is to read the CVS log messages. Even the release
- notes do not list every change made to the software.
-
- 1.10) What documentation is available?
-
- PostgreSQL includes extensive documentation, including a large manual,
- manual pages, and some test examples. See the /doc directory. You can
- also browse the manuals online at http://www.postgresql.org/docs.
-
- There are two PostgreSQL books available online at
- http://www.postgresql.org/docs/books/awbook.html and
- http://www.commandprompt.com/ppbook/. There are a number of PostgreSQL
- books available for purchase. One of the most popular ones is by Korry
- Douglas. A list of book reviews can be found at
- http://www.postgresql.org/docs/books/. There is also a collection of
- PostgreSQL technical articles at
- http://wiki.postgresql.org/wiki/Community_Generated_Articles%2C_Guides
- %2C_and_Documentation.
-
- The command line client program psql has some \d commands to show
- information about types, operators, functions, aggregates, etc. - use
- \? to display the available commands.
-
- Our web site contains even more documentation.
-
- 1.11) How can I learn SQL?
-
- First, consider the PostgreSQL-specific books mentioned above. Many of
- our users also like The Practical SQL Handbook, Bowman, Judith S., et
- al., Addison-Wesley. Others like The Complete Reference SQL, Groff et
- al., McGraw-Hill.
-
- There are also many nice tutorials available online:
- * http://www.intermedia.net/support/sql/sqltut.shtm
- * http://sqlcourse.com
- * http://www.w3schools.com/sql/default.asp
- * http://mysite.verizon.net/Graeme_Birchall/id1.html
- * http://sqlzoo.net
-
- 1.12) How do I submit a patch or join the development team?
-
- See the Developer's FAQ.
-
- 1.13) How does PostgreSQL compare to other DBMSs? Can PostgreSQL be embedded?
-
- There are several ways of measuring software: features, performance,
- reliability, support, and price.
-
- Features
- PostgreSQL has most features present in large commercial DBMSs,
- like transactions, subselects, triggers, views, foreign key
- referential integrity, and sophisticated locking. We have some
- features they do not have, like user-defined types,
- inheritance, rules, and multi-version concurrency control to
- reduce lock contention.
-
- Performance
- PostgreSQL's performance is comparable to other commercial and
- open source databases. It is faster for some things, slower for
- others. Our performance is usually +/-10% compared to other
- databases.
-
- Reliability
- We realize that a DBMS must be reliable, or it is worthless. We
- strive to release well-tested, stable code that has a minimum
- of bugs. Each release has at least one month of beta testing,
- and our release history shows that we can provide stable, solid
- releases that are ready for production use. We believe we
- compare favorably to other database software in this area.
-
- Support
- Our mailing lists provide contact with a large group of
- developers and users to help resolve any problems encountered.
- While we cannot guarantee a fix, commercial DBMSs do not always
- supply a fix either. Direct access to developers, the user
- community, manuals, and the source code often make PostgreSQL
- support superior to other DBMSs. There is commercial
- per-incident support available for those who need it. (See FAQ
- section 1.7.)
-
- Price
- We are free for all use, both commercial and non-commercial.
- You can add our code to your product with no limitations,
- except those outlined in our BSD-style license stated above.
-
- PostgreSQL is designed as a client/server architecture, which requires
- separate processes for each client and server, and various helper
- processes. Many embedded architectures can support such requirements.
- However, if your embedded architecture requires the database server to
- run inside the application process, you cannot use Postgres and should
- select a lighter-weight database solution.
-
- 1.14) Will PostgreSQL handle recent daylight saving time changes in various
- countries?
-
- USA daylight saving time changes are included in PostgreSQL release
- 8.0.[4+], and all later major releases, e.g. 8.1. Canada and Western
- Australia changes are included in 8.0.[10+], 8.1.[6+], and all later
- major releases. PostgreSQL releases prior to 8.0 use the operating
- system's timezone database for daylight saving information.
-
- 1.15) How do I unsubscribe from the PostgreSQL email lists? How do I avoid
- receiving duplicate emails?
-
- The PostgreSQL Majordomo page allows subscribing or unsubscribing from
- any of the PostgreSQL email lists. (You might need to have your
- Majordomo password emailed to you to log in.)
-
- All PostgreSQL email lists are configured so a group reply goes to the
- email list and the original email author. This is done so users
- receive the quickest possible email replies. If you would prefer not
- to receive duplicate email from the list in cases where you already
- receive an email directly, check eliminatecc from the Majordomo Change
- Settings page. You can also prevent yourself from receiving copies of
- emails you post to the lists by unchecking selfcopy.
- _________________________________________________________________
-
- User Client Questions
-
- 2.1) What interfaces are available for PostgreSQL?
-
- The PostgreSQL install includes only the C and embedded C interfaces.
- All other interfaces are independent projects that are downloaded
- separately; being separate allows them to have their own release
- schedule and development teams.
-
- Some programming languages like PHP include an interface to
- PostgreSQL. Interfaces for languages like Perl, TCL, Python, and many
- others are available at http://pgfoundry.org.
-
- 2.2) What tools are available for using PostgreSQL with Web pages?
-
- A nice introduction to Database-backed Web pages can be seen at:
- http://www.webreview.com
-
- For Web integration, PHP (http://www.php.net) is an excellent
- interface.
-
- For complex cases, many use the Perl and DBD::Pg with CGI.pm or
- mod_perl.
-
- 2.3) Does PostgreSQL have a graphical user interface?
-
- There are a large number of GUI Tools that are available for
- PostgreSQL from both commercial and open source developers. A detailed
- list can be found in the Community Guide to PostgreSQL GUI Tools.
- _________________________________________________________________
-
- Administrative Questions
-
- 3.1) How do I install PostgreSQL somewhere other than /usr/local/pgsql?
-
- Specify the --prefix option when running configure.
-
- 3.2) How do I control connections from other hosts?
-
- By default, PostgreSQL only allows connections from the local machine
- using Unix domain sockets or TCP/IP connections. Other machines will
- not be able to connect unless you modify listen_addresses in the
- postgresql.conf file, enable host-based authentication by modifying
- the $PGDATA/pg_hba.conf file, and restart the database server.
-
- 3.3) How do I tune the database engine for better performance?
-
- There are three major areas for potential performance improvement:
-
- Query Changes
- This involves modifying queries to obtain better performance:
-
- + Creation of indexes, including expression and partial indexes
- + Use of COPY instead of multiple INSERTs
- + Grouping of multiple statements into a single transaction to
- reduce commit overhead
- + Use of CLUSTER when retrieving many rows from an index
- + Use of LIMIT for returning a subset of a query's output
- + Use of Prepared queries
- + Use of ANALYZE to maintain accurate optimizer statistics
- + Regular use of VACUUM or pg_autovacuum
- + Dropping of indexes during large data changes
-
- Server Configuration
- A number of postgresql.conf settings affect performance. For
- more details, see Administration Guide/Server Run-time
- Environment/Run-time Configuration for a full listing, and for
- commentary see
- http://www.varlena.com/varlena/GeneralBits/Tidbits/annotated_co
- nf_e.html and
- http://www.varlena.com/varlena/GeneralBits/Tidbits/perf.html.
-
- Hardware Selection
- The effect of hardware on performance is detailed in
- http://www.powerpostgresql.com/PerfList/ and
- http://momjian.us/main/writings/pgsql/hw_performance/index.html
- .
-
- 3.4) What debugging features are available?
-
- There are many log_* server configuration variables at
- http://www.postgresql.org/docs/current/interactive/runtime-config-logg
- ing.html that enable printing of query and process statistics which
- can be very useful for debugging and performance measurements.
-
- 3.5) Why do I get "Sorry, too many clients" when trying to connect?
-
- You have reached the default limit of 100 database sessions. You need
- to increase the server's limit on how many concurrent backend
- processes it can start by changing the max_connections value in
- postgresql.conf and restarting the server.
-
- 3.6) What is the upgrade process for PostgreSQL?
-
- See http://www.postgresql.org/support/versioning for a general
- discussion about upgrading, and
- http://www.postgresql.org/docs/current/static/install-upgrading.html
- for specific instructions.
-
- 3.7) What computer hardware should I use?
-
- 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
- quality motherboards are more reliable and have better performance
- than less expensive hardware. PostgreSQL will run on almost any
- hardware, but if reliability and performance are important it is wise
- to research your hardware options thoroughly. A disk controller with a
- battery-backed cache is also useful. Our email lists can be used to
- discuss hardware options and tradeoffs.
- _________________________________________________________________
-
- Operational Questions
-
- 4.1) How do I SELECT only the first few rows of a query? A random row?
-
- To retrieve only a few rows, if you know at the number of rows needed
- at the time of the SELECT use LIMIT . If an index matches the ORDER BY
- it is possible the entire query does not have to be executed. If you
- don't know the number of rows at SELECT time, use a cursor and FETCH.
-
- To SELECT a random row, use:
- SELECT col
- FROM tab
- ORDER BY random()
- LIMIT 1;
-
- 4.2) How do I find out what tables, indexes, databases, and users are
- defined? How do I see the queries used by psql to display them?
-
- Use the \dt command to see tables in psql. For a complete list of
- commands inside psql you can use \?. Alternatively you can read the
- source code for psql in file pgsql/src/bin/psql/describe.c, it
- contains SQL commands that generate the output for psql's backslash
- 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. It is also possible to use full
- text indexing for word searches.
-
- 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
- );
-
- Automatically created sequence are named <table>_<serialcolumn>_seq,
- where table and serialcolumn are the names of the table and SERIAL
- column, respectively. See the create_sequence manual page for more
- information about sequences.
-
- 4.11.2) How do I get the value of a SERIAL insert?
-
- The simplest way is to retrieve the assigned SERIAL value with
- RETURNING. Using the example table in 4.11.1, it would look like this:
- INSERT INTO person (name) VALUES ('Blaise Pascal') RETURNING id;
-
- You can also call nextval() and use that value in the INSERT, or call
- currval() after the INSERT.
-
- 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
- the server:
- 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://wiki.postgresql.org/wiki/Return_more_than_one_row_of_data_from_
- PL/pgSQL_functions.
-
- 4.19) Why do I get "relation with OID ##### does not exist" errors when
- accessing temporary tables in PL/PgSQL functions?
-
- In PostgreSQL versions < 8.3, 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.
-
- This problem does not occur in PostgreSQL 8.3 and later.
-
- 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 unrecognized 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
+++ /dev/null
- <!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
-<HTML>
- <HEAD>
- <META name="generator" content="HTML Tidy, see www.w3.org">
- <META http-equiv="Content-Type" content="text/html; charset=US-ASCII">
- <TITLE>PostgreSQL FAQ</TITLE>
- </HEAD>
-
- <BODY bgcolor="#ffffff" text="#000000" link="#ff0000" vlink="#a00000"
- alink="#0000ff">
- <H1>Frequently Asked Questions (FAQ) for PostgreSQL</H1>
-
- <P>Last updated: Tue Sep 23 16:19:49 EDT 2008</P>
-
- <P>Current maintainer: Bruce Momjian (<A href=
- "mailto:bruce@momjian.us">bruce@momjian.us</A>)
- </P>
-
- <P>The most recent version of this document can be viewed at <A href=
- "http://www.postgresql.org/files/documentation/faqs/FAQ.html">
- http://www.postgresql.org/files/documentation/faqs/FAQ.html</A>.</P>
-
- <P>Platform-specific questions are answered at <A href=
- "http://www.postgresql.org/docs/faq/">
- http://www.postgresql.org/docs/faq/</A>.</P>
- <HR>
-
- <H2 align="center">General Questions</H2>
- <A href="#item1.1">1.1</A>) What is PostgreSQL? How is it pronounced?
- What is Postgres?<BR>
- <A href="#item1.2">1.2</A>) Who controls PostgreSQL?<BR>
- <A href="#item1.3">1.3</A>) What is the copyright of PostgreSQL?<BR>
- <A href="#item1.4">1.4</A>) What platforms does PostgreSQL support?<BR>
- <A href="#item1.5">1.5</A>) Where can I get PostgreSQL?<BR>
- <A href="#item1.6">1.6</A>) What is the most recent release?<BR>
- <A href="#item1.7">1.7</A>) Where can I get support?<BR>
- <A href="#item1.8">1.8</A>) How do I submit a bug report?<BR>
- <A href="#item1.9">1.9</A>) How do I find out about known bugs or
- missing features?<BR>
- <A href="#item1.10">1.10</A>) What documentation is available?<BR>
- <A href="#item1.11">1.11</A>) How can I learn <SMALL>SQL</SMALL>?<BR>
- <A href="#item1.12">1.12</A>) How do I submit a patch or join the
- development team?<BR>
- <A href="#item1.13">1.13</A>) How does PostgreSQL compare to other
- <SMALL>DBMS</SMALL>s? Can PostgreSQL be embedded?<BR>
- <A href="#item1.14">1.14</A>) Will PostgreSQL handle recent daylight
- saving time changes in various countries?<BR>
- <A href="#item1.15">1.15</A>) How do I unsubscribe from the
- PostgreSQL email lists? How do I avoid receiving duplicate emails?<BR>
-
-
- <H2 align="center">User Client Questions</H2>
- <A href="#item2.1">2.1</A>) What interfaces are available for
- PostgreSQL?<BR>
- <A href="#item2.2">2.2</A>) What tools are available for using
- PostgreSQL with Web pages?<BR>
- <A href="#item2.3">2.3</A>) Does PostgreSQL have a graphical user
- interface?<BR>
-
-
- <H2 align="center">Administrative Questions</H2>
- <A href="#item3.1">3.1</A>) How do I install PostgreSQL somewhere other
- than <I>/usr/local/pgsql</I>?<BR>
- <A href="#item3.2">3.2</A>) How do I control connections from other
- hosts?<BR>
- <A href="#item3.3">3.3</A>) How do I tune the database engine for
- better performance?<BR>
- <A href="#item3.4">3.4</A>) What debugging features are available?<BR>
- <A href="#item3.5">3.5</A>) Why do I get <I>"Sorry, too many
- clients"</I> when trying to connect?<BR>
- <A href="#item3.6">3.6</A> What is the upgrade process for PostgreSQL?<BR>
- <A href="#item3.7">3.7</A>) What computer hardware should I use?<BR>
-
-
- <H2 align="center">Operational Questions</H2>
- <A href="#item4.1">4.1</A>) How do I <SMALL>SELECT</SMALL> only the
- first few rows of a query? A random row?<BR>
- <A href="#item4.2">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?<BR>
- <A href="#item4.3">4.3</A>) How do you change a column's data type?<BR>
- <A href="#item4.4">4.4</A>) What is the maximum size for a row, a
- table, and a database?<BR>
- <A href="#item4.5">4.5</A>) How much database disk space is required
- to store data from a typical text file?<BR>
- <A href="#item4.6">4.6</A>) Why are my queries slow? Why don't they
- use my indexes?<BR>
- <A href="#item4.7">4.7</A>) How do I see how the query optimizer is
- evaluating my query?<BR>
- <A href="#item4.8">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?<BR>
- <A href="#item4.9">4.9</A>) In a query, how do I detect if a field
- is <SMALL>NULL</SMALL>? How do I concatenate possible <SMALL>NULL</SMALL>s?
- How can I sort on whether a field is <SMALL> NULL</SMALL> or not?<BR>
- <A href="#item4.10">4.10</A>) What is the difference between the
- various character types?<BR>
- <A href="#item4.11.1">4.11.1</A>) How do I create a
- serial/auto-incrementing field?<BR>
- <A href="#item4.11.2">4.11.2</A>) How do I get the value of a
- <SMALL>SERIAL</SMALL> insert?<BR>
- <A href="#item4.11.3">4.11.3</A>) Doesn't <I>currval()</I>
- lead to a race condition with other users?<BR>
- <A href="#item4.11.4">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?<BR>
- <A href="#item4.12">4.12</A>) What is an <SMALL>OID</SMALL>? What is a
- <SMALL>CTID</SMALL>?<BR>
- <A href="#item4.13">4.13</A>) Why do I get the error <I>"ERROR: Memory
- exhausted in AllocSetAlloc()"</I>?<BR>
- <A href="#item4.14">4.14</A>) How do I tell what PostgreSQL version I
- am running?<BR>
- <A href="#item4.15">4.15</A>) How do I create a column that will
- default to the current time?<BR>
- <A href="#item4.16">4.16</A>) How do I perform an outer join?<BR>
- <A href="#item4.17">4.17</A>) How do I perform queries using multiple
- databases?<BR>
- <A href="#item4.18">4.18</A>) How do I return multiple rows or columns
- from a function?<BR>
- <A href="#item4.19">4.19</A>) Why do I get "relation with OID #####
- does not exist" errors when accessing temporary tables in PL/PgSQL
- functions?<BR>
- <A href="#item4.20">4.20</A>) What replication solutions are available?<BR>
- <A href="#item4.21">4.21</A>) Why are my table and column names not
- recognized in my query? Why is capitalization not preserved?<BR>
-
-
- <HR>
-
- <H2 align="center">General Questions</H2>
-
- <H3 id="item1.1">1.1) What is PostgreSQL? How is it pronounced? What is Postgres?</H3>
-
- <P>PostgreSQL is pronounced <I>Post-Gres-Q-L</I>. (For those curious
- about how to say "PostgreSQL", an <a
- href="http://www.postgresql.org/files/postgresql.mp3">audio file</a>
- is available.)</P>
-
- <P>PostgreSQL is an object-relational database system that has the
- features of traditional commercial database systems with
- enhancements to be found in next-generation <SMALL>DBMS</SMALL>
- systems. PostgreSQL is free and the complete source code is
- available.</P>
-
- <P>PostgreSQL development is performed by a team of mostly volunteer
- developers spread throughout the world and communicating via the
- Internet. It is a community project and is not controlled by any
- company. To get involved, see the developer's FAQ at <A href=
- "http://www.postgresql.org/docs/faqs.FAQ_DEV.html">
- http://www.postgresql.org/docs/faqs.FAQ_DEV.html</A>
- </P>
-
- <P>Postgres is a widely-used nickname for PostgreSQL. It was the
- original name of the project at Berkeley and is strongly preferred
- over other nicknames. If you find 'PostgreSQL' hard to pronounce, call
- it 'Postgres' instead.</P>
-
- <H3 id="item1.2">1.2) Who controls PostgreSQL?<BR></H3>
-
- <P>If you are looking for a PostgreSQL gatekeeper, central committee,
- or controlling company, give up --- there isn't one. We do have a
- core committee and CVS committers, but these groups are more for
- administrative purposes than control. The project is directed by
- the community of developers and users, which anyone can join. All
- you need to do is subscribe to the mailing lists and participate in the
- discussions. (See the <a href="http://wiki.postgresql.org/wiki/Developer_FAQ">
- Developer's FAQ</A> for information on how to get involved in PostgreSQL
- development.)</P>
-
- <H3 id="item1.3">1.3) What is the copyright of
- PostgreSQL?</H3>
-
- <P>PostgreSQL is distributed under the classic BSD license. Basically,
- it allows users to do anything they want with the code, including
- reselling binaries without the source code. The only restriction is
- that you not hold us legally liable for problems with the software.
- There is also the requirement that this copyright appear in all copies
- of the software. Here is the actual BSD license we use:</P>
-
- <P>PostgreSQL Data Base Management System</P>
-
- <P>Portions Copyright (c) 1996-2009, PostgreSQL Global Development Group
- Portions Copyright (c) 1994-1996 Regents of the University of California</P>
-
- <P>Permission to use, copy, modify, and distribute this software
- and its documentation for any purpose, without fee, and without a
- written agreement is hereby granted, provided that the above
- copyright notice and this paragraph and the following two
- paragraphs appear in all copies.</P>
-
- <P>IN NO EVENT SHALL THE UNIVERSITY OF CALIFORNIA BE LIABLE TO ANY
- PARTY FOR DIRECT, INDIRECT, SPECIAL, INCIDENTAL, OR CONSEQUENTIAL
- DAMAGES, INCLUDING LOST PROFITS, ARISING OUT OF THE USE OF THIS
- SOFTWARE AND ITS DOCUMENTATION, EVEN IF THE UNIVERSITY OF
- CALIFORNIA HAS BEEN ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.</P>
-
- <P>THE UNIVERSITY OF CALIFORNIA SPECIFICALLY DISCLAIMS ANY
- WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
- OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE. THE
- SOFTWARE PROVIDED HEREUNDER IS ON AN "AS IS" BASIS, AND THE
- UNIVERSITY OF CALIFORNIA HAS NO OBLIGATIONS TO PROVIDE MAINTENANCE,
- SUPPORT, UPDATES, ENHANCEMENTS, OR MODIFICATIONS.</P>
-
- <H3 id="item1.4">1.4) 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
- the time of release are listed in the installation
- instructions.</P>
-
- <P>PostgreSQL also runs natively on Microsoft Windows NT-based operating
- systems like Win2000 SP4, WinXP, and Win2003. A prepackaged installer is
- available at <a href= "http://www.postgresql.org/download/windows">
- http://www.postgresql.org/download/windows</a>. MSDOS-based versions
- of Windows (Win95, Win98, WinMe) can run PostgreSQL using Cygwin.</P>
-
- <p>There is also a Novell Netware 6 port at
- <a href="http://developer.novell.com/wiki/index.php/Postgresql">http://developer.novell.com/wiki/index.php/Postgresql</a>,
- and an OS/2 (eComStation) version at <a href=
- "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>
-
- <H3 id="item1.5">1.5) 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>
-
- <H3 id="item1.6">1.6) What is the most recent release?</H3>
-
- <P>The latest release of PostgreSQL is version 8.3.3.</P>
-
- <P>We plan to have a major release every year, with minor releases
- every few months.</P>
-
- <H3 id="item1.7">1.7) 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
- <a href="http://www.postgresql.org/community/lists/">
- http://www.postgresql.org/community/lists/</a>. The <I>general</I>
- or <I>bugs</I> lists are a good place to start.</P>
-
- <P>The major IRC channel is <I>#postgresql</I> on Freenode
- (<I>irc.freenode.net</I>). To connect you can use the Unix
- program <CODE>irc -c '#postgresql' "$USER" irc.freenode.net</CODE>
- or use any other IRC clients. A Spanish one also exists
- on the same network, (<I>#postgresql-es</I>), a French one,
- (<I>#postgresqlfr</I>), and a Brazilian one, (<I>#postgresql-br</I>).
- There is also a PostgreSQL channel on EFNet.</P>
-
- <P>A list of commercial support companies is available at <A href=
- "http://www.postgresql.org/support/professional_support">
- http://www.postgresql.org/support/professional_support</A>.</P>
-
- <H3 id="item1.8">1.8) How do I submit a bug report?</H3>
-
- <P>Visit the PostgreSQL bug form at <A href=
- "http://www.postgresql.org/support/submitbug">
- http://www.postgresql.org/support/submitbug</A>.
- Also check out our ftp site <A href=
- "ftp://ftp.postgresql.org/pub/">ftp://ftp.postgresql.org/pub/</A> to
- see if there is a more recent PostgreSQL version.</P>
-
- <P>Bugs submitted using the bug form or posted to any PostgreSQL mailing
- list typically generates one of the following replies:</P>
- <ul>
- <li>It is not a bug, and why</li>
- <li>It is a known bug and is already on the
- <A href="http://www.postgresql.org/docs/faqs.TODO.html">TODO</A> list</li>
- <li>The bug has been fixed in the current release</li>
- <li>The bug has been fixed but is not packaged yet in an official
- release</li>
- <li>A request is made for more detailed information:
- <ul>
- <li>Operating system</li>
- <li>PostgreSQL version</li>
- <li>Reproducible test case</li>
- <li>Debugging information</li>
- <li>Debugger backtrace output</li>
- </ul>
- </li>
- <li>The bug is new. The following might happen:
- <ul>
- <li>A patch is created and will be included in the next major
- or minor release</li>
- <li>The bug cannot be fixed immediately and is added
- to the <A href="http://www.postgresql.org/docs/faqs.TODO.html">TODO</A>
- list</li>
- </ul>
- </li>
- </ul>
-
- <H3 id="item1.9">1.9) How do I find out about known bugs or
- missing features?</H3>
-
- <P>PostgreSQL supports an extended subset of <SMALL>SQL:2003</SMALL>.
- See our <A href="http://www.postgresql.org/docs/faqs.TODO.html">TODO</A>
- list for known bugs, missing features, and future plans.</P>
-
- <P>A feature request usually results in one of the following
- replies:</P>
- <ul>
- <li>The feature is already on the
- <A href="http://www.postgresql.org/docs/faqs.TODO.html">TODO</A>
- list</li>
- <li>The feature is not desired because:
- <ul>
- <li>It duplicates existing functionality that already
- follows the SQL standard</li>
- <li>The feature would increase code complexity but add little
- benefit</li>
- <li>The feature would be insecure or unreliable</li>
- </ul>
- </li>
- <li>The new feature is added to the
- <A href="http://www.postgresql.org/docs/faqs.TODO.html">TODO</A> list</li>
- </ul>
-
- <P>PostgreSQL does not use a bug tracking system because we find
- it more efficient to respond directly to email and keep the
- <A href="http://www.postgresql.org/docs/faqs.TODO.html">TODO</A>
- list up-to-date. In practice, bugs don't last very long in the
- software, and bugs that affect a large number of users are fixed
- rapidly. The only place to find all changes, improvements, and
- fixes in a PostgreSQL release is to read the
- <a href="http://www.postgresql.org/developer/sourcecode/">CVS</a>
- log messages. Even the release notes do not list every change
- made to the software.</P>
-
- <H3 id="item1.10">1.10) 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>
- directory. You can also browse the manuals online at <A href=
- "http://www.postgresql.org/docs">http://www.postgresql.org/docs</A>.
- </P>
-
- <P>There are two PostgreSQL books available online at <A href=
- "http://www.postgresql.org/docs/books/awbook.html">http://www.postgresql.org/docs/books/awbook.html</A>
- and <A href=
- "http://www.commandprompt.com/ppbook/">http://www.commandprompt.com/ppbook/</A>.
- There are a number of PostgreSQL books available for purchase. One
- of the most popular ones is by Korry Douglas. A list of book reviews
- can be found at <A href=
- "http://www.postgresql.org/docs/books/">http://www.postgresql.org/docs/books/</A>.
- There is also a collection of PostgreSQL technical articles at <A
- href=
- "http://wiki.postgresql.org/wiki/Community_Generated_Articles%2C_Guides%2C_and_Documentation">http://wiki.postgresql.org/wiki/Community_Generated_Articles%2C_Guides%2C_and_Documentation</A>.</P>
-
- <P>The command line client program <I>psql</I> has some \d commands to show
- information about types, operators, functions, aggregates, etc. - use \? to
- display the available commands.</P>
-
- <P>Our web site contains even more documentation.</P>
-
- <H3 id="item1.11">1.11) How can I learn
- <SMALL>SQL</SMALL>?</H3>
-
- <P>First, consider the PostgreSQL-specific books mentioned above.
- Many of our users also like <I>The Practical SQL Handbook</I>,
- Bowman, Judith S., et al., Addison-Wesley. Others like <I>The
- Complete Reference SQL</I>, Groff et al., McGraw-Hill.</P>
-
- <P>There are also many nice tutorials available online:
- <UL>
- <LI> <A href="http://www.intermedia.net/support/sql/sqltut.shtm">http://www.intermedia.net/support/sql/sqltut.shtm</A>
- </LI>
- <LI>
- <A href="http://sqlcourse.com/">http://sqlcourse.com</A>
- </LI>
- <LI>
- <A href="http://www.w3schools.com/sql/default.asp">http://www.w3schools.com/sql/default.asp</A>
- </LI>
- <LI><A href=
- "http://mysite.verizon.net/Graeme_Birchall/id1.html">http://mysite.verizon.net/Graeme_Birchall/id1.html</A>
- </LI>
- <LI><A href=
- "http://sqlzoo.net">http://sqlzoo.net</A>
- </LI>
- </UL>
-
- <H3 id="item1.12">1.12) How do I submit a patch or join the development
- team?</H3>
-
- <P>See the <a href="http://www.postgresql.org/docs/faqs.FAQ_DEV.html">
- Developer's FAQ</A>.</P>
-
- <H3 id="item1.13">1.13) How does PostgreSQL compare to other
- <SMALL>DBMS</SMALL>s? Can PostgreSQL be embedded?</H3>
-
- <P>There are several ways of measuring software: features,
- performance, reliability, support, and price.</P>
-
- <DL>
- <DT><B>Features</B></DT>
-
- <DD>PostgreSQL has most features present in large commercial
- <SMALL>DBMS</SMALL>s, like transactions, subselects, triggers,
- views, foreign key referential integrity, and sophisticated
- locking. We have some features they do not have, like
- user-defined types, inheritance, rules, and multi-version
- concurrency control to reduce lock contention.<BR>
- <BR>
- </DD>
-
- <DT><B>Performance</B></DT>
-
- <DD>PostgreSQL's performance is comparable to other commercial and
- open source databases. It is faster for some things, slower for
- others. Our performance is usually +/-10% compared to other databases.
- <BR>
- </DD>
-
- <DT><B>Reliability</B></DT>
-
- <DD>We realize that a <SMALL>DBMS</SMALL> must be reliable, or it
- is worthless. We strive to release well-tested, stable code that
- has a minimum of bugs. Each release has at least one month of
- beta testing, and our release history shows that we can provide
- stable, solid releases that are ready for production use. We
- believe we compare favorably to other database software in this
- area.<BR>
- <BR>
- </DD>
-
- <DT><B>Support</B></DT>
-
- <DD>Our mailing lists provide contact with a large group of developers
- and users to help resolve any problems encountered. While we cannot
- guarantee a fix, commercial <SMALL>DBMS</SMALL>s do not always
- supply a fix either. Direct access to developers, the user
- community, manuals, and the source code often make PostgreSQL
- support superior to other <SMALL>DBMS</SMALL>s. There is
- commercial per-incident support available for those who need it.
- (See <A href="#item1.7">FAQ section 1.7</A>.)<BR>
- <BR>
- </DD>
-
- <DT><B>Price</B></DT>
-
- <DD>We are free for all use, both commercial and non-commercial.
- You can add our code to your product with no limitations, except
- those outlined in our BSD-style license stated above.<BR>
- <BR>
- </DD>
- </DL>
-
- <P>PostgreSQL is designed as a client/server architecture, which
- requires separate processes for each client and server, and various
- helper processes. Many embedded architectures can support such
- requirements. However, if your embedded architecture requires the
- database server to run inside the application process, you cannot use
- Postgres and should select a lighter-weight database solution.</P>
-
- <H3 id="item1.14">1.14) Will PostgreSQL handle recent daylight
- saving time changes in various countries?</H3>
-
- <P>USA daylight saving time changes are included in PostgreSQL release
- 8.0.[4+], and all later major releases, e.g. 8.1. Canada and Western
- Australia changes are included in 8.0.[10+], 8.1.[6+], and all later
- major releases. PostgreSQL releases prior to 8.0 use the operating
- system's timezone database for daylight saving information.</P>
-
- <H3 id="item1.15">1.15) How do I unsubscribe from the
- PostgreSQL email lists? How do I avoid receiving duplicate emails?</H3>
-
- <P>The PostgreSQL <a
- href="http://mail.postgresql.org/mj/mj_wwwusr/domain=postgresql.org?user=&passw=&func=login">Majordomo</a>
- page allows subscribing or unsubscribing from any of the PostgreSQL
- email lists. (You might need to have your Majordomo password emailed
- to you to log in.)</P>
-
- <P>All PostgreSQL email lists are configured so a group reply goes to
- the email list <I>and</I> the original email author. This is done so
- users receive the quickest possible email replies. If you would prefer
- not to receive duplicate email from the list in cases where you already
- receive an email directly, check <I>eliminatecc</I> from the Majordomo
- <I>Change Settings</I> page. You can also prevent yourself from
- receiving copies of emails you post to the lists by unchecking
- <I>selfcopy</I>.</P>
-
-
- <HR>
-
- <H2 align="center">User Client Questions</H2>
-
- <H3 id="item2.1">2.1) 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
- that are downloaded separately; being separate allows them to have their
- own release schedule and development teams.</P>
-
- <P>Some programming languages like <SMALL>PHP</SMALL> include an
- interface to PostgreSQL. Interfaces for languages like Perl,
- <SMALL>TCL</SMALL>, Python, and many others are available at
- <a href="http://pgfoundry.org">http://pgfoundry.org</A>.</P>
-
- <H3 id="item2.2">2.2) 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>
-
- <P>For Web integration, PHP (<A
- href="http://www.php.net">http://www.php.net</A>) is an excellent
- interface.</P>
-
- <P>For complex cases, many use the Perl and DBD::Pg with CGI.pm or
- mod_perl.</P>
-
- <H3 id="item2.3">2.3) Does PostgreSQL have a graphical user
- interface?</H3>
-
- <P>There are a large number of GUI Tools that are available for PostgreSQL
- from both commercial and open source developers. A detailed list can be
- found in the <A href="http://wiki.postgresql.org/wiki/Community_Guide_to_PostgreSQL_GUI_Tools">
- Community Guide to PostgreSQL GUI Tools</A>.</P>
-
- <HR>
-
-
- <H2 align="center">Administrative Questions</H2>
-
- <H3 id="item3.1">3.1) 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>
-
- <H3 id="item3.2">3.2) 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
- machines will not be able to connect unless you modify
- <I>listen_addresses</I> in the <I>postgresql.conf</I> file, enable
- host-based authentication by modifying the
- <I>$PGDATA/pg_hba.conf</I> file, and restart the database server.</P>
-
- <H3 id="item3.3">3.3) How do I tune the database engine for
- better performance?</H3>
-
- <P>There are three major areas for potential performance
- improvement:</P>
-
- <DL>
- <DT><B>Query Changes</B></DT>
-
- <DD>This involves modifying queries to obtain better
- performance:
- <ul>
- <li>Creation of indexes, including expression and partial
- indexes</li>
- <li>Use of COPY instead of multiple <SMALL>INSERT</SMALL>s</li>
- <li>Grouping of multiple statements into a single transaction to
- reduce commit overhead</li>
- <li>Use of <SMALL>CLUSTER</SMALL> when retrieving many rows from an
- index</li>
- <li>Use of <SMALL>LIMIT</SMALL> for returning a subset of a query's
- output</li>
- <li>Use of Prepared queries</li>
- <li>Use of <SMALL>ANALYZE</SMALL> to maintain accurate optimizer
- statistics</li>
- <li>Regular use of <SMALL>VACUUM</SMALL> or <I>pg_autovacuum</I></li>
- <li>Dropping of indexes during large data changes</li>
- </ul><BR>
- <BR>
- </DD>
-
- <DT><B>Server Configuration</B></DT>
-
- <DD>A number of <I>postgresql.conf</I> settings affect performance.
- For more details, see <a href=
- "http://www.postgresql.org/docs/current/static/runtime-config.html">
- Administration Guide/Server Run-time Environment/Run-time
- Configuration</a> for a full listing, and for commentary see <a
- href="http://www.varlena.com/varlena/GeneralBits/Tidbits/annotated_conf_e.html">
- http://www.varlena.com/varlena/GeneralBits/Tidbits/annotated_conf_e.html</a>
- and <a href="http://www.varlena.com/varlena/GeneralBits/Tidbits/perf.html">
- http://www.varlena.com/varlena/GeneralBits/Tidbits/perf.html</a>.
- <BR>
- <BR>
- </DD>
-
- <DT><B>Hardware Selection</B></DT>
-
- <DD>The effect of hardware on performance is detailed in
- <a href="http://www.powerpostgresql.com/PerfList/">
- http://www.powerpostgresql.com/PerfList/</a> and <a
- href="http://momjian.us/main/writings/pgsql/hw_performance/index.html">
- http://momjian.us/main/writings/pgsql/hw_performance/index.html</a>.
- <BR>
- <BR>
- </DD>
- </DL>
-
- <H3 id="item3.4">3.4) What debugging features are
- available?</H3>
-
- <P>There are many <CODE>log_*</CODE> server configuration
- variables at <a
- href="http://www.postgresql.org/docs/current/interactive/runtime-config-logging.html">http://www.postgresql.org/docs/current/interactive/runtime-config-logging.html</a>
- that enable printing of query and process statistics which
- can be very useful for debugging and performance measurements.</P>
-
- <H3 id="item3.5">3.5) Why do I get <I>"Sorry, too many
- clients"</I> when trying to connect?</H3>
-
- <P>You have reached the default limit of 100 database sessions. You
- need to increase the server's limit on how many
- concurrent backend processes it can start by changing the
- <I>max_connections</I> value in <I>postgresql.conf</I> and
- restarting the server.</P>
-
- <H3 id="item3.6">3.6) What is the upgrade process for PostgreSQL?</H3>
-
- <P>See <a
- href="http://www.postgresql.org/support/versioning">http://www.postgresql.org/support/versioning</a>
- for a general discussion about upgrading, and <a href=
- "http://www.postgresql.org/docs/current/static/install-upgrading.html">
- http://www.postgresql.org/docs/current/static/install-upgrading.html</a>
- for specific instructions.</P>
-
- <H3 id="item3.7">3.7) 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
- quality motherboards are more reliable and have better performance than
- less expensive hardware. PostgreSQL will run on almost any hardware,
- but if reliability and performance are important it is wise to
- research your hardware options thoroughly. A disk controller with a
- battery-backed cache is also useful. Our email lists can be used
- to discuss hardware options and tradeoffs.</P>
-
- <HR>
-
- <H2 align="center">Operational Questions</H2>
-
- <H3 id="item4.1">4.1) 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
- <SMALL>LIMIT</SMALL> . If an index matches the <SMALL>ORDER
- BY</SMALL> it is possible the entire query does not have to be
- executed. If you don't know the number of rows at
- <SMALL>SELECT</SMALL> time, use a cursor and
- <SMALL>FETCH</SMALL>.</P>
-
- <P>To <SMALL>SELECT</SMALL> a random row, use:</P>
-<PRE>
- SELECT col
- FROM tab
- ORDER BY random()
- LIMIT 1;
-</PRE>
-
- <H3 id="item4.2">4.2) 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?</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
- code for <I>psql</I> in file <I>pgsql/src/bin/psql/describe.c</I>, it
- contains <SMALL>SQL</SMALL> commands that generate the output for
- <I>psql</I>'s backslash commands. You can also start <I>psql</I> with the
- <I>-E</I> option so it will print out the queries it uses to execute the
- commands you give. PostgreSQL also provides an <SMALL>SQL</SMALL> compliant
- INFORMATION SCHEMA interface you can query to get information about the
- database.</P>
-
- <P>There are also system tables beginning with <I>pg_</I> that describe
- these too.</P>
-
- <P>Use <I>psql -l</I> will list all databases.</P>
-
- <P>Also try the file <I>pgsql/src/tutorial/syscat.source</I>. It
- illustrates many of the <SMALL>SELECT</SMALL>s needed to get
- information from the database system tables.</P>
-
- <H3 id="item4.3">4.3) 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>
-
- <P>In earlier releases, do this:</P>
-<PRE>
- BEGIN;
- ALTER TABLE tab ADD COLUMN new_col <i>new_data_type</i>;
- UPDATE tab SET new_col = CAST(old_col AS <i>new_data_type</i>);
- ALTER TABLE tab DROP COLUMN old_col;
- COMMIT;
-</PRE>
- <P>You might then want to do <I>VACUUM FULL tab</I> to reclaim the
- disk space used by the expired rows.</P>
-
- <H3 id="item4.4">4.4) What is the maximum size for a row, a
- table, and a database?</H3>
-
- <P>These are the limits:</P>
-<BLOCKQUOTE>
-<TABLE>
-<TR><TD>Maximum size for a database?</TD><TD>unlimited (32 TB databases
-exist)</TD></TR>
-<TR><TD>Maximum size for a table?</TD><TD>32 TB</TD></TR>
-<TR><TD>Maximum size for a row?</TD><TD>400 GB</TD></TR>
-<TR><TD>Maximum size for a field?</TD><TD>1 GB</TD></TR>
-<TR><TD>Maximum number of rows in a table?</TD><TD>unlimited</TD></TR>
-<TR><TD>Maximum number of columns in a table?</TD><TD>250-1600 depending
-on column types</TD></TR>
-<TR><TD>Maximum number of indexes on a
-table?</TD><TD>unlimited</TD></TR>
-</TABLE>
-</BLOCKQUOTE>
-
- <P>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.</P>
-
- <P>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.</P>
-
- <P>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.</P>
-
- <P>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.</P>
-
- <H3 id="item4.5">4.5) 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>
-
- <P>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:</P>
-<PRE>
- 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)
-</PRE>
-
- <P>Indexes do not require as much overhead, but do contain the data
- that is being indexed, so they can be large also.</P>
-
- <P><SMALL>NULL</SMALL>s are stored as bitmaps, so they
- use very little space.</P>
-
- <H3 id="item4.6">4.6) 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
- 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. </P>
-
- <P>To determine if an index should be used, PostgreSQL must have
- statistics about the table. These statistics are collected using
- <SMALL>VACUUM ANALYZE</SMALL>, or simply <SMALL>ANALYZE</SMALL>.
- 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.</P>
-
- <P>Indexes are normally not used for <SMALL>ORDER BY</SMALL> or to
- perform joins. A sequential scan followed by an explicit sort is
- usually faster than an index scan of a large table.
- However, <SMALL>LIMIT</SMALL> combined with <SMALL>ORDER BY</SMALL>
- often will use an index because only a small portion of the table
- is returned.</P>
-
- <P>If you believe the optimizer is incorrect in choosing a
- sequential scan, use <CODE>SET enable_seqscan TO 'off'</CODE> and
- run query again to see if an index scan is indeed faster.</P>
-
- <P>When using wild-card operators such as <SMALL>LIKE</SMALL> or
- <I>~</I>, indexes can only be used in certain circumstances:</P>
- <UL>
- <LI>The beginning of the search string must be anchored to the start
- of the string, i.e.
- <UL>
- <LI><SMALL>LIKE</SMALL> patterns must not start with <I>%</I>.</LI>
- <LI><I>~</I> (regular expression) patterns must start with
- <I>^</I>.</LI>
- </UL></LI>
- <LI>The search string can not start with a character class,
- e.g. [a-e].</LI>
- <LI>Case-insensitive searches such as <SMALL>ILIKE</SMALL> and
- <I>~*</I> do not utilize indexes. Instead, use expression
- indexes, which are described in section <a href="#item4.8">4.8</a>.</LI>
- <LI>The default <I>C</I> locale must be used during
- <i>initdb</i> because it is not possible to know the next-greatest
- character in a non-C locale. You can create a special
- <CODE>text_pattern_ops</CODE> index for such cases that work only
- for <SMALL>LIKE</SMALL> indexing. It is also possible to use
- full text indexing for word searches.
- </LI>
- </UL>
-
- <H3 id="item4.7">4.7) How do I see how the query optimizer is
- evaluating my query?</H3>
-
- <P>See the <SMALL>EXPLAIN</SMALL> manual page.</P>
-
- <H3 id="item4.8">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?</H3>
-
- <P>The <I>~</I> operator does regular expression matching, and
- <I>~*</I> does case-insensitive regular expression matching. The
- case-insensitive variant of <SMALL>LIKE</SMALL> is called
- <SMALL>ILIKE</SMALL>.</P>
-
- <P>Case-insensitive equality comparisons are normally expressed
- as:</P>
-<PRE>
- SELECT *
- FROM tab
- WHERE lower(col) = 'abc';
-</PRE>
- This will not use an standard index. However, if you create an
- expression index, it will be used:
-<PRE>
- CREATE INDEX tabindex ON tab (lower(col));
-</PRE>
- <P>If the above index is created as <SMALL>UNIQUE</SMALL>, 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 <SMALL>CHECK</SMALL>
- constraint or a trigger.</P>
-
- <H3 id="item4.9">4.9) In a query, how do I detect if a field
- is <SMALL>NULL</SMALL>? How do I concatenate possible <SMALL>NULL</SMALL>s?
- How can I sort on whether a field is <SMALL> NULL</SMALL> or not?</H3>
-
- <P>You test the column with <SMALL>IS NULL</SMALL> and <SMALL>IS
- NOT NULL</SMALL>, like this:</P>
-
-<PRE>
- SELECT *
- FROM tab
- WHERE col IS NULL;
-</PRE>
-
- <P>To concatentate with possible <SMALL>NULL</SMALL>s, use <I>COALESCE()</I>,
- like this:</P>
-<PRE>
- SELECT COALESCE(col1, '') || COALESCE(col2, '')
- FROM tab
-</PRE>
-
- <P>To sort by the <SMALL>NULL</SMALL> status, use the <SMALL>IS NULL</SMALL>
- and <SMALL>IS NOT NULL</SMALL> modifiers in your <SMALL>ORDER BY</SMALL> clause.
- Things that are <I>true</I> will sort higher than things that are <I>false</I>,
- so the following will put NULL entries at the top of the resulting list:</P>
-
-<PRE>
- SELECT *
- FROM tab
- ORDER BY (col IS NOT NULL)
-</PRE>
-
- <H3 id="item4.10">4.10) What is the difference between the
- various character types?</H3>
-<BLOCKQUOTE>
-<TABLE>
-<TR><TH>Type</TH><TH>Internal Name</TH><TH>Notes</TH></TR>
-<TR><TD>VARCHAR(n)</TD><TD>varchar</TD><TD>size specifies maximum
-length, no padding</TD></TR>
-<TR><TD>CHAR(n)</TD><TD>bpchar</TD><TD>blank padded to the specified
-fixed length</TD></TR>
-<TR><TD>TEXT</TD><TD>text</TD><TD>no specific upper limit on
-length</TD></TR>
-<TR><TD>BYTEA</TD><TD>bytea</TD><TD>variable-length byte array
-(null-byte safe)</TD></TR>
-<TR><TD>"char"</TD><TD>char</TD><TD>one character</TD></TR>
-</TABLE>
-</BLOCKQUOTE>
-
- <P>You will see the internal name when examining system catalogs
- and in some error messages.</P>
-
- <P>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.</P>
-
- <SMALL>VARCHAR(n)</SMALL> is best when storing variable-length
- strings and it limits how long a string can be. <SMALL>TEXT</SMALL>
- is for strings of unlimited length, with a maximum of one gigabyte.
- <P><SMALL>CHAR(n)</SMALL> is for storing strings that are all the
- same length. <SMALL>CHAR(n)</SMALL> pads with blanks to the specified
- length, while <SMALL>VARCHAR(n)</SMALL> only stores the characters
- supplied. <SMALL>BYTEA</SMALL> is for storing binary data,
- particularly values that include <SMALL>NULL</SMALL> bytes. All the
- types described here have similar performance characteristics.</P>
-
- <H3 id="item4.11.1">4.11.1) 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, this:</P>
-<PRE>
- CREATE TABLE person (
- id SERIAL,
- name TEXT
- );
-</PRE>
-
- is automatically translated into this:
-
-<PRE>
- CREATE SEQUENCE person_id_seq;
- CREATE TABLE person (
- id INT4 NOT NULL DEFAULT nextval('person_id_seq'),
- name TEXT
- );
-</PRE>
-
- <P>Automatically created sequence are named
- <<I>table</I>>_<<I>serialcolumn</I>>_<I>seq</I>, where
- <I>table</I> and <I>serialcolumn</I> are the names of the table and
- <SMALL>SERIAL</SMALL> column, respectively. See the
- <I>create_sequence</I> manual page for more information about
- sequences.</P>
-
- <H3 id="item4.11.2">4.11.2) How do I get the value of a
- <SMALL>SERIAL</SMALL> insert?</H3>
-
- <P>The simplest way is to retrieve the assigned <SMALL>SERIAL</SMALL>
- value with <SMALL>RETURNING</SMALL>. Using the example table in <A
- href="#item4.11.1">4.11.1</A>, it would look like this:</P>
-
-<PRE>
- INSERT INTO person (name) VALUES ('Blaise Pascal') RETURNING id;
-</PRE>
-
- You can also call <I>nextval()</I> and use that value in the
- <SMALL>INSERT</SMALL>, or call <I>currval()</I> <I>after</I> the
- <SMALL>INSERT</SMALL>.
-
- <H3 id="item4.11.3">4.11.3) 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>
-
- <H3 id="item4.11.4">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?</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>
-
- <H3 id="item4.12">4.12) What is an <SMALL>OID</SMALL>? What is
- a <SMALL>CTID</SMALL>?</H3>
-
- <P>If a table is created <SMALL>WITH OIDS</SMALL>, each row
- gets a unique a <SMALL>OID</SMALL>. O<SMALL>ID</SMALL>s are
- automatically assigned unique 4-byte integers that are unique
- across the entire installation. However, they overflow at 4
- billion, and then the O<SMALL>ID</SMALL>s start being duplicated.
- PostgreSQL uses <SMALL>OID</SMALL>s to link its internal system
- tables together.</P>
-
- <P>To uniquely number rows in user tables, it is best to use
- <SMALL>SERIAL</SMALL> rather than O<SMALL>ID</SMALL>s because
- <SMALL>SERIAL</SMALL> sequences are unique only within a single
- table. and are therefore less likely to overflow.
- <SMALL>SERIAL8</SMALL> is available for storing eight-byte sequence
- values.</P>
-
- <P>C<SMALL>TID</SMALL>s are used to identify specific physical rows
- with block and offset values. C<SMALL>TID</SMALL>s change after rows
- are modified or reloaded. They are used by index entries to point
- to physical rows.</P>
-
- <H3 id="item4.13">4.13) 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
- before starting the server:</P>
-<PRE>
- ulimit -d 262144
- limit datasize 256m
-</PRE>
-
- 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 <SMALL>SQL</SMALL> client because the
- backend is returning too much data, try it before starting the
- client.
-
- <H3 id="item4.14">4.14) How do I tell what PostgreSQL version
- I am running?</H3>
-
- <P>From <I>psql</I>, type <CODE>SELECT version();</CODE></P>
-
- <H3 id="item4.15">4.15) 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>
-
- <H3 id="item4.16">4.16) How do I perform an outer join?</H3>
-
- <P>PostgreSQL supports outer joins using the SQL standard syntax.
- Here are two examples:</P>
-<PRE>
- SELECT *
- FROM t1 LEFT OUTER JOIN t2 ON (t1.col = t2.col);
-</PRE>
- or
-<PRE>
- SELECT *
- FROM t1 LEFT OUTER JOIN t2 USING (col);
-</PRE>
-
- <P>These identical queries join t1.col to t2.col, and also return
- any unjoined rows in t1 (those with no match in t2). A
- <SMALL>RIGHT</SMALL> join would add unjoined rows of t2. A
- <SMALL>FULL</SMALL> join would return the matched rows plus all
- unjoined rows from t1 and t2. The word <SMALL>OUTER</SMALL> is
- optional and is assumed in <SMALL>LEFT</SMALL>,
- <SMALL>RIGHT</SMALL>, and <SMALL>FULL</SMALL> joins. Ordinary joins
- are called <SMALL>INNER</SMALL> joins.</P>
-
- <H3 id="item4.17">4.17) 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
- uncertain how a cross-database query should even behave.</P>
-
- <P><I>contrib/dblink</I> 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.</P>
-
- <H3 id="item4.18">4.18) How do I return multiple rows or
- columns from a function?</H3>
-
- <P>It is easy using set-returning functions,
- <a href="http://wiki.postgresql.org/wiki/Return_more_than_one_row_of_data_from_PL/pgSQL_functions">
- http://wiki.postgresql.org/wiki/Return_more_than_one_row_of_data_from_PL/pgSQL_functions</a>.</P>
-
- <H3 id="item4.19">4.19) Why do I get "relation with OID #####
- does not exist" errors when accessing temporary tables in PL/PgSQL
- functions?</H3>
-
- <P>In PostgreSQL versions < 8.3, 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 <SMALL>EXECUTE</SMALL> for temporary table access in
- PL/PgSQL. This will cause the query to be reparsed every time.</P>
-
- <P>This problem does not occur in PostgreSQL 8.3 and later.</P>
-
- <H3 id="item4.20">4.20) What replication solutions are available?
- </H3>
-
- <P>Though "replication" is a single term, there are several technologies
- for doing replication, with advantages and disadvantages for each.</P>
-
- <P>Master/slave replication allows a single master to receive read/write
- queries, while slaves can only accept read/<SMALL>SELECT</SMALL>
- queries. The most popular freely available master-slave PostgreSQL
- replication solution is <A
- href="http://main.slony.info/">
- Slony-I</A>.</P>
-
- <P>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.
- <A href="http://pgfoundry.org/projects/pgcluster/">PGCluster</a> is the
- most popular such solution freely available for PostgreSQL.</P>
-
- <P>There are also commercial and hardware-based replication solutions
- available supporting a variety of replication models.</P>
-
- <H3 id="item4.21">4.21) Why are my table and column names not
- recognized in my query? Why is capitalization not preserved?</H3>
-
- <P>The most common cause of unrecognized 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 <a
- href="http://www.postgresql.org/docs/current/static/sql-syntax-lexical.html#SQL-SYNTAX-IDENTIFIERS">
- case-sensitive</a>, 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:
- <UL>
- <LI>Avoid double-quoting identifiers when creating tables</LI>
- <LI>Use only lowercase characters in identifiers</LI>
- <LI>Double-quote identifiers when referencing them in queries</LI>
- </UL>
-
- </BODY>
-</HTML>
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