From: Bruce Momjian Date: Tue, 1 Feb 2005 02:42:03 +0000 (+0000) Subject: Backpatch FAQ changes to 8.0.X. X-Git-Tag: REL8_0_2~101 X-Git-Url: http://git.ipfire.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=925320fe3d31fd974a51c3257d4e6845b3d73066;p=thirdparty%2Fpostgresql.git Backpatch FAQ changes to 8.0.X. --- diff --git a/doc/FAQ b/doc/FAQ index 8ddab0a3258..58842e75b90 100644 --- a/doc/FAQ +++ b/doc/FAQ @@ -1,7 +1,7 @@ Frequently Asked Questions (FAQ) for PostgreSQL - Last updated: Sat Jan 29 23:44:48 EST 2005 + Last updated: Mon Jan 31 21:40:28 EST 2005 Current maintainer: Bruce Momjian (pgman@candle.pha.pa.us) @@ -15,18 +15,17 @@ General Questions 1.1) What is PostgreSQL? How is it pronounced? - 1.2) What is the copyright on PostgreSQL? + 1.2) What is the copyright of PostgreSQL? 1.3) What platforms does PostgreSQL support? 1.4) Where can I get PostgreSQL? 1.5) Where can I get support? - 1.6) What is the latest release? - 1.7) What documentation is available? - 1.8) How do I find out about known bugs or missing features? - 1.9) How can I learn SQL? - 1.10) How do I join the development team? - 1.11) How do I submit a bug report? + 1.6) How do I submit a bug report? + 1.7) What is the latest release? + 1.8) What documentation is available? + 1.9) How do I find out about known bugs or missing features? + 1.10) How can I learn SQL? + 1.11) How do I join the development team? 1.12) How does PostgreSQL compare to other DBMSs? - 1.13) How can I financially assist PostgreSQL? User Client Questions @@ -52,7 +51,7 @@ 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 remove a column from a table, or change its data type? + 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? @@ -69,19 +68,18 @@ 4.11.3) 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 TID? - 4.13) What is the meaning of some of the terms used in PostgreSQL? - 4.14) Why do I get the error "ERROR: Memory exhausted in + 4.13) Why do I get the error "ERROR: Memory exhausted in AllocSetAlloc()"? - 4.15) How do I tell what PostgreSQL version I am running? - 4.16) Why does my large-object operations get "invalid large obj + 4.14) How do I tell what PostgreSQL version I am running? + 4.15) Why does my large-object operations get "invalid large obj descriptor"? - 4.17) How do I create a column that will default to the current time? - 4.18) How do I perform an outer join? - 4.19) How do I perform queries using multiple databases? - 4.20) How do I return multiple rows or columns from a function? - 4.21) Why can't I reliably create/drop temporary tables in PL/PgSQL + 4.16) How do I create a column that will default to the current time? + 4.17) How do I perform an outer join? + 4.18) How do I perform queries using multiple databases? + 4.19) How do I return multiple rows or columns from a function? + 4.20) Why can't I reliably create/drop temporary tables in PL/PgSQL functions? - 4.22) What encryption options are available? + 4.21) What encryption options are available? Extending PostgreSQL @@ -98,38 +96,26 @@ 1.1) What is PostgreSQL? How is it pronounced? - PostgreSQL is pronounced Post-Gres-Q-L. + PostgreSQL is pronounced Post-Gres-Q-L, also called just Postgres. - PostgreSQL is an enhancement of the POSTGRES database management - system (and is still sometimes reffered to as simply "Postgres"), a - next-generation DBMS research prototype. While PostgreSQL retains the - powerful data model and rich data types of POSTGRES, it replaces the - PostQuel query language with an extended subset of SQL. PostgreSQL is - free and the complete source 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 developers who all - subscribe to the PostgreSQL development mailing list. The current - coordinator is Marc G. Fournier (scrappy@PostgreSQL.org). (See section - 1.6 on how to join). This team is now responsible for all development - of PostgreSQL. It is a community project and is not controlled by any + 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/files/documentation/faqs/FAQ_DEV.html - The authors of PostgreSQL 1.01 were Andrew Yu and Jolly Chen. Many - others have contributed to the porting, testing, debugging, and - enhancement of the code. The original Postgres code, from which - PostgreSQL is derived, was the effort of many graduate students, - undergraduate students, and staff programmers working under the - direction of Professor Michael Stonebraker at the University of - California, Berkeley. - - The original name of the software at Berkeley was Postgres. When SQL - functionality was added in 1995, its name was changed to Postgres95. - The name was changed at the end of 1996 to PostgreSQL. - - 1.2) What is the copyright on PostgreSQL? + 1.2) What is the copyright of PostgreSQL? - PostgreSQL is subject to the following COPYRIGHT: + PostgreSQL is distributed under the classic BSD license. It has no + restrictions on how the source code can be used. We like it and have + no intention of changing it. + + This is the BSD license we use: PostgreSQL Data Base Management System @@ -155,10 +141,6 @@ CALIFORNIA HAS NO OBLIGATIONS TO PROVIDE MAINTENANCE, SUPPORT, UPDATES, ENHANCEMENTS, OR MODIFICATIONS. - The above is the BSD license, the classic open-source license. It has - no restrictions on how the source code may be used. We like it and - have no intention of changing it. - 1.3) What platforms does PostgreSQL support? In general, any modern Unix-compatible platform should be able to run @@ -183,60 +165,40 @@ 1.5) Where can I get support? - The main mailing list is: pgsql-general@PostgreSQL.org. It is - available for discussion of matters pertaining to PostgreSQL. To - subscribe, send mail with the following lines in the body (not the - subject line): - subscribe - end - - to pgsql-general-request@PostgreSQL.org. - - There is also a digest list available. To subscribe to this list, send - email to: pgsql-general-digest-request@PostgreSQL.org with a body of: - subscribe - end - - Digests are sent out to members of this list whenever the main list - has received around 30k of messages. + 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 bugs mailing list is available. To subscribe to this list, send - email to pgsql-bugs-request@PostgreSQL.org with a body of: - subscribe - end - - There is also a developers discussion mailing list available. To - subscribe to this list, send email to - pgsql-hackers-request@PostgreSQL.org with a body of: - subscribe - end - - Additional mailing lists and information about PostgreSQL can be found - via the PostgreSQL WWW home page at: - - http://www.PostgreSQL.org - The major IRC channel is #postgresql on Freenode (irc.freenode.net). - To connect you can use the Unix command irc -c '#postgresql' "$USER" + To connect you can use the Unix program irc -c '#postgresql' "$USER" irc.freenode.net or use any of the other popular IRC clients. A Spanish one also exists on the same network, (#postgresql-es), and a French one, (#postgresqlfr). There is also a PostgreSQL channel on EFNet. A list of commercial support companies is available at - http://techdocs.postgresql.org/companies.php. + http://techdocs.postg resql.org/companies.php. + + 1.6) How do I submit a bug report? + + Visit the PostgreSQL bug form at + http://www.postgresql.org/support/submitbug. - 1.6) What is the latest release? + Also check out our ftp site ftp://ftp.PostgreSQL.org/pub to see if + there is a more recent PostgreSQL version. + + 1.7) What is the latest release? The latest release of PostgreSQL is version 8.0.0. - We plan to have major releases every six to eight months. + We plan to have major releases every ten to twelve months. - 1.7) What documentation is available? + 1.8) What documentation is available? - Several manuals, manual pages, and some small test examples are - included in the distribution. See the /doc directory. You can also - browse the manuals online at http://www.PostgreSQL.org/docs. + 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/awbook.html and @@ -252,12 +214,12 @@ Our web site contains even more documentation. - 1.8) How do I find out about known bugs or missing features? + 1.9) How do I find out about known bugs or missing features? PostgreSQL supports an extended subset of SQL-92. See our TODO list for known bugs, missing features, and future plans. - 1.9) How can I learn SQL? + 1.10) How can I learn SQL? The PostgreSQL book at http://www.PostgreSQL.org/docs/awbook.html teaches SQL. There is another PostgreSQL book at @@ -273,10 +235,10 @@ et al., Addison-Wesley. Others like The Complete Reference SQL, Groff et al., McGraw-Hill. - 1.10) How do I join the development team? + 1.11) How do I join the development team? First, download the latest source and read the PostgreSQL Developers - documentation on our web site, or in the distribution. Second, + FAQ and documentation on our web site, or in the distribution. Second, subscribe to the pgsql-hackers and pgsql-patches mailing lists. Third, submit high quality patches to pgsql-patches. @@ -286,14 +248,6 @@ and we had confidence that patches they committed were of high quality. - 1.11) 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 or patches. - 1.12) How does PostgreSQL compare to other DBMSs? There are several ways of measuring software: features, performance, @@ -340,29 +294,6 @@ 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. - - 1.13) How can I financially assist PostgreSQL? - - PostgreSQL has had a first-class infrastructure since we started in - 1996. This is all thanks to Marc Fournier, who has created and managed - this infrastructure over the years. - - Quality infrastructure is very important to an open-source project. It - prevents disruptions that can greatly delay forward movement of the - project. - - Of course, this infrastructure is not cheap. There are a variety of - monthly and one-time expenses that are required to keep it going. If - you or your company has money it can donate to help fund this effort, - please go to http://store.pgsql.com/shopping/ and make a donation. - - Although the web page mentions PostgreSQL, Inc, the "contributions" - item is solely to support the PostgreSQL project and does not fund any - specific company. If you prefer, you can also send a check to the - contact address. - - Also, if you have a success story about PostgreSQL, please email it to - our advocacy list at pgsql-advocacy@postgresql.org. _________________________________________________________________ User Client Questions @@ -384,16 +315,16 @@ A nice introduction to Database-backed Web pages can be seen at: http://www.webreview.com - For Web integration, PHP is an excellent interface. It is at - http://www.php.net. + For Web integration, PHP (http://www.php.net) is an excellent + interface. - For complex cases, many use the Perl interface and CGI.pm or mod_perl. + For complex cases, many use the Perl and CGI.pm or mod_perl. 2.3) Does PostgreSQL have a graphical user interface? Yes, there are several graphical interfaces to PostgreSQL available. - These include PgAccess http://www.pgaccess.org), pgAdmin III - (http://www.pgadmin.org, RHDB Admin (http://sources.redhat.com/rhdb/ + These include pgAdmin III (http://www.pgadmin.org, PgAccess + http://www.pgaccess.org), RHDB Admin (http://sources.redhat.com/rhd b/ ), TORA (http://www.globecom.net/tora/, partly commercial), and Rekall ( http://www.rekallrevealed.org/). There is also PhpPgAdmin ( http://phppgadmin.sourceforge.net/ ), a web-based interface to @@ -414,61 +345,58 @@ 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 and enable host-based authentication by modifying the - file $PGDATA/pg_hba.conf accordingly. + postgresql.conf file, enable host-based authentication by modifying + the $PGDATA/pg_hba.conf file, and restart the server. 3.3) How do I tune the database engine for better performance? - Certainly, indexes can speed up queries. The EXPLAIN ANALYZE command - allows you to see how PostgreSQL is interpreting your query, and which - indexes are being used. - - If you are doing many INSERTs, consider doing them in a large batch - using the COPY command. This is much faster than individual INSERTS. - Second, statements not in a BEGIN WORK/COMMIT transaction block are - considered to be in their own transaction. Consider performing several - statements in a single transaction block. This reduces the transaction - overhead. Also, consider dropping and recreating indexes when making - large data changes. - - There are several tuning options in the Administration Guide/Server - Run-time Environment/Run-time Configuration. You can disable fsync() - by using fsync option. This will prevent fsync()s from flushing to - disk after every transaction. - - You can use the shared_buffers option to increase the number of shared - memory buffers used by the backend processes. If you make this - parameter too high, the postmaster may not start because you have - exceeded your kernel's limit on shared memory space. Each buffer is 8K - and the default is 1000 buffers. - - You can also use the sort_mem (from PostgreSQL 8.0: work_mem) options - to increase the maximum amount of memory used by the backend processes - for each temporary sort. The default is 1024 (i.e. 1MB). - - You can also use the CLUSTER command to group data in tables to match - an index. See the CLUSTER manual page for more details. + 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://candle.pha.pa.us/main/writings/pgsql/hw_performance/inde + x.html. + 3.4) What debugging features are available? - PostgreSQL has several features that report status information that - can be valuable for debugging purposes. + There are many log_* server configuration variables that enable + printing of query and process statistics which can be very useful for + debugging and performance measurements. - First, by running configure with the --enable-cassert option, many - assert()s monitor the progress of the backend and halt the program - when something unexpected occurs. + The following detailed debug instructions are to be used to provide + more detailed information for server developers debugging a problem. - Both postmaster and postgres have several debug options available. - First, whenever you start postmaster, make sure you send the standard - output and error to a log file, like: - cd /usr/local/pgsql - ./bin/postmaster >server.log 2>&1 & - - This will put a server.log file in the top-level PostgreSQL directory. - This file contains useful information about problems or errors - encountered by the server. Postmaster has a -d option that allows even - more detailed information to be reported. The -d option takes a number - that specifies the debug level. Be warned that high debug level values + It is also possible to debug the server if it isn't operating + properly. First, by running configure with the --enable-cassert + option, many assert()s monitor the progress of the backend and halt + the program when something unexpected occurs. + + The postmaster has a -d option that allows even more detailed + information to be reported. The -d option takes a number that + specifies the debug level. Be warned that high debug level values generate large log files. If postmaster is not running, you can actually run the postgres @@ -489,10 +417,6 @@ process with the debugger, set any breakpoints, and continue through the startup sequence. - There are several log_* server configuration variables that enable - printing of process statistics which can be very useful for debugging - and performance measurements. - You can also compile with profiling to see what functions are taking execution time. The backend profile files will be deposited in the pgsql/data/base/dbname directory. The client profile file will be put @@ -501,23 +425,10 @@ 3.5) Why do I get "Sorry, too many clients" when trying to connect? - You need to increase postmaster's limit on how many concurrent backend - processes it can start. - - The default limit is 32 processes. You can increase it by restarting - postmaster with a suitable -N value or modifying postgresql.conf. - - Note that if you make -N larger than 32, you must also increase -B - beyond its default of 64; -B must be at least twice -N, and probably - should be more than that for best performance. For large numbers of - backend processes, you are also likely to find that you need to - increase various Unix kernel configuration parameters. Things to check - include the maximum size of shared memory blocks, SHMMAX; the maximum - number of semaphores, SEMMNS and SEMMNI; the maximum number of - processes, NPROC; the maximum number of processes per user, MAXUPRC; - and the maximum number of open files, NFILE and NINODE. The reason - that PostgreSQL has a limit on the number of allowed backend processes - is so your system won't run out of resources. + You have reached the default limit is 100 database sessions. You need + to increase the postmaster's limit on how many concurrent backend + processes it can start by changing the max_connections value in + postgresql.conf and restarting the postmaster. 3.6) What is in the pgsql_tmp directory? @@ -534,17 +445,13 @@ PostgreSQL releases? The PostgreSQL team makes only small changes between minor releases, - so upgrading from 7.2 to 7.2.1 does not require a dump and restore. - However, major releases (e.g. from 7.2 to 7.3) often change the + so upgrading from 7.4 to 7.4.1 does not require a dump and restore. + However, major releases (e.g. from 7.3 to 7.4) often change the internal format of system tables and data files. These changes are - often complex, so we don't maintain backward compatability for data + often complex, so we don't maintain backward compatibility for data files. A dump outputs data in a generic format that can then be loaded in using the new internal format. - In releases where the on-disk format does not change, the pg_upgrade - script can be used to upgrade without a dump/restore. The release - notes mention whether pg_upgrade is available for the release. - 3.8) What computer hardware should I use? Because PC hardware is mostly compatible, people tend to believe that @@ -560,13 +467,10 @@ 4.1) How do I SELECT only the first few rows of a query? A random row? - See the FETCH manual page, or use SELECT ... LIMIT.... - - The entire query may have to be evaluated, even if you only want the - first few rows. Consider using a query that has an ORDER BY. If there - is an index that matches the ORDER BY, PostgreSQL may be able to - evaluate only the first few records requested, or the entire query may - have to be evaluated until the desired rows have been generated. + 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 @@ -587,25 +491,16 @@ 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. + 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 remove a column from a table, or change its data type? + 4.3) How do you change a column's data type? - DROP COLUMN functionality was added in release 7.3 with ALTER TABLE - DROP COLUMN. In earlier versions, you can do this: - BEGIN; - LOCK TABLE old_table; - SELECT ... -- select all columns but the one you want to remove - INTO TABLE new_table - FROM old_table; - DROP TABLE old_table; - ALTER TABLE new_table RENAME TO old_table; - COMMIT; - Changing the data type of a column can be done easily in 8.0 and later with ALTER TABLE ALTER COLUMN TYPE. @@ -622,14 +517,16 @@ 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? 1.6TB - 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 - + + Maximum size for a database? unlimited (32 TB databases exist) + Maximum size for a table? 32 TB + Maximum size for a row? 1.6TB + 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. @@ -716,15 +613,15 @@ * 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 functional indexes, which are described in - section 4.10. + 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-greater character in a non-C locale. You - can create a special text_pattern_ops index for such cases that - work only for LIKE indexing. + possible to know the next-greatest character in a non-C locale. + You can create a special text_pattern_ops index for such cases + that work only for LIKE indexing. In pre-8.0 releases, indexes often can not be used unless the data - types exactly match the index's column types. This is particularly + types exactly match the index's column types. This was particularly true of int2, int8, and numeric column indexes. 4.7) How do I see how the query optimizer is evaluating my query? @@ -745,7 +642,7 @@ WHERE lower(col) = 'abc'; This will not use an standard index. However, if you create a - functional index, it will be used: + expresssion index, it will be used: CREATE INDEX tabindex ON tab (lower(col)); 4.9) In a query, how do I detect if a field is NULL? @@ -754,14 +651,13 @@ 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 - + 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. @@ -797,9 +693,7 @@ BYTEA bytea variable-length byte array (null-byte safe) ); See the create_sequence manual page for more information about - sequences. You can also use each row's OID field as a unique value. - However, if you need to dump and reload the database, you need to use - pg_dump's -o option or COPY WITH OIDS option to preserve the OIDs. + sequences. 4.11.2) How do I get the value of a SERIAL insert? @@ -821,16 +715,10 @@ BYTEA bytea variable-length byte array (null-byte safe) execute("INSERT INTO person (name) VALUES ('Blaise Pascal')"); new_id = execute("SELECT currval('person_id_seq')"); - Finally, you could use the OID returned from the INSERT statement to - look up the default value, though this is probably the least portable - approach, and the oid value will wrap around when it reaches 4 - billion. In Perl, using DBI with the DBD::Pg module, the oid value is - made available via $sth->{pg_oid_status} after $sth->execute(). - 4.11.3) Doesn't currval() lead to a race condition with other users? - No. currval() returns the current value assigned by your backend, not - by all 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? @@ -856,25 +744,7 @@ BYTEA bytea variable-length byte array (null-byte safe) values. TIDs change after rows are modified or reloaded. They are used by index entries to point to physical rows. - 4.13) What is the meaning of some of the terms used in PostgreSQL? - - Some of the source code and older documentation use terms that have - more common usage. Here are some: - * table, relation, class - * row, record, tuple - * column, field, attribute - * retrieve, select - * replace, update - * append, insert - * OID, serial value - * portal, cursor - * range variable, table name, table alias - - A list of general database terms can be found at: - http://hea-www.harvard.edu/MST/simul/software/docs/pkgs/pgsql/glossary - /glossary.html - - 4.14) Why do I get the error "ERROR: Memory exhausted in AllocSetAlloc()"? + 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 @@ -889,11 +759,11 @@ BYTEA bytea variable-length byte array (null-byte safe) problem with the SQL client because the backend is returning too much data, try it before starting the client. - 4.15) How do I tell what PostgreSQL version I am running? + 4.14) How do I tell what PostgreSQL version I am running? From psql, type SELECT version(); - 4.16) Why does my large-object operations get "invalid large obj + 4.15) Why does my large-object operations get "invalid large obj descriptor"? You need to put BEGIN WORK and COMMIT around any use of a large object @@ -908,12 +778,12 @@ BYTEA bytea variable-length byte array (null-byte safe) If you are using a client interface like ODBC you may need to set auto-commit off. - 4.17) How do I create a column that will default to the current time? + 4.16) 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 ); + CREATE TABLE test (x int, modtime TIMESTAMP DEFAULT CURRENT_TIMESTAMP ); - 4.18) How do I perform an outer join? + 4.17) How do I perform an outer join? PostgreSQL supports outer joins using the SQL standard syntax. Here are two examples: @@ -931,37 +801,25 @@ BYTEA bytea variable-length byte array (null-byte safe) is assumed in LEFT, RIGHT, and FULL joins. Ordinary joins are called INNER joins. - In previous releases, outer joins can be simulated using UNION and NOT - IN. For example, when joining tab1 and tab2, the following query does - an outer join of the two tables: - SELECT tab1.col1, tab2.col2 - FROM tab1, tab2 - WHERE tab1.col1 = tab2.col1 - UNION ALL - SELECT tab1.col1, NULL - FROM tab1 - WHERE tab1.col1 NOT IN (SELECT tab2.col1 FROM tab2) - ORDER BY col1 - - 4.19) How do I perform queries using multiple databases? + 4.18) 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 make simultaneous connections to different + course, a client can also make simultaneous connections to different databases and merge the results on the client side. - 4.20) How do I return multiple rows or columns from a function? + 4.19) How do I return multiple rows or columns from a function? - In 7.3, you can easily return multiple rows or columns from a - function, http://techdocs.postgresql.org/guides/SetReturningFunctions. + It is easy using set-returning functions, + http://techdocs.postgresql.org/guides/SetReturningFunctions. - 4.21) Why can't I reliably create/drop temporary tables in PL/PgSQL + 4.20) Why can't I reliably create/drop temporary tables in PL/PgSQL functions? - PL/PgSQL caches function contents, and an unfortunate side effect is + 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 @@ -969,7 +827,7 @@ BYTEA bytea variable-length byte array (null-byte safe) table access in PL/PgSQL. This will cause the query to be reparsed every time. - 4.22) What encryption options are available? + 4.21) What encryption options are available? * contrib/pgcrypto contains many encryption functions for use in SQL queries. @@ -980,8 +838,7 @@ BYTEA bytea variable-length byte array (null-byte safe) possible to use a third-party encrypted transport, such as stunnel or ssh, rather than PostgreSQL's native SSL connections.) * Database user passwords are automatically encrypted when stored in - version 7.3. In previous versions, you must enable the option - PASSWORD_ENCRYPTION in postgresql.conf. + the system tables. * The server can run using an encrypted file system. _________________________________________________________________ diff --git a/doc/src/FAQ/FAQ.html b/doc/src/FAQ/FAQ.html index 65fbc0446c8..b0ccf2853f3 100644 --- a/doc/src/FAQ/FAQ.html +++ b/doc/src/FAQ/FAQ.html @@ -10,7 +10,7 @@ alink="#0000ff">

Frequently Asked Questions (FAQ) for PostgreSQL

-

Last updated: Sat Jan 29 23:44:48 EST 2005

+

Last updated: Mon Jan 31 21:40:28 EST 2005

Current maintainer: Bruce Momjian (pgman@candle.pha.pa.us) @@ -27,21 +27,19 @@

General Questions

1.1) What is PostgreSQL? How is it pronounced?
- 1.2) What is the copyright on PostgreSQL?
+ 1.2) What is the copyright of PostgreSQL?
1.3) What platforms does PostgreSQL support?
1.4) Where can I get PostgreSQL?
1.5) Where can I get support?
- 1.6) What is the latest release?
- 1.7) What documentation is available?
- 1.8) How do I find out about known bugs or + 1.6) How do I submit a bug report?
+ 1.7) What is the latest release?
+ 1.8) What documentation is available?
+ 1.9) How do I find out about known bugs or missing features?
- 1.9) How can I learn SQL?
- 1.10) How do I join the development team?
- 1.11) How do I submit a bug report?
+ 1.10) How can I learn SQL?
+ 1.11) How do I join the development team?
1.12) How does PostgreSQL compare to other DBMSs?
- 1.13) How can I financially assist - PostgreSQL?

User Client Questions

@@ -76,8 +74,7 @@ 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 remove a column from a - table, or change its data type?
+ 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 @@ -104,24 +101,22 @@ my sequence/SERIAL column?
4.12) What is an OID? What is a TID?
- 4.13) What is the meaning of some of the terms - used in PostgreSQL?
- 4.14) Why do I get the error "ERROR: Memory + 4.13) Why do I get the error "ERROR: Memory exhausted in AllocSetAlloc()"?
- 4.15) How do I tell what PostgreSQL version I + 4.14) How do I tell what PostgreSQL version I am running?
- 4.16) Why does my large-object operations get + 4.15) Why does my large-object operations get "invalid large obj descriptor"?
- 4.17) How do I create a column that will + 4.16) How do I create a column that will default to the current time?
- 4.18) How do I perform an outer join?
- 4.19) How do I perform queries using multiple + 4.17) How do I perform an outer join?
+ 4.18) How do I perform queries using multiple databases?
- 4.20) How do I return multiple rows or columns + 4.19) How do I return multiple rows or columns from a function?
- 4.21) Why can't I reliably create/drop + 4.20) Why can't I reliably create/drop temporary tables in PL/PgSQL functions?
- 4.22) What encryption options are available?
+ 4.21) What encryption options are available?

Extending PostgreSQL

@@ -140,45 +135,31 @@

1.1) What is PostgreSQL? How is it pronounced?

-

PostgreSQL is pronounced Post-Gres-Q-L.

- -

PostgreSQL is an enhancement of the POSTGRES database management - system (and is still sometimes reffered to as simply "Postgres"), - a next-generation DBMS research prototype. - While PostgreSQL retains the powerful data model and rich data - types of POSTGRES, it replaces the PostQuel query language with an - extended subset of SQL. PostgreSQL is free and the - complete source is available.

- -

PostgreSQL development is performed by a team of - developers who all subscribe to the PostgreSQL development mailing - list. The current coordinator is Marc G. Fournier (scrappy@PostgreSQL.org). (See - section 1.6 on how to join). This team is now - responsible for all development of PostgreSQL. It is a community - project and is not controlled by any company. To get involved, see - the developer's FAQ at PostgreSQL is pronounced Post-Gres-Q-L, also called just + Postgres.

+ +

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/files/documentation/faqs/FAQ_DEV.html

-

The authors of PostgreSQL 1.01 were Andrew Yu and Jolly Chen. - Many others have contributed to the porting, testing, debugging, - and enhancement of the code. The original Postgres code, from which - PostgreSQL is derived, was the effort of many graduate students, - undergraduate students, and staff programmers working under the - direction of Professor Michael Stonebraker at the University of - California, Berkeley.

- -

The original name of the software at Berkeley was Postgres. When - SQL functionality was added in 1995, its name was - changed to Postgres95. The name was changed at the end of 1996 to - PostgreSQL.

- -

1.2) What is the copyright on +

1.2) What is the copyright of PostgreSQL?

-

PostgreSQL is subject to the following COPYRIGHT:

+

PostgreSQL is distributed under the classic BSD license. It has + no restrictions on how the source code can be used. We like it and + have no intention of changing it.

+ +

This is the BSD license we use:

PostgreSQL Data Base Management System

@@ -204,10 +185,6 @@ UNIVERSITY OF CALIFORNIA HAS NO OBLIGATIONS TO PROVIDE MAINTENANCE, SUPPORT, UPDATES, ENHANCEMENTS, OR MODIFICATIONS.

-

The above is the BSD license, the classic open-source license. - It has no restrictions on how the source code may be used. We like - it and have no intention of changing it.

-

1.3) What platforms does PostgreSQL support?

In general, any modern Unix-compatible platform should be able to @@ -236,78 +213,46 @@

1.5) Where can I get support?

-

The main mailing list is: pgsql-general@PostgreSQL.org. - It is available for discussion of matters pertaining to PostgreSQL. - To subscribe, send mail with the following lines in the body (not - the subject line):

-
-    subscribe
-    end
-
- -

to pgsql-general-request@PostgreSQL.org.

- -

There is also a digest list available. To subscribe to this - list, send email to: pgsql-general-digest-request@PostgreSQL.org - with a body of:

-
-    subscribe
-    end
-
- - Digests are sent out to members of this list whenever the main list - has received around 30k of messages. - -

The bugs mailing list is available. To subscribe to this list, - send email to pgsql-bugs-request@PostgreSQL.org - with a body of:

-
-    subscribe
-    end
-
- - There is also a developers discussion mailing list available. To - subscribe to this list, send email to pgsql-hackers-request@PostgreSQL.org - with a body of: -
-    subscribe
-    end
-
- -

Additional mailing lists and information about PostgreSQL can be - found via the PostgreSQL WWW home page at:

- -
- http://www.PostgreSQL.org -
+

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 - command irc -c '#postgresql' "$USER" irc.freenode.net + program irc -c '#postgresql' "$USER" irc.freenode.net or use any of the other popular IRC clients. A Spanish one also exists on the same network, (#postgresql-es), and a French one, (#postgresqlfr). There is also a PostgreSQL channel on EFNet.

A list of commercial support companies is available at http://techdocs.postgresql.org/companies.php.

+ "http://techdocs.postgresql.org/companies.php">http://techdocs.postg + resql.org/companies.php.

-

1.6) What is the latest release?

+

1.6) 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.

+ +

1.7) What is the latest release?

The latest release of PostgreSQL is version 8.0.0.

-

We plan to have major releases every six to eight months.

+

We plan to have major releases every ten to twelve months.

-

1.7) What documentation is available?

+

1.8) What documentation is available?

-

Several manuals, manual pages, and some small test examples are - included in the distribution. See the /doc directory. You - can also browse the manuals online at http://www.PostgreSQL.org/docs.

+

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/awbook.html @@ -326,14 +271,14 @@

Our web site contains even more documentation.

-

1.8) How do I find out about known bugs or +

1.9) How do I find out about known bugs or missing features?

PostgreSQL supports an extended subset of SQL-92. See our TODO list for known bugs, missing features, and future plans.

-

1.9) How can I learn +

1.10) How can I learn SQL?

The PostgreSQL book at The Complete Reference SQL, Groff et al., McGraw-Hill.

-

1.10) How do I join the development +

1.11) How do I join the development team?

First, download the latest source and read the PostgreSQL - Developers documentation on our web site, or in the distribution. - Second, subscribe to the pgsql-hackers and + Developers FAQ and documentation on our web site, or in the + distribution. Second, subscribe to the pgsql-hackers and pgsql-patches mailing lists. Third, submit high quality patches to pgsql-patches.

@@ -372,16 +317,6 @@ committers to keep up, and we had confidence that patches they committed were of high quality.

-

1.11) 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 or patches.

-

1.12) How does PostgreSQL compare to other DBMSs?

@@ -448,31 +383,6 @@ -

1.13) How can I financially assist - PostgreSQL?

- -

PostgreSQL has had a first-class infrastructure since we started - in 1996. This is all thanks to Marc Fournier, who has created - and managed this infrastructure over the years.

- -

Quality infrastructure is very important to an open-source - project. It prevents disruptions that can greatly delay forward - movement of the project.

- -

Of course, this infrastructure is not cheap. There are a variety - of monthly and one-time expenses that are required to keep it - going. If you or your company has money it can donate to help fund - this effort, please go to http://store.pgsql.com/shopping/ - and make a donation.

- -

Although the web page mentions PostgreSQL, Inc, the - "contributions" item is solely to support the PostgreSQL project - and does not fund any specific company. If you prefer, you can also - send a check to the contact address.

- -

Also, if you have a success story about PostgreSQL, please email - it to our advocacy list at - pgsql-advocacy@postgresql.org.

@@ -499,27 +409,33 @@

A nice introduction to Database-backed Web pages can be seen at: http://www.webreview.com

-

For Web integration, PHP is an excellent interface. It is at http://www.php.net.

+

For Web integration, PHP (http://www.php.net) is an excellent + interface.

-

For complex cases, many use the Perl interface and CGI.pm or mod_perl.

+

For complex cases, many use the Perl and CGI.pm or mod_perl.

2.3) Does PostgreSQL have a graphical user interface?

-

Yes, there are several graphical interfaces to PostgreSQL available. - These include PgAccess - http://www.pgaccess.org), pgAdmin III (http://www.pgadmin.org, RHDB Admin (http://sources.redhat.com/rhdb/ - ), TORA (http://www.globecom.net/tora/, - partly commercial), and Rekall ( - http://www.rekallrevealed.org/). There is also PhpPgAdmin - ( - http://phppgadmin.sourceforge.net/ ), a web-based interface to +

Yes, there are several graphical interfaces to PostgreSQL + available. These include pgAdmin III (http://www.pgadmin.org, PgAccess + http://www.pgaccess.org), + RHDB Admin (http://sources.redhat.com/rhd + b/ ), TORA (http://www.globecom.net/tora/, + partly commercial), and Rekall ( + http://www.rekallrevealed.org/). There is also PhpPgAdmin ( + http://phppgadmin.sourceforge.net/ ), a web-based interface to PostgreSQL.

-

See http://techdocs.postgresql.org/guides/GUITools for a more detailed list.

+

See + http://techdocs.postgresql.org/guides/GUITools for a more + detailed list.

@@ -538,75 +454,85 @@

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 and enable - host-based authentication by modifying the file - $PGDATA/pg_hba.conf accordingly.

+ listen_addresses in the postgresql.conf file, enable + host-based authentication by modifying the + $PGDATA/pg_hba.conf file, and restart the server.

3.3) How do I tune the database engine for better performance?

-

Certainly, indexes can speed up queries. The - EXPLAIN ANALYZE command allows you to see how - PostgreSQL is interpreting your query, and which indexes are - being used.

- -

If you are doing many INSERTs, consider doing - them in a large batch using the COPY command. This - is much faster than individual INSERTS. Second, - statements not in a BEGIN WORK/COMMIT transaction - block are considered to be in their own transaction. Consider - performing several statements in a single transaction block. This - reduces the transaction overhead. Also, consider dropping and - recreating indexes when making large data changes.

- -

There are several tuning options in the 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. - You can disable fsync() by using fsync option. This will - prevent fsync()s from flushing to disk after every - transaction.

- -

You can use the shared_buffers option to - increase the number of shared memory buffers used by the backend - processes. If you make this parameter too high, the - postmaster may not start because you have exceeded your - kernel's limit on shared memory space. Each buffer is 8K and the - default is 1000 buffers.

- -

You can also use the sort_mem (from PostgreSQL 8.0: work_mem) - options to increase the maximum amount of memory used by the backend - processes for each temporary sort. The default is 1024 (i.e. 1MB).

- -

You can also use the CLUSTER command to group - data in tables to match an index. See the CLUSTER - manual page for more details.

+ 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_conf_e.html + and + http://www.varlena.com/varlena/GeneralBits/Tidbits/perf.html. +
+
+
+ +
Hardware Selection
+ +
The effect of hardware on performance is detailed in + http://candle.pha.pa.us/main/writings/pgsql/hw_performance/index.html. +
+
+
+

3.4) What debugging features are available?

-

PostgreSQL has several features that report status information - that can be valuable for debugging purposes.

+

There are many log_* server configuration variables + that enable printing of query and process statistics which can be + very useful for debugging and performance measurements.

+ +

The following detailed debug instructions are to be used to + provide more detailed information for server developers debugging a + problem.

-

First, by running configure with the --enable-cassert +

It is also possible to debug the server if it isn't operating + properly. First, by running configure with the --enable-cassert option, many assert()s monitor the progress of the backend and halt the program when something unexpected occurs.

-

Both postmaster and postgres have several debug - options available. First, whenever you start postmaster, - make sure you send the standard output and error to a log file, - like:

-
-    cd /usr/local/pgsql
-    ./bin/postmaster >server.log 2>&1 &
-
- -

This will put a server.log file in the top-level PostgreSQL - directory. This file contains useful information about problems or - errors encountered by the server. Postmaster has a -d - option that allows even more detailed information to be reported. - The -d option takes a number that specifies the debug level. - Be warned that high debug level values generate large log - files.

+

The postmaster has a -d option that allows even more + detailed information to be reported. The -d option takes a + number that specifies the debug level. Be warned that high debug + level values generate large log files.

If postmaster is not running, you can actually run the postgres backend from the command line, and type your @@ -630,10 +556,6 @@ the debugger, set any breakpoints, and continue through the startup sequence.

-

There are several log_* server configuration variables - that enable printing of process statistics which can be very useful - for debugging and performance measurements.

-

You can also compile with profiling to see what functions are taking execution time. The backend profile files will be deposited in the pgsql/data/base/dbname directory. The client profile @@ -643,27 +565,11 @@

3.5) Why do I get "Sorry, too many clients" when trying to connect?

-

You need to increase postmaster's limit on how many - concurrent backend processes it can start.

- -

The default limit is 32 processes. You can increase it by - restarting postmaster with a suitable -N value or - modifying postgresql.conf.

- -

Note that if you make -N larger than 32, you must also - increase -B beyond its default of 64; -B must be at - least twice -N, and probably should be more than that for - best performance. For large numbers of backend processes, you are - also likely to find that you need to increase various Unix kernel - configuration parameters. Things to check include the maximum size - of shared memory blocks, SHMMAX; the maximum number - of semaphores, SEMMNS and SEMMNI; the - maximum number of processes, NPROC; the maximum - number of processes per user, MAXUPRC; and the - maximum number of open files, NFILE and - NINODE. The reason that PostgreSQL has a limit on - the number of allowed backend processes is so your system won't run - out of resources.

+

You have reached the default limit is 100 database sessions. You + need to increase the postmaster's limit on how many + concurrent backend processes it can start by changing the + max_connections value in postgresql.conf and + restarting the postmaster.

3.6) What is in the pgsql_tmp directory?

@@ -681,18 +587,13 @@ to upgrade between major PostgreSQL releases?

The PostgreSQL team makes only small changes between minor releases, - so upgrading from 7.2 to 7.2.1 does not require a dump and restore. - However, major releases (e.g. from 7.2 to 7.3) often change the internal + so upgrading from 7.4 to 7.4.1 does not require a dump and restore. + However, major releases (e.g. from 7.3 to 7.4) often change the internal format of system tables and data files. These changes are often complex, - so we don't maintain backward compatability for data files. A dump outputs + so we don't maintain backward compatibility for data files. A dump outputs data in a generic format that can then be loaded in using the new internal format.

-

In releases where the on-disk format does not change, the - pg_upgrade script can be used to upgrade without a dump/restore. - The release notes mention whether pg_upgrade is available for the - release.

-

3.8) What computer hardware should I use?

Because PC hardware is mostly compatible, people tend to believe that @@ -710,15 +611,13 @@

4.1) How do I SELECT only the first few rows of a query? A random row?

-

See the FETCH manual page, or use - SELECT ... LIMIT....

- -

The entire query may have to be evaluated, even if you only want - the first few rows. Consider using a query that has an ORDER - BY. If there is an index that matches the ORDER - BY, PostgreSQL may be able to evaluate only the first few - records requested, or the entire query may have to be evaluated - until the desired rows have been generated.

+

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: 

@@ -743,28 +642,15 @@
     database.

 
     
There are also system tables beginning with pg_ that describe
-    these too. Use psql -l will list all databases.

+    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 remove a column from a
-    table, or change its data type?

-
-    
DROP COLUMN functionality was added in release 7.3
-    with ALTER TABLE DROP COLUMN.  In earlier versions,
-    you can do this:

-
-    BEGIN;
-    LOCK TABLE old_table;
-    SELECT ...  -- select all columns but the one you want to remove
-    INTO TABLE new_table
-    FROM old_table;
-    DROP TABLE old_table;
-    ALTER TABLE new_table RENAME TO old_table;
-    COMMIT;
-

+    
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.
@@ -784,19 +670,25 @@
     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?                  1.6TB
-    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
+

+
+
+
+
+
+
+
+
+
Maximum size for a database?unlimited (32 TB databases
+exist)
Maximum size for a table?32 TB
Maximum size for a row?1.6TB
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. 
+    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
@@ -893,10 +785,10 @@
     
  • 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 functional
-    indexes, which are described in section 4.10.
    • 
+    ~* 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-greater
+    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.
@@ -904,7 +796,7 @@
     
 
     
    In pre-8.0 releases, indexes often can not be used unless the data
-    types exactly match the index's column types.  This is particularly
+    types exactly match the index's column types.  This was particularly
     true of int2, int8, and numeric column indexes.
    
 
     
    4.7) How do I see how the query optimizer is
@@ -930,7 +822,7 @@
    • This will not use an standard index. However, if you create a - functional index, it will be used: + expresssion index, it will be used: 
         CREATE INDEX tabindex ON tab (lower(col));
    @@ -943,16 +835,20 @@

    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
-
    - +
    + + + + + + + +
    TypeInternal NameNotes
    VARCHAR(n)varcharsize specifies maximum +length, no padding
    CHAR(n)bpcharblank padded to the specified +fixed length
    TEXTtextno specific upper limit on +length
    BYTEAbyteavariable-length byte array +(null-byte safe)
    "char"charone character
    +

    You will see the internal name when examining system catalogs and in some error messages.

    @@ -996,10 +892,7 @@ BYTEA bytea variable-length byte array (null-byte safe) See the create_sequence manual page for more information - about sequences. You can also use each row's OID field as a - unique value. However, if you need to dump and reload the database, - you need to use pg_dump's -o option or COPY - WITH OIDS option to preserve the OIDs. + about sequences.

    4.11.2) How do I get the value of a SERIAL insert?

    @@ -1030,19 +923,11 @@ BYTEA bytea variable-length byte array (null-byte safe) new_id = execute("SELECT currval('person_id_seq')"); -

    Finally, you could use the OID - returned from the INSERT statement to look up the - default value, though this is probably the least portable approach, - and the oid value will wrap around when it reaches 4 billion. - In Perl, using DBI with the DBD::Pg module, the oid value is made - available via $sth->{pg_oid_status} after - $sth->execute().

    -

    4.11.3) Doesn't currval() lead to a race condition with other users?

    No. currval() returns the current value assigned by your - backend, not by all users.

    + 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 @@ -1076,36 +961,7 @@ BYTEA bytea variable-length byte array (null-byte safe) are modified or reloaded. They are used by index entries to point to physical rows.

    -

    4.13) What is the meaning of some of the - terms used in PostgreSQL?

    - -

    Some of the source code and older documentation use terms that - have more common usage. Here are some:

    - - - -

    A list of general database terms can be found at: http://hea-www.harvard.edu/MST/simul/software/docs/pkgs/pgsql/glossary/glossary.html

    - -

    4.14) Why do I get the error "ERROR: +

    4.13) Why do I get the error "ERROR: Memory exhausted in AllocSetAlloc()"?

    You probably have run out of virtual memory on your system, @@ -1124,12 +980,12 @@ BYTEA bytea variable-length byte array (null-byte safe) backend is returning too much data, try it before starting the client. -

    4.15) How do I tell what PostgreSQL version +

    4.14) How do I tell what PostgreSQL version I am running?

    From psql, type SELECT version();

    -

    4.16) Why does my large-object operations +

    4.15) Why does my large-object operations get "invalid large obj descriptor"?

    You need to put BEGIN WORK and COMMIT @@ -1145,15 +1001,15 @@ BYTEA bytea variable-length byte array (null-byte safe)

    If you are using a client interface like ODBC you may need to set auto-commit off.

    -

    4.17) How do I create a column that will +

    4.16) 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 );
+    CREATE TABLE test (x int, modtime TIMESTAMP DEFAULT CURRENT_TIMESTAMP );
    -

    4.18) How do I perform an outer join?

    +

    4.17) How do I perform an outer join?

    PostgreSQL supports outer joins using the SQL standard syntax. Here are two examples:

    @@ -1176,24 +1032,7 @@ BYTEA bytea variable-length byte array (null-byte safe) RIGHT, and FULL joins. Ordinary joins are called INNER joins.

    -

    In previous releases, outer joins can be simulated using - UNION and NOT IN. For example, when - joining tab1 and tab2, the following query does an - outer join of the two tables:
    -
    -

    -
    -    SELECT tab1.col1, tab2.col2
-    FROM tab1, tab2
-    WHERE tab1.col1 = tab2.col1
-    UNION ALL
-    SELECT tab1.col1, NULL
-    FROM tab1
-    WHERE tab1.col1 NOT IN (SELECT tab2.col1 FROM tab2)
-    ORDER BY col1
-
    - -

    4.19) How do I perform queries using +

    4.18) How do I perform queries using multiple databases?

    There is no way to query a database other than the current one. @@ -1201,29 +1040,29 @@ BYTEA bytea variable-length byte array (null-byte safe) uncertain how a cross-database query should even behave.

    contrib/dblink allows cross-database queries using - function calls. Of course, a client can make simultaneous + function calls. Of course, a client can also make simultaneous connections to different databases and merge the results on the client side.

    -

    4.20) How do I return multiple rows or +

    4.19) How do I return multiple rows or columns from a function?

    -

    In 7.3, you can easily return multiple rows or columns from a - function, +

    It is easy using set-returning functions, http://techdocs.postgresql.org/guides/SetReturningFunctions. -

    4.21) Why can't I reliably create/drop +

    4.20) Why can't I reliably create/drop temporary tables in PL/PgSQL functions?

    -

    PL/PgSQL caches function contents, and an unfortunate side effect + +

    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 + table is later dropped and recreated, and the function called again, + the function will fail because the cached function contents still + point to the old temporary table. The solution is to use EXECUTE for temporary table access in PL/PgSQL. This will cause the query to be reparsed every time.

    -

    4.22) What encryption options are available? +

    4.21) What encryption options are available?

    • contrib/pgcrypto contains many encryption functions for @@ -1236,8 +1075,7 @@ BYTEA bytea variable-length byte array (null-byte safe) encrypted transport, such as stunnel or ssh, rather than PostgreSQL's native SSL connections.)
    • Database user passwords are automatically encrypted when stored in - version 7.3. In previous versions, you must enable the option - PASSWORD_ENCRYPTION in postgresql.conf.
      • + the system tables.
    • The server can run using an encrypted file system.
    @@ -1277,4 +1115,3 @@ BYTEA bytea variable-length byte array (null-byte safe) compiler compute the dependencies automatically.

    -