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RedHat 9 (Linux i386) - man page for vacuum (redhat section 7)

VACUUM(7)					     SQL Commands					    VACUUM(7)

VACUUM - garbage-collect and optionally analyze a database
VACUUM [ FULL ] [ FREEZE ] [ VERBOSE ] [ table ] VACUUM [ FULL ] [ FREEZE ] [ VERBOSE ] ANALYZE [ table [ (column [, ...] ) ] ] INPUTS FULL Selects ``full'' vacuum, which may reclaim more space, but takes much longer and exclusively locks the table. FREEZE Selects aggressive ``freezing'' of tuples. VERBOSE Prints a detailed vacuum activity report for each table. ANALYZE Updates statistics used by the optimizer to determine the most efficient way to execute a query. table The name (optionally schema-qualified) of a specific table to vacuum. Defaults to all tables in the current database. column The name of a specific column to analyze. Defaults to all columns. OUTPUTS VACUUM The command is complete. INFO: --Relation table-- The report header for table. INFO: Pages 98: Changed 25, Reapped 74, Empty 0, New 0; The analysis for table itself. INFO: Index index: Pages 28; The analysis for an index on the target table.
VACUUM reclaims storage occupied by deleted tuples. In normal PostgreSQL operation, tuples that are deleted or obsoleted by UPDATE are not physically removed from their table; they remain present until a VACUUM is done. Therefore it's necessary to do VACUUM periodically, especially on frequently-updated tables. With no parameter, VACUUM processes every table in the current database. With a parameter, VACUUM processes only that table. VACUUM ANALYZE performs a VACUUM and then an ANALYZE for each selected table. This is a handy combination form for routine maintenance scripts. See ANALYZE [analyze(7)] for more details about its processing. Plain VACUUM (without FULL) simply reclaims space and makes it available for re-use. This form of the command can operate in parallel with normal reading and writing of the table, as an exclusive lock is not obtained. VACUUM FULL does more extensive processing, including moving of tuples across blocks to try to compact the ta- ble to the minimum number of disk blocks. This form is much slower and requires an exclusive lock on each ta- ble while it is being processed. FREEZE is a special-purpose option that causes tuples to be marked ``frozen'' as soon as possible, rather than waiting until they are quite old. If this is done when there are no other open transactions in the same data- base, then it is guaranteed that all tuples in the database are ``frozen'' and will not be subject to transac- tion ID wraparound problems, no matter how long the database is left un-vacuumed. FREEZE is not recommended for routine use. Its only intended usage is in connection with preparation of user-defined template databases, or other databases that are completely read-only and will not receive routine maintenance VACUUM operations. See the Administrator's Guide for details. NOTES We recommend that active production databases be VACUUM-ed frequently (at least nightly), in order to remove expired rows. After adding or deleting a large number of records, it may be a good idea to issue a VACUUM ANA- LYZE command for the affected table. This will update the system catalogs with the results of all recent changes, and allow the PostgreSQL query optimizer to make better choices in planning user queries. The FULL option is not recommended for routine use, but may be useful in special cases. An example is when you have deleted most of the rows in a table and would like the table to physically shrink to occupy less disk space. VACUUM FULL will usually shrink the table more than a plain VACUUM would.
The following is an example from running VACUUM on a table in the regression database: regression=> VACUUM VERBOSE ANALYZE onek; INFO: --Relation onek-- INFO: Index onek_unique1: Pages 14; Tuples 1000: Deleted 3000. CPU 0.00s/0.11u sec elapsed 0.12 sec. INFO: Index onek_unique2: Pages 16; Tuples 1000: Deleted 3000. CPU 0.00s/0.10u sec elapsed 0.10 sec. INFO: Index onek_hundred: Pages 13; Tuples 1000: Deleted 3000. CPU 0.00s/0.10u sec elapsed 0.10 sec. INFO: Index onek_stringu1: Pages 31; Tuples 1000: Deleted 3000. CPU 0.01s/0.09u sec elapsed 0.10 sec. INFO: Removed 3000 tuples in 70 pages. CPU 0.02s/0.04u sec elapsed 0.07 sec. INFO: Pages 94: Changed 0, Empty 0; Tup 1000: Vac 3000, Keep 0, UnUsed 0. Total CPU 0.05s/0.45u sec elapsed 0.59 sec. INFO: Analyzing onek VACUUM
SQL92 There is no VACUUM statement in SQL92. SQL - Language Statements 2002-11-22 VACUUM(7)

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