VACUUM(l) SQL - Language Statements (2002-11-22) VACUUM(l)
NAME
VACUUM - garbage-collect and optionally analyze a database
SYNOPSIS
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.
DESCRIPTION
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
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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(l)] 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 table to the minimum
number of disk blocks. This form is much slower and requires
an exclusive lock on each table 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 database, then it is
guaranteed that all tuples in the database are ``frozen''
and will not be subject to transaction 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 ANALYZE 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.
USAGE
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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
COMPATIBILITY
SQL92
There is no VACUUM statement in SQL92.
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