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VACUUM

Name

VACUUM — Clean and analyze a Postgres database
VACUUM [ VERBOSE ] [ ANALYZE ] [ table ]
VACUUM [ VERBOSE ] ANALYZE [ table [ (column [, ...] ) ] ]

Inputs

VERBOSE

Prints a detailed vacuum activity report for each table.

ANALYZE

Updates column statistics used by the optimizer to determine the most efficient way to execute a query. The statistics represent the disbursion of the data in each column. This information is valuable when several execution paths are possible.

table

The name of a specific table to vacuum. Defaults to all tables.

column

The name of a specific column to analyze. Defaults to all columns.

Outputs

VACUUM

The command has been accepted and the database is being cleaned.

NOTICE: --Relation table--

The report header for table.

NOTICE: Pages 98: Changed 25, Reapped 74, Empty 0, New 0; Tup 1000: Vac 3000, Crash 0, UnUsed 0, MinLen 188, MaxLen 188; Re-using: Free/Avail. Space 586952/586952; EndEmpty/Avail. Pages 0/74. Elapsed 0/0 sec.

The analysis for table itself.

NOTICE: Index index: Pages 28; Tuples 1000: Deleted 3000. Elapsed 0/0 sec.

The analysis for an index on the target table.

Description

VACUUM serves two purposes in Postgres as both a means to reclaim storage and also a means to collect information for the optimizer.

VACUUM opens every class in the database, cleans out records from rolled back transactions, and updates statistics in the system catalogs. The statistics maintained include the number of tuples and number of pages stored in all classes. Running VACUUM periodically will increase the speed of the database in processing user queries.

Notes

The open database is target for VACUUM.

We recommend that active production databases be cleaned nightly, in order to keep statistics relatively current. The VACUUM query may be executed at any time, however. In particular, after copying a large class into Postgres or after deleting a large number of records, it may be a good idea to issue a VACUUM query. This will update the system catalogs with the results of all recent changes, and allow the Postgres query optimizer to make better choices in planning user queries.

If the server crashes during a VACUUM command, chances are it will leave a lock file hanging around. Attempts to re-run the VACUUM command result in an error message about the creation of a lock file. If you are sure VACUUM is not running, remove the pg_vlock file in your database directory (i.e. PGDATA/base/dbname/pg_vlock).

Usage

The following is an example from running VACUUM on a table in the regression database:

regression=> vacuum verbose analyze onek;
NOTICE:  --Relation onek--
NOTICE:  Pages 98: Changed 25, Reapped 74, Empty 0, New 0;
         Tup 1000: Vac 3000, Crash 0, UnUsed 0, MinLen 188, MaxLen 188;
         Re-using: Free/Avail. Space 586952/586952; EndEmpty/Avail. Pages 0/74.
         Elapsed 0/0 sec.
NOTICE:  Index onek_stringu1: Pages 28; Tuples 1000: Deleted 3000. Elapsed 0/0 sec.
NOTICE:  Index onek_hundred: Pages 12; Tuples 1000: Deleted 3000. Elapsed 0/0 sec.
NOTICE:  Index onek_unique2: Pages 19; Tuples 1000: Deleted 3000. Elapsed 0/0 sec.
NOTICE:  Index onek_unique1: Pages 17; Tuples 1000: Deleted 3000. Elapsed 0/0 sec.
NOTICE:  Rel onek: Pages: 98 --> 25; Tuple(s) moved: 1000. Elapsed 0/1 sec.
NOTICE:  Index onek_stringu1: Pages 28; Tuples 1000: Deleted 1000. Elapsed 0/0 sec.
NOTICE:  Index onek_hundred: Pages 12; Tuples 1000: Deleted 1000. Elapsed 0/0 sec.
NOTICE:  Index onek_unique2: Pages 19; Tuples 1000: Deleted 1000. Elapsed 0/0 sec.
NOTICE:  Index onek_unique1: Pages 17; Tuples 1000: Deleted 1000. Elapsed 0/0 sec.
VACUUM

Compatibility

SQL92

There is no VACUUM statement in SQL92.

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