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VACUUM

Name

VACUUM  --  Clean and analyze a Postgres database

Synopsis

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.

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 table 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 tables.

VACUUM ANALYZE collects statistics representing the dispersion of the data in each column. This information is valuable when several query execution paths are possible.

Running VACUUM periodically will increase the speed of the database in processing user queries.

Notes

The open database is the target for VACUUM.

We recommend that active production databases be VACUUM-ed nightly, in order to remove expired rows. After copying a large table into Postgres or after deleting a large number of records, it may be a good idea to issue a VACUUM ANALYZE 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.

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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