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Databases - Practical PostgreSQL
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Using VACUUM

The syntax for the VACUUM SQL command is as follows:

  VACUUM [ VERBOSE ] [ ANALYZE ] [ 
table
 ]
  VACUUM [ VERBOSE ] ANALYZE [ 
table
 [ ( 
column
 [, ...] ) ] ]

Used without any of the optional keywords or identifiers, a VACUUM statement will clean up each table in the presently connected database, one at a time, deleting temporary data and recovering disk space. This use of VACUUM is primarily to maximize free disk space.

An optional table identifier may be specified if you want the VACUUM to clean a single table in the connected database, rather than all tables. It will also update statistics in the system catalogs pertaining to the number of records and amount of data stored in each table. Example 9-14 shows the use of a VACUUM statement in the booktown database, on the books table.

Example 9-14. Using VACUUM on a table

booktown=# 
VACUUM books;

VACUUM

The VACUUM message returned in Example 9-14 indicates that the process finished successfully. If a specified table cannot be found, you will instead receive the following notice:

NOTICE:  Vacuum: table not found

With the use of the optional ANALYZE keyword, PostgreSQL examines the allocation of data in each column for each table (or the specified table, if provided), and uses the information to prepare the query optimizer for better planning. With the use of the ANALYZE keyword, you also have the option to analyze only specified columns. Example 9-15 shows the use of the VACUUM ANALYZE command on the entire booktown database.

Example 9-15. Using VACUUM ANALYZE on a database

booktown=# 
VACUUM ANALYZE;

VACUUM

Finally, the optional VERBOSE keyword may be applied if you are interested in seeing a detailed internal report of the findings of the VACUUM statement. This is most likely not of interest to anyone not actively developing the PostgreSQL engine, or related software.

Databases - Practical PostgreSQL
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