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## SQL As Understood By SQLite

### VACUUM

sql-statement ::= | VACUUM

The VACUUM command is an SQLite extension modeled after a similar command found in PostgreSQL. If VACUUM is invoked with the name of a table or index then it is suppose to clean up the named table or index. In version 1.0 of SQLite, the VACUUM command would invoke gdbm_reorganize() to clean up the backend database file.

VACUUM became a no-op when the GDBM backend was removed from SQLITE in version 2.0.0. VACUUM was reimplemented in version 2.8.1.

When an object (table, index, or trigger) is dropped from the database, it leaves behind empty space. This makes the database file larger than it needs to be, but can speed up inserts. In time inserts and deletes can leave the database file structure fragmented, which slows down disk access to the database contents. The VACUUM command cleans the main database by copying its contents to a temporary database file and reloading the original database file from the copy. This eliminates free pages, aligns table data to be contiguous, and otherwise cleans up the database file structure.

The VACUUM command may change the ROWID of entires in tables that do not have an explicit INTEGER PRIMARY KEY.

VACUUM only works on the main database. It is not possible to VACUUM an attached database file.

The VACUUM command will fail if there is an active transaction. The VACUUM command is a no-op for in-memory databases.

As of SQLite version 3.1, an alternative to using the VACUUM command is auto-vacuum mode, enabled using the auto_vacuum pragma. When auto-vacuum is enabled for a database, large deletes cause the size of the database file to shrink. However, auto-vacuum also causes excess fragmentation of the database file. And auto-vacuum does not compact partially filled pages of the database as VACUUM does.

This page last modified 2007/12/13 19:54:34 UTC