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## SQL As Understood By SQLite

### VACUUM

sql-statement ::= | VACUUM

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 2008/08/12 18:08:17 UTC