SQL As Understood By SQLite

TYPES

sql-statement ::=

TYPES [datatype name][,datatype name][,datatype name][,...] ; select stmt

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. The index or table name argument is now ignored.

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. It is not possible to perform the same process on an attached database file.

This command will fail if there is an active transaction. This command has no effect on an in-memory database.

As of SQLite version 3.1, an alternative to using the VACUUM command is auto-vacuum mode, enabled using the auto_vacuum pragma.

 

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