TRUNCATE TABLE

Syntax

TRUNCATE [TABLE] tbl_name

Description

TRUNCATE TABLE empties a table completely. It requires the DROP privilege as of MySQL 5.1.16. (Before 5.1.16, it requires the DELETE privilege.) See GRANT.

tbl_name can also be specified in the form db_name.tbl_name (see Identifier Qualifiers).

Logically, TRUNCATE TABLE is equivalent to a DELETE statement that deletes all rows, but there are practical differences under some circumstances.

TRUNCATE TABLE is faster than DELETE, because it drops and re-creates a table. With XtraDB/InnoDB, this is faster if the innodb_file_per_table variable is ON.

For an InnoDB table, InnoDB processes TRUNCATE TABLE by deleting rows one by one if there are any FOREIGN KEY constraints that reference the table. If there are no FOREIGN KEY constraints, InnoDB performs fast truncation by dropping the original table and creating an empty one with the same definition, which is much faster than deleting rows one by one. The AUTO_INCREMENT counter is reset by TRUNCATE TABLE, regardless of whether there is a FOREIGN KEY constraint.

In the case that FOREIGN KEY constraints reference the table, InnoDB deletes rows one by one and processes the constraints on each one. If the FOREIGN KEY constraint specifies DELETE CASCADE, rows from the child (referenced) table are deleted, and the truncated table becomes empty. If the FOREIGN KEY constraint does not specify CASCADE, the TRUNCATE statement deletes rows one by one and stops if it encounters a parent row that is referenced by the child, returning this error:

ERROR 1451 (23000): Cannot delete or update a parent row: a foreign
key constraint fails (`test`.`child`, CONSTRAINT `child_ibfk_1`
FOREIGN KEY (`parent_id`) REFERENCES `parent` (`id`))

This is the same as a DELETE statement with no WHERE clause.

The count of rows affected by TRUNCATE TABLE is accurate only when it is mapped to a DELETE statement.

For other storage engines, TRUNCATE TABLE differs from DELETE in the following ways in MariaDB 5.1:

• Truncate operations drop and re-create the table, which is much faster than deleting rows one by one, particularly for large tables.
• Truncate operations cause an implicit commit.
• Truncation operations cannot be performed if the session holds an active table lock.
• Truncation operations do not return a meaningful value for the number of deleted rows. The usual result is "0 rows affected," which should be interpreted as "no information."
• As long as the table format file tbl_name.frm is valid, the table can be re-created as an empty table with TRUNCATE TABLE, even if the data or index files have become corrupted.
• The table handler does not remember the last used AUTO_INCREMENT value, but starts counting from the beginning. This is true even for MyISAM and InnoDB, which normally do not reuse sequence values.
• When used with partitioned tables, TRUNCATE TABLE preserves the partitioning; that is, the data and index files are dropped and re-created, while the partition definitions (.par) file is unaffected.
• Since truncation of a table does not make any use of DELETE, the TRUNCATE statement does not invoke ON DELETE triggers.

TRUNCATE TABLE does not work on views.