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.

When you use this fast truncation technique with the innodb_file_per_table option enabled, the operating system can reuse the freed disk space. For users of the InnoDB Plugin, the space is reclaimed automatically, as described in TRUNCATE TABLE Reclaims Space. If you do not have the InnoDB Plugin installed, issue the OPTIMIZE TABLE statement to free the disk space for the table.

If there are any FOREIGN KEY constraints that reference the table, InnoDB processes TRUNCATE TABLE by deleting rows one by one, processing the constraints as it proceeds. 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 TABLE 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`))

The AUTO_INCREMENT counter is reset to zero by TRUNCATE TABLE, regardless of whether there is a FOREIGN KEY constraint.

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 TABLE statement does not invoke ON DELETE triggers.