13.1.34 TRUNCATE TABLE Statement

TRUNCATE [TABLE] tbl_name

TRUNCATE TABLE empties a table completely. It requires the DROP privilege.

Logically, TRUNCATE TABLE is similar to a DELETE statement that deletes all rows, or a sequence of DROP TABLE and CREATE TABLE statements. To achieve high performance, it bypasses the DML method of deleting data. Thus, it cannot be rolled back, it does not cause ON DELETE triggers to fire, and it cannot be performed for InnoDB tables with parent-child foreign key relationships.

Although TRUNCATE TABLE is similar to DELETE, it is classified as a DDL statement rather than a DML statement. It differs from DELETE in the following ways:

• 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, and so cannot be rolled back. See Section 13.3.3, “Statements That Cause an Implicit Commit”.

• Truncation operations cannot be performed if the session holds an active table lock.

• TRUNCATE TABLE fails for an InnoDB table or NDB table if there are any FOREIGN KEY constraints from other tables that reference the table. Foreign key constraints between columns of the same table are permitted.

• 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.

• Any AUTO_INCREMENT value is reset to its start value. 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.

• The TRUNCATE TABLE statement does not invoke ON DELETE triggers.

TRUNCATE TABLE is treated for purposes of binary logging and replication as DDL rather than DML, and is always logged as a statement.

TRUNCATE TABLE for a table closes all handlers for the table that were opened with HANDLER OPEN.

On a system with a large InnoDB buffer pool and innodb_adaptive_hash_index enabled, TRUNCATE TABLE operations may cause a temporary drop in system performance due to an LRU scan that occurs when removing an InnoDB table's adaptive hash index entries. The problem was addressed for DROP TABLE in MySQL 5.5.23 (Bug #13704145, Bug #64284) but remains a known issue for TRUNCATE TABLE (Bug #68184).

TRUNCATE TABLE can be used with Performance Schema summary tables, but the effect is to reset the summary columns to 0 or NULL, not to remove rows. See Section 25.12.15, “Performance Schema Summary Tables”.