MySQL 5.0 Reference Manual  /  ...  /  TRUNCATE TABLE Syntax

13.1.21 TRUNCATE TABLE Syntax


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

For an InnoDB table before version 5.0.3, InnoDB processes TRUNCATE TABLE by deleting rows one by one. As of MySQL 5.0.3, row by row deletion is used only 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. (When fast truncation is used, it resets any AUTO_INCREMENT counter to zero. From MySQL 5.0.13 on, the AUTO_INCREMENT counter is reset to zero 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 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`))

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 MySQL 5.0:

  • Truncate operations drop and re-create the table, which is much faster than deleting rows one by one, particularly for large tables.

  • As of MySQL 5.0.8, truncate operations cause an implicit commit. Before 5.0.8, truncate operations are not transaction-safe; an error occurs when attempting one in the course of an active transaction.

  • 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.

  • Since truncation of a table does not make any use of DELETE, the TRUNCATE TABLE statement does not invoke ON DELETE triggers.

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User Comments
  Posted by Yi Peng on August 29, 2010
On Windows, MySQL server 5.1 defaults to safe update mode, even though safe-updates is not enabled in my.ini (a bug possibly?). Truncating table results in error listed below.

Message: You are using safe update mode and you tried to update a table without a WHERE that uses a KEY column

I found that the easiest way to disable the safe mode is to execute this SQL statement first to disable safe update mode temporarily.

SET sql_safe_updates=0

Defaulting to safe update mode is not bad after all.

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