13.7.2.3. CHECKSUM TABLE Syntax

CHECKSUM TABLE tbl_name [, tbl_name] ... [ QUICK | EXTENDED ]

CHECKSUM TABLE reports a checksum for the contents of a table. You can use this statement to verify that the contents are the same before and after a backup, rollback, or other operation that is intended to put the data back to a known state. This statement requires the SELECT privilege for the table.

Performance Considerations

By default, the entire table is read row by row and the checksum is calculated. For large tables, this could take a long time, thus you would only perform this operation occasionally. This row-by-row calculation is what you get with the EXTENDED clause, with InnoDB and all other storage engines other than MyISAM, and with MyISAM tables not created with the CHECKSUM=1 clause.

For MyISAM tables created with the CHECKSUM=1 clause, CHECKSUM TABLE or CHECKSUM TABLE ... QUICK returns the "live" table checksum that can be returned very fast. If the table does not meet all these conditions, the QUICK method returns NULL. See Section 13.1.14, "CREATE TABLE Syntax" for the syntax of the CHECKSUM clause.

For a nonexistent table, CHECKSUM TABLE returns NULL and generates a warning.

The checksum value depends on the table row format. If the row format changes, the checksum also changes. For example, the storage format for VARCHAR changed between MySQL 4.1 and 5.0, so if a 4.1 table is upgraded to MySQL 5.0, the checksum value may change.

Important

If the checksums for two tables are different, then it is almost certain that the tables are different in some way. However, because the hashing function used by CHECKSUM TABLE is not guaranteed to be collision-free, there is a slight chance that two tables which are not identical can produce the same checksum.




Spec-Zone.ru - all specs in one place