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13.7.3.3 CHECKSUM TABLE Syntax
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.
This statement is not supported for views. If you run
CHECKSUM TABLE against a view,
the Checksum value is always
NULL, and a warning is returned.
For a nonexistent table, CHECKSUM
TABLE returns NULL and generates a
warning.
During the checksum operation, the table is locked with a read
lock for InnoDB and
MyISAM.
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. The QUICK
method is not supported with InnoDB tables.
See Section 13.1.20, “CREATE TABLE Syntax” for the syntax of the
CHECKSUM clause.
The checksum value depends on the table row format. If the row
format changes, the checksum also changes. For example, the
storage format for temporal types such as
TIME,
DATETIME, and
TIMESTAMP changed in MySQL 5.6
prior to MySQL 5.6.5, so if a 5.5 table is upgraded to MySQL
5.6, the checksum value may change.
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.
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Document créé le 26/06/2006, dernière modification le 26/10/2018
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