CHECKSUM TABLEtbl_name[,tbl_name] ... [ QUICK | EXTENDED ]
CHECKSUM TABLE reports a table
checksum. This statement requires the
SELECT privilege for the table.
With QUICK, the live table checksum is
reported if it is available, or NULL
otherwise. This is very fast. A live checksum is enabled by
specifying the CHECKSUM=1 table option when
you create the table; currently, this is supported only for
MyISAM tables. See
Section 12.1.14, “CREATE TABLE Syntax”.
With EXTENDED, the entire table is read row
by row and the checksum is calculated. This can be very slow for
large tables.
If neither QUICK nor
EXTENDED is specified, MySQL returns a live
checksum if the table storage engine supports it and scans the
table otherwise.
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.
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.