18.2.4.1 Corrupted MyISAM Tables

Even though the MyISAM table format is very reliable (all changes to a table made by an SQL statement are written before the statement returns), you can still get corrupted tables if any of the following events occur:

• The mysqld process is killed in the middle of a write.

• An unexpected computer shutdown occurs (for example, the computer is turned off).

• Hardware failures.

• You are using an external program (such as myisamchk) to modify a table that is being modified by the server at the same time.

• A software bug in the MySQL or MyISAM code.

Typical symptoms of a corrupt table are:

• You get the following error while selecting data from the table:

  Incorrect key file for table: '...'. Try to repair it

• Queries don't find rows in the table or return incomplete results.

You can check the health of a MyISAM table using the CHECK TABLE statement, and repair a corrupted MyISAM table with REPAIR TABLE. When mysqld is not running, you can also check or repair a table with the myisamchk command. See Section 15.7.3.2, “CHECK TABLE Statement”, Section 15.7.3.5, “REPAIR TABLE Statement”, and Section 6.6.4, “myisamchk — MyISAM Table-Maintenance Utility”.

If your tables become corrupted frequently, you should try to determine why this is happening. The most important thing to know is whether the table became corrupted as a result of an unexpected server exit. You can verify this easily by looking for a recent restarted mysqld message in the error log. If there is such a message, it is likely that table corruption is a result of the server dying. Otherwise, corruption may have occurred during normal operation. This is a bug. You should try to create a reproducible test case that demonstrates the problem. See Section B.3.3.3, “What to Do If MySQL Keeps Crashing”, and Section 7.9, “Debugging MySQL”.