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myisamchk's default action with no options is --check, so those commands are equivalent.
The normal check method is usually sufficient to identify problems. If it reports no errors but you still suspect damage (perhaps because queries do not seem to be working properly), you can perform a more extensive check by specifying the --extend-check option. This can be very slow, but it is extremely thorough; for each record in the table's data file, the associated key for every index in the index file is checked to make sure it really points to the correct record. (myisamchk also supports a --medium-check option to perform an intermediate check that is less thorough but faster than extended checking.)
If no errors are reported for a check with --extend-check, you can be sure your table is okay. If you still have problems with the table, the cause must lie elsewhere. Re-examine any queries that seem to yield problematic results to verify that they are written correctly. If you believe the problem may be with the MySQL server, consider filing a bug report or upgrading to a newer version.
If myisamchk reports that a table has errors, you should try to repair it.
Repairing Tables with myisamchk
Table repair is an ugly business, made more so by the fact that the details tend to be very incident-specific. Nevertheless, there are general guidelines and procedures you can follow to significantly increase your chances of being able to fix the tables. Generally, you begin with the fastest repair method to see if that will correct the damage. If you find that it is not sufficient, you can escalate to more thorough (but slower) repair methods until either the damage has been repaired or you cannot escalate further. (In practice, most problems are fixable without going to more extensive and slower repair modes.) If the table cannot be repaired, you'll need to restore the table from your backups. Instructions for recovery using backup files and log files are given later in this chapter.
To perform a standard repair operation on a table, use the following procedure:
Try to fix the table using the --recover option, and use the --quick option as well to attempt recovery based only on the contents of the index file. This will repair the table without touching the data file:
% myisamchk --recover --quick tbl_name
If problems remain, rerun the command without the --quick option to allow myisamchk to go ahead and modify the data file, too:
% myisamchk --recover tbl_name
If that doesn't work, try the --safe-recover repair mode. This is slower than regular recovery mode, but is capable of fixing a few problems that --recover mode will not:
% myisamchk --safe-recover tbl_name
It's possible when you run these commands that myisamchk will stop with an error message of the form Can't create new temp file: file_name. If that happens, repeat the command and add the --force option to force removal of the temporary file that may have been left around from a previous failed repair attempt.
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