MySQL Performance Optimization Part II
You can speed up index scans if you periodically perform a void ALTER TABLE operation, which causes MySQL to rebuild the table:
ALTER TABLE tbl_name ENGINE=INNODB; or ALTER TABLE tbl_name ENGINE=MYISAM;
Another way to perform a defragmentation operation is to use mysqldump to dump the table to a text file, drop the table, and reload it from the dump file.
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Please feel free to use the comments form below if you have any questions or need more explanation on anything. I recommend thoroughy testing on a production-like test system first before moving to production.