There are some bugs in how it handles its MySQL database. Some of the frequently-used tables can develop corruption over time and need to be repaired. This script should be run periodically out of cron to repair the damage. It should be run at a time when you expect not to be watching or recording anything, as the repair operations cause some kind of mutex hang if MythTV is reading or writing them at the same time.
My recordedmarkup table was badly damaged, leading to bizarre playback behavior. Jumping more than a minute would lag. The total-time field of the status window would show over 1200 hours. Searching would sometimes cause the front end to hang. I don't even want to imagine how badly corrupted an extraction would have been.