I help look after the forum at middle-age-shred.com (website for old/ old skool skateboarders), which has proved to be a victim of it's own success with loads of traffic., and hence swelling of the MySql database. I'm not sure where the database is hosted but they don't allow much disk space for the package so it is important to keep an eye on it. Despite "pruning" of the forum (e.g. removing posts older than x days), the table sizes didn't seem to be reducing much, so I wondered if they behave a bit like disk storage where deleting information doesn't always free up space. This turned out to be the case so I did a bit of googling and found this:-

Running the optimise command reduced the tables by about a third :-)