This feature seemed especially likely to be useful to me for my older blogs, the ones that had gone through imports and exports and lots of different plugins. At first I wasn’t sure what you were supposed to do besides pasting the code
define('WP_ALLOW_REPAIR', true); into your config.php file. (And when I pasted it the first time, I messed up, because for some reason I had backticks in there instead of straight single quotes.)
I found the answer in Technosailor’s 10 Things You Need to Know About WordPress 2.9 post. After you update your config.php file, you need to visit a special URL in order to run the repair script. That URL is http://yourdomain/yourWPinstalldirectory/wp-admin/maint/repair.php.
Continue reading at How to Repair and Optimize Your Database in WordPress 2.9