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.
Once you get there, you’ll see the following message:
I hit “Repair and Optimize” and was pleased (if mildly surprised) to see that all my tables were both okay and optimized already.
After the script finishes running, you get this message:
Repairs complete. Please remove the following line from wp-config.php to prevent this page from being used by unauthorized users.
You might want to just comment the line out so it’s ready for you next time you want to run the script.