Adding a [`BTREE` or `HASH` index][1] shouldn't break anything, and I imagine that is what you want. At worst you could end up [with multiple indexes, or less than optimal ones, which might negatively effect performance][2]. I assume that if you are going to do this you will be evaluating the change to see if it actually does help, rather than hurt. I guess you could run into trouble with a `UNIQUE` index, since WordPress does use a couple of those. [1]: http://dev.mysql.com/doc/refman/5.5/en/index-btree-hash.html [2]: https://stackoverflow.com/questions/4120160/mysql-too-many-indexes