How to choose between multi column indexes and single column indexes in MySQL
===

I have one table called `users` with a PRIMARY key on `users.zbid`. However, all `SELECT` queries run on it include the `WHERE` clause `cid='$cid' AND zbid='$zbid'`. Should I add an index for `cid` or a multi-column index for `cid,zbid` ? I know the `WHERE` clause on `cid` seems redundant as `zbid` is `PRIMARY`, but it's mainly there as a precautionary step, each `cid` designates a different forum, and in case something went wrong I don't want every forum to be effected.

Secondly, I have another table called `notifications`. The only query that is run on this table uses the where clause `cid='$cid' AND zbid='$zbid'`, should I add them both as individual indexes or just one? Why?