>I think you are still reading it too fast. What is wrong with using it as a candidate key? What other kind of key is there?

My point of view is: a candidate key is just a second primary key. That is, you could just pick one or the other as a primary key without affecting the uniqueness of the primary key. So, what definition apply to a primary key apply by default to a candidate key.

But it's just a purist point of view, since there's no real problem as using a composite index as candidate key.