Database performance for polymorphic associations under high load

Hey guys,

I've been thinking lately about polymorphic associations, and I wonder: is there a noticeable or tangible performance "hit" from doing polymorphic associations when you've got your database under really high query load?

So for example, maybe I have a "message" model, a "user" model and an "administrator" model, without any subclassing or STI going on. Message has a polymorphic interface called "message_author", and both "User" and "Administrator" use it. Say there wind up being ~1 million messages in that message table. Assuming everything's properly indexed, do polymorphic associations add any significant overhead to conducting database queries?

Thanks guys, appreciate the input.

I would stay away from polymorphic as there is no way to enforce DB FK relationships.

That's been my "official" stance for a while as well, and I'm fully aware of that - but I'm also wondering about its performance, so I can get a better perspective of the "full picture", if you follow my logic.

Still, your point is quite valid and has been taken under advisement. Thank you.