Chad,

>I can apply a FK on a PK and unique constraint, but not to
>a unique index?

- the target for a FK must be a constraint (an index won't do).
- when you define an unique constraint, an unique index is implicitly
created.
- when you define an unique index, you have practically the same
effect as defining an unique constraint, minus the ability to have FKs
point to it.

I only define unique indexes for performance reasons, and define
unique constraints for business rules related reasons.

HTH
--
Nando Dessena
http://www.flamerobin.org