Subject Unique constraint vs unique index, why do they have to be different?
Author David Garamond
I know they are "conceptually" different, but apart from the first one
being able to be used in FK relation and the other cannot, what are the
other practical differences between the two? Aren't UC always
"implemented" using UI? (And by using UI, don't we effectively achieve UC?)

Why can't/won't the Firebird engine behave like this: if a UI is added,
UC is also automatically be declared, and vice versa. Why do we still
need to separate both? Historical reason?