Re: Difference between UNIQUE constraint vs index

"Jim C. Nasby" <jim(at)nasby(dot)net> writes:
> In some databases if you know that an index just happens to be unique
> you might gain some query performance by defining the index as unique,
> but I don't think the PostgreSQL planner is that smart.

Actually, the planner only pays attention to whether indexes are unique;
the notion of a unique constraint is outside its bounds. In PG a unique
constraint is implemented by creating a unique index, and so there is
really not any interesting difference.

I would imagine that other DBMSes also enforce uniqueness by means of
indexes, because it'd be awful darn expensive to enforce the constraint
without one; but I'm only guessing here, not having looked. Can anyone
point to a real system that enforces unique constraints without an
underlying index?

regards, tom lane