"Jim C. Nasby" <jim@xxxxxxxxx> 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
