AFAICS, Oracle as well.
On 2/28/07, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
> "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
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