Hannu Krosing <hannu(at)tm(dot)ee> writes:
> If we have unique index on t.i and we define additional unique
> constraint on (t.i, t.j), then we don't need the extra unique index to
> be created - the index on t.i is enough to quarantee the uniqueness of
> (t.i,t.j) or any set of columns that includes t.i.
You missed the point: we are concerned about existence of a row, not only
uniqueness.
regards, tom lane