Submitted By: robocop
I noticed it in 1999 (IB6 in private beta testing) but I'm not sure who discovered it. It still exists and may be rather old.
create table mau(a int);
create UNIQUE index idx_mau on mau(a);
Whether this was left as a transition from Paradox (that accepts NULL PKs) or it's an oversight is what I want to confirm.
As a comparation, you cannot declarate a PK or UNIQUE constraint if you don't include the NOT NULL clause on the affected field(s), so the underlying automatic index for such constraint will be always on non-nullable column(s).