Gregory Stark <stark(at)enterprisedb(dot)com> writes:
> I think Postgres can't do better because there could be a NULL in the
> subquery. If there's a NULL in the subquery then no record would match.
Yeah. NOT IN does not have the right semantics to become an antijoin.
FWIW, you can use a NOT EXISTS:
select a.a from a where not exists (select 1 from b where a.a = b.a);
In HEAD this should give the identical plan to the leftjoin/is null
hack.
regards, tom lane