reduce_outer_joins() already recognizes that a LEFT JOIN is really an anti-join when an upper qual forces a nullable-side Var to be NULL while that Var is actually non-null in every matching row. In that case only the null-extended (unmatched) rows can satisfy the upper qual, which is exactly anti-join semantics, so we switch JOIN_LEFT to JOIN_ANTI. This is worth detecting because an anti-join is usually much cheaper than computing the outer join and then filtering the result with the IS NULL clause.
While you’re at it, any chance of changing it so that "a LEFT JOIN b WHERE b IS NULL" is guaranteed not to have worse performance than " a LEFT JOIN b WHERE b.f IS NULL"? I've had this and asked about it here and it was suggested that I should specify the primary key fields of b; but surely if I want an antijoin it's clearer and better implicit documentation if I don't zero in on specific fields. I think somebody else is suggesting "a LEFT ANTI JOIN b" which would be even better.
