The extra "AND expr" in the join clause makes it more strict, which means the outer join can produce more nulls in the joined tuple. In a sequence like A left

The extra "AND expr" in the join clause makes it more strict, which means the outer join can produce more nulls in the joined tuple. In a sequence like A left join B left join C, moving the extra join filter between the two joins is the difference between getting tuples like (a..., null..., null...) vs (a..., b..., null...).