Same here; I've always intuited that this would limit the generated tuples. I'm too lazy to do it now, but I wonder if other DB engines also perform this optimization that effectively makes filtering in JOIN conditions equivalent to filtering in WHERE clauses. I'd also be interested in some example queries that were hand-optimized to the point of obvious obscurity--my guess is it's harder to do this in SQL than in something like C.
Any non-toy SQL engine is going to do condition pullup and pushdown. (Well, I know at least one that only does explicit pullup, but does pushdown more implicitly because it doesn't use a tree. The effect is largely the same, modulo some shenanigans around outer joins.)
Pushdown is actually a more subtle problem than people give it credit for; it inhabits this weird space where things are too trivial to be covered in papers but is too hard to cover properly in textbooks (e.g., no coverage of multiple equalities or outer joins).
