Re: WIP: Upper planner pathification

Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> writes:
>>>> Is there some reason why hash and nestloop are safe but merge isn't?

> To make hash and nestloop work in parallel queries, we just push those
> nodes below gather node. Refer code
> paths match_unsorted_outer()->consider_parallel_nestloop()
> and hash_inner_and_outer()->try_partial_hashjoin_path().

AFAICS, those are about generating partial paths, which is a completely
different thing from whether a regular path is parallel-safe or not.
(I think, anyway. It would be nice if this stuff were documented better.
It would also likely be a good thing if partial-ness of a path were marked
in the path itself, which does not seem to be the case now. Or at the
very least, it'd be a good thing if create_foo_path and the underlying
costing functions were told it was a partial path, because how the heck
can they generate sane cost numbers without that knowledge?)

regards, tom lane