On Mon, Mar 7, 2016 at 9:13 AM, Tom Lane <tgl@sss.pgh.pa.us> wrote: > > Amit Kapila <amit.kapila16@gmail.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.
>
Okay, but the main point I wanted to convey is that I think setting parallel_degree = 0 in mergejoin path is not necessarily a copy-paste error. If you see the other path generation code like create_index_path(), create_samplescan_path(), etc. there we set parallel_degree to zero even though the parallel-safety is determined based on reloptinfo. And I don't see any use of setting parallel_degree for path which can't be pushed beneath gather (aka can be executed by multiple workers).