On Thu, 23 Apr 2020 at 11:11, Tom Lane <t...@sss.pgh.pa.us> wrote: > > David Rowley <dgrowle...@gmail.com> writes: > > Given the same set of paths, when would a non-parallel append be > > cheaper than a parallel one? > > Well, anytime the parallel startup cost is significant, for starters. > But maybe we account for that at some other point, like when building > the Gather?
Yeah. There's no mention of parallel_setup_cost or parallel_tuple_cost in any of the Append costing code. Those are only applied when we cost Gather / GatherMerge At the point Amit and I are talking about, we're only comparing two Append paths. No Gather/GatherMerge in sight yet, so any additional costs from those is not applicable. If there was some reason that a Parallel Append could come out more expensive, then maybe we could just create a non-parallel Append using the same subpath list and add_partial_path() it. I just don't quite see how that would ever win though. I'm willing to be proven wrong though. David
