On Thu, Feb 1, 2024 at 1:36 AM Richard Guo <guofengli...@gmail.com> wrote: > > > On Thu, Feb 1, 2024 at 11:37 AM David Rowley <dgrowle...@gmail.com> wrote: >> >> On Thu, 1 Feb 2024 at 16:29, Richard Guo <guofengli...@gmail.com> wrote: >> > On Thu, Feb 1, 2024 at 10:04 AM James Coleman <jtc...@gmail.com> wrote: >> >> I don't see any inherent reason why we must always assume that >> >> gather_grouping_paths will always result in having at least one entry >> >> in pathlist. If, for example, we've disabled incremental sort and the >> >> cheapest partial path happens to already be sorted, then I don't >> >> believe we'll add any paths. And if that happens then set_cheapest >> >> will error with the message "could not devise a query plan for the >> >> given query". So I propose we add a similar guard to this call point. >> > >> > >> > I don't believe that would happen. It seems to me that we should, at >> > the very least, have a path which is Gather on top of the cheapest >> > partial path (see generate_gather_paths), as long as the >> > partially_grouped_rel has partial paths. >> >> It will have partial paths because it's nested inside "if >> (partially_grouped_rel && partially_grouped_rel->partial_pathlist)" > > > Right. And that leads to the conclusion that gather_grouping_paths will > always generate at least one path for partially_grouped_rel. So it > seems to me that the check added in the patch is not necessary. But > maybe we can add an Assert or a comment clarifying that the pathlist of > partially_grouped_rel cannot be empty here.
Yes, that's true currently. I don't particularly love that we have the knowledge of that baked in at this level, but it won't break anything currently. I'll put this as a patch in the parallelization patch series referenced earlier since in that series my changes result in generate_gather_paths() not necessarily always being able to build a path. Regards, James Coleman
