Re: set_cheapest without checking pathlist

On Thu, Feb 1, 2024 at 11:37 AM David Rowley <dgrowleyml(at)gmail(dot)com> wrote:

> On Thu, 1 Feb 2024 at 16:29, Richard Guo <guofenglinux(at)gmail(dot)com> wrote:
> > On Thu, Feb 1, 2024 at 10:04 AM James Coleman <jtc331(at)gmail(dot)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.

Thanks
Richard