Re: set_cheapest without checking pathlist

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.

Thanks
Richard