Re: set_cheapest without checking pathlist

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