Re: Wrong rows estimations with joins of CTEs slows queries by more than factor 500

Richard Guo <guofenglinux(at)gmail(dot)com> writes:
> On Fri, Nov 17, 2023 at 11:38 AM Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
>> That line of argument also leads to the conclusion that it'd be
>> okay to expose info about the ordering of the CTE result to the
>> upper planner.

> In the light of this conclusion, I had a go at propagating the pathkeys
> from CTEs up to the outer planner and came up with the attached.

Oh, nice! I remembered we had code already to do this for regular
SubqueryScans, but I thought we'd need to do some refactoring to
apply it to CTEs. I think you are right though that
convert_subquery_pathkeys can be used as-is. Some thoughts:

* Do we really need to use make_tlist_from_pathtarget? Why isn't
the tlist of the cteplan good enough (indeed, more so)?

* I don't love having this code assume that it knows how to find
the Path the cteplan was made from. It'd be better to make
SS_process_ctes save that somewhere, maybe in a list paralleling
root->cte_plan_ids.

Alternatively: maybe it's time to do what the comments in
SS_process_ctes vaguely speculate about, and just save the Path
at that point, with construction of the plan left for createplan()?
That might be a lot of refactoring for not much gain, so not sure.

regards, tom lane