Re: Removing redundant grouping columns

On Sat, 31 Dec 2022 at 02:32, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
>
> I wrote:
> > Richard Guo <guofenglinux(at)gmail(dot)com> writes:
> >> While we are here, I wonder if we can do the same trick for
> >> distinctClause, to cope with cases like
> >> select distinct a.x, b.y from a, b where a.x = b.y;
>
> > We do that already, no?
>
> Oh, wait, I see what you mean: we are smart in code paths that rely
> on distinct_pathkeys, but not in the hash-based code paths. Right,
> that can be fixed the same way. 0001 attached is the same as before,
> 0002 adds similar logic for the distinctClause.
>
> The plan change in expected/pg_trgm.out is surprising at first
> glance, but I believe it's correct: the item that is being
> dropped is a parameterless STABLE function, so its value is not
> supposed to change for the duration of the scan.

The patch does not apply on top of HEAD as in [1], please post a rebased patch:
=== Applying patches on top of PostgreSQL commit ID
ff23b592ad6621563d3128b26860bcb41daf9542 ===
=== applying patch ./v3-0002-remove-redundant-DISTINCT.patch
....
Hunk #4 FAILED at 4704.
....
1 out of 10 hunks FAILED -- saving rejects to file
src/backend/optimizer/plan/planner.c.rej

[1] - http://cfbot.cputube.org/patch_41_4083.log

Regards,
Vignesh