Re: enable_incremental_sort changes query behavior

On Tue, Oct 06, 2020 at 09:37:31AM -0400, James Coleman wrote:
>On Tue, Oct 6, 2020 at 9:28 AM Jaime Casanova
><jaime(dot)casanova(at)2ndquadrant(dot)com> wrote:
>> Can you please create an entry in the commitfest for this one so we
>> don't lose track of it?
>

We're not too far from the next minor release, so I've been looking at
this fix again and I intend to get it committed shortly (on Monday or
so). Attached is a slightly modified version of the v4 patch that:

(a) Removes comments about projections added to code that is not
directly related to the fix. I'm not against adding such comments
separately.

(b) Fixes comment in expected output of incremental_sort test.

(c) Removes else branch from find_em_expr_usable_for_sorting_rel. It's
not quite needed thanks to the "return" in the "if" branch. IMO this
makes it more elegant.

I do have two questions about find_em_expr_usable_for_sorting_rel.

(a) Where does the em_is_const check come from? The comment claims we
should not be trying to sort by equivalence class members, but I can't
convince myself it's actually true and I don't see it discussed in this
thread.

(b) In find_em_expr_for_rel, which was what was used before, the
condition on relids was this:

if (bms_is_subset(em->em_relids, rel->relids) &&
!bms_is_empty(em->em_relids))
{
return em->em_expr;
}

but here we're using only

if (!bms_is_subset(em->em_relids, rel->relids))
continue;

Isn't this missing the second bms_is_empty condition?

Of course, an alternative to this fix would be reverting ba3e76cc571
(completely or just the part introducing generate_useful_gather_paths).
If anyone thinks that's what we should do, please speak now.

regards

--
Tomas Vondra http://www.2ndQuadrant.com
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services