Re: enable_incremental_sort changes query behavior - Mailing list pgsql-hackers

On Thu, Oct 29, 2020 at 6:06 PM Tomas Vondra
<tomas.vondra@2ndquadrant.com> wrote:
>
> 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.casanova@2ndquadrant.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.

Seems fine. Do you want to commit them at the same time (just a
separate commit)? Or have a separate patch? It seems a bit overkill to
start a new thread just for those.

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

Thanks.

> (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.

No objection.

> 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.

That comes from find_ec_member_for_tle which is called by
prepare_sort_from_pathkeys which we note in the comments contains the
set of rules we need to mirror.

> (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?

I definitely remember intentionally removing that condition. For one,
it's not present in prepare_sort_from_pathkeys. My memory is a bit
fuzzy on all of the whys, but isn't it the case that if we've already
excluded const expressions, that we must have vars (and therefore
rels) present, and therefore the relids bms must be non-empty?
Probably more importantly, we're going to check that an actual em expr
matches, which means the bms subset check is really just an
optimization to avoid unnecessarily scanning the exprs for equality.
Perhaps it's worth adding a comment saying as such?

By the way, the fact that this is parallel to
prepare_sort_from_pathkeys is the reason the XXX comment is still
there about possibly needing to remove relabel types (since
prepare_sort_from_pathkeys does that). I don't think it's a hard
requirement: the worst case is that by not digging into relabel types
we're being unnecessarily strict.

Both of these I can add if desired.

James