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

On Fri, Oct 30, 2020 at 01:26:08PM -0400, James Coleman wrote:
>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.
>

Probably sometime later, as a separate patch. I haven't thought very
much about those comments, it just seemed unrelated to the fix so I've
removed that for now. Let's not bother with this until after the minor
release.

>> (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.
>

Thanks for the pointer. I'll take a look.

>> (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.
>

Yeah, it'd be good to explain the reasoning why it's fine to have the
conditions like this. Thanks.


regards


-- 
Tomas Vondra                  http://www.2ndQuadrant.com
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