Re: POC: GROUP BY optimization

From: Andrei Lepikhov <a(dot)lepikhov(at)postgrespro(dot)ru>
To: Alexander Korotkov <aekorotkov(at)gmail(dot)com>
Cc: Pavel Borisov <pashkin(dot)elfe(at)gmail(dot)com>, vignesh C <vignesh21(at)gmail(dot)com>, PostgreSQL Developers <pgsql-hackers(at)lists(dot)postgresql(dot)org>, Tomas Vondra <tomas(dot)vondra(at)enterprisedb(dot)com>, Teodor Sigaev <teodor(at)sigaev(dot)ru>, David Rowley <dgrowleyml(at)gmail(dot)com>, "a(dot)rybakina" <a(dot)rybakina(at)postgrespro(dot)ru>, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Subject: Re: POC: GROUP BY optimization
Date: 2024-01-14 12:14:13
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On 13/1/2024 22:00, Alexander Korotkov wrote:
> On Sat, Jan 13, 2024 at 11:09 AM Andrei Lepikhov
> <a(dot)lepikhov(at)postgrespro(dot)ru> wrote:
>> On 11/1/2024 18:30, Alexander Korotkov wrote:
>>> On Tue, Jan 9, 2024 at 1:14 PM Pavel Borisov <pashkin(dot)elfe(at)gmail(dot)com> wrote:
>>>>> Hmm, I don't see this old code in these patches. Resend 0002-* because
>>>>> of trailing spaces.
>>>> AFAIK, cfbot does not seek old versions of patchset parts in previous messages. So for it to run correctly, a new version of the whole patchset should be sent even if most patches are unchanged.
>>> Please, find the revised patchset with some refactoring and comments
>>> improvement from me. I'll continue looking into this.
>> The patch looks better, thanks to your refactoring.
>> I propose additional comments and tests to make the code more
>> understandable (see attachment).
>> I intended to look into this part of the code more, but the tests show a
>> difference between PostgreSQL v.15 and v.16, which causes changes in the
>> code of this feature.
> Makes sense. I've incorporated your changes into the patchset.
One more improvement. To underpin code change:

- return cur_ec; /* Match! */
+ {
+ /*
+ * Match!
+ *
+ * Copy the sortref if it wasn't set yet. That may happen if
+ * the ec was constructed from WHERE clause, i.e. it doesn't
+ * have a target reference at all.
+ */
+ if (cur_ec->ec_sortref == 0 && sortref > 0)
+ cur_ec->ec_sortref = sortref;
+ return cur_ec;
+ }

I propose the test (see attachment). It shows why we introduce this
change: GROUP-BY should juggle not only pathkeys generated by explicit
sort operators but also planner-made, likewise in this example, by

Andrei Lepikhov
Postgres Professional

Attachment Content-Type Size
details-2.txt text/plain 2.7 KB

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