Re: POC: GROUP BY optimization

On 13/1/2024 22:00, Alexander Korotkov wrote:
> On Sat, Jan 13, 2024 at 11:09 AM Andrei Lepikhov
> <a.lepikhov@postgrespro.ru> wrote:
>> On 11/1/2024 18:30, Alexander Korotkov wrote:
>>> On Tue, Jan 9, 2024 at 1:14 PM Pavel Borisov <pashkin.elfe@gmail.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
versionof 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 
MergeJoin.

-- 
regards,
Andrei Lepikhov
Postgres Professional