Re: POC: GROUP BY optimization

On 15.01.2024 12:46, Andrei Lepikhov wrote:
> On 15/1/2024 13:42, Richard Guo wrote:
>>
>> On Mon, Jan 15, 2024 at 8:20 AM Alexander Korotkov
>> <aekorotkov(at)gmail(dot)com <mailto:aekorotkov(at)gmail(dot)com>> wrote:
>>
>>     Thank you for providing the test case relevant for this code change.
>>     The revised patch incorporating this change is attached. Now the
>>     patchset looks good to me.  I'm going to push it if there are no
>>     objections.
>>
>>
>> Seems I'm late for the party.  Can we hold for several more days?  I'd
>> like to have a review on this patch.
> Get on board! It looks like this feature needs as much review as
> possible (likewise SJE).
>
Hi! Thank you for your work on this issue! I believe that this will help
the scheduler to make a more optimal query plan here and therefore speed
up their execution.

I have reviewed patches and noticed that we can add some code
refactoring. I have attached a diff file (group_by.diff) to this email.

The changes involve spelling corrections, renaming variables and porting
some common parts.

In addition, I have a few questions, since some points in the code
remained unclear to me.

1.  I didn't understand why we have a question in the comment next to
the enable_group_by_reordering variable in
src/backend/optimizer/path/pathkeys.c file, I assumed it was spelling
and fixed it in the diff file.

2. Why do we set the variable (path = path_save) here
(add_paths_to_grouping_rel function) if we change its variable below and
we can pass path_save as a parameter?

foreach(lc2, pathkey_orderings)
{
    PathKeyInfo *info = (PathKeyInfo *) lfirst(lc2);

    /* restore the path (we replace it in the loop) */
    path = path_save;

    path = make_ordered_path(root,
                             grouped_rel,
                             path,
                             cheapest_path,
                             info->pathkeys);
    if (path == NULL)
        continue;

--
Regards,
Alena Rybakina
Postgres Professional: http://www.postgrespro.com
The Russian Postgres Company