Re: POC: GROUP BY optimization

Here is a new version of GROUP-BY optimization without sort model.

On 21/12/2023 17:53, Alexander Korotkov wrote:
> I'd like to make some notes.
>
> 1) As already mentioned, there is clearly a repetitive pattern for the
> code following after get_useful_group_keys_orderings() calls. I think
> it would be good to extract it into a separate function. Please, do
> this as a separate patch coming before the group-by patch. That would
> simplify the review.
Done. See patch 0001-*. Unfortunately, extraction of whole cycle isn't
practical, because it blows out the interface of the routine.

> 2) I wonder what planning overhead this patch could introduce? Could
> you try to measure the worst case? What if we have a table with a lot
> of indexes and a long list of group-by clauses partially patching
> every index. This should give us an understanding on whether we need
> a separate GUC to control this feature.
In current implementation I don't anticipate any significant overhead.
GUC is needed here to allow users adhere their own ordering and to
disable feature in the case of problems.

> 4) I think we can do some optimizations when enable_incremental_sort
> == off. Then in get_useful_group_keys_orderings() we should only deal
> with input_path fully matching the group-by clause, and try only full
> match of group-by output to the required order.
Hm, is it really make sense in current implementation?

--
regards,
Andrei Lepikhov
Postgres Professional