Re: POC: GROUP BY optimization

Hi!

On Sun, Oct 1, 2023 at 11:45 AM Andrei Lepikhov
<a(dot)lepikhov(at)postgrespro(dot)ru> wrote:
>
> New version of the patch. Fixed minor inconsistencies and rebased onto
> current master.

Thank you (and other authors) for working on this subject. Indeed to
GROUP BY clauses are order-agnostic. Reordering them in the most
suitable order could give up significant query planning benefits. I
went through the thread: I see significant work has been already made
on this patch, the code is quite polished.

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.

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.

3) I see that get_useful_group_keys_orderings() makes 3 calls to
get_cheapest_group_keys_order() function. Each time
get_cheapest_group_keys_order() performs the cost estimate and
reorders the free keys. However, cost estimation implies the system
catalog lookups (that is quite expensive). I wonder if we could
change the algorithm. Could we just sort the group-by keys by cost
once, save this ordering and then just re-use it. So, every time we
need to reorder a group by, we can just pull the required keys to the
top and use saved ordering for the rest. I also wonder if we could do
this once for add_paths_to_grouping_rel() and
create_partial_grouping_paths() calls. So, it probably should be
somewhere in create_ordinary_grouping_paths().

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.

------
Regards,
Alexander Korotkov