Re: POC: GROUP BY optimization

On Thu, 31 Mar 2022 at 12:19, Tomas Vondra
<tomas(dot)vondra(at)enterprisedb(dot)com> wrote:
> Pushed, after going through the patch once more, running check-world
> under valgrind, and updating the commit message.

I'm still working in this area and I noticed that db0d67db2 updated
some regression tests in partition_aggregate.out without any care as
to what the test was testing.

The comment above the test reads:

-- Without ORDER BY clause, to test Gather at top-most path

and you've changed the expected plan from being a parallel plan with a
Gather to being a serial plan. So it looks like the test might have
become useless.

I see that the original plan appears to come back with some
adjustments to parallel_setup_cost and parallel_tuple_cost. It seems a
bit strange to me that the changes with this patch would cause a
change of plan for this. There is only 1 GROUP BY column in the query
in question. There's no rearrangement to do with a single column GROUP
BY.

David