Re: POC: GROUP BY optimization

On 7/15/22 07:18, David Rowley wrote:
> 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 agree this is a mistake in db0d67db2 that makes the test 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.
>

It might seem a bit strange, but the patch tweaked the costing a bit, so
it's not entirely unexpected. I'd bet the plan cost changed just a teeny
bit, but enough to change the cheapest plan. The costing changed for all
group counts, including a single group.

regards

--
Tomas Vondra
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