Re: POC: GROUP BY optimization

Hi, Andrei!

On Thu, Apr 18, 2024 at 11:54 AM Andrei Lepikhov
<a(dot)lepikhov(at)postgrespro(dot)ru> wrote:
> On 4/12/24 06:44, Tom Lane wrote:
> > * Speaking of pathnodes.h, PathKeyInfo is a pretty uninformative node
> > type name, and the comments provided for it are not going to educate
> > anybody. What is the "association" between the pathkeys and clauses?
> > I guess the clauses are supposed to be SortGroupClauses, but not all
> > pathkeys match up to SortGroupClauses, so what then? This is very
> > underspecified, and fuzzy thinking about data structures tends to lead
> > to bugs.
> I'm not the best in English documentation and naming style. So, feel
> free to check my version.
> >
> > So I'm quite afraid that there are still bugs lurking here.
> > What's more, given that the plans this patch makes apparently
> > seldom win when you don't put a thumb on the scales, it might
> > take a very long time to isolate those bugs. If the patch
> > produces a faulty plan (e.g. not sorted properly) we'd never
> > notice if that plan isn't chosen, and even if it is chosen
> > it probably wouldn't produce anything as obviously wrong as
> > a crash.
> I added checkings on the proper order of pathkeys and clauses.
> If you really care about that, we should spend additional time and
> rewrite the code to generate an order of clauses somewhere in the plan
> creation phase. For example, during the create_group_plan call, we could
> use the processed_groupClause list, cycle through subpath->pathkeys, set
> the order, and detect whether the pathkeys list corresponds to the
> group-by or is enough to build a grouping plan.
> Anyway, make this part of code more resistant to blunders is another story.

Thank you for the fixes you've proposed. I didn't look much into
details yet, but I think the main concern Tom expressed in [1] is
whether the feature is reasonable at all. I think at this stage the
most important thing is to come up with convincing examples showing
how huge performance benefits it could cause. I will return to this
later today and will try to provide some convincing examples.

Links
1. https://www.postgresql.org/message-id/266850.1712879082%40sss.pgh.pa.us

------
Regards,
Alexander Korotkov