Re: POC: GROUP BY optimization

On 7/24/23 04:10, Andrey Lepikhov wrote:
> On 20/7/2023 18:46, Tomas Vondra wrote:
>> On 7/20/23 08:37, Andrey Lepikhov wrote:
>>> On 3/10/2022 21:56, Tom Lane wrote:
>>>> Revert "Optimize order of GROUP BY keys".
>>>>
>>>> This reverts commit db0d67db2401eb6238ccc04c6407a4fd4f985832 and
>>>> several follow-on fixes.
>>>> ...
>>>> Since we're hard up against the release deadline for v15, let's
>>>> revert these changes for now.  We can always try again later.
>>>
>>> It may be time to restart the project. As a first step, I rebased the
>>> patch on the current master. It wasn't trivial because of some latest
>>> optimizations (a29eab, 1349d27 and 8d83a5d).
>>> Now, Let's repeat the review and rewrite the current path according to
>>> the reasons uttered in the revert commit.
>>
>> I think the fundamental task is to make the costing more reliable, and
>> the commit message 443df6e2db points out a couple challenges in this
>> area. Not sure how feasible it is to address enough of them ...
>>
>> 1) procost = 1.0 - I guess we could make this more realistic by doing
>> some microbenchmarks and tuning the costs for the most expensive cases.
>>
>> 2) estimating quicksort comparisons - This relies on ndistinct
>> estimates, and I'm not sure how much more reliable we can make those.
>> Probably not much :-( Not sure what to do about this, the only thing I
>> can think of is to track "reliability" of the estimates and only do the
>> reordering if we have high confidence in the estimates. That means we'll
>> miss some optimization opportunities, but it should limit the risk.
> I read up on the history of this thread.
> As I see, all the problems mentioned above can be beaten by excluding
> the new cost model at all. We can sort GROUP BY columns according to the
> 'ndistinct' value.
> I see the reason for introducing the cost model in [1]. The main
> supporting point here is that with this patch, people couldn't optimize
> the query by themselves, organizing the order of the columns in a more
> optimal way. But now we have at least the GUC to switch off the
> behaviour introduced here. Also, some extensions, like the well-known
> pg_hint_plan, can help with automation.

I think the main concern is that if we reorder the group keys and get it
wrong, it's a regression. If that happens often (due to costing based on
poor stats), it's a problem. Yes, there's a GUC, but that's a rather
blunt instrument, unfortunately.

> So, how about committing of the undoubted part of the feature and
> working on the cost model in a new thread?
> 

But Tom's commit message says this:

    Worse, to arrive at estimates of the number of calls made to the
    lower-order-column comparison functions, the code needs to make
    estimates of the numbers of distinct values of multiple columns,
    which are necessarily even less trustworthy than per-column stats.

so I'm not sure this really counts as "undoubted".


regards

-- 
Tomas Vondra
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