Re: [HACKERS] Runtime Partition Pruning

On Wed, Oct 7, 2020 at 7:00 PM Andy Fan <zhihui(dot)fan1213(at)gmail(dot)com> wrote:
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> On Wed, Oct 7, 2020 at 5:05 PM Andy Fan <zhihui(dot)fan1213(at)gmail(dot)com> wrote:
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>> On Sun, Oct 4, 2020 at 3:10 PM Andy Fan <zhihui(dot)fan1213(at)gmail(dot)com> wrote:
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>>>> Now, in my experience, the current system for custom plans vs. generic
>>>> plans doesn't approach the problem in this way at all, and in my
>>>> experience that results in some pretty terrible behavior. It will do
>>>> things like form a custom plan every time because the estimated cost
>>>> of the custom plan is lower than the estimated cost of the generic
>>>> plan even though the two plans are structurally identical; only the
>>>> estimates differ. It will waste gobs of CPU cycles by replanning a
>>>> primary key lookup 5 times just on the off chance that a lookup on the
>>>> primary key index isn't the best option. But this patch isn't going
>>>> to fix any of that. The best we can probably do is try to adjust the
>>>> costing for Append paths in some way that reflects the costs and
>>>> benefits of pruning. I'm tentatively in favor of trying to do
>>>> something modest in that area, but I don't have a detailed proposal.
>>>>
>>>
>>> I just realized this issue recently and reported it at [1], then Amit pointed
>>> me to this issue being discussed here, so I would like to continue this topic
>>> here.
>>>
>>> I think we can split the issue into 2 issues. One is the partition prune in initial
>>> partition prune, which maybe happen in custom plan case only and caused
>>> the above issue. The other one happens in the "Run-Time" partition prune,
>>> I admit that is an important issue to resolve as well, but looks harder. So I
>>> think we can fix the first one at first.
>>>
>>> ... When we count for the cost of a
>>> generic plan, we can reduce the cost based on such information.
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>> This way doesn't work since after the initial partition prune, not only the
>> cost of the Append node should be reduced, the cost of other plans should
>> be reduced as well [1]
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>> However I think if we can use partition prune information from a custom plan
>> at the cost_append_path stage, it looks the issue can be fixed. If so, the idea
>> is similar to David's idea in [2], however Robert didn't agree with this[2].
>> Can anyone elaborate this objection? for a partkey > $1 or BETWEEN cases,
>> some real results from the past are probably better than some hard-coded
>> assumptions IMO.
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> I can understand Robert's idea now, he intended to resolve both the
> "initial-partition-prune" case and "runtime partition prune" case while I always think
> about the former case (Amit reminded me about that at the beginning, but I just
> wake up right now..)
>
> With the Robert's idea, I think we can do some hack at create_append_path,
> we can figure out the pruneinfo (it was done at create_append_plan now) at that
> stage, and then did a fix_append_path with such pruneinfo. We need to fix not
> only the cost of AppendPath, but also rows of AppendPath, For example:
> SELECT .. FROM t1, t2, p where t1.a = p.partkey and t1.b = t2.b, if the
> plan is:
> Nest Loop
> Nest Loop
> t1
> Append (p)
> t2
>
> Then the rows of Append (p) will be very important.
>
> For this idea, how to estimate how many partitions/rows can be pruned is a key
> part. Robert said "I was thinking, rather, that if we know for example that
> we've doing pruning on partition_column = $1, then we know that only
> one partition will match. That's probably a common case. If we've
> got partition_column > $1, we could assume that, say, 75% of the
> partitions would match. partition_column BETWEEN $1 and $2 is
> probably a bit more selective, so maybe we assume 50% of the
> partitions would match.". I think we can't say the 75% or 50% is a good
> number, but the keypoint may be "partition_column = $1" is a common
> case. And for the others case, we probably don't make it worse.
>
> I think we need to do something here, any thoughts? Personally I'm more
> like this idea now.

Yes, I think we have to do something on those lines. But I am
wondering why our stats machinery is failing to do that already. For
equality I think it's straight forward. But even for other operators
we should use the statistics from the partitioned table to estimate
the selectivity and then adjust number of scanned partitions = (number
of partitions * fraction of rows scanned). That might be better than
50% or 75%.

--
Best Wishes,
Ashutosh Bapat