Re: Partial aggregates pushdown

Robert Haas писал 2023-11-21 20:16:

>> > I don't think the patch does a good job explaining why HAVING,
>> > DISTINCT, and ORDER BY are a problem. It seems to me that HAVING
>> > shouldn't really be a problem, because HAVING is basically a WHERE
>> > clause that occurs after aggregation is complete, and whether or not
>> > the aggregation is safe shouldn't depend on what we're going to do
>> > with the value afterward. The HAVING clause can't necessarily be
>> > pushed to the remote side, but I don't see how or why it could make
>> > the aggregate itself unsafe to push down. DISTINCT and ORDER BY are a
>> > little trickier: if we pushed down DISTINCT, we'd still have to
>> > re-DISTINCT-ify when combining locally, and if we pushed down ORDER
>> > BY, we'd have to do a merge pass to combine the returned values unless
>> > we could prove that the partitions were non-overlapping ranges that
>> > would be visited in the correct order. Although that all sounds
>> > doable, I think it's probably a good thing that the current patch
>> > doesn't try to handle it -- this is complicated already. But it should
>> > explain why it's not handling it and maybe even a bit about how it
>> > could be handling in the future, rather than just saying "well, this
>> > kind of thing is not safe." The trouble with that explanation is that
>> > it does nothing to help the reader understand whether the thing in
>> > question is *fundamentally* unsafe or whether we just don't have the
>> > right code to make it work.
>>
>> Makes sense.
>
> Actually, I think I was wrong about this. We can't handle ORDER BY or
> DISTINCT because we can't distinct-ify or order after we've already
> partially aggregated. At least not in general, and not without
> additional aggregate support functions. So what I said above was wrong
> with respect to those. Or so I believe, anyway. But I still don't see
> why HAVING should be a problem.

Hi. HAVING is also a problem. Consider the following query

SELECT count(a) FROM t HAVING count(a) > 10 - we can't push it down to
foreign server as HAVING needs full aggregate result, but foreign server
don't know it.

--
Best regards,
Alexander Pyhalov,
Postgres Professional