Re: Partial aggregates pushdown

On 3/22/22 01:49, Andres Freund wrote:
> On 2022-01-17 15:27:53 +0300, Alexander Pyhalov wrote:
>> Alexander Pyhalov писал 2022-01-17 15:26:
>>> Updated patch.
>>
>> Sorry, missed attachment.
>
> Needs another update: http://cfbot.cputube.org/patch_37_3369.log
>
> Marked as waiting on author.
>

TBH I'm still not convinced this is the right approach. I've voiced this
opinion before, but to reiterate the main arguments:

1) It's not clear to me how could this get extended to aggregates with
more complex aggregate states, to support e.g. avg() and similar fairly
common aggregates.

2) I'm not sure relying on aggpartialpushdownsafe without any version
checks etc. is sufficient. I mean, how would we know the remote node has
the same idea of representing the aggregate state. I wonder how this
aligns with assumptions we do e.g. for functions etc.

Aside from that, there's a couple review comments:

1) should not remove the comment in foreign_expr_walker

2) comment in deparseAggref is obsolete/inaccurate

3) comment for partial_agg_ok should probably explain when we consider
aggregate OK to be pushed down

4) I'm not sure why get_rcvd_attinmeta comment talks about "return type
bytea" and "real input type".

5) Talking about "partial" aggregates is a bit confusing, because that
suggests this is related to actual "partial aggregates". But it's not.

6) Can add_foreign_grouping_paths do without the new 'partial'
parameter? Clearly, it can be deduced from extra->patype, no?

7) There's no docs for PARTIALCONVERTERFUNC / PARTIAL_PUSHDOWN_SAFE in
CREATE AGGREGATE sgml docs.

8) I don't think "serialize" in the converter functions is the right
term, considering those functions are not "serializing" anything. If
anything, it's the remote node that is serializing the agg state and the
local not is deserializing it. Or maybe I just misunderstand where are
the converter functions executed?

regards

--
Tomas Vondra
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