Re: Partial aggregates pushdown

On Thu, Dec 15, 2022 at 10:23:05PM +0000, Fujii(dot)Yuki(at)df(dot)MitsubishiElectric(dot)co(dot)jp wrote:
> Hi Mr.Freund.
>
> > cfbot complains about some compiler warnings when building with clang:
> > https://cirrus-ci.com/task/6606268580757504
> >
> > deparse.c:3459:22: error: equality comparison with extraneous parentheses
> > [-Werror,-Wparentheses-equality]
> > if ((node->aggsplit == AGGSPLIT_SIMPLE)) {
> > ~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~
> > deparse.c:3459:22: note: remove extraneous parentheses around the
> > comparison to silence this warning
> > if ((node->aggsplit == AGGSPLIT_SIMPLE)) {
> > ~ ^ ~
> > deparse.c:3459:22: note: use '=' to turn this equality comparison into an
> > assignment
> > if ((node->aggsplit == AGGSPLIT_SIMPLE)) {
> > ^~
> > =
> I fixed this error.

Considering we only have a week left before feature freeze, I wanted to
review the patch from this commitfest item:

https://commitfest.postgresql.org/42/4019/

The most recent patch attached.

This feature has been in development since 2021, and it is something
that will allow new workloads for Postgres, specifically data warehouse
sharding workloads.

We currently allow parallel aggregates when the table is on the same
machine, and we allow partitonwise aggregates on FDWs only with GROUP BY
keys matching partition keys. The first is possible since we can share
data structures between background workers, and the second is possible
because if the GROUP BY includes the partition key, we are really just
appending aggregate rows, not combining aggregate computations.

What we can't do without this patch is to push aggregates that require
partial aggregate computations (no partition key GROUP BY) to FDW
partitions because we don't have a clean way to pass such information
from the remote FDW server to the finalize backend. I think that is
what this patch does.

First, am I correct? Second, how far away is this from being committable
and/or what work needs to be done to get it committable, either for PG 16
or 17?

--
Bruce Momjian <bruce(at)momjian(dot)us> https://momjian.us
EDB https://enterprisedb.com

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