RE: Partial aggregates pushdown

Hi Mr.Pyhalov.

> One more issue I started to think about - now we don't check
> partialagg_minversion for "simple" aggregates at all. Is it correct? It seems that ,
> for example, we could try to pushdown bit_or(int8) to old servers, but it didn't
> exist, for example, in 8.4. I think it's a broader issue (it would be also the case
> already if we push down
> aggregates) and shouldn't be fixed here. But there is an issue -
> is_shippable() is too optimistic.
I think it is correct for now.
F.38.7 of [1] says "A limitation however is that postgres_fdw generally assumes that
immutable built-in functions and operators are safe to send to the remote server for
execution, if they appear in a WHERE clause for a foreign table." and says that we can
avoid this limitation by rewriting query.
It looks that postgres_fdw follows this policy in case of UPPERREL_GROUP_AGG aggregate pushdown.
If a aggreagate has not internal aggtranstype and has not aggfinalfn ,
partialaggfn of it is equal to itself.
So I think that it is adequate to follow this policy in case of partial aggregate pushdown
for such aggregates.

> >> 2) Do we really have to look at pg_proc in partial_agg_ok() and
> >> deparseAggref()? Perhaps, looking at aggtranstype is enough?
> > You are right. I fixed according to your comment.
> >
>
> partial_agg_ok() still looks at pg_proc.
Sorry for taking up your time. I fixed it.

[1] https://www.postgresql.org/docs/current/postgres-fdw.html

Sincerely yours,
Yuuki Fujii

--
Yuuki Fujii
Information Technology R&D Center Mitsubishi Electric Corporation