Re: Partial aggregates pushdown

On Tue, 11 Jun 2024 at 13:40, Fujii(dot)Yuki(at)df(dot)MitsubishiElectric(dot)co(dot)jp
<Fujii(dot)Yuki(at)df(dot)mitsubishielectric(dot)co(dot)jp> wrote:
> > From: Bruce Momjian <bruce(at)momjian(dot)us>
> > So, we need import/export text representation for the partial aggregate mode for these eight, and call the base data type
> > text import/export functions for the zero ones when in this mode?
>
> I think that you are basically right.
> But, I think, in a perfect world we should also add an import/export function for the following
> two category.
>
> Category1. Validation Chek is needed for Safety.
> For example, I think a validation check is needed for avg(float4),
> whose transition type is not internal. (See p.18 in [1])
> I plan to add import functions of avg, count (See p.18, p.19 in [1]).
> Category1. Transition type is a pseudo data type.
> Aggregate functions of this category needs to accept many actual data types,
> including user-defined types. So I think that it is hard to implement import/export functions.
> Consequently, I do not plan to support these category. (See p.19 in [1])

How about instead of trying to serialize the output of
serialfn/deserialfn, instead we don't use the "internal" type and
create actual types in pg_type for these transtypes? Then we can
simply use the in/out and recv/send functions of those types to
serialize the values of the partial aggregate over the network.
Instead of having to rely on serialfn/deserialfn to be network-safe
(which they probably aren't).

That indeed still leaves the pseudo types. Since non of those
pseudotypes have a working in/recv function (they always error by
definition), I agree that we can simply not support those.

Basically that would mean that any aggregate with a non-internal and
non-pseudotype as a transtype could be used in this multi-node partial
aggregate pushdown.