Re: Partial aggregates pushdown

On Tue, 11 Jun 2024 at 13:40, Fujii.Yuki@df.MitsubishiElectric.co.jp
<Fujii.Yuki@df.mitsubishielectric.co.jp> wrote:
> > From: Bruce Momjian <bruce@momjian.us>
> > So, we need import/export text representation for the partial aggregate mode for these eight, and call the base
datatype
 
> > 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.