Re: Partial aggregates pushdown

On Wed, Jun 5, 2024 at 08:19:04AM +0300, Alexander Pyhalov wrote:
> > * Passes unsafe binary data from the foreign server.
> >
> > Can someone show me where that last item is in the patch, and why can't
> > we just pass back values cast to text?
>
> In finalize_aggregate() when we see partial aggregate with
> peragg->aggref->aggtranstype = INTERNALOID
> we call aggregate's serialization function and return it as bytea.
>
> The issue is that this internal representation isn't guaranteed to be
> compatible between servers
> of different versions (or architectures?). So, likely, we instead should
> have called some export function for aggregate
> and later - some import function on postgres_fdw side. It doesn't matter
> much, what this export function
> generates - text, json or some portable binary format,
> 1) export/import functions should just "understand" it,
> 2) it should be a stable representation.

Okay, so looking at the serialization output functions already defined, I
see many zeros, which I assume means just the base data type, and eight
more:

SELECT DISTINCT aggserialfn from pg_aggregate WHERE aggserialfn::oid != 0;
aggserialfn
---------------------------
numeric_avg_serialize
string_agg_serialize
array_agg_array_serialize
numeric_serialize
int8_avg_serialize
array_agg_serialize
interval_avg_serialize
numeric_poly_serialize

I realize we need to return the sum and count for average, so that makes
sense.

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?

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

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