Re: Partial aggregates pushdown

On Thu, Aug 22, 2024 at 08:31:11PM +0200, Tomas Vondra wrote:
> > My question is related to #3 and #4. For #3, if we are going to be
> > building infrastructure to handle passing int128 for AVG, wouldn't it be
> > wiser to create an int128 type and an int128 array type, and then use
> > method #2 to handle those, rather than creating custom code just to
> > read/write int128 values for FDWs aggregate passing alone.
> >
>
> Yep, adding int128 as a data type would extend this to aggregates that
> store state as int128 (or array of int128).

Great, I am not too far off then.

> > For #4, can we use or improve the RECORD data type to handle #4 --- that
> > seems preferable to creating custom FDWs aggregate passing code.
> >
> > I know the open question was whether we should create custom FDWs
> > aggregate passing functions or custom data types for FDWs aggregate
> > passing, but I am asking if we can improve existing facilities, like
> > int128 or record passing, to reduce the need for some of these.
> >
>
> But which code would produce the record? AFAIK it can't happen in some
> generic executor code, because that only sees "internal" for each
> aggregate. The exact structure of the aggstate is private within the
> code of each aggregate - the record would have to be built there, no?
>
> I imagine we'd add this for each aggregate as a new pair of functions to
> build/parse the record, but that's kinda the serial/deserial way we
> discussed earlier.
>
> Or are you suggesting we'd actually say:
>
> CREATE AGGREGATE xyz(...) (
> STYPE = record,
> ...
> )

So my idea from the email I just sent is to create a
pg_proc.proargtypes-like column (data type oidvector) for pg_aggregate
which stores the oids of the values we want to return, so AVG(interval)
would have an array of the oids for interval and int8, e.g.:

SELECT oid FROM pg_type WHERE typname = 'interval';
oid
------
1186

SELECT oid FROM pg_type WHERE typname = 'int8';
oid
-----
20

SELECT '1186 20'::oidvector;
oidvector
-----------
1186 20

It seems all four methods could use this, again assuming we create
int128/int16 and whatever other types we need.

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

Only you can decide what is important to you.