Re: Partial aggregates pushdown

On Mon, Jul 10, 2023 at 07:35:27AM +0000, Fujii(dot)Yuki(at)df(dot)MitsubishiElectric(dot)co(dot)jp wrote:
> > > I will add a postgres_fdw option "check_partial_aggregate_support".
> > > This option is false, default.
> > > Only if this option is true, postgres_fdw connect to the remote server and get the version of the remote server.
> > > And if the version of the remote server is less than PG17, then partial aggregate push down to the remote server is
> > disable.
> >
> > Great!
> I have modified the program except for the point "if the version of the remote server is less than PG17".
> Instead, we have addressed the following.
> "If check_partial_aggregate_support is true and the remote server version is older than the local server
> version, postgres_fdw does not assume that the partial aggregate function is on the remote server unless
> the partial aggregate function and the aggregate function match."
> The reason for this is to maintain compatibility with any aggregate function that does not support partial
> aggregate in one version of V1 (V1 is PG17 or higher), even if the next version supports partial aggregate.
> For example, string_agg does not support partial aggregation in PG15, but it will support partial aggregation
> in PG16.

Just to clarify, I think you are saying:

If check_partial_aggregate_support is true and the remote server
version is older than the local server version, postgres_fdw
checks if the partial aggregate function exists on the remote
server during planning and only uses it if it does.

I tried to phrase it in a positive way, and mentioned the plan time
distinction. Also, I am sorry I was away for most of July and am just
getting to this.

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

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