Date: Mon, 7 Aug 2023 14:30:54 -0400
From: Bruce Momjian <bruce@momjian.us>
To: "Fujii.Yuki@df.MitsubishiElectric.co.jp" <Fujii.Yuki@df.mitsubishielectric.co.jp>
Cc: Alexander Pyhalov <a.pyhalov@postgrespro.ru>,
	PostgreSQL-development <pgsql-hackers@postgresql.org>,
	Andres Freund <andres@anarazel.de>, Tom Lane <tgl@sss.pgh.pa.us>,
	Tomas Vondra <tomas.vondra@enterprisedb.com>,
	Julien Rouhaud <rjuju123@gmail.com>,
	Daniel Gustafsson <daniel@yesql.se>,
	Ilya Gladyshev <i.gladyshev@postgrespro.ru>
Subject: Re: Partial aggregates pushdown
Message-ID: <ZNE4XiLsV/Wo4egh@momjian.us>
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On Mon, Jul 10, 2023 at 07:35:27AM +0000, Fujii.Yuki@df.MitsubishiElectric.co.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.

-- 
  Bruce Momjian  <bruce@momjian.us>        https://momjian.us
  EDB                                      https://enterprisedb.com

  Only you can decide what is important to you.