RE: Partial aggregates pushdown

Hi Mr.Momjian.

Thank you for advises.

> From: Bruce Momjian <bruce@momjian.us>
> Sent: Monday, June 12, 2023 10:38 PM
> > I understand what the problem is. I will put a mechanism maintaining compatibility into the patch.
> > I believe there are three approaches.
> > Approach 1-1 is preferable because it does not require additional options for postgres_fdw.
> > I will revise the patch according to Approach 1-1, unless otherwise commented.
> >
> > Approach1:
> > I ensure that postgres_fdw retrieves the version of each remote server
> > and does not partial aggregate pushd down if the server version is less than 17.
> > There are two approaches to obtaining remote server versions.
> > Approach1-1: postgres_fdw connects a remote server and use PQserverVersion().
> > Approach1-2: Adding a postgres_fdw option about a remote server version (like "server_version").
> >
> > Approach2:
> > Adding a postgres_fdw option for partial aggregate pushdown is enable
> > or not (like enable_partial_aggregate_pushdown).
>
> These are good questions.  Adding a postgres_fdw option called enable_partial_aggregate_pushdown helps make the
> purpose of the option clear, but remote_version can be used for future breakage as well.
>
> I think remote_version is the best idea, and in the documention for the option, let's explcitly say it is useful to
disable
> partial aggreates pushdown on pre-PG 17 servers.  If we need to use the option for other cases, we can just update
the
> documentation.  When the option is blank, the default, everything is pushed down.
>
> I see remote_version a logical addition to match our "extensions" option that controls what extension functions can
be
> pushed down.

Thank you for your perspective.
So, of the approaches I have presented, you think that approach 1-2 is
preferable and that the option name remote_server is preferable?
Indeed, the option of a remote version may have other uses.
However, this information can be obtained by connecting to a remote server,
I'm concerned that some people may find this option redundant.

Is the problem with approach 1-1 because the user cannot decide whether to include the compatibility check in the
decisionto do partial aggregate pushdown or not? 
# If Approach 1-1 is taken, the problem is that this feature cannot be used for all buit-in aggregate functions
# when the remote server is older than PG17.
If so, Approache1-3 below seem more desirable.
Would it be possible for us to hear your thoughts?

Approache1-3：We add a postgres_fdw option about a compatibility check for partial aggregate pushdown
(like "enable_aggpartialfunc_compatibility_check"). This option is false, the default.
When this option is true, postgres_fdw obtains the remote server version by connecting the remote server and using
PQserverVersion(). 
And if the remote server version is older than PG17, then the partial aggregate pushdown feature is enable for all
buit-inaggregate functions. 
Otherwise the partial aggregate pushdown feature is disable for all buit-in aggregate functions.

Sincerely yours,
Yuuki Fujii

--
Yuuki Fujii
Information Technology R&D Center Mitsubishi Electric Corporation