Re: Partial aggregates pushdown

On Mon, Sep 25, 2023 at 03:18:13AM +0000, Fujii.Yuki@df.MitsubishiElectric.co.jp wrote:
> Hi Mr.Bruce, Mr.Pyhalov, Mr.Finnerty, hackers.
> 
> Thank you for your valuable comments. I sincerely apologize for the very late reply.
> Here is a response to your comments or a fix to the patch.
> 
> Tuesday, August 8, 2023 at 3:31 Bruce Momjian
> > > 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.
> Thanks for your comment. In the documentation, the description of check_partial_aggregate_support is as follows
> (please see postgres-fdw.sgml).
> --
> check_partial_aggregate_support (boolean)
> Only if this option is true, during query planning, postgres_fdw connects to the remote server and check if the
remoteserver version is older than the local server version. If so, postgres_fdw assumes that for each built-in
aggregatefunction, the partial aggregate function is not defined on the remote server unless the partial aggregate
functionand the aggregate function match. The default is false.
 
> --

My point is that there are three behaviors:

*  false - no check
*  true, remote version >= sender - no check
*  true, remove version < sender - check

Here is your code:

    + * Check that a buit-in aggpartialfunc exists on the remote server. If
    + * check_partial_aggregate_support is false, we assume the partial aggregate
    + * function exsits on the remote server. Otherwise we assume the partial
    + * aggregate function exsits on the remote server only if the remote server
    + * version is not less than the local server version.
    + */
    +static bool
    +is_builtin_aggpartialfunc_shippable(Oid aggpartialfn, PgFdwRelationInfo *fpinfo)
    +{
    +       bool            shippable = true;
    +
    +       if (fpinfo->check_partial_aggregate_support)
    +       {
    +               if (fpinfo->remoteversion == 0)
    +               {
    +                       PGconn     *conn = GetConnection(fpinfo->user, false, NULL);
    +
    +                       fpinfo->remoteversion = PQserverVersion(conn);
    +               }
    +               if (fpinfo->remoteversion < PG_VERSION_NUM)
    +                       shippable = false;
    +       }
    +       return shippable;
    +}

I think this needs to be explained in the docs.  I am ready to adjust
the patch to improve the wording whenever you are ready.  Should I do it
now and post an updated version for you to use?

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

  Only you can decide what is important to you.