Re: Partial aggregates pushdown

On Tue, Jun 6, 2023 at 03:08:50AM +0000, Fujii(dot)Yuki(at)df(dot)MitsubishiElectric(dot)co(dot)jp wrote:
> > I've seen this message. But introduction of new built-in function will break
> > requests to old servers only if this new function is used in the request (this
> > means that query changes). However, this patch changes the behavior of old
> > queries, which worked prior to update. This seems to be different to me.
>
> You are right.
> However, for now, partial aggregates pushdown is mainly used when using built-in sharding in PostgreSQL.
> I believe when using built-in sharding in PostgreSQL, the version of the primary node server and
> the version of the remote server will usually be the same.
> So I think it would be sufficient to include in the documentation a note about such problem
> and a workaround for them.

I agree that this feature is designed for built-in sharding, but it is
possible people could be using aggregates on partitions backed by
foreign tables without sharding. Adding a requirement for non-sharding
setups to need PG 17+ servers might be unreasonable.

Looking at previous release note incompatibilities, we don't normally
change non-administrative functions in a way that causes errors if run
on older servers. Based on Alexander's observations, I wonder if we
need to re-add the postgres_fdw option to control partial aggregate
pushdown, and default it to enabled.

If we ever add more function breakage we might need more postgres_fdw
options. Fortunately, such changes are rare.

Yuki, thank you for writing and updating this patch, and Alexander,
thank you for helping with this patch.

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

Only you can decide what is important to you.