Re: Partial aggregates pushdown

On Mon, Nov 27, 2023 at 3:59 PM Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
> Even if the partial-aggregate serialization value isn't "internal"
> but some more-narrowly-defined type, it is still an internal
> implementation detail of the aggregate. You have no right to assume
> that the remote server implements the aggregate the same way the
> local one does. If we start making such an assumption then we'll
> be unable to revise the implementation of an aggregate ever again.
>
> TBH, I think this entire proposal is dead in the water. Which is
> sad from a performance standpoint, but I can't see any way that
> we would not regret shipping a feature that makes such assumptions.

I think it's ridiculous to just hold our breath and pretend like this
feature isn't needed -- it's at least half a decade overdue. We engage
in endless hand-wringing over local-remote symmetry in cases where
other systems seem to effortlessly make that assumption and then get
on with building new features. It's not that I disagree with the
concern; we're *already* doing stuff that is unprincipled in a bunch
of different areas and that could and occasionally does cause queries
that push things to the remote side to return wrong answers, and I
hate that. But the response to that can't be to refuse to add new
features and maybe rip out the features we already have. Users don't
like it when pushdown causes queries to return wrong answers, but they
like it even less when the pushdown doesn't happen in the first place
and the query runs until the heat death of the universe. I'm not
entirely sure what the right design ideas are here, but giving up and
refusing to add features ought to be completely off the table.

--
Robert Haas
EDB: http://www.enterprisedb.com