Re: Combining Aggregates

On 20 January 2016 at 10:54, Robert Haas <robertmhaas(at)gmail(dot)com> wrote:

> On Tue, Jan 19, 2016 at 4:50 PM, David Rowley
> <david(dot)rowley(at)2ndquadrant(dot)com> wrote:
> >> Oh, one more point: is there any reason why all of this needs to be a
> >> single (giant) patch? I feel like the handling of internal states
> >> could be a separate patch from the basic patch to allow partial
> >> aggregation and aggregate-combining, and that the latter could be
> >> committed first if we had a reasonably final version of it. That
> >> seems like it would be easier from a review standpoint, and might
> >> allow more development to proceed in parallel, too.
> >
> > I didn't ever really imagine that I'd include any actual new combinefns
> or
> > serialfn/deserialfn in this patch. One set has of course now ended up in
> > there, I can move these off to the test patch for now.
> >
> > Did you imagine that the first patch in the series would only add the
> > aggcombinefn column to pg_aggregate and leave the aggserialfn stuff until
> > later?
>
> I think that would be a sensible way to proceed.
>
> > I thought it seemed better to get the infrastructure committed in one
> > hit, then add a bunch of new combinefn, serialfn, deserialfns for
> existing
> > aggregates in follow-on commits.
>
> To my mind, priority #1 ought to be putting this fine new
> functionality to some use. Expanding it to every aggregate we've got
> seems like a distinctly second priority. That's not to say that it's
> absolutely gotta go down that way, but those would be my priorities.

Agreed. So I've attached a version of the patch which does not have any of
the serialise/deserialise stuff in it.

I've also attached a test patch which modifies the grouping planner to add
a Partial Aggregate node, and a final aggregate node when it's possible.
Running the regression tests with this patch only shows up variances in the
EXPLAIN outputs, which is of course expected.

--
David Rowley http://www.2ndQuadrant.com/
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