Re: Aggregate transition state merging vs. hypothetical set functions

David Rowley <david.rowley@2ndquadrant.com> writes:
> On 13 October 2017 at 12:41, Tom Lane <tgl@sss.pgh.pa.us> wrote:
>> Yeah, we would probably also want to check the flag in nodeWindowAgg.
>> Not sure exactly how that should play out --- maybe we end up with
>> a tri-valued property "works as normal agg without merging, works
>> as normal agg with merging, works as window agg".

> hmm, maybe I'm lacking imagination here, but surely the final function
> is either destructive or it's not? I can't understand what the
> difference between nodeAgg.c calling the finalfn multiple times on the
> same state and nodeWindowAgg.c doing it. Maybe there's something I'm
> not accounting for that you are?

nodeWindowAgg is doing something more: not only is it calling the finalfn
repeatedly, but it's continuing to mutate the transition state in between.
The ordered-set aggs provide a counterexample to considering that to be
equivalent to state merging.  The OSAs can cope with state merging as
long as they have a flag to make sure only the first finalfn does
tuplesort_performsort ... but that's not good enough to make them workable
as window aggs.  Once we sort, we can't absorb more rows into the
tuplesort object.

			regards, tom lane