Re: Aggregate transition state merging vs. hypothetical set functions

On 13 October 2017 at 12:41, Tom Lane <tgl@sss.pgh.pa.us> wrote:
> David Rowley <david.rowley@2ndquadrant.com> writes:
>> If the user defines their normal aggregate as not safe for merging,
>> then surely it'll not be suitable to be used as a window function
>> either, since the final function will also be called there multiple
>> times per state.
>
> 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".  But this would
> arguably be an improvement over the current situation.  Right now
> I'm sure there are user-written aggs out there that will just crash
> if used as a window agg, and the authors don't have much choice because
> the performance costs of not modifying the transition state in the
> finalfn are higher than they're willing to bear.  At least with a
> flag they could ensure that the case will fail cleanly.

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?

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