Re: [PATCH] Negative Transition Aggregate Functions (WIP)

Dean Rasheed <dean(dot)a(dot)rasheed(at)gmail(dot)com> writes:
> Reading over this, I realised that there is a problem with NaN
> handling --- once the state becomes NaN, it can never recover. So the
> results using the inverse transition function don't match HEAD in
> cases like this:

Ouch! That takes out numeric, float4, and float8 in one fell swoop.

Given the relative infrequency of NaNs in most data, it seems like
it might still be possible to get a speedup if we could use inverse
transitions until we hit a NaN, then do it the hard way until the
NaN is outside the window, then go back to inverse transitions.
I'm not sure though if this is at all practical from an implementation
standpoint. We certainly don't want the core code knowing about
anything as datatype-specific as a NaN, but maybe the inverse transition
function could have an API that allows reporting "I can't do it here,
fall back to the hard way".

regards, tom lane