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

Florian Pflug  writes:
> I think it'd be worthwile to get this into 9.4, if that's still an option,
> even if we only support COUNT.

My thought is

(1) we can certainly implement inverse transitions for COUNT() and the
integer variants of SUM(), and that alone would be a killer feature for
some people.

(2) the float and numeric variants should be implemented under nondefault
names (I'm thinking FAST_SUM(), but bikeshed away).  People who need
extra speed and don't mind the slightly different results can alter
their queries to use these variants.

One reason I'm thinking this is that whatever we do to ameliorate
the semantic issues is going to slow down the forward transition
function --- to no benefit unless the aggregate is being used as a
window function in a moving window.  So I'm less than convinced
that we *should* implement any of these designs in the default
aggregates, even if we get to the point where we arguably *could*
do it with little risk of functional differences.
		regards, tom lane