Re: Planner regression in 9.1: min(x) cannot use partial index with NOT NULL

So it's a clever hack that we used to allow the partial indexes to be
used. It relied on the implicit assumption that min(x) and max(x)
where the only values of x where NULL were both NULL.

It would be nice if we were clever enough to support *any* strict
aggregate using partial indexes on WHERE NOT NULL since they'll all
have that property.