Re: Have the planner convert COUNT(1) / COUNT(not_null_col) to COUNT(*)

It may be possible to apply transformations to other aggregate
functions too, but I don't want to discuss that here. I mostly want to
determine if the infrastructure is ok and do the count(*) one because
it seems like the most likely one to be useful.

One thing I wasn't too sure about was if we should make it possible
for the support function to return something that's not an Aggref.  In
theory, something like COUNT(NULL) could just return '0'::bigint.
While that does seem an optimisation that wouldn't be applied very
often, I have opted to leave it so that such an optimisation *could*
be done by the support function. I also happen to test that that
doesn't entirely break the query, as ordinarily it would if we didn't
have Query.hasAggs (It's not too dissimilar to removing unused columns
from a subquery)

Should we do this?

David