I was going to rebase my HashAgg patch, and got some conflicts related to the grouping sets patch. I could probably sort them out, but I think that may be the tipping point where we want to break up nodeAgg.c into nodeSortedAgg.c and nodeHashAgg.c, and probably a common file as well.
This would also (I hope) be convenient for Simon and David Rowley, who have been hacking on aggregates in general.
That would be more inconvenient right now as I have a pending patch which makes quite a number of changes which are all over nodeAgg.c. https://commitfest.postgresql.org/5/271/
