Two important changes must be made to HashAgg:
1) It should have a flag for whether or not to recompute the hash table on calls to re-open. In the case where the HashAgg is on the right side of an NLJ and there are no correlated variables below it (or used in the HashAgg itself), we can safely skip any re-computation. This will be an important optimization to have for the way $lookup will be translated.
2) It should have an option to "look up" a particular group key chosen at runtime. That is, on re-opens, when the hash table is already built (and does not need to be re-built) the stage should simply position itself at this key and then return EOF.