In HashAggStage, there is a map storing the keys of the groups and their pairs. There is an iterator on this map that is used to track and return results to the caller. The records that have been consumed are not deleted from the map but kept in memory for the whole execution of the query. We should release memory by deleting the consumed records, provided that this will not affect the performance of the queries. (The same is true for the BlockHashAggStage)