Minor Description
For partial hash aggregation (code-gen path), we have two level of hash map for aggregation. First level is from `RowBasedHashMapGenerator`, which is computation faster compared to the second level from `UnsafeFixedWidthAggregationMap`. The introducing of two level hash map can help improve CPU performance of query as the first level hash map normally fits in hardware cache and has cheaper hash function for key lookup.
For final hash aggregation, we can also support two level of hash map, to improve query performance further.
