Github user jianqiao commented on the issue:
https://github.com/apache/incubator-quickstep/pull/179
For the question about `PartitionedHashTablePool` and `HashTablePool`. Note
that their use patterns are different so perhaps it is not natural to merge
them into one class.
`PartitionedHashTablePool` creates **a fixed number** of hash tables **on
its construction**. The use pattern is that every `AggregationWorkOrder`
updates **all** of these hash tables and every `FinalizeAggregationWorkOrder`
updates one of these hash tables.
`HashTablePool` creates hash tables **on demand**. The current use pattern
is that every `AggregationWorkOrder` checkouts **exclusively** one hash table,
updates the hash table, and returns the hash table back to the pool. Then only
one `FinalizeAggregationWorkOrder` is created to merge all the tables in the
pool to the final hash table.
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