HashAggregateExec current creates one HashPartition per input partition for the initial aggregate per partition, and then explicitly calls MergeExec and then creates another HashPartition for the final reduce operation.
This is fine for in-memory queries in DataFusion but is not extensible. For example, it is not possible to provide a different MergeExec implementation that would distribute queries to a cluster.
A better design would be to move the logic into the query planner so that the physical plan contains explicit steps such as:
This would then make it easier to customize the plan in other projects, to support distributed execution: