If there is an Aggregate on top of an Aggregate, and the top aggregate's group key is a subset of the lower aggregate's group key, and the aggregates are expansions of rollups, then a rule would convert into a single aggregate.
For example, SUM of SUM becomes SUM; SUM of COUNT becomes COUNT; MAX of MAX becomes MAX; MIN of MIN becomes MIN. AVG of AVG would not match, nor would COUNT of COUNT.
The rule would convert
The rule would not work if there was anything between the two Aggregate operators.
The implementation would use SqlSplittableAggFunction to deduce rollups, and in fact would be practically the converse of AggregateExchangeTransposeRule proposed in CALCITE-907.