Add support for derived queries of the form:
SELECT * FROM ( SELECT company, revenue FROM Company ORDER BY revenue) LIMIT 10
Adding support for this requires a compile time change as well as a runtime execution change. The first version of the compile-time change could limit aggregation to only be allowed in the inner or the outer query, but not both. In this case, the inner and outer queries can be combined into a single query with the outer select becoming just a remapping of a subset of the projection from the inner select. The second version of the compile-time change could handle aggregation in the inner and outer select by performing client side (this is likely a less common scenario).
For the runtime execution, change the UngroupedAggregateRegionObserver would be modified to look for a new "TopNLimit" attribute with an int value in the Scan. This would control the maximum number of values for the coprocessor to hold on to as the scan is performed. Then the GroupedAggregatingResultIterator would be modified to handle keeping the topN values received back from all the child iterators.