The current aggregation visitors (AverageVisitor, CountVisitor, MaxVisitor, MedianVisitor, MinVisitor, StandardDeviationVisitor and SumVisitor) don’t support a “group by” behavior. For example, we are able to compute the average electrical consumption per building. But we are not able to ask the average electrical consumption per building type.
The idea is to add new "group by" visitor that will be able to group features by type and apply a visitor on them. The JDBCDataStore will be extend to natively support the "group by" behavior. The existing Aggregator process will also be extended to support the "group by" behavior.
All of this will be done preserving backwards compatibility.
Mailing list discussion about this: