public class ExecReducer
ExecReducer is the generic Reducer class for Hive. Together with ExecMapper it is
the bridge between the map-reduce framework and the Hive operator pipeline at
execution time. It's main responsibilities are:
- Load and setup the operator pipeline from XML
- Run the pipeline by transforming key, value pairs to records and forwarding them to the operators
- Sending start and end group messages to separate records with same key from one another
- Catch and handle errors during execution of the operators.