public GenericUDAFEvaluator getWindowingEvaluator(WindowFrameDef wFrmDef) {
    return null;
  }
When evaluating an aggregates over a fixed Window, the naive way to compute
results is to compute the aggregate for each row. But often there is a way
to compute results in a more efficient manner. This method enables the
basic evaluator to provide a function object that does the job in a more
efficient manner.
<p>
This method is called after this Evaluator is initialized. The returned
Function must be initialized. It is passed the 'window' of aggregation for
each row.

@param wFrmDef
         the Window definition in play for this evaluation.
@return null implies that this fn cannot be processed in Streaming mode. So
        each row is evaluated independently.
ql/src/java/org/apache/hadoop/hive/ql/udf/generic/GenericUDAFEvaluator.java