@Override
  public void reset(AggregationBuffer agg) throws HiveException {
    StreamingState ss = (StreamingState) agg;
    wrappedEval.reset(ss.wrappedBuf);
    ss.reset();
  }
For the cases "X preceding and Y preceding" or the number of processed rows
is more than the size of FOLLOWING window, we are able to generate a PTF result
for a previous row.
@return
ql/src/java/org/apache/hadoop/hive/ql/udf/generic/GenericUDAFStreamingEvaluator.java