@Override
  public void merge(AggregationBuffer agg, Object partial) throws HiveException {
    throw new HiveException(getClass().getSimpleName()
        + ": merge not supported");
  }
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