@Override
  public Object getNextResult(AggregationBuffer agg) throws HiveException {
    StreamingState ss = (StreamingState) agg;
    if (!ss.results.isEmpty()) {
      T1 res = ss.results.remove(0);
      if (res == null) {
        return ISupportStreamingModeForWindowing.NULL_RESULT;
      }
      return res;
    }
    return null;
  }
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