@Override
  public ObjectInspector init(Mode m, ObjectInspector[] parameters)
      throws HiveException {
    return wrappedEval.init(m, parameters);
  }
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