@Override
  protected Double plus(Double number1, Double number2) {
    return VectorPTFEvaluatorHelper.plus(number1, number2);
  }
This class evaluates double avg() for a PTF group.

Sum up non-null column values; group result is sum / non-null count.
ql/src/java/org/apache/hadoop/hive/ql/exec/vector/ptf/VectorPTFEvaluatorDoubleAvg.java