@Override
  public HiveDecimalWritable minus(HiveDecimalWritable t1, HiveDecimalWritable t2) {
    return VectorPTFEvaluatorHelper.minus(t1, t2);
  }
This class evaluates HiveDecimal 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/VectorPTFEvaluatorDecimalAvg.java