@Override
  public Type getResultColumnVectorType() {
    return Type.DECIMAL;
  }
This class evaluates streaming HiveDecimal avg() for a PTF group.

Stream average non-null column values and output sum / non-null count as we go along.
ql/src/java/org/apache/hadoop/hive/ql/exec/vector/ptf/VectorPTFEvaluatorStreamingDecimalAvg.java