@Override
  public Object getGroupResult() {
    return lastValue;
  }
This class evaluates long first_value() for a PTF group.

We capture the last value from the last batch.  It can be NULL.
It becomes the group value.
ql/src/java/org/apache/hadoop/hive/ql/exec/vector/ptf/VectorPTFEvaluatorLongLastValue.java