@Override
  public Type getResultColumnVectorType() {
    return Type.LONG;
  }
This class evaluates long first_value() for a PTF group.

We capture the first value from the first batch.  It can be NULL.
We then set (stream) the output column with that value as repeated in each batch.
ql/src/java/org/apache/hadoop/hive/ql/exec/vector/ptf/VectorPTFEvaluatorLongFirstValue.java