@Override
  public Object getGroupResult() {
    return Long.valueOf(uniqueObjects.size());
  }
This class evaluates count(column) for a PTF group where a distinct keyword is applied to the
partitioning column itself, e.g.:

SELECT
  txt1,
  txt2,
  count(distinct txt1) over(partition by txt1) as n,
  count(distinct txt2) over(partition by txt2) as m
FROM example;

In this case, the framework is still supposed to ensure sorting
on the key (let's say txt1 for the first Reducer stage), but the original
VectorPTFEvaluatorCount is not aware that a distinct keyword was applied
to the key column. This case would be simple, because such function should
return 1 every time. However, that's just a corner-case, a real scenario is
when the partitioning column is not the same. In such cases, a real count
distinct implementation is needed:

SELECT
  txt1,
  txt2,
  count(distinct txt2) over(partition by txt1) as n,
  count(distinct txt1) over(partition by txt2) as m
FROM example;
ql/src/java/org/apache/hadoop/hive/ql/exec/vector/ptf/VectorPTFEvaluatorCountDistinct.java