@Override
  public Object computeValue(Object row) throws HiveException {
    Object[] objs = new Object[orderDef.getExpressions().size()];
    for (int i = 0; i < objs.length; i++) {
      Object o = orderDef.getExpressions().get(i).getExprEvaluator().evaluate(row);
      objs[i] = ObjectInspectorUtils.copyToStandardObject(o, orderDef.getExpressions().get(i).getOI());
    }
    return objs;
  }
Simply returns the Object[] returned by VectorPTFGroupBatches.getAt(i), which contains the
value of the ordering columns.
ql/src/java/org/apache/hadoop/hive/ql/udf/ptf/ValueBoundaryScanner.java