protected int computeEndFollowing(int rowIdx, PTFPartition p) throws HiveException {
    int amt = end.getAmt();
    if ( amt == BoundarySpec.UNBOUNDED_AMOUNT ) {
      return p.size();
    }
    throw new HiveException(
            "FOLLOWING needs UNBOUNDED for RANGE with multiple expressions in ORDER BY");
  }
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