@Override
  public int computeStart(int rowIdx, PTFPartition p) throws HiveException {
    switch(start.getDirection()) {
    case PRECEDING:
      return computeStartPreceding(rowIdx, p);
    case CURRENT:
      return computeStartCurrentRow(rowIdx, p);
    case FOLLOWING:
      default:
        throw new HiveException(
                "FOLLOWING not allowed for starting 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