protected int computeStartCurrentRow(int rowIdx, PTFPartition p) throws HiveException {
    Object sortKey = computeValueUseCache(rowIdx, p);
    Object rowVal = sortKey;
    int r = rowIdx;

    while (r >= 0 && isEqual(rowVal, sortKey) ) {
      Pair<Integer, Object> stepResult = skipOrStepBack(r, p);
      r = stepResult.getLeft();
      rowVal = stepResult.getRight();
    }
    return r + 1;
  }
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