private void assignStringNullsNoRepeatingSelection(int keyIndex, int index, int size,
      BytesColumnVector columnVector, int[] selected) {
    for(int i=0; i<size; ++i) {
      int row = selected[i];
      if (columnVector.isNull[row]) {
        vectorHashKeyWrappers[i].assignNullString(keyIndex, index);
      } else {
        vectorHashKeyWrappers[i].assignString(
            index,
            columnVector.vector[row],
            columnVector.start[row],
            columnVector.length[row]);
      }
    }
  }
Helper method to assign values from a vector column into the key wrapper.
Optimized for string type, possible nulls, no repeat values, batch selection vector.
ql/src/java/org/apache/hadoop/hive/ql/exec/vector/wrapper/VectorHashKeyWrapperBatch.java