@Override
  protected void childFlatten(boolean useSelected, int[] selected, int size) {
    keys.flatten(useSelected, selected, size);
    values.flatten(useSelected, selected, size);
  }
Constructor for MapColumnVector

@param len Vector length
@param keys The keys column vector
@param values The values column vector
storage-api/src/java/org/apache/hadoop/hive/ql/exec/vector/MapColumnVector.java