@Override
  public void flatten(boolean selectedInUse, int[] sel, int size) {
    flattenPush();
    for(int i=0; i < fields.length; ++i) {
      fields[i].flatten(selectedInUse, sel, size);
    }
    flattenNoNulls(selectedInUse, sel, size);
  }
Constructor for UnionColumnVector

@param len Vector length
@param fields the field column vectors
storage-api/src/java/org/apache/hadoop/hive/ql/exec/vector/UnionColumnVector.java