@Override
  protected OrcUnion createUnionObject(List<TypeInfo> childrenTypes, Object previous) {
    return (previous instanceof OrcUnion) ? (OrcUnion)previous : new OrcUnion();
  }
BatchToRowReader that returns the rows readable by ORC IOs.
ql/src/java/org/apache/hadoop/hive/ql/io/orc/OrcOiBatchToRowReader.java