@Override
    public AbstractRowContainer.RowIterator<List<Object>> rowIter() throws HiveException {
      return this;
    }
Sometimes, when container is empty in multi-table mapjoin, we need to add a dummy row.
This container does not normally support adding rows; this is for the dummy row.
ql/src/java/org/apache/hadoop/hive/ql/exec/persistence/MapJoinBytesTableContainer.java