@Override
    public MapJoinRowContainer copy() throws HiveException {
      return this; // Independent of hashtable and can be modified, no need to copy.
    }
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