@Override
    public int rowCount() throws HiveException {
      // For performance reasons we do not want to chase the values to the end to determine
      // the count.  Use hasRows and isSingleRow instead.
      throw new UnsupportedOperationException("Getting the row count not supported");
    }
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