@Override
    public void addRow(List<Object> t) {
      if (dummyRow != null || hashMapResult.hasRows()) {
        throw new RuntimeException("Cannot add rows when not empty");
      }
      dummyRow = t;
      wasFirstCalledOnDummyRow = false;
    }
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