@Override
    public void clearRows() {
      // Doesn't clear underlying hashtable
      hashMapResult.forget();
      dummyRow = null;
      aliasFilter = (byte) 0xff;
    }
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/HybridHashTableContainer.java