code: @Override
    public void close() {
      if (isClosed())
        return;
  
      vertexBaseClass = null;
      edgeBaseClass = null;
  
      checkOpeness();
      rollback();
  
      getMetadata().close();
      ((ODatabaseRaw) underlying.getUnderlying()).callOnCloseListeners();
      getLevel1Cache().clear();
  
      final OGraphDatabasePool pool = ownerPool;
      ownerPool = null;
      pool.release(this);
    }
docs: Avoid to close it but rather release itself to the owner pool.