Github user jianqiao commented on a diff in the pull request:
https://github.com/apache/incubator-quickstep/pull/326#discussion_r154230379
--- Diff: query_optimizer/rules/ReferencedBaseRelations.hpp ---
@@ -0,0 +1,78 @@
+/**
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements. See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership. The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License. You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing,
+ * software distributed under the License is distributed on an
+ * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+ * KIND, either express or implied. See the License for the
+ * specific language governing permissions and limitations
+ * under the License.
+ **/
+
+#ifndef QUICKSTEP_QUERY_OPTIMIZER_RULES_REFERENCED_BASE_RELATIONS_HPP_
+#define QUICKSTEP_QUERY_OPTIMIZER_RULES_REFERENCED_BASE_RELATIONS_HPP_
+
+#include <string>
+#include <vector>
+
+#include "catalog/CatalogTypedefs.hpp"
+#include "query_optimizer/logical/Logical.hpp"
+#include "query_optimizer/rules/DFSTraversal.hpp"
+#include "utility/Macros.hpp"
+
+namespace quickstep {
+
+class CatalogRelation;
+
+namespace optimizer {
+
+class OptimizerContext;
+
+class ReferencedBaseRelations : public DFSTraversal<logical::Logical> {
--- End diff --
Since this class overrides the `apply` method, we can just inherit
`Rule<logical::Logical>`.
---
