Github user zuyu commented on a diff in the pull request:
https://github.com/apache/incubator-quickstep/pull/237#discussion_r112847625
--- Diff: query_optimizer/expressions/CommonSubexpression.hpp ---
@@ -0,0 +1,141 @@
+/**
+ * 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_EXPRESSIONS_COMMON_SUBEXPRESSION_HPP_
+#define QUICKSTEP_QUERY_OPTIMIZER_EXPRESSIONS_COMMON_SUBEXPRESSION_HPP_
+
+#include <memory>
+#include <string>
+#include <unordered_map>
+#include <vector>
+
+#include "query_optimizer/expressions/AttributeReference.hpp"
+#include "query_optimizer/expressions/ExprId.hpp"
+#include "query_optimizer/expressions/Expression.hpp"
+#include "query_optimizer/expressions/ExpressionType.hpp"
+#include "query_optimizer/expressions/Scalar.hpp"
+#include "utility/Macros.hpp"
+
+#include "glog/logging.h"
+
+namespace quickstep {
+
+class Scalar;
+class Type;
+
+namespace optimizer {
+namespace expressions {
+
+/** \addtogroup OptimizerExpressions
+ * @{
+ */
+
+class CommonSubexpression;
+typedef std::shared_ptr<const CommonSubexpression> CommonSubexpressionPtr;
+
+/**
+ * @brief A wrapper expression that represents a common subexpression.
+ */
+class CommonSubexpression : public Scalar {
+ public:
+ ExpressionType getExpressionType() const override {
+ return ExpressionType::kCommonSubexpression;
+ }
+
+ std::string getName() const override {
+ return "CommonSubexpression";
+ }
+
+ bool isConstant() const override {
+ return operand_->isConstant();
+ }
+
+ /**
+ * @return The unique ID for the equivalence class that this common
subexpression
+ * belongs to.
+ */
+ inline ExprId common_subexpression_id() const {
+ return common_subexpression_id_;
+ }
+
+ /**
+ * @return The operand that represents the underlying subexpression.
+ */
+ const ScalarPtr& operand() const {
+ return operand_;
+ }
+
+ const Type& getValueType() const override {
+ return operand_->getValueType();
+ }
+
+ ExpressionPtr copyWithNewChildren(
+ const std::vector<ExpressionPtr> &new_children) const override;
+
+ std::vector<AttributeReferencePtr> getReferencedAttributes() const
override {
+ return operand_->getReferencedAttributes();
+ }
+
+ ::quickstep::Scalar* concretize(
+ const std::unordered_map<ExprId, const CatalogAttribute*>
&substitution_map) const override;
+
+ /**
+ * @brief Creates an immutable CommonSubexpression.
+ *
+ * @param common_subexpression_id A unique ID for the equivalence class
that
+ * this common subexpressions belongs to.
+ * @param operand The operand that represents the underlying
subexpression.
+ * @return An immutable CommonSubexpression.
+ */
+ static CommonSubexpressionPtr Create(const ExprId
common_subexpression_id,
+ const ScalarPtr &operand) {
+ return CommonSubexpressionPtr(
+ new CommonSubexpression(common_subexpression_id, operand));
+ }
+
+ protected:
+ void getFieldStringItems(
+ std::vector<std::string> *inline_field_names,
+ std::vector<std::string> *inline_field_values,
+ std::vector<std::string> *non_container_child_field_names,
+ std::vector<OptimizerTreeBaseNodePtr> *non_container_child_fields,
+ std::vector<std::string> *container_child_field_names,
+ std::vector<std::vector<OptimizerTreeBaseNodePtr>>
*container_child_fields) const override;
+
+ private:
+ CommonSubexpression(const ExprId common_subexpression_id,
+ const ScalarPtr &operand)
+ : common_subexpression_id_(common_subexpression_id),
+ operand_(operand) {
+ addChild(operand);
+ }
+
+ ExprId common_subexpression_id_;
+ ScalarPtr operand_;
--- End diff --
Mark both `const`.
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