Github user zuyu commented on a diff in the pull request:

    https://github.com/apache/incubator-quickstep/pull/237#discussion_r112828010
  
    --- Diff: expressions/scalar/ScalarSharedExpression.hpp ---
    @@ -0,0 +1,127 @@
    +/**
    + * 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_EXPRESSIONS_SCALAR_SCALAR_SHARED_EXPRESSION_HPP_
    +#define QUICKSTEP_EXPRESSIONS_SCALAR_SCALAR_SHARED_EXPRESSION_HPP_
    +
    +#include <memory>
    +#include <string>
    +#include <utility>
    +#include <vector>
    +
    +#include "catalog/CatalogTypedefs.hpp"
    +#include "expressions/Expressions.pb.h"
    +#include "expressions/scalar/Scalar.hpp"
    +#include "storage/StorageBlockInfo.hpp"
    +#include "types/TypedValue.hpp"
    +#include "types/containers/ColumnVector.hpp"
    +#include "utility/Macros.hpp"
    +
    +namespace quickstep {
    +
    +class ColumnVectorCache;
    +class ValueAccessor;
    +
    +struct SubBlocksReference;
    +
    +/** \addtogroup Expressions
    + *  @{
    + */
    +
    +/**
    + * @brief Scalars that represent common subexpressions whose results are 
cached
    + *        and shared.
    + **/
    +class ScalarSharedExpression : public Scalar {
    + public:
    +  /**
    +   * @brief Constructor.
    +   *
    +   * @param share_id The unique integer identifier for each equivalence 
class of
    +   *        common subexpressions.
    +   * @param operand The underlying scalar subexpression.
    --- End diff --
    
    Add comments stating that it takes ownership of `operand`.


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