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

    https://github.com/apache/incubator-quickstep/pull/237#discussion_r112828368
  
    --- Diff: query_optimizer/expressions/Scalar.hpp ---
    @@ -65,10 +67,49 @@ class Scalar : public Expression {
           const std::unordered_map<ExprId, const CatalogAttribute*>& 
substitution_map)
           const = 0;
     
    +  /**
    +   * @brief Check whether this scalar is semantically equivalent to \p 
other.
    +   *
    +   * @note The fact that two scalars are semantically equal brings more
    +   *       optimization opportunities, e.g. common subexpression 
elimination.
    +   *       Meanwhile, it is always safe to assume that two scalars are not 
equal.
    +   *
    +   * @return True if this scalar equals \p other; false otherwise.
    +   */
    +  virtual bool equals(const ScalarPtr &other) const {
    +    return false;
    +  }
    +
    +  /**
    +   * @brief Get a hash of this scalar.
    +   *
    +   * @return A hash of this scalar.
    +   */
    +  std::size_t hash() const {
    +    if (hash_cache_ == nullptr) {
    +      hash_cache_ = std::make_unique<std::size_t>(computeHash());
    +    }
    +    return *hash_cache_;
    +  }
    +
      protected:
       Scalar() {}
     
    +  /**
    +   * @brief Compute a hash of this scalar.
    +   *
    +   * @note Override this method to actually compute the hash. Note that the
    +   *       public method hash() is a caching wrapper for this method.
    +   *
    +   * @return A hash of this scalar.
    +   */
    +  virtual std::size_t computeHash() const {
    +    throw HashNotSupported("Unsupported computeHash() in " + getName());
    --- End diff --
    
    Suggest to change to a pure virtual method.


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