Github user jianqiao commented on a diff in the pull request:
https://github.com/apache/incubator-quickstep/pull/237#discussion_r113070668
--- 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_;
--- End diff --
There is no performance consideration here and the pointer makes it clear
for the "null" case, since `0` itself can be a valid hash value.
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