progress on column_expression and table_column_expression

include/matador/query/criteria/abstract_column_criteria.hpp

@@ -15,7 +15,7 @@ public:
     [[nodiscard]] const table_column& col() const;
 
 protected:
-    table_column column_;
+    const table_column& column_;
 };
 
 }

include/matador/query/criteria/binary_criteria.hpp

@@ -34,7 +34,7 @@ private:
 class binary_column_criteria final : public abstract_criteria {
 public:
     binary_column_criteria() = delete;
-    binary_column_criteria(table_column  left_column, binary_operator operand, table_column  right_column);
+    binary_column_criteria(const table_column& left_column, binary_operator operand, const table_column& right_column);
 
     void accept(criteria_visitor& visitor) const override;
 
@@ -43,9 +43,9 @@ public:
     [[nodiscard]] const table_column& right_column() const;
 
 private:
-    table_column left_column_;
+    const table_column& left_column_;
     binary_operator operator_{};
-    table_column right_column_;
+    const table_column& right_column_;
 };
 }
 #endif //CRITERIA_BINARY_CRITERIA_NODE_HPP

include/matador/query/expression/table_column_expression.hpp

@@ -8,14 +8,14 @@ namespace matador::query {
 class table_column_expression : public abstract_column_expression {
 public:
   table_column_expression() = delete;
-  explicit table_column_expression(table_column col);
+  explicit table_column_expression(const table_column& col);
 
   void accept(expression_visitor& visitor) const override;
 
   [[nodiscard]] const table_column& col() const;
 
 private:
-  table_column column_;
+  const table_column& column_;
 };
 }
 #endif //MATADOR_TABLE_COLUMN_EXPRESSION_HPP