feat: add read_only attribute for columns.

include/yugawara/storage/column_feature.h

@@ -17,17 +17,24 @@ namespace yugawara::storage {
 enum class column_feature {
     /**
      * @brief the target column is defined by the system (NOT explicitly defined by a user).
-     * @details users must not treat these columns directly.
+     * @details users cannot access these columns.
      */
     synthesized = 0,
 
     /**
      * @brief the target column is hidden from the user.
-     * @details hidden columns are ignored the following operations:
+     * @details these columns are ignored the following operations:
      * - `SELECT *`
      * - `NATURAL JOIN`, and similar column name based matching (e.g. `UNION CORRESPONDING`)
      */
     hidden = 1,
+
+    /**
+     * @brief the target column is read-only.
+     * @details The read-only columns are created or update only with its default value.
+     * @attention The compiler does not treat this feature.
+     */
+    read_only = 2,
 };
 
 /**
@@ -36,7 +43,7 @@ enum class column_feature {
 using column_feature_set = ::takatori::util::enum_set<
         column_feature,
         column_feature::synthesized,
-        column_feature::hidden>;
+        column_feature::read_only>;
 
 /**
  * @brief returns string representation of the value.
@@ -49,6 +56,7 @@ inline constexpr std::string_view to_string_view(column_feature value) noexcept
     switch (value) {
         case kind::synthesized: return "synthesized"sv;
         case kind::hidden: return "hidden"sv;
+        case kind::read_only: return "read_only"sv;
     }
     std::abort();
 }