cleanup: key parts comparison

Engine specific code moved to engine.

sql/handler.h

@@ -2936,6 +2936,13 @@ public:
   virtual ~Handler_share() {}
 };
 
+enum class Compare_keys : uint32_t
+{
+  Equal,
+  EqualButKeyPartLength,
+  EqualButComment,
+  NotEqual
+};
 
 /**
   The handler class is the interface for dynamically loadable
@@ -4877,6 +4884,13 @@ public:
     return false;
   }
 
+  /* Used for ALTER TABLE.
+  Some engines can handle some differences in indexes by themself. */
+  virtual Compare_keys compare_key_parts(const Field &old_field,
+                                         const Column_definition &new_field,
+                                         const KEY_PART_INFO &old_part,
+                                         const KEY_PART_INFO &new_part) const;
+
 protected:
   Handler_share *get_ha_share_ptr();
   void set_ha_share_ptr(Handler_share *arg_ha_share);