BUG#6676: Added comment for the fix

sql/log_event.cc

@@ -2528,6 +2528,11 @@ int User_var_log_event::exec_event(struct st_relay_log_info* rli)
     0 can be passed as last argument (reference on item)
   */
   e.fix_fields(thd, 0, 0);
+  /*
+    A variable can just be considered as a table with
+    a single record and with a single column. Thus, like
+    a column value, it could always have IMPLICIT derivation.
+   */
   e.update_hash(val, val_len, type, charset, DERIVATION_IMPLICIT);
   free_root(thd->mem_root,0);