Use SELECT 1, instead SELECT COUNT(*) to ask for notes existency

app/services/system_note_service.rb

@@ -347,7 +347,7 @@ module SystemNoteService
       notes = notes.where(noteable_id: noteable.id)
     end
 
-    notes_for_mentioner(mentioner, noteable, notes).count > 0
+    notes_for_mentioner(mentioner, noteable, notes).exists?
   end
 
   # Build an Array of lines detailing each commit added in a merge request