MySQL statement optimization ranking

First variable stroke using the following basic rules

While using this value to a variable assignment:

set @rownum:=0;
select actor_id,@rownum:=@rownum+1 as rownum from actor limit 10;

Use custom variables to rank the results:

set @film_num:=0;
select *,@film_num:=@film_num+1 as num from(
select actor_id,count(*) as cnt from film_actor GROUP BY actor_id ORDER BY cnt DESC limit 10)t