使用覆盖索引

Question

actor 表在 last_name 列上有一个索引 idx_actor_last_name。以下哪个查询可以使用此索引作为覆盖索引？

Answer 1

选择不同的 last_name 从 actor;

Answer 2

select last_name, count(*) from actor group by last_name;

Answer 3

select last_name, last_update from actor order by last_name desc;

Answer 4

select last_name from actor order by last_update;