mysql怎么使用索引？

SELECT user_name, city, age FROM user_test ORDER BY user_name, sex;

SELECT user_name, city, age FROM user_test ORDER BY user_name ASC, city DESC;

SELECT user_name, city, age, sex FROM user_test ORDER BY user_name;

SELECT user_name, city, age FROM user_test WHERE user_name LIKE 'feinik%' ORDER BY city;