mysql 联合索引作用范围

有联合索引 idx_f1234 (f1, f2, f3, f4)
查询语句 select * from ~ where f1= ~ and f2 = ~ order by f3, f4 (或group by f3, f4) 联合索引是生效的

show INDEX from practice_answer;
EXPLAIN
SELECT * from practice_answer where `status` = 0 and practice_id = 509 limit 100;

EXPLAIN 
SELECT * from practice_answer where `status` = 0 and practice_id = 509 ORDER BY create_time, user_id desc limit 100;

等有时间细化