mysql group by 字段多，怎么优化？

Question

比如现在有个sql语句是这样的：select count(score) from students group by school_name,grade,student_name
students里面大概一百万条数据，怎么添加索引才能优化查询？

Answer 1

给 school_name,grade,student_name 做个索引就完事了

Answer 2

alter TABLE student add index idx(school_name,grade,student_name)