MySQL 数据库 索引优化
现有sql语句:
①select * from tab1 where number = '5' and mode >= 1 and mode <= 21 order by id desc limit 100;
② select * from (select * from tab2 where mode >= 1 and mode <= 21 and name = '5' order by id desc limit 100) A order by id;
请问这两句sql该如何建立索引以达到优化的目的?对于①,我建立了联合索引,key(number,id)但是explain的时候提示没有用到该索引,用的是主键索引。