百万级别表的limit优化：

select * from code order by id limit 1000000,10
执行时间：0.622

select a.* from code a inner join (select id from code order by id limit 1000000,10) b USING(id)

执行时间：0.235s

select * from code where id>=(select id from code order by id limit 1000000,1) limit 10;

执行时间：0.235s