MySQL 分页优化

select * from table1 limit m, n

随着偏移量 m 的增大，MySQL 需要花费大量时间来扫描需要丢弃的数据

优化方法：延迟关联，通过使用覆盖索引查询返回需要的主键，再根据这些主键和原表做一次关联操作获得需要的行，这可以减少回表次数，同时减少 MySQL 扫描那些需要丢弃的行数

select * from table1 where id >= (select id from table1 limit m, 1) limit 10

select * from table1 a join (select id from table1 limit m, 10) b on a.id = b.id

select * from table1 inner join (select id from table1 limit m, 10) as x using id

执行子查询时，MySQL 需要创建临时表，查询完毕后再删除这些临时表，所以，子查询的速度会比关联查询低

key (k)

select * from table1 a, 
(select id from table1 where k > 1000 limit m, 10) b 
where a.id = b.id

select * from table1 a, 
(select id from table1 where k > 1000 and id > 100000 order by id limit 10) b 
where a.id = b.id

select * from (
    select id from table1
    where k > 1000 order by id limit 300000,10
) a left join table1 b on a.id = b.id

这里最好 key(k, id)