Question

select active from websites where id = (select max(id) from websites where url = 'google.com')

id  select_type table     type   possible_keys  key      key_len  ref    rows  Extra
1   PRIMARY     websites  const  PRIMARY        PRIMARY  4        const  1   
2   SUBQUERY    websites  ref    url            url      767             1867  Using where

如何优化此查询？ url字段是索引，id是主键。那么为什么它会遍历所有行？

Answer 1

MAX始终处理所有行 - 使用order by和limit - 所以查询将以这种方式查看

 SELECT * FROM wbsites WHERE url = '...' ORDER BY id DESC LIMIT 1

对于这种情况，不需要子查询

编辑：忘记网址

Answer 2

考虑

alter table websites add index a (url, active);
select active 
    from websites 
    where url = 'google.com' 
    order by id desc 
    limit 1;

查询优化：子查询中的max（）

2 个答案: