欢迎进入Linux社区论坛,与200万技术人员互动交流 >>进入 建表及插入数据: create table a(id int primary key); create table a_info(id int, title char(1), key(id)); insert into a value(1),(2); insert into a_info value(1, 'a')
欢迎进入Linux社区论坛,与200万技术人员互动交流 >>进入
建表及插入数据: create table a(id int primary key); create table a_info(id int, title char(1), key(id)); insert into a value(1),(2); insert into a_info value(1, 'a'),(2, 'b'); 现在 a_info 表 id 列变为普通索引(非唯一性索引): mysql> explain select * from a join a_info using(id) where a.id=1;
…+--------+-------+…
…| table | type |…
…+--------+-------+…
…| a | const |…
…| a_info | ref |…
…+--------+-------+… a_info 表 type 变为 ref 类型了。所以,唯一性索引才会出现 eq_ref (非唯一性索引会出现 ref ),因为唯一,所以最多只返回一条记录,找到后无需继续查找,因此比 ref 更快。
const 被称为"常量",这个词不好理解,不过出现 const 的话就表示发生下面两种情况:在整个查询过程中这个表最多只会有一条匹配的行,比如主键 id=1 就肯定只有一行,只需读取一次表数据便能取得所需的结果,且表数据在分解执行计划时读取。返回值直接放在 select 语句中,类似 select 1 AS f .可以通过 extended 选择查看内部过程:
建表及插入数据: create table a(id int primary key, c1 char(20) not null, c2 text not null, c3 text not null); insert into a values(1, 'asdfasdf', 'asdfasdf', 'asdfasdf'), (2, 'asdfasdf', 'asdfasdf', 'asdfasdf'); mysql> explain extended select * from a where id=1\G
…
type: const
possible_keys: PRIMARY
key: PRIMARY
… 用 show warnings 查看 MySQL 是如何优化的: mysql> show warnings\G
…
Message: select '1' AS `id`,'asdfasdf' AS `c1`,'asdfasdf' AS `c2`,'asdfasdf' AS
`c3` from `test`.`a` where 1 查询返回的结果为: mysql> select * from a where id=1;
+----+----------+----------+----------+
| id | c1 | c2 | c3 |
+----+----------+----------+----------+
| 1 | asdfasdf | asdfasdf | asdfasdf |
+----+----------+----------+----------+ 可以看出,返回结果中的字段值都以"值 AS 字段名"的形式直接出现在优化后的 select 语句中。修改一下查询: mysql> explain select * from a where id in(1,2)\G
…
type: range … 当返回结果超过 1 条时, type 便不再为 const 了。重新建表及插入数据: create table a (id int not null); insert into a value(1),(2),(3); mysql> explain select * from a where id=1\G
…
type: ALL 目前表中只有一条 id=1 的记录,但 type 已为 ALL ,因为只有唯一性索引才能保证表中最多只有一条记录,只有这样 type 才有可能为 const .为 id 加普通索引后, type 变为 ref ,改为加唯一或主键索引后, type 便变为 const 了。
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