mysql的explain

explain 一般用于分析sql.  
如下

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drop table if exists test1; CREATE TABLE test1 (     id INT NOT NULL primary key  auto_increment, -- 自动递增     username varchar(5) not null -- 用户名 ) ENGINE=innodb ;   insert into test1 (username) values (concat('test',rand())); insert into test1 (username)   select username from test1; insert into test1 (username)   select username from test1; insert into test1 (username)   select username from test1; insert into test1 (username)   select username from test1; insert into test1 (username)   select username from test1; insert into test1 (username)   select username from test1; insert into test1 (username)   select username from test1; insert into test1 (username)   select username from test1; insert into test1 (username)   select username from test1; insert into test1 (username)   select username from test1; insert into test1 (username)   select username from test1; insert into test1 (username)   select username from test1; insert into test1 (username)   select username from test1; insert into test1 (username)   select username from test1; insert into test1 (username)   select username from test1; insert into test1 (username)   select username from test1; insert into test1 (username)   select username from test1; insert into test1 (username)   select username from test1; insert into test1 (username)   select username from test1; insert into test1 (username)   select username from test1; insert into test1 (username)   select username from test1; insert into test1 (username)   select username from test1;

先创建一部分数据. 然后执行

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1	explain select * from test1;

结果

explain select username from test1;
+----+-------------+-------+------+---------------+------+---------+------+---------+-------+
| id | select_type | table | type | possible_keys | key  | key_len | ref  | rows    | Extra |
+----+-------------+-------+------+---------------+------+---------+------+---------+-------+
|  1 | SIMPLE      | test1 | ALL  | NULL          | NULL | NULL    | NULL | 4187248 | NULL  |
+----+-------------+-------+------+---------------+------+---------+------+---------+-------+
1 row in set (0.00 sec)

再执行

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1	explain select username from test1 where username = 'test';

+----+-------------+-------+------+---------------+------+---------+------+---------+-------------+
| id | select_type | table | type | possible_keys | key  | key_len | ref  | rows    | Extra       |
+----+-------------+-------+------+---------------+------+---------+------+---------+-------------+
|  1 | SIMPLE      | test1 | ALL  | NULL          | NULL | NULL    | NULL | 4187248 | Using where |
+----+-------------+-------+------+---------------+------+---------+------+---------+-------------+
1 row in set (0.00 sec)

接下来再给表的username创建一个索引

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1	ALTER TABLE `test1` ADD INDEX `i_name` (`username`) comment '';

   过程 会比较 卡.需要等待几秒到几十秒.取决于机器. 也可以少插入几条数据. 然后再执行刚才的两条语句

mysql> explain select username from test1 where username = 'test';
+----+-------------+-------+------+---------------+--------+---------+-------+------+--------------------------+
| id | select_type | table | type | possible_keys | key    | key_len | ref   | rows | Extra                    |
+----+-------------+-------+------+---------------+--------+---------+-------+------+--------------------------+
|  1 | SIMPLE      | test1 | ref  | i_name        | i_name | 7       | const |    1 | Using where; Using index |
+----+-------------+-------+------+---------------+--------+---------+-------+------+--------------------------+
1 row in set (0.00 sec)

mysql> explain select username from test1;
+----+-------------+-------+-------+---------------+--------+---------+------+---------+-------------+
| id | select_type | table | type  | possible_keys | key    | key_len | ref  | rows    | Extra       |
+----+-------------+-------+-------+---------------+--------+---------+------+---------+-------------+
|  1 | SIMPLE      | test1 | index | NULL          | i_name | 7       | NULL | 4187248 | Using index |
+----+-------------+-------+-------+---------------+--------+---------+------+---------+-------------+
1 row in set (0.00 sec)

发现 type 变了   key也变了.  extra也不一样了. 对比一下就了解情况

其中，

type=const表示通过索引一次就找到了；

key=primary的话，表示使用了主键；

type=all,表示为全表扫描；

key=null表示没用到索引。
type=ref,因为这时认为是多个匹配行，在联合查询中，一般为REF。

经过分析后就可以判断出哪些字段需要建 索引,哪些条件可以优化等. 以及条数等



system > const > eq_ref > ref > fulltext > ref_or_null > index_merge > unique_subquery > index_subquery > range > index > ALL