[MySQL] 查看explain中的key_len判断究竟使用了哪个索引

在一张表里有多个索引 , 我们where字段里条件有多个索引的值 , 那么究竟使用的哪个呢?

我们可以使用explain来查看 , 其中的key_len字段可以看得出来

比如下面这条sql

explain select * from ent_calendar_diary where email='xxxx' and diary_id=1784;

+----+-------------+--------------------+------------+-------+-------------------------+---------+---------+-------+------+----------+-------+
| id | select_type | table              | partitions | type  | possible_keys           | key     | key_len | ref   | rows | filtered | Extra |
+----+-------------+--------------------+------------+-------+-------------------------+---------+---------+-------+------+----------+-------+
|  1 | SIMPLE      | ent_calendar_diary | NULL       | const | PRIMARY,idx_email_stime | PRIMARY | 4       | const |    1 |   100.00 | NULL  |
+----+-------------+--------------------+------------+-------+-------------------------+---------+---------+-------+------+----------+-------+

possible_keys里面有两个索引字段 , 但是看key_len  是4个字节 

列类型	key_len	备注
id int	key_len = 4+1 = 5	允许NULL，加1-byte
id int not null	key_len = 4	不允许NULL
user char(30) utf8	key_len = 30*3+1	允许NULL
user varchar(30) not null utf8	key_len = 30*3+2	动态列类型，加2-bytes
user varchar(30) utf8	key_len = 30*3+2+1	动态列类型，加2-bytes；允许NULL，再加1-byte
detail text(10) utf8	key_len = 30*3+2+1	TEXT列截取部分，被视为动态列类型，加2-bytes；且允许NULL

备注，key_len 只指示了WHERE中用于条件过滤时被选中的索引列，是不包含 ORDER BY/GROUP BY 

int类型并且not null 是4个字节 , 因此上面的sql是使用的主键索引

explain select * from ent_calendar_diary where email='xxxx';

+----+-------------+--------------------+------------+------+-----------------+-----------------+---------+-------+------+----------+-------+
| id | select_type | table              | partitions | type | possible_keys   | key             | key_len | ref   | rows | filtered | Extra |
+----+-------------+--------------------+------------+------+-----------------+-----------------+---------+-------+------+----------+-------+
|  1 | SIMPLE      | ent_calendar_diary | NULL       | ref  | idx_email_stime | idx_email_stime | 767     | const |  111 |   100.00 | NULL  |
+----+-------------+--------------------+------------+------+-----------------+-----------------+---------+-------+------+----------+-------+

这个是767个字节 , varchar(255)  not null   255 * 3 +2正好符合 , 因此是使用的email那个普通索引

CREATE TABLE `ent_calendar_diary` (
`diary_id` int(10) unsigned NOT NULL AUTO_INCREMENT,
`email` varchar(255) NOT NULL,
`title` varchar(100) NOT NULL,
`summary` varchar(500) NOT NULL DEFAULT '',
`stime` bigint(11) NOT NULL DEFAULT '0',
`ctime` int(10) unsigned NOT NULL DEFAULT '0',
PRIMARY KEY (`diary_id`),
KEY `idx_email_stime` (`email`,`stime`)
) ENGINE=InnoDB AUTO_INCREMENT=1809 DEFAULT CHARSET=utf8