[MySQL] 查看explain中的key

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

参考链接