查看explain中的key_len判断究竟用了哪个索引

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?

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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

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

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