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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?
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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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