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@H_404_15@ | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+--------------------+------------+-------+-------------------------+---------+---------+-------+------+----------+-------+
| 1@H_404_15@ | SIMPLE | ent_calendar_diary | NULL | const | PRIMARY,idx_email_stime | PRIMARY | 4@H_404_15@ | const | 1@H_404_15@ | 100.00@H_404_15@ | 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@H_404_15@ | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+--------------------+------------+------+-----------------+-----------------+---------+-------+------+----------+-------+
| 1@H_404_15@ | SIMPLE | ent_calendar_diary | NULL | ref | idx_email_stime | idx_email_stime | 767@H_404_15@ | const | 111@H_404_15@ | 100.00@H_404_15@ | 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