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一、联合索引
- 联合索引又称复合索引,是指由多个字段组成的索引
二、最左匹配原则
- 最左匹配原则,MySQL 会一直向右匹配,如果遇到范围查询 (>、<、between、like) 就会停止匹配,如:
a = 1 and b = 2 and c > 3 and d = 4
,如果建立(a, b, c, d)
顺序的联合索引,那么 d 就使用不了联合索引了;如果建立(a, b, d, c)
顺序的联合索引,则都可以用的到,a、b、d 的顺序可以任意调整 - = 和 in 可以乱序,比如:
a = 1 and b = 2 and c = 3
,如果建立(a, b, c)
顺序的联合索引,则可以任意顺序,MySQL 的查询优化器会帮我们将其优化成索引可识别的形式
二、最左匹配原则成因
- MySQL 创建联合索引的规则,是首先对联合索引最左边 (第一个) 的索引字段的数据进行排序,然后在第一个字段数据排序的基础上,再对第二个索引字段的数据进行排序,以此类推,相当于实现了类似
order by field1 filed2 ... filedN
这样的排序规则 - 因此在没有使用第一个字段的情况下,直接使用第二个或后面的字段进行条件判断是无法使用索引的,这就是 MySQL 为什么强调联合索引最左匹配原则的原因
三、举例说明
- 创建学生表
DROP TABLE IF EXISTS tbl_student_left;
CREATE TABLE tbl_student_left (
id BIGINT(20) UNSIGNED NOT NULL AUTO_INCREMENT,
uuid VARCHAR(40) NOT NULL COMMENT '学生唯一标识',
name VARCHAR(20) NOT NULL COMMENT '学生姓名',
age INT(3) UNSIGNED NOT NULL COMMENT '学生年龄',
sex TINYINT(1) UNSIGNED NOT NULL COMMENT '学生性别: 1=男,0=女',
PRIMARY KEY (id),
KEY idx_uuid_name_age (uuid, name, age)
) ENGINE=InnoDB DEFAULT CHARSET=utf8 COMMENT='学生信息表';
- 如上所示,我们建立了
(uuid, name, age)
顺序的联合索引idx_uuid_name_age_sex
,按照最左匹配原则,实际相当于是建立了(uuid)
、(uuid, name)
、(uuid, age)
、(uuid, name, age)
四个索引 - 其他类似于
(name)
、(age)
、(name, age)
等组合则使用不了联合索引 - 这里我们插入 1000 条数据来作为分析说明
- 使用联合索引示例
- 示例一
(uuid)
mysql> explain select * from tbl_student_left where uuid = '426fcfd1-c8de-11ea-84aa-00163e0d9873';
+----+-------------+------------------+------------+------+-------------------+-------------------+---------+-------+------+----------+-------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+------------------+------------+------+-------------------+-------------------+---------+-------+------+----------+-------+
| 1 | SIMPLE | tbl_student_left | NULL | ref | idx_uuid_name_age | idx_uuid_name_age | 122 | const | 1 | 100.00 | NULL |
+----+-------------+------------------+------------+------+-------------------+-------------------+---------+-------+------+----------+-------+
1 row in set, 1 warning (0.00 sec)
- 示例二
(uuid, name)
mysql> explain select * from tbl_student_left where uuid = '426fcfd1-c8de-11ea-84aa-00163e0d9873' and name = 'NrDHrYmKurDVyRvMc';
+----+-------------+------------------+------------+------+-------------------+-------------------+---------+-------------+------+----------+-------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+------------------+------------+------+-------------------+-------------------+---------+-------------+------+----------+-------+
| 1 | SIMPLE | tbl_student_left | NULL | ref | idx_uuid_name_age | idx_uuid_name_age | 184 | const,const | 1 | 100.00 | NULL |
+----+-------------+------------------+------------+------+-------------------+-------------------+---------+-------------+------+----------+-------+
1 row in set, 1 warning (0.00 sec)
- 示例三
(uuid, age)
mysql> explain select * from tbl_student_left where uuid = '426fcfd1-c8de-11ea-84aa-00163e0d9873' and age = 22;
+----+-------------+------------------+------------+------+-------------------+-------------------+---------+-------+------+----------+-----------------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+------------------+------------+------+-------------------+-------------------+---------+-------+------+----------+-----------------------+
| 1 | SIMPLE | tbl_student_left | NULL | ref | idx_uuid_name_age | idx_uuid_name_age | 122 | const | 1 | 10.00 | Using index condition |
+----+-------------+------------------+------------+------+-------------------+-------------------+---------+-------+------+----------+-----------------------+
1 row in set, 1 warning (0.00 sec)
- 示例四
(uuid, name, age)
mysql> explain select * from tbl_student_left where uuid = '426fcfd1-c8de-11ea-84aa-00163e0d9873' and name = 'NrDHrYmKurDVyRvMc' and age = 22;
+----+-------------+------------------+------------+------+-------------------+-------------------+---------+-------------------+------+----------+-------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+------------------+------------+------+-------------------+-------------------+---------+-------------------+------+----------+-------+
| 1 | SIMPLE | tbl_student_left | NULL | ref | idx_uuid_name_age | idx_uuid_name_age | 188 | const,const,const | 1 | 100.00 | NULL |
+----+-------------+------------------+------------+------+-------------------+-------------------+---------+-------------------+------+----------+-------+
1 row in set, 1 warning (0.00 sec)
- 无法使用联合索引示例
- 示例一
(name)
mysql> explain select * from tbl_student_left where name = 'NrDHrYmKurDVyRvMc';
+----+-------------+------------------+------------+------+---------------+------+---------+------+------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+------------------+------------+------+---------------+------+---------+------+------+----------+-------------+
| 1 | SIMPLE | tbl_student_left | NULL | ALL | NULL | NULL | NULL | NULL | 1000 | 10.00 | Using where |
+----+-------------+------------------+------------+------+---------------+------+---------+------+------+----------+-------------+
1 row in set, 1 warning (0.00 sec)
- 示例二
(age)
mysql> explain select * from tbl_student_left where age = 22;
+----+-------------+------------------+------------+------+---------------+------+---------+------+------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+------------------+------------+------+---------------+------+---------+------+------+----------+-------------+
| 1 | SIMPLE | tbl_student_left | NULL | ALL | NULL | NULL | NULL | NULL | 1000 | 10.00 | Using where |
+----+-------------+------------------+------------+------+---------------+------+---------+------+------+----------+-------------+
1 row in set, 1 warning (0.00 sec)
- 示例三
(name, age)
mysql> explain select * from tbl_student_left where name = 'NrDHrYmKurDVyRvMc' and age = 22;
+----+-------------+------------------+------------+------+---------------+------+---------+------+------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+------------------+------------+------+---------------+------+---------+------+------+----------+-------------+
| 1 | SIMPLE | tbl_student_left | NULL | ALL | NULL | NULL | NULL | NULL | 1000 | 1.00 | Using where |
+----+-------------+------------------+------------+------+---------------+------+---------+------+------+----------+-------------+
1 row in set, 1 warning (0.01 sec)