MySQL 联合索引测试
搭建测试环境
1：创建表
CREATE TABLE tab_index
(id int(5),
age int(3),
dte datetime);
2：插入测试数据
INSERT INTO tab_index
VALUES(1,'2012-05-13',23);
INSERT INTO tab_index
VALUES(2,'2012-05-13',23);
INSERT INTO tab_index
VALUES(3,'2012-05-13',31);
INSERT INTO tab_index
VALUES(4,'2012-05-13',32);
INSERT INTO tab_index
VALUES(5,'2012-05-13',33);
INSERT INTO tab_index
VALUES(6,'2012-06-13',34);
INSERT INTO tab_index
VALUES(7,'2012-07-13',35);
INSERT INTO tab_index
VALUES(8,'2012-08-13',36);
INSERT INTO tab_index
VALUES(9,'2012-09-13',37);
INSERT INTO tab_index
VALUES(10,'2012-05-17',38);
INSERT INTO tab_index
VALUES(11,'2012-05-19',39);
INSERT INTO tab_index
VALUES(1,'2012-05-13',23);
INSERT INTO tab_index
VALUES(2,'2012-05-13',23);
INSERT INTO tab_index
VALUES(3,'2012-05-13',31);
INSERT INTO tab_index
VALUES(4,'2012-05-13',32);
INSERT INTO tab_index
VALUES(5,'2012-05-13',33);
INSERT INTO tab_index
VALUES(6,'2012-06-13',34);
INSERT INTO tab_index
VALUES(7,'2012-07-13',35);
INSERT INTO tab_index
VALUES(8,'2012-08-13',36);
INSERT INTO tab_index
VALUES(9,'2012-09-13',37);
INSERT INTO tab_index
VALUES(10,'2012-05-17',38);
INSERT INTO tab_index
VALUES(11,'2012-05-19',39);
INSERT INTO tab_index
VALUES(1,'2012-05-13',23);
INSERT INTO tab_index
VALUES(2,'2015-05-13',23);
INSERT INTO tab_index
VALUES(3,'2012-05-13',31);
INSERT INTO tab_index
VALUES(4,'2012-05-13',32);
INSERT INTO tab_index
VALUES(5,'2012-05-13',33);
INSERT INTO tab_index
VALUES(6,'2012-06-13',34);
INSERT INTO tab_index
VALUES(7,'2013-07-13',35);
INSERT INTO tab_index
VALUES(8,'2012-08-13',36);
INSERT INTO tab_index
VALUES(9,'2012-09-13',37);
INSERT INTO tab_index
VALUES(10,'2012-05-17',38);
INSERT INTO tab_index
VALUES(11,'2011-05-19',39);
INSERT INTO tab_index
VALUES(1,'2012-05-13',23);
INSERT INTO tab_index
VALUES(2,'2012-05-13',23);
INSERT INTO tab_index
VALUES(3,'2010-05-13',31);
INSERT INTO tab_index
VALUES(4,'2012-05-13',32);
INSERT INTO tab_index
VALUES(5,'2010-05-13',33);
INSERT INTO tab_index
VALUES(6,'2010-06-13',34);
INSERT INTO tab_index
VALUES(7,'2012-07-13',35);
INSERT INTO tab_index
VALUES(8,'2012-08-13',36);
INSERT INTO tab_index
VALUES(9,'2011-09-13',37);
INSERT INTO tab_index
VALUES(10,'2012-05-17',38);
INSERT INTO tab_index
VALUES(11,'2012-05-19',39);
INSERT INTO tab_index
VALUES(1,'2012-05-13',23);
INSERT INTO tab_index
VALUES(2,'2012-05-13',23);
INSERT INTO tab_index
VALUES(3,'2012-05-13',31);
INSERT INTO tab_index
VALUES(4,'2012-05-13',32);
INSERT INTO tab_index
VALUES(5,'2012-05-13',33);
INSERT INTO tab_index
VALUES(6,'2012-06-13',34);
INSERT INTO tab_index
VALUES(7,'2014-07-13',35);
INSERT INTO tab_index
VALUES(8,'2012-08-13',36);
INSERT INTO tab_index
VALUES(9,'2011-09-13',37);
INSERT INTO tab_index
VALUES(10,'2012-05-17',38);
INSERT INTO tab_index
VALUES(11,'2012-05-19',39);
3：创建id和age上的联合索引
CREATE INDEX idx1
ON tab_index(id,age);
4：开始测试
mysql> EXPLAIN SELECT id,dte,age
-> FROM tab_index
-> WHERE id=3;
+----+-------------+-----------+------+---------------+------+---------+-------+------+-------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-----------+------+---------------+------+---------+-------+------+-------------+
| 1 | SIMPLE | tab_index | ref | idx1 | idx1 | 5 | const | 5 | Using where |
+----+-------------+-----------+------+---------------+------+---------+-------+------+-------------+
1 row in set (0.00 sec)
id=3走索引,age=31不走索引。很容易理解
mysql> EXPLAIN SELECT id,dte,age
-> FROM tab_index
-> WHERE age=31;
+----+-------------+-----------+------+---------------+------+---------+------+------+-------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-----------+------+---------------+------+---------+------+------+-------------+
| 1 | SIMPLE | tab_index | ALL | NULL | NULL | NULL | NULL | 55 | Using where |
+----+-------------+-----------+------+---------------+------+---------+------+------+-------------+
1 row in set (0.00 sec)
mysql> EXPLAIN SELECT id,dte,age
-> FROM tab_index
-> WHERE id=3 AND age=31;
+----+-------------+-----------+------+---------------+------+---------+-------------+------+-------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-----------+------+---------------+------+---------+-------------+------+-------------+
| 1 | SIMPLE | tab_index | ref | idx1 | idx1 | 10 | const,const | 1 | Using where |
+----+-------------+-----------+------+---------------+------+---------+-------------+------+-------------+
1 row in set (0.00 sec)
id=3 AND age=31 和 age=31 AND id=3都走索引了，但是索引长度跟之前不同
mysql> EXPLAIN SELECT id,dte,age
-> FROM tab_index
-> WHERE age=31 AND id=3 ;
+----+-------------+-----------+------+---------------+------+---------+-------------+------+-------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-----------+------+---------------+------+---------+-------------+------+-------------+
| 1 | SIMPLE | tab_index | ref | idx1 | idx1 | 10 | const,const | 1 | Using where |
+----+-------------+-----------+------+---------------+------+---------+-------------+------+-------------+
1 row in set (0.00 sec)
mysql> EXPLAIN SELECT id,dte,age
-> FROM tab_index
-> WHERE id=3 AND age=31 AND dte BETWEEN '2011-05-13 00:00:00' AND '2013-05-13 00:00:00';
+----+-------------+-----------+------+---------------+------+---------+-------------+------+-------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-----------+------+---------------+------+---------+-------------+------+-------------+
| 1 | SIMPLE | tab_index | ref | idx1 | idx1 | 10 | const,const | 1 | Using where |
+----+-------------+-----------+------+---------------+------+---------+-------------+------+-------------+
1 row in set (0.00 sec)
mysql> EXPLAIN SELECT id,dte,age
-> FROM tab_index
-> WHERE id=3 AND dte BETWEEN '2011-05-13 00:00:00' AND '2013-05-13 00:00:00' AND age=31;
+----+-------------+-----------+------+---------------+------+---------+-------------+------+-------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-----------+------+---------------+------+---------+-------------+------+-------------+
| 1 | SIMPLE | tab_index | ref | idx1 | idx1 | 10 | const,const | 1 | Using where |
+----+-------------+-----------+------+---------------+------+---------+-------------+------+-------------+
1 row in set (0.00 sec)
mysql> EXPLAIN SELECT id,dte,age
-> FROM tab_index
-> WHERE dte BETWEEN '2011-05-13 00:00:00' AND '2013-05-13 00:00:00' AND id=3 AND age=31;
+----+-------------+-----------+------+---------------+------+---------+-------------+------+-------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-----------+------+---------------+------+---------+-------------+------+-------------+
| 1 | SIMPLE | tab_index | ref | idx1 | idx1 | 10 | const,const | 1 | Using where |
+----+-------------+-----------+------+---------------+------+---------+-------------+------+-------------+
1 row in set (0.00 sec)
mysql> EXPLAIN SELECT id,dte,age
-> FROM tab_index
-> WHERE age=31 AND id=3 AND dte BETWEEN '2011-05-13 00:00:00' AND '2013-05-13 00:00:00';
+----+-------------+-----------+------+---------------+------+---------+-------------+------+-------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-----------+------+---------------+------+---------+-------------+------+-------------+
| 1 | SIMPLE | tab_index | ref | idx1 | idx1 | 10 | const,const | 1 | Using where |
+----+-------------+-----------+------+---------------+------+---------+-------------+------+-------------+
1 row in set (0.00 sec)
mysql> EXPLAIN SELECT id,dte,age
-> FROM tab_index
-> WHERE age=31 AND dte BETWEEN '2011-05-13 00:00:00' AND '2013-05-13 00:00:00' AND id=3 ;
+----+-------------+-----------+------+---------------+------+---------+-------------+------+-------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-----------+------+---------------+------+---------+-------------+------+-------------+
| 1 | SIMPLE | tab_index | ref | idx1 | idx1 | 10 | const,const | 1 | Using where |
+----+-------------+-----------+------+---------------+------+---------+-------------+------+-------------+
1 row in set (0.00 sec)
mysql> EXPLAIN SELECT id,dte,age
-> FROM tab_index
-> WHERE dte BETWEEN '2011-05-13 00:00:00' AND '2013-05-13 00:00:00' AND age=31 AND id=3 ;
+----+-------------+-----------+------+---------------+------+---------+-------------+------+-------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-----------+------+---------------+------+---------+-------------+------+-------------+
| 1 | SIMPLE | tab_index | ref | idx1 | idx1 | 10 | const,const | 1 | Using where |
+----+-------------+-----------+------+---------------+------+---------+-------------+------+-------------+
1 row in set (0.00 sec)
另外其他几种情况，同学们自己尝试下以加深印象。
总结如下：
(id)走索引，且索引长度最短
(id,dte)走索引，且索引长度最短
(id,age)走索引，且索引长度最长
(id,age,dte)走索引，且索引长度最长
(id,dte,age)走索引，且索引长度最长
(dte,id,age)走索引，且索引长度最长
(dte,age,id)走索引，且索引长度最长
(age,dte,id)走索引，且索引长度最长
(age,id,dte)走索引，且索引长度最长
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