什么时候使用组合索引，什么时候使用单独索引
一.前期数据准备
1.建表
CREATE TABLE `user` (
`uid` int(11) NOT NULL AUTO_INCREMENT,
`name` varchar(50) DEFAULT NULL,
`pwd` varchar(50) DEFAULT NULL,
`create_time` datetime DEFAULT NULL,
`modify_time` timestamp NULL DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP,
`rids` varchar(15) DEFAULT NULL,
`nickname` varchar(45) DEFAULT NULL,
`company` varchar(15) DEFAULT NULL,
PRIMARY KEY (`uid`),
UNIQUE KEY `name_UNIQUE` (`name`)
) ENGINE=InnoDB AUTO_INCREMENT=2 DEFAULT CHARSET=utf8
2.插入数据
INSERT INTO `monitor`.`user`(`uid`, `name`, `pwd`, `create_time`, `modify_time`, `rids`, `nickname`, `company`) VALUES (1, 'rocker', 'rocker', NULL, '2019-10-08 11:05:02', '1', 'rocker', 'rocker');
INSERT INTO `monitor`.`user`(`uid`, `name`, `pwd`, `create_time`, `modify_time`, `rids`, `nickname`, `company`) VALUES (2, 'danny', 'danny', NULL, '2019-10-08 11:31:36', '2', 'rocker', 'danny');
INSERT INTO `monitor`.`user`(`uid`, `name`, `pwd`, `create_time`, `modify_time`, `rids`, `nickname`, `company`) VALUES (3, 'tom', 'tom', NULL, '2019-10-08 11:31:39', '1', 'tom', 'rocker');
INSERT INTO `monitor`.`user`(`uid`, `name`, `pwd`, `create_time`, `modify_time`, `rids`, `nickname`, `company`) VALUES (4, 'messi', 'messi', NULL, '2019-10-08 11:31:21', '2', 'messi', 'messi');
INSERT INTO `monitor`.`user`(`uid`, `name`, `pwd`, `create_time`, `modify_time`, `rids`, `nickname`, `company`) VALUES (5, 'wenger', 'wenger', NULL, '2019-10-08 11:29:38', '1', 'wenger', 'rocker');
INSERT INTO `monitor`.`user`(`uid`, `name`, `pwd`, `create_time`, `modify_time`, `rids`, `nickname`, `company`) VALUES (6, 'henry', 'henry', NULL, '2019-10-08 11:30:46', '2', 'henry', 'henry');
INSERT INTO `monitor`.`user`(`uid`, `name`, `pwd`, `create_time`, `modify_time`, `rids`, `nickname`, `company`) VALUES (7, 'ronaldo', 'ronaldo', NULL, '2019-10-08 11:30:49', '1', 'ronaldo', 'ronaldo');
INSERT INTO `monitor`.`user`(`uid`, `name`, `pwd`, `create_time`, `modify_time`, `rids`, `nickname`, `company`) VALUES (8, 'kaka', 'kaka', NULL, '2019-10-08 11:29:45', '2', 'kaka', 'rocker');
二.分析
1.不加索引
首先在'nickname'和‘company’这俩字段不加索引的情况下执行一个查询语句，并分析
mysql> explain select * from user where nickname = 'rocker' and company = 'rocker';
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------------+
| 1 | SIMPLE | user | NULL | ALL | NULL | NULL | NULL | NULL | 8 | 12.50 | Using where |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------------+
1 row in set, 1 warning (0.00 sec)
可以看到，没有走索引，总共查询了8条数据，而表中总共也是8条数据，相当于全表扫描了。
mysql> explain select * from user where company = 'rocker' or nickname = 'rocker';
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------------+
| 1 | SIMPLE | user | NULL | ALL | NULL | NULL | NULL | NULL | 8 | 23.44 | Using where |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------------+
1 row in set, 1 warning (0.00 sec)
可以看到：不加任何索引的情况下，不管是and还是or，都是全表扫描，没有索引。
2.单独索引
给nickname和company分别加上索引，再执行and和or的sql查询
alter table user add index `idx_nickname` (`nickname`);
alter table user add index `idx_company` (`company`);
执行查询语句and
mysql> explain select * from user where nickname = 'rocker' and company = 'rocker';
+----+-------------+-------+------+--------------------------+--------------+---------+-------+------+-------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------+------+--------------------------+--------------+---------+-------+------+-------------+
| 1 | SIMPLE | user | ref | idx_nickname,idx_company | idx_nickname | 138 | const | 2 | Using where |
+----+-------------+-------+------+--------------------------+--------------+---------+-------+------+-------------+
1 row in set (0.05 sec)
执行查询语句or
mysql> explain select * from user where company = 'rocker' or nickname = 'rocker';
+----+-------------+-------+------+--------------------------+------+---------+------+------+-------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------+------+--------------------------+------+---------+------+------+-------------+
| 1 | SIMPLE | user | ALL | idx_nickname,idx_company | NULL | NULL | NULL | 8 | Using where |
+----+-------------+-------+------+--------------------------+------+---------+------+------+-------------+
1 row in set (0.00 sec)
可以看到：加上索引后and查询是可以走索引的，但是只有一个索引起作用，对于另一个索引字段还是要进行遍历，而且and查询会根据关联性高(符合该条件的行数少)选择具体走哪个索引
or查询不走索引
3.组合索引
删除原先的单独索引，新增组合索引
alter table user drop index `idx_nickname`
alter table user drop index `idx_company`
alter table user add index `idx_composition` (`nickname`,`company`);
执行查询语句and
mysql> explain select * from user where nickname = 'rocker' and company = 'rocker';
+----+-------------+-------+------+-----------------+-----------------+---------+-------------+------+-------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------+------+-----------------+-----------------+---------+-------------+------+-------------+
| 1 | SIMPLE | user | ref | idx_composition | idx_composition | 186 | const,const | 1 | Using where |
+----+-------------+-------+------+-----------------+-----------------+---------+-------------+------+-------------+
1 row in set (0.00 sec)
执行查询语句or
mysql> explain select * from user where company = 'rocker' or nickname = 'rocker';
+----+-------------+-------+------+-----------------+------+---------+------+------+-------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------+------+-----------------+------+---------+------+------+-------------+
| 1 | SIMPLE | user | ALL | idx_composition | NULL | NULL | NULL | 8 | Using where |
+----+-------------+-------+------+-----------------+------+---------+------+------+-------------+
1 row in set (0.00 sec)
可以看到：加上组合索引后，组合索引起作用，只需查询一条符合结果的数据，效率要比单独索引高，
但是复合索引对于or查询不起作用
4.组合索引查询单个索引列
对于组合索引为(nickname,company)这个顺序的情况
alter table user drop index `idx_composition`;
alter table user add index `idx_composition` (`nickname`,`company`);
mysql> explain select * from user where nickname = 'rocker';
+----+-------------+-------+------+-----------------+-----------------+---------+-------+------+-------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------+------+-----------------+-----------------+---------+-------+------+-------------+
| 1 | SIMPLE | user | ref | idx_composition | idx_composition | 138 | const | 2 | Using where |
+----+-------------+-------+------+-----------------+-----------------+---------+-------+------+-------------+
1 row in set (0.00 sec)
mysql> explain select * from user where company = 'rocker';
+----+-------------+-------+------+---------------+------+---------+------+------+-------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------+------+---------------+------+---------+------+------+-------------+
| 1 | SIMPLE | user | ALL | NULL | NULL | NULL | NULL | 8 | Using where |
+----+-------------+-------+------+---------------+------+---------+------+------+-------------+
1 row in set (0.00 sec)
可以看到：组合索引中nickname在前时，单独查询nickname会走索引，单独查询compamy不会走索引
对于组合索引为(company,nickname)这个顺序的情况
alter table user drop index `idx_composition`;
alter table user add index `idx_composition` (`company`,`nickname`);
mysql> explain select * from user where nickname = 'rocker';
+----+-------------+-------+------+---------------+------+---------+------+------+-------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------+------+---------------+------+---------+------+------+-------------+
| 1 | SIMPLE | user | ALL | NULL | NULL | NULL | NULL | 8 | Using where |
+----+-------------+-------+------+---------------+------+---------+------+------+-------------+
1 row in set (0.00 sec)
mysql> explain select * from user where company = 'rocker';
+----+-------------+-------+------+-----------------+-----------------+---------+-------+------+-------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------+------+-----------------+-----------------+---------+-------+------+-------------+
| 1 | SIMPLE | user | ref | idx_composition | idx_composition | 48 | const | 2 | Using where |
+----+-------------+-------+------+-----------------+-----------------+---------+-------+------+-------------+
1 row in set (0.00 sec)
可以看到：组合索引中compamy在前时，单独查询compamy会走索引，单独查询nickname不会走索引
如果组合索引是(A,B)，则对于条件A=a，是可以用上这个组合索引的，因为组合索引是先按照第一列进行排序的，所以没必要对A单独建立一个索引，但是对于B=b就用不上了，因为只有在第一列相同的情况下，才比较第二列，因而第二列相同的，可以分布在不同的节点上，没办法快速定位