组合索引与前缀索引

注意，这两种称呼是对建立索引技巧的一种称呼，并非索引的类型 。

前缀索引

如果索引列长度过长，这种列索引时将会产生很大的索引文件，不便于操作，可以使用前缀索引方式进行索引前缀索引应该控制在一个合适的点，控制在0.31黄金值即可（大于这个值就可以创建）。

# 什么时候适合创建前缀索引，当以下值 小于 0.31 时适合创建前缀索引
mysql> SELECT COUNT(DISTINCT(LEFT(name,10)))/COUNT(*) FROM students;
+-----------------------------------------+
| COUNT(DISTINCT(LEFT(name,10)))/COUNT(*) |
+-----------------------------------------+
|                                  1.0000 |
+-----------------------------------------+
1 row in set (0.00 sec)

# 创建前缀索引，根据 name 的前几个字符创建前缀索引
mysql> alter table test add INDEX(name(4));


# 删除前缀索引
mysql> alter table test drop INDEX name;
Query OK, 0 rows affected (0.00 sec)
Records: 0  Duplicates: 0  Warnings: 0

组合索引

单列字段的记录不唯一，但多个字段的记录组合到一起，是唯一的 。

# 预备表
mysql> desc students;
+---------------+---------------------+------+-----+-------------------+----------------+
| Field         | Type                | Null | Key | Default           | Extra          |
+---------------+---------------------+------+-----+-------------------+----------------+
| id            | int(11)             | NO   | PRI | NULL              | auto_increment |
| name          | varchar(12)         | NO   |     | NULL              |                |
| age           | tinyint(3) unsigned | NO   |     | NULL              |                |
| gender        | enum('M','F')       | YES  |     | M                 |                |
| register_time | datetime            | YES  |     | CURRENT_TIMESTAMP |                |
| hobby         | varchar(32)         | YES  |     | NULL              |                |
| phone         | varchar(32)         | YES  |     | NULL              |                |
+---------------+---------------------+------+-----+-------------------+----------------+
7 rows in set (0.00 sec)

# 预备表数据
mysql> select * from students;
+----+------+-----+--------+---------------------+----------+--------+
| id | name | age | gender | register_time       | hobby    | phone  |
+----+------+-----+--------+---------------------+----------+--------+
|  1 | wqh  |  18 | M      | 2020-07-14 22:01:58 | abc      | 1      |
|  2 | wzh  |  18 | M      | 2020-07-14 22:02:04 | wzhhobby | 12     |
|  3 | djd  |  18 | M      | 2020-07-14 22:02:11 | djdhobby | 123    |
|  4 | tcy  |  18 | F      | 2020-07-14 22:02:27 | tcyhobby | 1234   |
|  5 | gsp  |  18 | M      | 2020-07-14 22:02:36 | gsphobby | 12345  |
|  6 | cjq  |  18 | M      | 2020-07-14 22:02:40 | cjqhobby | 12346  |
|  7 | wzd  |  12 | M      | 2020-07-17 10:13:45 | wzdhobby | 123467 |
+----+------+-----+--------+---------------------+----------+--------+
7 rows in set (0.00 sec)



# 添加组合（联合）索引
mysql> alter table students add index union_key(gender,hobby,phone); 

# 删除组合索引
mysql> alter table students drop index union_key;

# 查看索引
mysql> show index from students;
+----------+------------+-----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| Table    | Non_unique | Key_name  | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | Index_comment |
+----------+------------+-----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| students |          0 | PRIMARY   |            1 | id          | A         |           7 |     NULL | NULL   |      | BTREE      |         |               |
| students |          1 | union_key |            1 | gender      | A         |           7 |     NULL | NULL   | YES  | BTREE      |         |               |
| students |          1 | union_key |            2 | hobby       | A         |           7 |     NULL | NULL   | YES  | BTREE      |         |               |
| students |          1 | union_key |            3 | phone       | A         |           7 |     NULL | NULL   | YES  | BTREE      |         |               |
+----------+------------+-----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
4 rows in set (0.00 sec)


# 组合索引的使用
前提：使用 where 条件搜索的内容不超过全部记录的 15% 
走索引		gender,hobby,phone
                # hobby,phone,gender // 可以正序排列成上一条件
                gender,hobby
                # hobby,gender // 可以正序排列成上一条件
                gender
不走索引		gender,phone
                # phone,gender // 不能从头正序排列，和上一条件本质相同
                hobby,phone
                # phone,hobby // 不能从头正序排列，和上一条本件质相同
                hobby
                phone