MySQL高级-索引-使用规则-前缀索引

1、前缀索引

当字段类型为字符串（varchar，text等）时，有时候需要索引很长的字符串，这会让索引变得很大，查询时，浪费大量的磁盘IO，影响查询效率。此时可以只将字符串的一部分前缀，建立索引，这样可以大大节约索引空间，从而提高索引效率。

create index idx_xxxx on table_name(column(n))

2、前缀长度

可以根据索引的选择性来决定，而选择性是指不重复的索引值（基数）和数据表的记录总数的比值，索引选择性越高则查询效率越高，唯一索引的选择性是1，这是最好的索引选择性，性能页是最好的。

select count(distinct email)/count(*) from tb_user;
select count(distinct substring(email,1,5))/count(*) from tb_user;

3、查询表数据

mysql> select * from tb_user;
+----+--------+-------------+-----------------------+----------------------+------+--------+--------+---------------------+
| id | name   | phone       | email                 | profession           | age  | gender | status | createtime          |
+----+--------+-------------+-----------------------+----------------------+------+--------+--------+---------------------+
|  1 | 吕布   | 17799990000 | [e-mail]       | 软件工程             |   23 | 1      | 6      | 2001-02-02 00:00:00 |
|  2 | 曹操   | 17799990001 | [e-mail]      | 通讯工程             |   33 | 1      | 0      | 2001-03-05 00:00:00 |
|  3 | 赵云   | 17799990002 | [e-mail]      | 英语                 |   34 | 1      | 2      | 2002-03-02 00:00:00 |
|  4 | 孙悟空 | 17799990003 | [e-mail]     | 工程造价             |   54 | 1      | 0      | 2001-07-02 00:00:00 |
|  5 | 花木兰 | 17799990004 | [e-mail]     | 软件工程             |   23 | 2      | 1      | 2001-04-22 00:00:00 |
|  6 | 大乔   | 17799990005 | [e-mail]    | 舞蹈                 |   22 | 2      | 0      | 2001-02-07 00:00:00 |
|  7 | 露娜   | 17799990006 | [e-mail]    | 应用数学             |   24 | 2      | 0      | 2001-02-08 00:00:00 |
|  8 | 程咬金 | 17799990007 | [e-mail]   | 化工                 |   38 | 1      | 5      | 2001-05-23 00:00:00 |
|  9 | 项羽   | 17799990008 | [e-mail]      | 金属材料             |   43 | 1      | 0      | 2001-09-18 00:00:00 |
| 10 | 白起   | 17799990009 | [e-mail]     | 机械工程及其自动
化 |   27 | 1      | 2      | 2001-08-16 00:00:00 |
| 11 | 韩信   | 17799990010 | [e-mail]     | 无机非金属材料工
程 |   27 | 1      | 0      | 2001-06-12 00:00:00 |
| 12 | 荆轲   | 17799990011 | [e-mail]     | 会计                 |   29 | 1      | 0      | 2001-05-11 00:00:00 |
| 13 | 兰陵王 | 17799990012 | [e-mail] | 工程造价             |   44 | 1      | 1      | 2001-04-09 00:00:00 |
| 14 | 狂铁   | 17799990013 | [e-mail]     | 应用数学             |   43 | 1      | 2      | 2001-04-10 00:00:00 |
| 15 | 貂蝉   | 17799990014 | [e-mail]    | 软件工程             |   40 | 2      | 3      | 2001-02-12 00:00:00 |
| 16 | 妲己   | 17799990015 | [e-mail]     | 软件工程             |   31 | 2      | 0      | 2001-01-30 00:00:00 |
| 17 | 芈月   | 17799990016 | [e-mail]  | 工业经济             |   35 | 2      | 0      | 2000-05-03 00:00:00 |
| 18 | 嬴政   | 17799990017 | [e-mail]     | 化工                 |   38 | 1      | 1      | 2001-08-08 00:00:00 |
| 19 | 狄仁杰 | 17799990018 | [e-mail]  | 国际贸易             |   30 | 1      | 0      | 2007-03-12 00:00:00 |
| 20 | 安琪拉 | 17799990019 | [e-mail]       | 城市规划             |   51 | 2      | 0      | 2001-08-15 00:00:00 |
| 21 | 典韦   | 17799990020 | [e-mail]   | 城市规划             |   52 | 1      | 2      | 2000-04-12 00:00:00 |
| 22 | 廉颇   | 17799990021 | [e-mail]     | 土木工程             |   19 | 1      | 3      | 2002-07-18 00:00:00 |
| 23 | 后羿   | 17799990022 | [e-mail]    | 城市园林             |   20 | 1      | 0      | 2002-03-10 00:00:00 |
| 24 | 姜子牙 | 17799990023 | [e-mail]       | 工程造价             |   29 | 1      | 4      | 2003-05-26 00:00:00 |
+----+--------+-------------+-----------------------+----------------------+------+--------+--------+---------------------+
24 rows in set (0.00 sec)mysql>

4、查询表的记录总数

mysql> select count(*) from tb_user;
+----------+
| count(*) |
+----------+
|       24 |
+----------+
1 row in set (0.00 sec)mysql>

5、计算并返回具有电子邮件地址（email）的用户的数量

mysql> select count(email) from tb_user;
+--------------+
| count(email) |
+--------------+
|           24 |
+--------------+
1 row in set (0.00 sec)mysql>

6、从tb_user表中计算并返回具有不同电子邮件地址的用户的数量

mysql> select count(distinct email) from tb_user;
+-----------------------+
| count(distinct email) |
+-----------------------+
|                    24 |
+-----------------------+
1 row in set (0.00 sec)mysql>

7、计算唯一电子邮件地址（email）的比例相对于表中的总行数

mysql> select count(distinct email)/count(*) from tb_user;
+--------------------------------+
| count(distinct email)/count(*) |
+--------------------------------+
|                         1.0000 |
+--------------------------------+
1 row in set (0.00 sec)mysql>

• 其中1表示所有用户都有唯一的电子邮件地址，而0表示没有用户有电子邮件地址（尽管这在现实中不太可能）。
• 用来衡量 email 字段的去重比例，即表示不重复的 email 占总记录数的比例。
• 用来评估数据中电子邮件地址的唯一性程度，或者说检测是否存在大量的重复邮箱账户。如果结果接近1，说明几乎每个行都有一个唯一的电子邮件地址；如果远小于1，则表示有较多的电子邮件地址重复。

8、从每个电子邮件地址中提取前10个字符，并计算这些前10个字符唯一值的数量与总用户数量的比率。

mysql> select count(distinct substring(email,1,10))/count(*) from tb_user;
+------------------------------------------------+
| count(distinct substring(email,1,10))/count(*) |
+------------------------------------------------+
|                                         1.0000 |
+------------------------------------------------+
1 row in set (0.00 sec)mysql>

9、电子邮件地址的前9个字符的唯一值的数量与总用户数量的比率

mysql> select count(distinct substring(email,1,9))/count(*) from tb_user;
+-----------------------------------------------+
| count(distinct substring(email,1,9))/count(*) |
+-----------------------------------------------+
|                                        0.9583 |
+-----------------------------------------------+
1 row in set (0.00 sec)mysql>

10、电子邮件地址的前8个字符与前9个字符在唯一性方面的表现是相似的

mysql> select count(distinct substring(email,1,8))/count(*) from tb_user;
+-----------------------------------------------+
| count(distinct substring(email,1,8))/count(*) |
+-----------------------------------------------+
|                                        0.9583 |
+-----------------------------------------------+
1 row in set (0.00 sec)mysql>

11、前 6 个字符的不重复数量占总行数的比例

mysql> select count(distinct substring(email,1,6))/count(*) from tb_user;
+-----------------------------------------------+
| count(distinct substring(email,1,6))/count(*) |
+-----------------------------------------------+
|                                        0.9583 |
+-----------------------------------------------+
1 row in set (0.00 sec)mysql>

12、前 5 个字符的不重复数量占总行数的比例

mysql> select count(distinct substring(email,1,5))/count(*) from tb_user;
+-----------------------------------------------+
| count(distinct substring(email,1,5))/count(*) |
+-----------------------------------------------+
|                                        0.9583 |
+-----------------------------------------------+
1 row in set (0.00 sec)mysql>

13、随着截取长度的减少，电子邮件地址前缀的唯一性也在减少

mysql> select count(distinct substring(email,1,4))/count(*) from tb_user;
+-----------------------------------------------+
| count(distinct substring(email,1,4))/count(*) |
+-----------------------------------------------+
|                                        0.9167 |
+-----------------------------------------------+
1 row in set (0.00 sec)mysql>

14、查看MySQL中tb_user表的索引

mysql> show index from tb_user;
+---------+------------+----------------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+---------+------------+
| Table   | Non_unique | Key_name             | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | Index_comment | Visible | Expression |
+---------+------------+----------------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+---------+------------+
| tb_user |          0 | PRIMARY              |            1 | id          | A         |          24 |     NULL |   NULL |      | BTREE      |         |               | YES     | NULL       |
| tb_user |          1 | idx_user_pro_age_sta |            1 | profession  | A         |          16 |     NULL |   NULL | YES  | BTREE      |         |               | YES     | NULL       |
| tb_user |          1 | idx_user_pro_age_sta |            2 | age         | A         |          22 |     NULL |   NULL | YES  | BTREE      |         |               | YES     | NULL       |
| tb_user |          1 | idx_user_pro_age_sta |            3 | status      | A         |          24 |     NULL |   NULL | YES  | BTREE      |         |               | YES     | NULL       |
+---------+------------+----------------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+---------+------------+
4 rows in set (0.01 sec)mysql>

15、在tb_user表的email列上创建一个前缀索引，其中只包括email列的前5个字符

mysql> create index idx_email_5 on tb_user(email(5));
Query OK, 0 rows affected (0.05 sec)
Records: 0  Duplicates: 0  Warnings: 0mysql> show index from tb_user;
+---------+------------+----------------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+---------+------------+
| Table   | Non_unique | Key_name             | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | Index_comment | Visible | Expression |
+---------+------------+----------------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+---------+------------+
| tb_user |          0 | PRIMARY              |            1 | id          | A         |          24 |     NULL |   NULL |      | BTREE      |         |               | YES     | NULL       |
| tb_user |          1 | idx_user_pro_age_sta |            1 | profession  | A         |          16 |     NULL |   NULL | YES  | BTREE      |         |               | YES     | NULL       |
| tb_user |          1 | idx_user_pro_age_sta |            2 | age         | A         |          22 |     NULL |   NULL | YES  | BTREE      |         |               | YES     | NULL       |
| tb_user |          1 | idx_user_pro_age_sta |            3 | status      | A         |          24 |     NULL |   NULL | YES  | BTREE      |         |               | YES     | NULL       |
| tb_user |          1 | idx_email_5          |            1 | email       | A         |          23 |        5 |   NULL | YES  | BTREE      |         |               | YES     | NULL       |
+---------+------------+----------------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+---------+------------+
5 rows in set (0.01 sec)mysql>

16、查询 email=‘[e-mail]’ 的用户

mysql> select * from tb_user where email='[e-mail]';
+----+------+-------------+--------------------+------------+------+--------+--------+---------------------+
| id | name | phone       | email              | profession | age  | gender | status | createtime          |
+----+------+-------------+--------------------+------------+------+--------+--------+---------------------+
|  6 | 大乔 | 17799990005 | [e-mail] | 舞蹈       |   22 | 2      | 0      | 2001-02-07 00:00:00 |
+----+------+-------------+--------------------+------------+------+--------+--------+---------------------+
1 row in set (0.00 sec)mysql>

17、执行计划 email=‘[e-mail]’

mysql> explain select * from tb_user where email='[e-mail]';
+----+-------------+---------+------------+------+---------------+-------------+---------+-------+------+----------+-------------+
| id | select_type | table   | partitions | type | possible_keys | key         | key_len | ref   | rows | filtered | Extra       |
+----+-------------+---------+------------+------+---------------+-------------+---------+-------+------+----------+-------------+
|  1 | SIMPLE      | tb_user | NULL       | ref  | idx_email_5   | idx_email_5 | 23      | const |    1 |   100.00 | Using where |
+----+-------------+---------+------------+------+---------------+-------------+---------+-------+------+----------+-------------+
1 row in set, 1 warning (0.00 sec)mysql>