MySQL索引的学习和研究

这里所谈论只针对B-Tree类型索引，也是MySQL用的最多最普通的索引。创建索引的时候是按照字面量的顺序创建的，这个要特别注意。在B-Tree类型索引中，索引顺序是至关重要的。

高性能索引

索引要小而美

单列索引

前缀索引

多列索引

选择合适的索引顺序

覆盖索引

使用索引扫描来排序

首先构造1亿条数据

参照前面写的文章造数据

http://blog.csdn.net/csujiangyu/article/details/51096978

索引结构如下

mysql> show index from user;
+-------+------------+-------------------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| Table | Non_unique | Key_name                | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | Index_comment |
+-------+------------+-------------------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| user  |          0 | PRIMARY                 |            1 | id          | A         |    92123632 |     NULL | NULL   |      | BTREE      |         |               |
| user  |          1 | idx_last_first_name_age |            1 | last_name   | A         |      363474 |     NULL | NULL   | YES  | BTREE      |         |               |
| user  |          1 | idx_last_first_name_age |            2 | first_name  | A         |    92123840 |     NULL | NULL   | YES  | BTREE      |         |               |
| user  |          1 | idx_last_first_name_age |            3 | age         | A         |    87360984 |     NULL | NULL   | YES  | BTREE      |         |               |
| user  |          1 | idx_phone               |            1 | phone       | A         |    92123840 |     NULL | NULL   | YES  | BTREE      |         |               |
| user  |          1 | idx_create_time         |            1 | create_time | A         |       88503 |     NULL | NULL   | YES  | BTREE      |         |               |
+-------+------------+-------------------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
6 rows in set (0.00 sec)

user表一共有4个索引，包括一个主键id，2个单列索引(phone, create_time), 1个多列索引(last_name+first_name+age)

通过explain来查看使用索引情况

主要关注如下：

type：访问类型，性能从差到好为：

ALL -> index -> range -> ref -> eq_ref -> const,system -> NULL

index：使用的索引

可以使用索引的情况

单列索引全值匹配

这是最简单的方式

mysql> explain SELECT * from user where phone = '13901986429';
+----+-------------+-------+------------+------+---------------+-----------+---------+-------+------+----------+-------+
| id | select_type | table | partitions | type | possible_keys | key       | key_len | ref   | rows | filtered | Extra |
+----+-------------+-------+------------+------+---------------+-----------+---------+-------+------+----------+-------+
|  1 | SIMPLE      | user  | NULL       | ref  | idx_phone     | idx_phone | 14      | const |    1 |   100.00 | NULL  |
+----+-------------+-------+------------+------+---------------+-----------+---------+-------+------+----------+-------+
1 row in set, 1 warning (0.01 sec)

从type可以得知访问类型ref，使用索引key为idx_phone。

多列索引全值匹配

mysql> explain SELECT * FROM  user where last_name='last_name_110' and first_name='first_name_115' and sex='F';
+----+-------------+-------+------------+------+-------------------------+-------------------------+---------+-------------+------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys           | key                     | key_len | ref         | rows | filtered | Extra       |
+----+-------------+-------+------------+------+-------------------------+-------------------------+---------+-------------+------+----------+-------------+
|  1 | SIMPLE      | user  | NULL       | ref  | idx_last_first_name_age | idx_last_first_name_age | 96      | const,const |    1 |    10.00 | Using where |
+----+-------------+-------+------------+------+-------------------------+-------------------------+---------+-------------+------+----------+-------------+
1 row in set, 1 warning (0.00 sec)

前缀索引

如果是单列索引，可以匹配列的最左前缀

mysql> explain SELECT * from user where phone like '139019864%';
+----+-------------+-------+------------+-------+---------------+-----------+---------+------+------+----------+-----------------------+
| id | select_type | table | partitions | type  | possible_keys | key       | key_len | ref  | rows | filtered | Extra                 |
+----+-------------+-------+------------+-------+---------------+-----------+---------+------+------+----------+-----------------------+
|  1 | SIMPLE      | user  | NULL       | range | idx_phone     | idx_phone | 14      | NULL |  100 |   100.00 | Using index condition |
+----+-------------+-------+------------+-------+---------------+-----------+---------+------+------+----------+-----------------------+
1 row in set, 1 warning (0.01 sec)

如果是多列索引，可以从左开始组合索引

mysql> explain SELECT * FROM  user where last_name='last_name_110' and first_name='first_name_115';
+----+-------------+-------+------------+------+-------------------------+-------------------------+---------+-------------+------+----------+-------+
| id | select_type | table | partitions | type | possible_keys           | key                     | key_len | ref         | rows | filtered | Extra |
+----+-------------+-------+------------+------+-------------------------+-------------------------+---------+-------------+------+----------+-------+
|  1 | SIMPLE      | user  | NULL       | ref  | idx_last_first_name_age | idx_last_first_name_age | 96      | const,const |    1 |   100.00 | NULL  |
+----+-------------+-------+------------+------+-------------------------+-------------------------+---------+-------------+------+----------+-------+
1 row in set, 1 warning (0.01 sec)

匹配索引值的范围

使用between

mysql> explain select * from user where id between 1 and 10;
+----+-------------+-------+------------+-------+---------------+---------+---------+------+------+----------+-------------+
| id | select_type | table | partitions | type  | possible_keys | key     | key_len | ref  | rows | filtered | Extra       |
+----+-------------+-------+------------+-------+---------------+---------+---------+------+------+----------+-------------+
|  1 | SIMPLE      | user  | NULL       | range | PRIMARY       | PRIMARY | 4       | NULL |   10 |   100.00 | Using where |
+----+-------------+-------+------------+-------+---------------+---------+---------+------+------+----------+-------------+
1 row in set, 1 warning (0.00 sec)

使用in

mysql> explain select * from user where id in (1, 2, 3);
+----+-------------+-------+------------+-------+---------------+---------+---------+------+------+----------+-------------+
| id | select_type | table | partitions | type  | possible_keys | key     | key_len | ref  | rows | filtered | Extra       |
+----+-------------+-------+------------+-------+---------------+---------+---------+------+------+----------+-------------+
|  1 | SIMPLE      | user  | NULL       | range | PRIMARY       | PRIMARY | 4       | NULL |    3 |   100.00 | Using where |
+----+-------------+-------+------------+-------+---------------+---------+---------+------+------+----------+-------------+
1 row in set, 1 warning (0.01 sec)

使用or

mysql> explain select * from user where id=1 or id=10;
+----+-------------+-------+------------+-------+---------------+---------+---------+------+------+----------+-------------+
| id | select_type | table | partitions | type  | possible_keys | key     | key_len | ref  | rows | filtered | Extra       |
+----+-------------+-------+------------+-------+---------------+---------+---------+------+------+----------+-------------+
|  1 | SIMPLE      | user  | NULL       | range | PRIMARY       | PRIMARY | 4       | NULL |    2 |   100.00 | Using where |
+----+-------------+-------+------------+-------+---------------+---------+---------+------+------+----------+-------------+
1 row in set, 1 warning (0.00 sec)

覆盖索引

Extra字段为Using index，可以看出返回数据只需要查索引即可，不需要回表查其他字段数据。

mysql> explain select phone from user where phone='13900123456';
+----+-------------+-------+------------+------+---------------+-----------+---------+-------+------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key       | key_len | ref   | rows | filtered | Extra       |
+----+-------------+-------+------------+------+---------------+-----------+---------+-------+------+----------+-------------+
|  1 | SIMPLE      | user  | NULL       | ref  | idx_phone     | idx_phone | 14      | const |    1 |   100.00 | Using index |
+----+-------------+-------+------------+------+---------------+-----------+---------+-------+------+----------+-------------+
1 row in set, 1 warning (0.00 sec)

不能使用索引的情况

从type为ALL可以看出进行了全表扫描。

没有使用左前缀索引

phone是索引列，格式为’%XXX’不能是使用索引

mysql> explain SELECT * from user where phone like '%39019864%';
+----+-------------+-------+------------+------+---------------+------+---------+------+---------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key  | key_len | ref  | rows    | filtered | Extra       |
+----+-------------+-------+------------+------+---------------+------+---------+------+---------+----------+-------------+
|  1 | SIMPLE      | user  | NULL       | ALL  | NULL          | NULL | NULL    | NULL | 9698255 |    11.11 | Using where |
+----+-------------+-------+------------+------+---------------+------+---------+------+---------+----------+-------------+
1 row in set, 1 warning (0.00 sec)

没有采用最左组合索引

last_name + first_name + sex联合组成了多列索引，采用最左匹配， 只有last_name，last_name + first_name，last_name + first_name + sex才能使用索引。

mysql> explain SELECT * FROM  user where first_name='first_name_115' and sex='F';
+----+-------------+-------+------------+------+---------------+------+---------+------+---------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key  | key_len | ref  | rows    | filtered | Extra       |
+----+-------------+-------+------------+------+---------------+------+---------+------+---------+----------+-------------+
|  1 | SIMPLE      | user  | NULL       | ALL  | NULL          | NULL | NULL    | NULL | 9698255 |     1.00 | Using where |
+----+-------------+-------+------------+------+---------------+------+---------+------+---------+----------+-------------+
1 row in set, 1 warning (0.00 sec)

使用函数

mysql> explain SELECT * FROM  user where left(phone, 11) = '13900123456';
+----+-------------+-------+------------+------+---------------+------+---------+------+---------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key  | key_len | ref  | rows    | filtered | Extra       |
+----+-------------+-------+------------+------+---------------+------+---------+------+---------+----------+-------------+
|  1 | SIMPLE      | user  | NULL       | ALL  | NULL          | NULL | NULL    | NULL | 9698255 |   100.00 | Using where |
+----+-------------+-------+------------+------+---------------+------+---------+------+---------+----------+-------------+
1 row in set, 1 warning (0.00 sec)

用索引字段进行计算

mysql> explain select * from user where id + 1 = 2;
+----+-------------+-------+------------+------+---------------+------+---------+------+----------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key  | key_len | ref  | rows     | filtered | Extra       |
+----+-------------+-------+------------+------+---------------+------+---------+------+----------+----------+-------------+
|  1 | SIMPLE      | user  | NULL       | ALL  | NULL          | NULL | NULL    | NULL | 92123838 |   100.00 | Using where |
+----+-------------+-------+------------+------+---------------+------+---------+------+----------+----------+-------------+
1 row in set, 1 warning (0.00 sec)

or后面没有使用索引

mysql> explain select * from user where id=1 or age=10;
+----+-------------+-------+------------+------+---------------+------+---------+------+----------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key  | key_len | ref  | rows     | filtered | Extra       |
+----+-------------+-------+------------+------+---------------+------+---------+------+----------+----------+-------------+
|  1 | SIMPLE      | user  | NULL       | ALL  | PRIMARY       | NULL | NULL    | NULL | 92123838 |    10.00 | Using where |
+----+-------------+-------+------------+------+---------------+------+---------+------+----------+----------+-------------+
1 row in set, 1 warning (0.00 sec)

索引和数据占用空间的查询

-- 查看example数据库整体大小
SELECT CONCAT(table_schema,'.',table_name) AS 'Table Name', CONCAT(ROUND(table_rows/1000000,4),'M') AS 'Number of Rows', CONCAT(ROUND(data_length/(1024*1024*1024),4),'G') AS 'Data Size', CONCAT(ROUND(index_length/(1024*1024*1024),4),'G') AS 'Index Size', CONCAT(ROUND((data_length+index_length)/(1024*1024*1024),4),'G') AS'Total'FROM information_schema.TABLES WHERE table_schema = 'example';

从图可以看出，1亿条数据索引占用了9.69G，数据占用了10G，总共占用了将近20G。索引占用空间几乎和数据大小一样，因此我们需要慎用索引，在必要的情况下才使用，索引要小而美。