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。

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