MySQL查询的优化是个老生常谈的问题，方法更是多种多样，其中最直接的就是创建索引.
这里通过一个简单的demo来实际用一下索引，看看索引在百万级别查询中速率的提升效果如何
所需数据可以从我前面的一篇博客中获取:https://www.cnblogs.com/wangbaojun/p/11154515.html
有一张salaries,
查看表结构如下：
mysql> desc salaries; +-----------+---------+------+-----+---------+-------+ | Field | Type | Null | Key | Default | Extra | +-----------+---------+------+-----+---------+-------+ | emp_no | int(11) | NO | PRI | NULL | | | salary | int(11) | NO | | NULL | | | from_date | date | NO | PRI | NULL | | | to_date | date | NO | | NULL | | +-----------+---------+------+-----+---------+-------+
可以看到emp_no，from_date都是PRI（主键索引），这是在这个表中将这两个字段联合起来设置为主键，一张表中还是只能有一个主键
查看表的创建命令：
mysql> show create table salaries; +----------+------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+ | Table | Create Table | +----------+------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+ | salaries | CREATE TABLE `salaries` ( `emp_no` int(11) NOT NULL, `salary` int(11) NOT NULL, `from_date` date NOT NULL, `to_date` date NOT NULL, PRIMARY KEY (`emp_no`,`from_date`), CONSTRAINT `salaries_ibfk_1` FOREIGN KEY (`emp_no`) REFERENCES `employees` (`emp_no`) ON DELETE CASCADE ) ENGINE=InnoDB DEFAULT CHARSET=latin1 | +----------+------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+ 1 row in set (0.00 sec)
注意我标红的地方，将emp_no`,`from_date设置为组合主键,组合索引遵从左前缀原则，查询emp_no，或者查询(`emp_no`,`from_date`)会走索引，但是查from_date不会走索引,可以看一下用explain命令查看:
mysql> explain select * from salaries where emp_no=227694; +----+-------------+----------+------+---------------+---------+---------+-------+------+-------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+----------+------+---------------+---------+---------+-------+------+-------+ | 1 | SIMPLE | salaries | ref | PRIMARY | PRIMARY | 4 | const | 18 | NULL | # key为PRIMARY 走了索引 +----+-------------+----------+------+---------------+---------+---------+-------+------+-------+ 1 row in set (0.01 sec) mysql> explain select * from salaries where from_date = '1986-06-26'; +----+-------------+----------+------+---------------+------+---------+------+---------+-------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+----------+------+---------------+------+---------+------+---------+-------------+ | 1 | SIMPLE | salaries | ALL | NULL | NULL | NULL | NULL | 2838426 | Using where | key为Null，Extra 使用了where，没走索引 +----+-------------+----------+------+---------------+------+---------+------+---------+-------------+ 1 row in set (0.00 sec) mysql> explain select * from salaries where from_date = '1986-06-26' and emp_no=75047; +----+-------------+----------+-------+---------------+---------+---------+-------------+------+-------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+----------+-------+---------------+---------+---------+-------------+------+-------+ | 1 | SIMPLE | salaries | const | PRIMARY | PRIMARY | 7 | const,const | 1 | NULL | # key为PRIMARY 走了索引
+----+-------------+----------+-------+---------------+---------+---------+-------------+------+-------+
1 row in set (0.00 sec)
to_date字段没有设置索引，我们来测试一下加索引前后，该字段查询效率会不会有提升：
未加索引：
mysql> select * from salaries where to_date = '1986-06-26'; +--------+--------+------------+------------+ | emp_no | salary | from_date | to_date | +--------+--------+------------+------------+ | 25676 | 40000 | 1985-06-26 | 1986-06-26 | | 28757 | 40000 | 1985-06-26 | 1986-06-26 | | 30860 | 64620 | 1985-06-26 | 1986-06-26 | | 69209 | 40000 | 1985-06-26 | 1986-06-26 | | 80550 | 45292 | 1985-06-26 | 1986-06-26 | | 91204 | 47553 | 1985-06-26 | 1986-06-26 | | 96140 | 52908 | 1985-06-26 | 1986-06-26 | | 208352 | 42989 | 1985-06-26 | 1986-06-26 | | 213109 | 90133 | 1985-06-26 | 1986-06-26 | | 217498 | 80247 | 1985-06-26 | 1986-06-26 | | 219462 | 83880 | 1985-06-26 | 1986-06-26 | | 223150 | 40000 | 1985-06-26 | 1986-06-26 | | 227694 | 73897 | 1985-06-26 | 1986-06-26 | | 232856 | 73126 | 1985-06-26 | 1986-06-26 | | 237619 | 56982 | 1985-06-26 | 1986-06-26 | | 244087 | 40000 | 1985-06-26 | 1986-06-26 | | 253472 | 72004 | 1985-06-26 | 1986-06-26 | | 257395 | 40000 | 1985-06-26 | 1986-06-26 | | 261811 | 40000 | 1985-06-26 | 1986-06-26 | | 268968 | 40000 | 1985-06-26 | 1986-06-26 | | 269331 | 40000 | 1985-06-26 | 1986-06-26 | | 274805 | 40000 | 1985-06-26 | 1986-06-26 | | 279432 | 74530 | 1985-06-26 | 1986-06-26 | | 285685 | 83198 | 1985-06-26 | 1986-06-26 | | 286745 | 44082 | 1985-06-26 | 1986-06-26 | | 290901 | 49876 | 1985-06-26 | 1986-06-26 | | 400719 | 79168 | 1985-06-26 | 1986-06-26 | | 401448 | 49600 | 1985-06-26 | 1986-06-26 | | 427374 | 40000 | 1985-06-26 | 1986-06-26 | | 432024 | 40000 | 1985-06-26 | 1986-06-26 | | 432654 | 40000 | 1985-06-26 | 1986-06-26 | | 438461 | 44451 | 1985-06-26 | 1986-06-26 | | 446228 | 42733 | 1985-06-26 | 1986-06-26 | | 447391 | 62381 | 1985-06-26 | 1986-06-26 | | 448823 | 40000 | 1985-06-26 | 1986-06-26 | | 452355 | 40000 | 1985-06-26 | 1986-06-26 | | 453590 | 61615 | 1985-06-26 | 1986-06-26 | | 456521 | 40000 | 1985-06-26 | 1986-06-26 | | 464415 | 48955 | 1985-06-26 | 1986-06-26 | | 467901 | 52349 | 1985-06-26 | 1986-06-26 | | 472895 | 40000 | 1985-06-26 | 1986-06-26 | | 476501 | 40000 | 1985-06-26 | 1986-06-26 | | 477079 | 40000 | 1985-06-26 | 1986-06-26 | | 478934 | 55054 | 1985-06-26 | 1986-06-26 | | 480301 | 44177 | 1985-06-26 | 1986-06-26 | | 484507 | 40000 | 1985-06-26 | 1986-06-26 | | 486187 | 40000 | 1985-06-26 | 1986-06-26 | | 491159 | 46034 | 1985-06-26 | 1986-06-26 | | 493154 | 40000 | 1985-06-26 | 1986-06-26 | | 498140 | 81909 | 1985-06-26 | 1986-06-26 | | 498565 | 72853 | 1985-06-26 | 1986-06-26 | +--------+--------+------------+------------+ 51 rows in set (1.08 sec)
用explain分析一下:
mysql> explain select * from salaries where to_date = '1986-06-26'; +----+-------------+----------+------+---------------+------+---------+------+---------+-------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+----------+------+---------------+------+---------+------+---------+-------------+ | 1 | SIMPLE | salaries | ALL | NULL | NULL | NULL | NULL | 2838426 | Using where | # Extra使用where。key为Null，在2838426条数据中找51条记录用时1.08s +----+-------------+----------+------+---------------+------+---------+------+---------+-------------+ 1 row in set (0.00 sec)
为to_date字段加索引：
mysql> create index to_date on salaries(to_date); Query OK, 0 rows affected (5.31 sec) Records: 0 Duplicates: 0 Warnings: 0 创建索引会耗时，索然提升了查询速率，但是更新添加动作会效率降低
现在看一下表结构：
mysql> desc salaries; +-----------+---------+------+-----+---------+-------+ | Field | Type | Null | Key | Default | Extra | +-----------+---------+------+-----+---------+-------+ | emp_no | int(11) | NO | PRI | NULL | | | salary | int(11) | NO | | NULL | | | from_date | date | NO | PRI | NULL | | | to_date | date | NO | MUL | NULL | | MUL表示非唯一索引 +-----------+---------+------+-----+---------+-------+ 4 rows in set (0.00 sec) mysql> show create table salaries; +----------+-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+ | Table | Create Table | +----------+-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+ | salaries | CREATE TABLE `salaries` ( `emp_no` int(11) NOT NULL, `salary` int(11) NOT NULL, `from_date` date NOT NULL, `to_date` date NOT NULL, PRIMARY KEY (`emp_no`,`from_date`), KEY `to_date` (`to_date`), # 创建了索引key为to_date CONSTRAINT `salaries_ibfk_1` FOREIGN KEY (`emp_no`) REFERENCES `employees` (`emp_no`) ON DELETE CASCADE ) ENGINE=InnoDB DEFAULT CHARSET=latin1 | +----------+-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+ 1 row in set (0.00 sec)
再次查询：
mysql> select * from salaries where to_date = '1986-06-26'; +--------+--------+------------+------------+ | emp_no | salary | from_date | to_date | +--------+--------+------------+------------+ | 25676 | 40000 | 1985-06-26 | 1986-06-26 | | 28757 | 40000 | 1985-06-26 | 1986-06-26 | | 30860 | 64620 | 1985-06-26 | 1986-06-26 | | 69209 | 40000 | 1985-06-26 | 1986-06-26 | | 80550 | 45292 | 1985-06-26 | 1986-06-26 | | 91204 | 47553 | 1985-06-26 | 1986-06-26 | | 96140 | 52908 | 1985-06-26 | 1986-06-26 | | 208352 | 42989 | 1985-06-26 | 1986-06-26 | | 213109 | 90133 | 1985-06-26 | 1986-06-26 | | 217498 | 80247 | 1985-06-26 | 1986-06-26 | | 219462 | 83880 | 1985-06-26 | 1986-06-26 | | 223150 | 40000 | 1985-06-26 | 1986-06-26 | | 227694 | 73897 | 1985-06-26 | 1986-06-26 | | 232856 | 73126 | 1985-06-26 | 1986-06-26 | | 237619 | 56982 | 1985-06-26 | 1986-06-26 | | 244087 | 40000 | 1985-06-26 | 1986-06-26 | | 253472 | 72004 | 1985-06-26 | 1986-06-26 | | 257395 | 40000 | 1985-06-26 | 1986-06-26 | | 261811 | 40000 | 1985-06-26 | 1986-06-26 | | 268968 | 40000 | 1985-06-26 | 1986-06-26 | | 269331 | 40000 | 1985-06-26 | 1986-06-26 | | 274805 | 40000 | 1985-06-26 | 1986-06-26 | | 279432 | 74530 | 1985-06-26 | 1986-06-26 | | 285685 | 83198 | 1985-06-26 | 1986-06-26 | | 286745 | 44082 | 1985-06-26 | 1986-06-26 | | 290901 | 49876 | 1985-06-26 | 1986-06-26 | | 400719 | 79168 | 1985-06-26 | 1986-06-26 | | 401448 | 49600 | 1985-06-26 | 1986-06-26 | | 427374 | 40000 | 1985-06-26 | 1986-06-26 | | 432024 | 40000 | 1985-06-26 | 1986-06-26 | | 432654 | 40000 | 1985-06-26 | 1986-06-26 | | 438461 | 44451 | 1985-06-26 | 1986-06-26 | | 446228 | 42733 | 1985-06-26 | 1986-06-26 | | 447391 | 62381 | 1985-06-26 | 1986-06-26 | | 448823 | 40000 | 1985-06-26 | 1986-06-26 | | 452355 | 40000 | 1985-06-26 | 1986-06-26 | | 453590 | 61615 | 1985-06-26 | 1986-06-26 | | 456521 | 40000 | 1985-06-26 | 1986-06-26 | | 464415 | 48955 | 1985-06-26 | 1986-06-26 | | 467901 | 52349 | 1985-06-26 | 1986-06-26 | | 472895 | 40000 | 1985-06-26 | 1986-06-26 | | 476501 | 40000 | 1985-06-26 | 1986-06-26 | | 477079 | 40000 | 1985-06-26 | 1986-06-26 | | 478934 | 55054 | 1985-06-26 | 1986-06-26 | | 480301 | 44177 | 1985-06-26 | 1986-06-26 | | 484507 | 40000 | 1985-06-26 | 1986-06-26 | | 486187 | 40000 | 1985-06-26 | 1986-06-26 | | 491159 | 46034 | 1985-06-26 | 1986-06-26 | | 493154 | 40000 | 1985-06-26 | 1986-06-26 | | 498140 | 81909 | 1985-06-26 | 1986-06-26 | | 498565 | 72853 | 1985-06-26 | 1986-06-26 | +--------+--------+------------+------------+ 51 rows in set (0.00 sec) # 创建索引后同样的查询条件从1.08s变为了0.00s，惊讶吧
explain分析：
mysql> explain select * from salaries where to_date = '1986-06-26'; +----+-------------+----------+------+---------------+---------+---------+-------+------+-------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+----------+------+---------------+---------+---------+-------+------+-------+ | 1 | SIMPLE | salaries | ref | to_date | to_date | 3 | const | 51 | NULL | key从Null变味了索引字段to_date，row从两百多万变为了51 +----+-------------+----------+------+---------------+---------+---------+-------+------+-------+ 1 row in set (0.00 sec)
这个demo从数据上直观的体现了索引带来的查询效率提升有多可观,但是索引也是有利必有害，更多索引的底层知识可以参考这位大牛的博客：https://www.cnblogs.com/Aiapple/p/5693239.html