mysql慢查询优化实例------建立索引

    我们都知道， 很多时候， 要对数据库增加索引， 可以提升查询速度， 有兴趣的可以看看数据库索引原理， 本文来实际测试一下。
       测试数据库记录大概10000条， 先看看没有索引的情况。 

       表结构为：

[sql]  view plain  copy

1. mysql> show create table tb_test;                              
2. +---------+------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+    
3. | Table   | Create Table                                                                                                                                                                                                                                                                                                                 |    
4. +---------+------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+    
5. | tb_test | CREATE TABLE `tb_test` (    
6.   `id` int(10) unsigned NOT NULL AUTO_INCREMENT COMMENT 'test',    
7.   `name` varchar(32) NOT NULL COMMENT 'test',    
8.   `score` int(10) unsigned NOT NULL DEFAULT '0' COMMENT 'test',    
9.   PRIMARY KEY (`id`)    
10. ) ENGINE=InnoDB AUTO_INCREMENT=10004 DEFAULT CHARSET=utf8 COMMENT='测试表， 无实际作用'          |    
11. +---------+------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+    
12. 1 row in set (0.00 sec)    

       开启profiling(命令是set profiling=1;),  然后进行两次 select操作， 分析结果如下：

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1. mysql> show profiles;                              
2. +----------+------------+----------------------------------------------------+    
3. | Query_ID | Duration   | Query                                              |    
4. +----------+------------+----------------------------------------------------+    
5. |    10021 | 0.00306534 | select * from tb_test where name = 'n1'            |    
6. |    10022 | 0.00298983 | select * from tb_test where score = 1              |    
7. +----------+------------+----------------------------------------------------+    

      

       将name字段设置为索引， 表结构为：

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1. mysql> show create table tb_test;                              
2. +---------+---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+    
3. | Table   | Create Table                                                                                                                                                                                                                                                                                                                                            |    
4. +---------+---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+    
5. | tb_test | CREATE TABLE `tb_test` (    
6.   `id` int(10) unsigned NOT NULL AUTO_INCREMENT COMMENT 'test',    
7.   `name` varchar(32) NOT NULL COMMENT 'test',    
8.   `score` int(10) unsigned NOT NULL DEFAULT '0' COMMENT 'test',    
9.   PRIMARY KEY (`id`),    
10.   KEY `idx_name` (`name`)    
11. ) ENGINE=InnoDB AUTO_INCREMENT=10004 DEFAULT CHARSET=utf8 COMMENT='测试表， 无实际作用'          |    
12. +---------+---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+    
13. 1 row in set (0.00 sec)    

       再次进行两次相同的select操作， 分析结果如下：

[sql]  view plain  copy

1. mysql> show profiles;                              
2. +----------+------------+----------------------------------------------------+    
3. | Query_ID | Duration   | Query                                              |    
4. +----------+------------+----------------------------------------------------+    
5. |    10021 | 0.00306534 | select * from tb_test where name = 'n1'            |    
6. |    10022 | 0.00298983 | select * from tb_test where score = 1              |    
7. |    10023 | 0.00074623 | select * from tb_test where name = 'n1'            |    
8. |    10024 | 0.00298101 | select * from tb_test where score = 1              |    
9. +----------+------------+----------------------------------------------------+    

       可见， Query_ID为10023的那条查询快了很多。

       继续将score字段设置为索引， 表结构为：

[sql]  view plain  copy

1. mysql> show create table tb_test;    
2. +---------+--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+    
3. | Table   | Create Table                                                                                                                                                                                                                                                                                                                                                                         |    
4. +---------+--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+    
5. | tb_test | CREATE TABLE `tb_test` (    
6.   `id` int(10) unsigned NOT NULL AUTO_INCREMENT COMMENT 'test',    
7.   `name` varchar(32) NOT NULL COMMENT 'test',    
8.   `score` int(10) unsigned NOT NULL DEFAULT '0' COMMENT 'test',    
9.   PRIMARY KEY (`id`),    
10.   KEY `idx_name` (`name`),    
11.   KEY `idx_score` (`score`)    
12. ) ENGINE=InnoDB AUTO_INCREMENT=10004 DEFAULT CHARSET=utf8 COMMENT='测试表， 无实际作用'          |    
13. +---------+--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+    
14. 1 row in set (0.00 sec)   

       进行两次相同的select操作， 分析结果如下：

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1. mysql> show profiles;                              
2. +----------+------------+----------------------------------------------------+    
3. | Query_ID | Duration   | Query                                              |    
4. +----------+------------+----------------------------------------------------+    
5. |    10021 | 0.00306534 | select * from tb_test where name = 'n1'            |    
6. |    10022 | 0.00298983 | select * from tb_test where score = 1              |    
7. |    10023 | 0.00074623 | select * from tb_test where name = 'n1'            |    
8. |    10024 | 0.00298101 | select * from tb_test where score = 1              |    
9. |    10027 | 0.00073865 | select * from tb_test where name = 'n1'            |    
10. |    10028 | 0.00072649 | select * from tb_test where score = 1              |    
11. +----------+------------+----------------------------------------------------+    

        可见， Query_ID为10027和10028d的那两条快了很多。



        综上所述： 
        1. 索引能提升查询速度。 当然， 索引也有弊端， 不能任意妄为。
        2. 如上记录仅仅是10000条， 如果记录条数为100万， 那么索引的效果就更能很好体现了。