MySQL 性能分析 之 聯合索引(複合索引)實踐分析

MySQL 性能分析 之 聯合索引(複合索引)實踐分析

做爲開發者，你們都知道，一個服務器、一個數據庫的性能是項目的重中之重，後臺架構、寫法與數據庫設計的好壞每每直接影響到整個項目的性能。

索引:是當你的業務完成後，跟據查詢條件來創建的。當你的數據量大（通常是10萬條數據）了以後，咱們會再把普通索引刪除，使用自建索引表。由於數據量大的時候你要批量修改（索引表也會修改）會變的很是的慢！

這裏給分析一下MySQL的索引；索引分：普通索引和聯合索引，而索引的關鍵相信不少人也知道，用非主鍵字段進行查詢的時候MySQL在查詢的時候就是掃錶行爲，若是有索引的話，狀況就獲得了大大的改善。

加索引的時候，先建議使用單列索引一個一個加！而後再改進使用聯合索引！並且最好是根據本身的項目業務查詢來統計 哪些字段的使用率是多少 哪些字段組合在一次使用了多少次，由於今天分析的就是聯合索引並非在任何狀況下都命中的

表結構

mysql> show create table m_user\G;  
*************************** 1. row ***************************  Table: m_user Create Table: CREATE TABLE `m_user` ( `id` int(10) unsigned NOT NULL AUTO_INCREMENT, `name` char(32) NOT NULL, `age` tinyint(4) NOT NULL, `school` char(128) NOT NULL, `status` tinyint(4) NOT NULL DEFAULT '1', PRIMARY KEY (`id`), KEY `name` (`name`,`age`,`status`) ) ENGINE=InnoDB AUTO_INCREMENT=1001 DEFAULT CHARSET=utf8 1 row in set (0.00 sec) 

聯合索引字段： 
KEY name (name,age,status)

模擬了數據後 咱們來查看結果

• [1 3 命中]

select * from m_user where name='leal' and status=1

mysql> desc select * from m_user where name='leal' and status=1 \G;  
*************************** 1. row ***************************  id: 1 select_type: SIMPLE  table: m_user partitions: NULL  type: ref possible_keys: name  key: name  key_len: 96  ref: const  rows: 1  filtered: 10.00  Extra: Using index condition 1 row in set, 1 warning (0.00 sec) 

• [1 3 order by 3 命中]

select * from m_user where name='leal' and status=1 order by status desc

mysql> desc select * from m_user where name='leal' and status=1 order by status desc \G;  
*************************** 1. row ***************************  id: 1 select_type: SIMPLE  table: m_user partitions: NULL  type: ref possible_keys: name  key: name  key_len: 96  ref: const  rows: 1  filtered: 10.00  Extra: Using index condition 1 row in set, 1 warning (0.00 sec) 

• [2 3 不命中]

select * from m_user where age=10 and status=1

mysql> desc select * from m_user where age=10 and status=1 \G;  
*************************** 1. row ***************************  id: 1 select_type: SIMPLE  table: m_user partitions: NULL  type: ALL possible_keys: NULL  key: NULL  key_len: NULL  ref: NULL  rows: 1000  filtered: 1.00  Extra: Using where 1 row in set, 1 warning (0.00 sec) 

• [1 in 命中]

select * from m_user where name in ('leal') and age<10

select * from m_user where name in ('leal') and age<10 order by school

mysql> desc select * from m_user where name in ('leal') and age<10 order by school *************************** 1. row *************************** id: 1 select_type: SIMPLE table: m_user partitions: NULL type: range possible_keys: name key: name key_len: 97 ref: NULL rows: 1 filtered: 100.00 Extra: Using index condition; Using filesort 1 row in set, 1 warning (0.00 sec) 

• [1 between 不命中]

select * from m_user where name between 'leal' and 'tom'

select * from m_user where name between 'leal' and 'tom' order by school desc

mysql> desc select * from m_user where name between 'leal' and  'tom' order by school desc \G;  
*************************** 1. row ***************************  id: 1 select_type: SIMPLE  table: m_user partitions: NULL  type: ALL possible_keys: name  key: NULL  key_len: NULL  ref: NULL  rows: 1000  filtered: 22.38  Extra: Using where; Using filesort 1 row in set, 1 warning (0.00 sec) 

• [1 <> 不命中]

select * from m_user where name<>'leal'

select * from m_user where name<>'leal' and age<10

select * from m_user where name<>'leal' and age<10 order by school desc

mysql> desc select * from m_user where name<>'leal' and age<10 order by school desc \G; *************************** 1. row *************************** id: 1 select_type: SIMPLE table: m_user partitions: NULL type: ALL possible_keys: name key: NULL key_len: NULL ref: NULL rows: 1000 filtered: 33.30 Extra: Using where; Using filesort 1 row in set, 1 warning (0.00 sec) -------------------------------------------------------------- mysql> desc select * from m_user where name<>'leal'\G; *************************** 1. row *************************** id: 1 select_type: SIMPLE table: m_user partitions: NULL type: ALL possible_keys: name key: NULL key_len: NULL ref: NULL rows: 1000 filtered: 99.90 Extra: Using where 1 row in set, 1 warning (0.00 sec) 

• [1 < 或 <= 命中]

select * from m_user where name < 'leal'

select * from m_user where name <= 'leal'

select * from m_user where name <= 'leal' and age<10

select * from m_user where name <= 'leal' and age<10 order by school desc

mysql> desc select * from m_user where name < 'leal' and age<10 order by school desc \G; *************************** 1. row *************************** id: 1 select_type: SIMPLE table: m_user partitions: NULL type: range possible_keys: name key: name key_len: 96 ref: NULL rows: 1 filtered: 33.33 Extra: Using index condition; Using filesort 1 row in set, 1 warning (0.00 sec) 

• [1 > 或 >= 不命中]

select * from m_user where name>'leal'

select * from m_user where name>='leal'

mysql> desc select * from m_user where name >= 'leal' and age<10 order by school desc \G; *************************** 1. row *************************** id: 1 select_type: SIMPLE table: m_user partitions: NULL type: ALL possible_keys: name key: NULL key_len: NULL ref: NULL rows: 1000 filtered: 33.30 Extra: Using where; Using filesort 1 row in set, 1 warning (0.00 sec) 

• [無where條件 直接order by 不命中]

select * from m_user order by name desc

mysql> explain select * from m_user order by name desc\G;  
*************************** 1. row ***************************  id: 1 select_type: SIMPLE  table: m_user partitions: NULL  type: ALL possible_keys: NULL  key: NULL  key_len: NULL  ref: NULL  rows: 1000  filtered: 100.00  Extra: Using filesort 1 row in set, 1 warning (0.13 sec) 

結束語

通常人和一個項目都是從小到大逐漸健壯的，前期通常的項目 直接加索引就夠了，保證命中率；當項目業務複雜到必定程度或者負載到必定程度的時候，就得刪除索引，自建索引表來管理數據庫的索引了。

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