MySQL使用不到索引的情況有很多，今天具體操作了一番，總結了些常見的情況

建立以下表，有四個欄位，其中name和password做了關聯索引，id為主鍵索引

mysql> show create table users \G
*************************** 1. row ***************************
       Table: users
Create Table: CREATE TABLE `users` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `name` varchar(255) DEFAULT NULL,
  `password` varchar(255) DEFAULT NULL,
  `email` varchar(255) DEFAULT NULL,
  PRIMARY KEY (`id`),
  KEY `loginIndex` (`name`,`password`)
) ENGINE=InnoDB AUTO_INCREMENT=6428544 DEFAULT CHARSET=utf8
1 row in set (0.00 sec)
 

mysql>  explain select * from users where name like '%abc' \G
*************************** 1. row ***************************
           id: 1
  select_type: SIMPLE
        table: users
         type: ALL
possible_keys: NULL
          key: NULL
      key_len: NULL
          ref: NULL
         rows: 6257258
        Extra: Using where
1 row in set (0.00 sec)
 

mysql> explain select * from users where password = '123' \G
*************************** 1. row ***************************
           id: 1
  select_type: SIMPLE
        table: users
         type: ALL
possible_keys: NULL
          key: NULL
      key_len: NULL
          ref: NULL
         rows: 6257258
        Extra: Using where
1 row in set (0.00 sec)
 

mysql> explain select * from users where name != 'aaa' \G
*************************** 1. row ***************************
           id: 1
  select_type: SIMPLE
        table: users
         type: ALL
possible_keys: loginIndex
          key: NULL
      key_len: NULL
          ref: NULL
         rows: 6257258
        Extra: Using where
1 row in set (0.04 sec)

mysql> explain select * from users where name not in ('aaa', 'bbb', 'ccc') \G
*************************** 1. row ***************************
           id: 1
  select_type: SIMPLE
        table: users
         type: ALL
possible_keys: loginIndex
          key: NULL
      key_len: NULL
          ref: NULL
         rows: 6257258
        Extra: Using where
1 row in set (0.04 sec)

mysql> explain select * from users where name = 123456 \G
*************************** 1. row ***************************
           id: 1
  select_type: SIMPLE
        table: users
         type: ALL
possible_keys: loginIndex
          key: NULL
      key_len: NULL
          ref: NULL
         rows: 6257258
        Extra: Using where
1 row in set (0.00 sec)

mysql> explain select * from users where name = 'a' or password = 'b' \G
*************************** 1. row ***************************
           id: 1
  select_type: SIMPLE
        table: users
         type: ALL
possible_keys: loginIndex
          key: NULL
      key_len: NULL
          ref: NULL
         rows: 6257258
        Extra: Using where
1 row in set (0.00 sec)