在查询时，如果使用到LIKE关键字，就要注意有没有使用到索引；

1. 没有使用索引前：

   mysql> explain select * from employees where first_name=‘Georgi‘\G;
   *************************** 1. row ***************************
              id: 1
     select_type: SIMPLE
           table: employees
            type: ALL
   possible_keys: NULL
             key: NULL
         key_len: NULL
             ref: NULL
            rows: 294382
           Extra: Using where
   1 row in set (0.00 sec)

2. 添加索引: create index first_index on employees (first_name);

3. 索引后：

    

   mysql> explain select * from employees where first_name=‘Georgi‘\G;
   *************************** 1. row ***************************
              id: 1
     select_type: SIMPLE
           table: employees
            type: ref
   possible_keys: first_index
             key: first_index
         key_len: 16
             ref: const
            rows: 253
           Extra: Using where
   1 row in set (0.00 sec)

4. 查询中使用LIKE：

   mysql> explain select * from employees where first_name like ‘%Georgi‘\G;
   *************************** 1. row ***************************
              id: 1
     select_type: SIMPLE
           table: employees
            type: ALL
   possible_keys: NULL
             key: NULL
         key_len: NULL
             ref: NULL
            rows: 294382
           Extra: Using where
   1 row in set (0.05 sec)

5. mysql> explain select * from employees where first_name like ‘%Georgi%‘\G;
   *************************** 1. row ***************************
              id: 1
     select_type: SIMPLE
           table: employees
            type: ALL
   possible_keys: NULL
             key: NULL
         key_len: NULL
             ref: NULL
            rows: 294382
           Extra: Using where
   1 row in set (0.00 sec)
   

6. mysql> explain select * from employees where first_name like ‘Georgi%‘\G;
   *************************** 1. row ***************************
              id: 1
     select_type: SIMPLE
           table: employees
            type: range
   possible_keys: first_index
             key: first_index
         key_len: 16
             ref: NULL
            rows: 253
           Extra: Using where
   1 row in set (0.00 sec)

可以发现只有在查询关键字后面的加%才可以用到索引；

还有使用了is not null,<>都不可以用到索引:

mysql> explain select * from employees where first_name is not null\G;
*************************** 1. row ***************************
           id: 1
  select_type: SIMPLE
        table: employees
         type: ALL
possible_keys: first_index
          key: NULL
      key_len: NULL
          ref: NULL
         rows: 294382
        Extra: Using where
1 row in set (0.00 sec)

mysql> explain select * from employees where first_name <> ‘Georgi‘\G;
*************************** 1. row ***************************
           id: 1
  select_type: SIMPLE
        table: employees
         type: ALL
possible_keys: first_index
          key: NULL
      key_len: NULL
          ref: NULL
         rows: 294382
        Extra: Using where
1 row in set (0.00 sec)

如果在索引的列上面用了函数或运算都不能引用到索引的：

mysql> explain select * from employees where UPPER(first_name)=‘Ult‘\G;
*************************** 1. row ***************************
           id: 1
  select_type: SIMPLE
        table: employees
         type: ALL
possible_keys: NULL
          key: NULL
      key_len: NULL
          ref: NULL
         rows: 294382
        Extra: Using where
1 row in set (0.00 sec)

在使用复合索引的时候也要注意第一列有没有在WHERE中；

正确使用索引的：

mysql> explain select * from employees where first_name=‘Georgi‘ and last_name=‘
Facello‘\G;
*************************** 1. row ***************************
           id: 1
  select_type: SIMPLE
        table: employees
         type: ref
possible_keys: name_index
          key: name_index
      key_len: 34
          ref: const,const
         rows: 2
        Extra: Using where
1 row in set (0.00 sec)

mysql> explain select * from employees where first_name=‘Georgi‘\G;
*************************** 1. row ***************************
           id: 1
  select_type: SIMPLE
        table: employees
         type: ref
possible_keys: name_index
          key: name_index
      key_len: 16
          ref: const
         rows: 253
        Extra: Using where
1 row in set (0.00 sec)

没有使用到索引的:

mysql> explain select * from employees where last_name=‘Facello‘\G;
*************************** 1. row ***************************
           id: 1
  select_type: SIMPLE
        table: employees
         type: ALL
possible_keys: NULL
          key: NULL
      key_len: NULL
          ref: NULL
         rows: 300252
        Extra: Using where
1 row in set (0.00 sec)

mysql> explain select * from employees where last_name=‘Facello‘ or first_name=‘
Georgi‘\G;
*************************** 1. row ***************************
           id: 1
  select_type: SIMPLE
        table: employees
         type: ALL
possible_keys: name_index
          key: NULL
      key_len: NULL
          ref: NULL
         rows: 300252
        Extra: Using where
1 row in set (0.00 sec)

第二个例子因为用了OR所以WHERE中也只是对first_name单独检索；