mysql 覆盖索引

覆盖索引定义 http://zhangzhibinshuai.diandian.com/post/2012-03-31/14761710

测试： 

表结构 表中数据只有5行

mysql> desc aa;
+-------+-------------+------+-----+---------+-------+
| Field | Type        | Null | Key | Default | Extra |
+-------+-------------+------+-----+---------+-------+
| id    | int(11)     | YES  |     | NULL    |       |
| name  | varchar(16) | YES  |     | NULL    |       |
| other | varchar(16) | YES  |     | NULL    |       |
+-------+-------------+------+-----+---------+-------+

+------+--------+--------+
| id   | name   | other  |
+------+--------+--------+
|    1 | asdf   | dfdfd  |
|    2 | sdf    | fdfd   |
|    3 | dfasdf | dfdasd |
|    4 | fasdf  | fdasd  |
|    5 | asdf   | dasd   |
+------+--------+--------+

 在id 和 name 上建立联合索引后 第一种查询用到了覆盖索引(Extra using index)(select name ... where id)

mysql> create index aa_id_name on aa(id,name);
Query OK, 5 rows affected (0.14 sec)
Records: 5  Duplicates: 0  Warnings: 0

mysql> explain select name from aa where id = 5\G
*************************** 1. row ***************************
           id: 1
  select_type: SIMPLE
        table: aa
         type: ref
possible_keys: aa_id_name
          key: aa_id_name
      key_len: 5
          ref: const
         rows: 1
        Extra: Using where; Using index
1 row in set (0.00 sec)

mysql> explain select other from aa where id = 5\G
*************************** 1. row ***************************
           id: 1
  select_type: SIMPLE
        table: aa
         type: ref
possible_keys: aa_id_name
          key: aa_id_name
      key_len: 5
          ref: const
         rows: 1
        Extra: Using where
1 row in set (0.00 sec)

 创建id 和 name联合索引后的表结构：

mysql> desc aa;
+-------+-------------+------+-----+---------+-------+
| Field | Type        | Null | Key | Default | Extra |
+-------+-------------+------+-----+---------+-------+
| id    | int(11)     | YES  | MUL | NULL    |       |
| name  | varchar(16) | YES  |     | NULL    |       |
| other | varchar(16) | YES  |     | NULL    |       |
+-------+-------------+------+-----+---------+-------+

 索引顺序是id->name

①查询时将name 放到where条件中（select id ... where name），根据下边信息知道

http://www.chenyajun.com/2009/01/02/1581

mysql> explain select id from aa where name = 'asdf'\G
*************************** 1. row ***************************
           id: 1
  select_type: SIMPLE
        table: aa
         type: index
possible_keys: NULL　　　　　　//根据sql语句mysql根据最左前缀规则是用不到索引的
          key: aa_id_name 　　//mysql优化时会找一个覆盖索引来进行查询
      key_len: 24
          ref: NULL
         rows: 5
        Extra: Using where; Using index

② 当然下边两条sql执行计划信息表明这条第一条SQL语句肯定没有用到索引（select other ... where name 和select other ... where id）

mysql> explain select other from aa where name = 'asdf'\G

*************************** 1. row ***************************
           id: 1
  select_type: SIMPLE
        table: aa
         type: ALL
possible_keys: NULL
          key: NULL
      key_len: NULL
          ref: NULL
         rows: 5
        Extra: Using where
1 row in set (0.20 sec)

mysql> explain select other from aa where id = 5\G
*************************** 1. row ***************************
           id: 1
  select_type: SIMPLE
        table: aa
         type: ref
possible_keys: aa_id_name
          key: aa_id_name
      key_len: 5
          ref: const
         rows: 1
        Extra: Using where
1 row in set (0.00 sec)