mysql explain 查看查詢索引

數據庫查詢分爲兩種，全表掃描，索引掃描。

explain 關鍵字 判斷sql語句是否用到索引

explain select * from grains_resource where os_family="RedHat" and id = 4053;

+----+-------------+-----------------+-------+---------------+---------+---------+-------+------+-------+

| id | select_type | table           | type  | possible_keys | key     | key_len | ref   | rows | Extra |

+----+-------------+-----------------+-------+---------------+---------+---------+-------+------+-------+

|  1 | SIMPLE      | grains_resource | const | PRIMARY       | PRIMARY | 4       | const |    1 |       |

+----+-------------+-----------------+-------+---------------+---------+---------+-------+------+-------+

• type 爲const，意味着經過索引直接找到匹配行，因此優化器認爲它的時間複雜度爲常量。
  

• key爲PRIMARY，意味着此次查詢使用了主鍵索引。

  
  

---

explain select * from grains_resource where os_family="RedHat";

+----+-------------+-----------------+------+---------------+------+---------+------+-------+-------------+

| id | select_type | table           | type | possible_keys | key  | key_len | ref  | rows  | Extra       |

+----+-------------+-----------------+------+---------------+------+---------+------+-------+-------------+

|  1 | SIMPLE      | grains_resource | ALL  | NULL          | NULL | NULL    | NULL | 51752 | Using where |

+----+-------------+-----------------+------+---------------+------+---------+------+-------+-------------+

type是all，表明此次查詢是全表掃描，key爲null 沒用索引。

---

爲os_family字段加入索引

explain select * from grains_resource where os_family="RedHat";

+----+-------------+-----------------+------+---------------+-----------+---------+-------+-------+-------------+

| id | select_type | table           | type | possible_keys | key       | key_len | ref   | rows  | Extra       |

+----+-------------+-----------------+------+---------------+-----------+---------+-------+-------+-------------+

|  1 | SIMPLE      | grains_resource | ref  | os_family     | os_family | 138     | const | 26315 | Using where |

+----+-------------+-----------------+------+---------------+-----------+---------+-------+-------+-------------+

使用了os_family這個索引

---

explain select * from grains_resource where os_family like "%RedHat";

+----+-------------+-----------------+------+---------------+------+---------+------+-------+-------------+

| id | select_type | table           | type | possible_keys | key  | key_len | ref  | rows  | Extra       |

+----+-------------+-----------------+------+---------------+------+---------+------+-------+-------------+

|  1 | SIMPLE      | grains_resource | ALL  | NULL          | NULL | NULL    | NULL | 52631 | Using where |

+----+-------------+-----------------+------+---------------+------+---------+------+-------+-------------+

like不支持索引