MySQL EXPLAIN：Impossible WHERE noticed after re...

mysql 的 explain命令用於查看執行效果，很久沒有來分析sql了（ps:有段時間沒寫SQL了）無解……還好，有google。

mysql> create table t_william(id int primary key,name char(2))engine=innodb;
Query OK, 0 rows affected (0.13 sec)

/*上兩條數據*/

insert into t_william value(1,'ad');
insert into t_william value(2,'a2');

mysql> explain select *from t_willam where id=1;
+----+-------------+----------+-------+---------------+---------+---------+-------+------+-------+
| id | select_type | table    | type  | possible_keys | key     | key_len | ref   | rows | Extra |
+----+-------------+----------+-------+---------------+---------+---------+-------+------+-------+
|  1 | SIMPLE      | t_willam | const | PRIMARY       | PRIMARY | 4       | const |    1 |       |
+----+-------------+----------+-------+---------------+---------+---------+-------+------+-------+
1 row in set (0.00 sec)

對於已存在的記錄還正常。換一個不存在的主鍵id，則奇葩出現了，出現了，

mysql> explain select *from t_willam where id=3; 
+----+-------------+-------+------+---------------+------+---------+------+------+-----------------------------------------------------+
| id | select_type | table | type | possible_keys | key  | key_len | ref  | rows | Extra                                               |
+----+-------------+-------+------+---------------+------+---------+------+------+-----------------------------------------------------+
|  1 | SIMPLE      | NULL  | NULL | NULL          | NULL | NULL    | NULL | NULL | Impossible WHERE noticed after reading const tables |
+----+-------------+-------+------+---------------+------+---------+------+------+-----------------------------------------------------+

Impossible WHERE noticed after reading const tables。（聽說，這是由於mysql是首先對索引記錄進行查詢，而後在分析效果。）

附錄：

知識回顧：

Explain的type顯示的是訪問類型，是較爲重要的一個指標，結果值從好到壞依次是：

system > const > eq_ref > ref > fulltext > ref_or_null > index_merge > unique_subquery > index_subquery > range > index > ALL

參考：

http://wangyuanzju.blog.163.com/blog/static/13029200691114035831/

http://hi.baidu.com/thinkinginlamp/item/88fd66da58ff0ae4795daad3