Mysql Explain 這里做一個資料的全面整理。

一.語法

explain < table_name >

例如: explain select * from t3 where id=3952602;

二.explain輸出解釋

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

1.id

  我的理解是SQL執行的順利的標識,SQL從大到小的執行.

例如:

mysql> explain select * from (select * from ( select * from t3 where id=3952602) a) b;
+----+-------------+------------+--------+-------------------+---------+---------+------+------+-------+
| id | select_type | table      | type   | possible_keys     | key     | key_len | ref  | rows | Extra |
+----+-------------+------------+--------+-------------------+---------+---------+------+------+-------+
|  1 | PRIMARY     | <derived2> | system | NULL              | NULL    | NULL    | NULL |    1 |       |
|  2 | DERIVED     | <derived3> | system | NULL              | NULL    | NULL    | NULL |    1 |       |
|  3 | DERIVED     | t3         | const  | PRIMARY,idx_t3_id | PRIMARY | 4       |      |    1 |       |
+----+-------------+------------+--------+-------------------+---------+---------+------+------+-------+

很顯然這條SQL是從里向外的執行,就是從id=3 向上執行.

2. select_type

就是select類型,可以有以下

(1) SIMPLE

簡單SELECT(不使用UNION或子查詢等) 例如:

mysql> explain select * from t3 where id=3952602;

+----+-------------+-------+-------+-------------------+---------+---------+-------+------+-------+
| id | select_type | table | type  | possible_keys     | key     | key_len | ref   | rows | Extra |
+----+-------------+-------+-------+-------------------+---------+---------+-------+------+-------+
|  1 | SIMPLE      | t3    | const | PRIMARY,idx_t3_id | PRIMARY | 4       | const |    1 |       |
+----+-------------+-------+-------+-------------------+---------+---------+-------+------+-------+