MySQL中如何分析查詢語句

Oracle中有explain for，mysql中也有同樣的功能，那便是explain，舉例如下：

mysql> explain select (case (select count(*) from stock) when 0 then 0 else max(id)+1 end) from stock;
+----+-------------+-------+-------+---------------+---------+---------+------+------+------------------------------+
| id | select_type | table | type  | possible_keys | key     | key_len | ref  | rows | Extra                        |
+----+-------------+-------+-------+---------------+---------+---------+------+------+------------------------------+
|  1 | PRIMARY     | NULL  | NULL  | NULL          | NULL    | NULL    | NULL | NULL | Select tables optimized away |
|  2 | SUBQUERY    | stock | index | NULL          | PRIMARY | 4       | NULL | 3696 | Using index                  |
+----+-------------+-------+-------+---------------+---------+---------+------+------+------------------------------+
2 rows in set (0.00 sec)

desc也是同樣的功能：

mysql> desc select (case (select count(*) from stock) when 0 then 0 else max(id)+1 end) from stock;
+----+-------------+-------+-------+---------------+---------+---------+------+------+------------------------------+
| id | select_type | table | type  | possible_keys | key     | key_len | ref  | rows | Extra                        |
+----+-------------+-------+-------+---------------+---------+---------+------+------+------------------------------+
|  1 | PRIMARY     | NULL  | NULL  | NULL          | NULL    | NULL    | NULL | NULL | Select tables optimized away |
|  2 | SUBQUERY    | stock | index | NULL          | PRIMARY | 4       | NULL | 3696 | Using index                  |
+----+-------------+-------+-------+---------------+---------+---------+------+------+------------------------------+
2 rows in set (0.00 sec)

其中rows可以當cost那樣看。

--2020年5月2日--