MySQL中的嵌套视图有什么问题吗

让一个视图引用另一个视图有什么不对吗？例如，假设我有一个 用户表

CREATE TABLE `users` (
`id` int(11) unsigned NOT NULL AUTO_INCREMENT,
`first_name` varchar(255) NOT NULL,
`last_name` varchar(255) NOT NULL,
PRIMARY KEY (`id`)
);

然后，为了进行论证，需要一个只显示所有用户的视图

CREATE VIEW all_users AS SELECT * FROM users

然后是一个只返回他们的名字和姓氏

CREATE VIEW full_names AS SELECT first_name, last_name FROM all_users

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基于一个视图而非另一个视图是否存在性能问题？让我们假设这是最简单的示例，现实世界的场景会复杂得多，但基于一个视图的另一个视图的一般概念是一样的。

这取决于所使用的场景。诱惑可能非常昂贵，而合并应该与直接使用表相同，因此不会造成损失。

这取决于使用的表。诱惑可能非常昂贵，而合并应该与直接使用表相同，因此不会造成损失。

对于您的示例，是相同的：

mysql> explain select * from users;
+----+-------------+-------+--------+---------------+------+---------+------+------+---------------------+
| id | select_type | table | type   | possible_keys | key  | key_len | ref  | rows | Extra               |
+----+-------------+-------+--------+---------------+------+---------+------+------+---------------------+
|  1 | SIMPLE      | users | system | NULL          | NULL | NULL    | NULL |    0 | const row not found |
+----+-------------+-------+--------+---------------+------+---------+------+------+---------------------+
1 row in set (0.01 sec)

mysql> explain select * from all_users;
+----+-------------+-------+--------+---------------+------+---------+------+------+---------------------+
| id | select_type | table | type   | possible_keys | key  | key_len | ref  | rows | Extra               |
+----+-------------+-------+--------+---------------+------+---------+------+------+---------------------+
|  1 | SIMPLE      | users | system | NULL          | NULL | NULL    | NULL |    0 | const row not found |
+----+-------------+-------+--------+---------------+------+---------+------+------+---------------------+
1 row in set (0.00 sec)

mysql> explain select * from full_names ;
+----+-------------+-------+--------+---------------+------+---------+------+------+---------------------+
| id | select_type | table | type   | possible_keys | key  | key_len | ref  | rows | Extra               |
+----+-------------+-------+--------+---------------+------+---------+------+------+---------------------+
|  1 | SIMPLE      | users | system | NULL          | NULL | NULL    | NULL |    0 | const row not found |
+----+-------------+-------+--------+---------------+------+---------+------+------+---------------------+
1 row in set (0.02 sec)

---

你的例子也是一样的：

mysql> explain select * from users;
+----+-------------+-------+--------+---------------+------+---------+------+------+---------------------+
| id | select_type | table | type   | possible_keys | key  | key_len | ref  | rows | Extra               |
+----+-------------+-------+--------+---------------+------+---------+------+------+---------------------+
|  1 | SIMPLE      | users | system | NULL          | NULL | NULL    | NULL |    0 | const row not found |
+----+-------------+-------+--------+---------------+------+---------+------+------+---------------------+
1 row in set (0.01 sec)

mysql> explain select * from all_users;
+----+-------------+-------+--------+---------------+------+---------+------+------+---------------------+
| id | select_type | table | type   | possible_keys | key  | key_len | ref  | rows | Extra               |
+----+-------------+-------+--------+---------------+------+---------+------+------+---------------------+
|  1 | SIMPLE      | users | system | NULL          | NULL | NULL    | NULL |    0 | const row not found |
+----+-------------+-------+--------+---------------+------+---------+------+------+---------------------+
1 row in set (0.00 sec)

mysql> explain select * from full_names ;
+----+-------------+-------+--------+---------------+------+---------+------+------+---------------------+
| id | select_type | table | type   | possible_keys | key  | key_len | ref  | rows | Extra               |
+----+-------------+-------+--------+---------------+------+---------+------+------+---------------------+
|  1 | SIMPLE      | users | system | NULL          | NULL | NULL    | NULL |    0 | const row not found |
+----+-------------+-------+--------+---------------+------+---------+------+------+---------------------+
1 row in set (0.02 sec)

---

通常情况下，性能比直接根据表进行选择差，但最好尝试一下，看看它是否适合您的用例。删除主键（id）后的分号。另外，通过不为NULL来更改OT NULL通常，性能比直接根据表进行选择差，但最好尝试一下，看看是否适合您的用例。删除主键（id）后的分号。也可以通过NOT NULL更改OT NULL