1、视图是一个虚拟表，可以认为对原表封装了一下，一般情况下，可以把视图当做表来对待。
2、视图的实现由两种策略：临时表算法与合并算法。临时表算法：把视图对原表的查询结果放在一个临时表中，以后对视图的操作就是对临时表的操作。合并算法：把对试图的操作转化为对原表的操作。
3、举例来说，mysql> create view bigAgeStuView as select * from stu where age>25;
对于查询 mysql> select * from bigAgeStuView where age<28;
如果采用临时表算法，Mysql会把select * from stu where age>25的结果放入一张临时表，再操作临时表，查找age>28
如果采用合并算法，Mysql会把对视图的查询转化为select * from bigAgeStuView where 25<age and age<28;
显然，临时表算法会存在严重的性能问题。
4、对于create view bigAgeStuView as select * from stu where age>25; 可以指定视图的算法，格式如下：
mysql> create algorithm=temptable view bigAgeStuView as select * from stu where age>25;
Query OK, 0 rows affected
mysql> desc extended select * from bigAgeStuView;
+----+-------------+------------+------+---------------+------+---------+------+------+----------+-------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+------------+------+---------------+------+---------+------+------+----------+-------------+
| 1 | PRIMARY | <derived2> | ALL | NULL | NULL | NULL | NULL | 6 | 100 | |
| 2 | DERIVED | stu | ALL | NULL | NULL | NULL | NULL | 1138 | 100 | Using where |
+----+-------------+------------+------+---------------+------+---------+------+------+----------+-------------+
2 rows in set
temptable 执行计划显示，使用了派生表（临时表）
mysql> create algorithm=merge view bigAgeStuView as select * from stu where age>25;
Query OK, 0 rows affected
mysql> desc extended select * from bigAgeStuView;
+----+-------------+-------+------+---------------+------+---------+------+------+----------+-------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------+---------------+------+---------+------+------+----------+-------------+
| 1 | SIMPLE | stu | ALL | NULL | NULL | NULL | NULL | 1138 | 100 | Using where |
+----+-------------+-------+------+---------------+------+---------+------+------+----------+-------------+
1 row in set
merge 执行计划显示，没有使用派生表
mysql> create algorithm=undefined view bigAgeStuView as select * from stu where age>25;
Query OK, 0 rows affected
mysql> desc extended select * from bigAgeStuView;
+----+-------------+-------+------+---------------+------+---------+------+------+----------+-------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------+---------------+------+---------+------+------+----------+-------------+
| 1 | SIMPLE | stu | ALL | NULL | NULL | NULL | NULL | 1138 | 100 | Using where |
+----+-------------+-------+------+---------------+------+---------+------+------+----------+-------------+
1 row in set
undefined（由Mysql决定使用合适的算法）执行计划显示，没有使用派生表
5、需要注意的是，临时表算法存在性能问题，因此我们倾向于使用合并算法。但是，并不是每个视图都能使用合并算法，比如建立视图使用了group by, distinct,聚合函数,组合查询union,子查询，不能使用合并算法，因为视图记录无法与原表记录建立一一映射的关系。如下：
mysql> create algorithm=merge view bigAgeStuView as select max(age) from stu;
Query OK, 0 rows affected
mysql> show warnings;
+---------+------+-------------------------------------------------------------------------------+
| Level | Code | Message |
+---------+------+-------------------------------------------------------------------------------+
| Warning | 1354 | View merge algorithm can't be used here for now (assumed undefined algorithm) |
+---------+------+-------------------------------------------------------------------------------+
1 row in set
mysql> desc extended select * from bigAgeStuView;
+----+-------------+------------+--------+---------------+------+---------+------+------+----------+-------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+------------+--------+---------------+------+---------+------+------+----------+-------+
| 1 | PRIMARY | <derived2> | system | NULL | NULL | NULL | NULL | 1 | 100 | |
| 2 | DERIVED | stu | ALL | NULL | NULL | NULL | NULL | 1138 | 100 | |
+----+-------------+------------+--------+---------------+------+---------+------+------+----------+-------+
6、使用临时表算法，没有办法更新原表的记录，这个很好理解，因为临时表算法，操作的是临时表。