标签：
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：
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。
标签：
原文地址：http://www.cnblogs.com/nzbbody/p/4356415.html
