oracle view  学习小结

视图的定义就是其使用的查询语句，Oracle 将这个定义以文本形式存储
在数据字典中。当用户在 SQL 语句中引用了视图时，Oracle 将进行以
下工作： 
1.  将引用了视图的语句与视图的定义语句整合为（merge）一个语句
2.  在共享 SQL 区（shared SQL area）解析（parse）整合后的语句
3.  执行（execute）此语句

接下来用一个实验来证明下：

SQL> alter system flush shared_pool;

SQL> SELECT last_name
  2   FROM hr.employees, hr.departments
  3  WHERE employees.department_id = departments.department_id AND
  4  departments.department_id = 10 AND
  5  employees.employee_id = 9876;

no rows selected

Execution Plan
----------------------
Plan hash value: 1833546154

--------------------------------------------
-------------

| Id  | Operation                   | Name          | Rows  | Bytes | Cost (%CPU
)| Time     |

--------------------------------------------
-------------

|   0 | SELECT STATEMENT            |               |     1 |    15 |     1   (0
)| 00:00:01 |

|*  1 |  TABLE ACCESS BY INDEX ROWID| EMPLOYEES     |     1 |    15 |     1   (0
)| 00:00:01 |

|*  2 |   INDEX UNIQUE SCAN         | EMP_EMP_ID_PK |     1 |       |     0   (0
)| 00:00:01 |

--------------------------------------------
-------------

Predicate Information (identified by operation id):
---------------------------------------------------

   1 - filter("EMPLOYEES"."DEPARTMENT_ID"=10)
   2 - access("EMPLOYEES"."EMPLOYEE_ID"=9876)

Statistics
----------------------
        202  recursive calls
          0  db block gets
        290  consistent gets
          1  physical reads
          0  redo size
        337  bytes sent via SQL*Net to client
        512  bytes received via SQL*Net from client
          1  SQL*Net roundtrips to/from client
         13  sorts (memory)
          0  sorts (disk)
          0  rows processed

SQL> alter system flush shared_pool; 

System altered.

SQL> SELECT last_name 
  2  from hr.employees_view
  3  WHERE employee_id = 9876;

no rows selected

Execution Plan
----------------------
Plan hash value: 1833546154

--------------------------------------------
-------------

| Id  | Operation                   | Name          | Rows  | Bytes | Cost (%CPU
)| Time     |

--------------------------------------------
-------------

|   0 | SELECT STATEMENT   &nb