深入理解Oracle表(1)：ORDERED和USE_NL

       ORDERED好理解，就是表示根据 from 后面表的顺序join，从左到右，左边的表做驱动表
       use_nl（t1,t2）：表示对表t1、t2关联时采用嵌套循环连接,其并不能让优化器确定谁是驱动表或谁是被驱动的表
       USE_NL()，先看看oracle doc怎么说：
       
       In this statement, the USE_NL hint explicitly chooses a nested loops join with the customers table as the inner table:
       SELECT /*+ ORDERED USE_NL(customers) to get first row faster */
       accounts.balance, customers.last_name, customers.first_name
       FROM accounts, customers
       WHERE accounts.customer_id = customers.customer_id;
     
       customers 作为inner table，也就是说作为被驱动表。驱动表称为outer table
       如果指定的表是outer table（驱动表），则优化器会忽略这个hint
       如果非要强制它作为inner table，可以配上ordered参数
       oradered 表示根据from 后面表的顺序，从左到右join，左表做驱动表，3个或3个以上最有用
       也就是说use_nl如果只带了一个表名作为参数，则该表为被驱动表
       如果带了2个以上的参数，Oracle并没有指出use_nl(a,b)中哪个是驱动表，所以常使用ordered或者full()或者index()来强化我们的目标
       
       以下是测试：

hr@ORCL> select first_name,departments.department_id from employees,departments where employees.department_id=departments.department_id;

Execution Plan
----------------------------------------------------------
Plan hash value: 169719308

---------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
---------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 106 | 1484 | 3 (0)| 00:00:01 |
| 1 | NESTED LOOPS | | 106 | 1484 | 3 (0)| 00:00:01 |
| 2 | TABLE ACCESS FULL| EMPLOYEES | 107 | 1070 | 3 (0)| 00:00:01 |
|* 3 | INDEX UNIQUE SCAN| DEPT_ID_PK | 1 | 4 | 0 (0)| 00:00:01 |
---------------------------------------------------------------------------------
       此处优化器选择employees作为驱动表，因为departments上有索引，而且索引正好建立在连接列上

hr@ORCL> select /*+ use_nl(employees) */ first_name,departments.department_id from employees,departments where employees.department_id=departments.department_id;

Execution Plan
----------------------------------------------------------
Plan hash value: 169719308

---------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
---------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 106 | 1484 | 3 (0)| 00:00:01 |
| 1 | NESTED LOOPS | | 106 | 1484 | 3 (0)| 00:00:01 |
| 2 | TABLE ACCESS FULL| EMPLOYEES | 107 | 1070 | 3 (0)| 00:00:01 |
|* 3 | INDEX UNIQUE SCAN| DEPT_ID_PK | 1 | 4 | 0 (0)| 00:00:01 |
---------------------------------------------------------------------------------
       由于employees是作为驱动表，优化器会忽略hint提示

hr@ORCL> select /*+ ordered use_nl(employees) */ first_name,departments.department_id from departments,employees where employees.department_id=departments.department_id;

Execution Plan
----------------------------------------------------------
Plan hash value: 2677871237

-------------------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
-------------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 106 | 1484 | 8 (0)| 00:00:01 |
| 1 | TABLE ACCESS BY INDEX ROWID| EMPLOYEES | 4 | 40 | 1 (0)| 00:00:01 |
| 2 | NESTED LOOPS | | 106 | 1484 | 8 (0)| 00:00:01 |
| 3 | INDEX FULL SCAN | DEPT_ID_PK | 27 | 108 | 1 (0)| 00:00:01 |
|* 4 | INDEX RANGE SCAN | EMP_DEPARTMENT_IX | 10 | | 0 (0)| 00:00:01 |
-------------------------------------------------------------------------------------------------
       现在是departments作为驱动表了