ORDERED好理解，就是表示根據(jù) from 后面表的順序join，從左到右，左邊的表做驅(qū)動表

use_nl(t1,t2)：表示對表t1、t2關聯(lián)時采用嵌套循環(huán)連接,其并不能讓優(yōu)化器確定誰是驅(qū)動表或誰是被驅(qū)動的表

USE_NL()，先看看oracle doc怎么說：

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 作為inner table，也就是說作為被驅(qū)動表。驅(qū)動表稱為outer table

如果指定的表是outer table(驅(qū)動表)，則優(yōu)化器會忽略這個hint

如果非要強制它作為inner table，可以配上ordered參數(shù)

oradered 表示根據(jù)from 后面表的順序，從左到右join，左表做驅(qū)動表，3個或3個以上最有用

也就是說use_nl如果只帶了一個表名作為參數(shù)，則該表為被驅(qū)動表

如果帶了2個以上的參數(shù)，Oracle并沒有指出use_nl(a,b)中哪個是驅(qū)動表，所以常使用ordered或者full()或者index()來強化我們的目標

以下是測試：

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 |

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

此處優(yōu)化器選擇employees作為驅(qū)動表，因為departments上有索引，而且索引正好建立在連接列上

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是作為驅(qū)動表，優(yōu)化器會忽略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 |

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

現(xiàn)在是departments作為驅(qū)動表了

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