深入理解Oracle—ORDERED和USE_NL

忘是亡心i 2022-04-11 06:47 169阅读 0赞

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。

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