Hint：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：[sql] 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[sql] 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[sql] 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.