scott@TEST>select /*+ use_hash(t1) , index(t2 pk_dept) */ t1.empno,t1.empno,t2.loc from emp t1,dept t2 where t1.deptno=t2.deptno;
14 rows selected.
Execution Plan
----------------------------------------------------------
Plan hash value: 615168685
---------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
---------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 14 | 518 | 7 (15)| 00:00:01 |
|* 1 | HASH JOIN | | 14 | 518 | 7 (15)| 00:00:01 |
| 2 | TABLE ACCESS FULL| DEPT | 4 | 44 | 3 (0)| 00:00:01 |
| 3 | TABLE ACCESS FULL| EMP | 14 | 364 | 3 (0)| 00:00:01 |
---------------------------------------------------------------------------
从执行计划中可以看出，仍然走的是HASH JOIN但是index(t2 pk_dept)失效了。因为','是Oracle的保留关键字，所以','后面的index(t2 pk_dept)失效了，再修改Hint如下并执行SQL：
scott@TEST>select /*+ comment use_hash(t1) index(t2 pk_dept) */ t1.empno,t1.empno,t2.loc from emp t1,dept t2 where t1.deptno=t2.deptno;
14 rows selected.
Execution Plan
----------------------------------------------------------
Plan hash value: 844388907
----------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
----------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 14 | 518 | 6 (17)| 00:00:01 |
| 1 | MERGE JOIN | | 14 | 518 | 6 (17)| 00:00:01 |
| 2 | TABLE ACCESS BY INDEX ROWID| DEPT | 4 | 44 | 2 (0)| 00:00:01 |
| 3 | INDEX FULL SCAN | PK_DEPT | 4 | | 1 (0)| 00:00:01 |
|* 4 | SORT JOIN | | 14 | 364 | 4 (25)| 00:00:01 |
| 5 | TABLE ACCESS FULL | EMP | 14 | 364 | 3 (0)| 00:00:01 |
----------------------------------------------------------------------------------------
从执行计划中看出，现在走的是跟一开始的执行计划一样，说明Hint中的两个都失效了，因为这两个都在Oracle保留关键字comment后面。再修改Hint如下再次执行SQL:
scott@TEST>select /*+ this use_hash(t1) index(t2 pk_dept) */ t1.empno,t1.empno,t2.loc from emp t1,dept t2 where t1.deptno=t2.deptno;
14 rows selected.
Execution Plan
----------------------------------------------------------
Plan hash value: 2622742753
----------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
----------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 14 | 518 | 6 (17)| 00:00:01 |
|* 1 | HASH JOIN | | 14 | 518 | 6 (17)| 00:00:01 |
| 2 | TABLE ACCESS BY INDEX ROWID| DEPT | 4 | 44 | 2 (0)| 00:00:01 |
| 3 | INDEX FULL SCAN | PK_DEPT | 4 | | 1 (0)| 00:00:01 |
| 4 | TABLE ACCESS FULL | EMP | 14 | 364 | 3 (0)| 00:00:01 |
----------------------------------------------------------------------------------------
现在执行计划又走出了Hint指定的样子，说明两个都生效了，这是因为this不是Oracle保留关键字。
以上介绍了5种Hint被Oracle忽略的情况，在实例使用过程中一定要注意使用方法，使用正确有效的Hint来提升SQL执行效率，避免Hint被Oracle忽略。