Hint可以影响优化器对于执行计划的选择,但这种影响不是强制性的,优化器在某些情况下可能会忽略目标SQL中的Hint。由于各种原因导致Hint被Oracle忽略后,Oracle并不会给出任何提示或者警告,更不会报错,目标SQL依然可以正常运行,这也符合Hint实际上是一种特殊注释的身份。注释本来就是可有可无的东西,不应该因为它的存在而而导致原先在没有Hint时可以正常执行的SQL因为加了Hint后而变得不能正常执行。
下面来看几种Hint被Oracle忽略的常见情形。
1 使用的Hint有语法或者拼写错误
一旦使用的Hint中有语法或者拼写错误,Oracle就会忽略该Hint,看几个示例SQL:
select * from emp;
select * from emp;
select * from emp;
select * from emp;
select * from emp;
select * from emp e;
select * from emp;
select t1.ename,t1.deptno from emp t1 where t1.deptno in (select t2.deptno from detp t where t2.loc='CHICAGO');
实际上,上述8条SQL中的Hint都是无效的,它们都会被Oracle忽略。
1是因为关键字应该是"index"而不是"ind"
2是因为漏掉了一个右括号
3是因为Hint中第一个*和+之间出现了空格
4是因为Hint出现的位置不对,它应该出现在*前面
5是因为emp表前面带上了SCHEME名称
6是因为没有emp表的别名
7是因为索引名称写错了
8是因为Hint跨了Query Block。Hint生效的范围公限于它本身所在的Query Block,如果将某个Hint生将范围扩展到它所在的Query Block之外而又没在该Hint中指定其生效的Query Block名称的话,Oracle就会忽略该Hint。
2 使用的Hint无效
即使语法是正确的,但如果由于某种原因导致Oracle认为这个Hint无效,则Oracle还是会忽略该Hint。
看几个实例
scott@TEST>set autotrace traceonly
scott@TEST>select deptno,dname from dept where loc='CHICAGO';
Execution Plan
----------------------------------------------------------
Plan hash value: 492093765
--------------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
--------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 10 | 300 | 2 (0)| 00:00:01 |
| 1 | TABLE ACCESS BY INDEX ROWID| DEPT | 10 | 300 | 2 (0)| 00:00:01 |
|* 2 | INDEX RANGE SCAN | IDX_DEPT_LOC | 4 | | 1 (0)| 00:00:01 |
--------------------------------------------------------------------------------------------
......
从上面的输出可以看出,上面的SQL的执行计划走的是对索引IDX_DEPT_LOC的索引范围扫描,说明Hint生效了,但是如果把where条件替换为与索引IDX_DEPT_LOC毫不相关的deptno=30,再来看执行情况
scott@TEST>select deptno,dname from dept where deptno=30;
Execution Plan
----------------------------------------------------------
Plan hash value: 2852011669
---------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
---------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 22 | 2 (0)| 00:00:01 |
| 1 | TABLE ACCESS BY INDEX ROWID| DEPT | 1 | 22 | 2 (0)| 00:00:01 |
|* 2 | INDEX UNIQUE SCAN | PK_DEPT | 1 | | 1 (0)| 00:00:01 |
---------------------------------------------------------------------------------------
......
从上面的输出可以看出,执行计划走的是对主键PK_DEPT的INDEX UNIQUE SCAN,面不是Hint里的IDX_DEPT_LOC。这就说明Hint在这个SQL失效了。
即使不改where条件,如果把索引IDX_DEPT_LOC删除,这个Hint也会失效:
scott@TEST>drop index idx_dept_loc;
Index dropped.
scott@TEST>select deptno,dname from dept where loc='CHICAGO';
Execution Plan
----------------------------------------------------------
Plan hash value: 3383998547
--------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
--------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 10 | 300 | 29 (0)| 00:00:01 |
|* 1 | TABLE ACCESS FULL| DEPT | 10 | 300 | 29 (0)| 00:00:01 |
--------------------------------------------------------------------------
从上面的执行计划可以看出走的是对表DEPT的TABLE ACCESS FULL,Hint也是失效的。
再来看一个使用组合Hint的例子,先看如下SQL的执行计划
scott@TEST>select deptno from dept;
Execution Plan
----------------------------------------------------------
Plan hash value: 587379989
--------------------------------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | TQ |IN-OUT| PQ Distrib |
--------------------------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1000 | 13000 | 16 (0)| 00:00:01 | | | |
| 1 | PX COORDINATOR | | | | | | | | |
| 2 | PX SEND QC (RANDOM)| :TQ10000 | 1000 | 13000 | 16 (0)| 00:00:01 | Q1,00 | P->S | QC (RAND) |
| 3 | PX BLOCK ITERATOR | | 1000 | 13000 | 16 (0)| 00:00:01 | Q1,00 | PCWC | |
| 4 | TABLE ACCESS FULL| DEPT | 1000 | 13000 | 16 (0)| 00:00:01 | Q1,00 | PCWP | |
--------------------------------------------------------------------------------------------------------------
......
从上面输出内容可以看出,现在是对表DEPT做的并行全表扫描,说明组合Hint中的两个都生效了,这个Hint的含义是既要全表扫描又要并行访问表DEPT,两者不矛盾,因为全表扫描可以并行执行。再看如下的SQL:
scott@TEST>select deptno from dept;
4 rows selected.
Execution Plan
----------------------------------------------------------
Plan hash value: 2913917002
----------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
----------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1000 | 13000 | 26 (0)| 00:00:01 |
| 1 | INDEX FULL SCAN | PK_DEPT | 1000 | 13000 | 26 (0)| 00:00:01 |
----------------------------------------------------------------------------
......
现在SQL走的是对索引PK_DEPT的索引全扫描,但是串行的,说明Hint中的parallel(dept 2)失效了,因为表DEPT上的主键索引PK_DEPT不是分区索引,而对于非分区索引而言,索引范围扫描或索引全扫描并不能并行执行,所以上述组合Hint中忽略了parallel(dept 2)。
再看一个HASH JOIN的例子:
下面的SQL中use_hash的Hint是生效的:
scott@TEST>select t1.empno,t1.empno,t2.loc from emp t1,dept t2 where t1.deptno=t2.deptno and t2.loc='CHICAGO';
6 rows selected.
Execution Plan
----------------------------------------------------------
Plan hash value: 615168685
---------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
---------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 5 | 185 | 7 (15)| 00:00:01 |
|* 1 | HASH JOIN | | 5 | 185 | 7 (15)| 00:00:01 |
|* 2 | TABLE ACCESS FULL| DEPT | 1 | 11 | 3 (0)| 00:00:01 |
| 3 | TABLE ACCESS FULL| EMP | 14 | 364 | 3 (0)| 00:00:01 |
--------------------------------------------------------------------------
-
但是如果把SQL修改为如下则use_hash的Hint就会被忽略
scott@TEST>select t1.empno,t1.empno,t2.loc from emp t1,dept t2 where t1.deptno>t2.deptno and t2.loc='CHICAGO';
no rows selected
Execution Plan
----------------------------------------------------------
Plan hash value: 4192419542
---------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
---------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 37 | 6 (0)| 00:00:01 |
| 1 | NESTED LOOPS | | 1 | 37 | 6 (0)| 00:00:01 |
|* 2 | TABLE ACCESS FULL| DEPT | 1 | 11 | 3 (0)| 00:00:01 |
|* 3 | TABLE ACCESS FULL| EMP | 1 | 26 | 3 (0)| 00:00:01 |
---------------------------------------------------------------------------
从上面的执行计划中看出use_hash确实是被Oracle忽略了,这是因为哈希连接只适用于等值连接条件,不等值的连接条件对哈希连接而言是没有意义的,所以上述Hint就被Oracle忽略了。
3 使用的Hint自相矛盾
如果使用的组合Hint是自相矛盾的,则这些自相矛盾的Hint都会被Oracle忽略。但Oracle只会将自相矛盾的Hint全部忽略掉,但如果使用的组合Hint中还有其他有效的Hint,则这些有效Hint不受影响。
看一个使用自相矛盾Hint的实例,先执行单个Hint的SQL
scott@TEST>select deptno from dept;
4 rows selected.
Execution Plan
----------------------------------------------------------
Plan hash value: 2578398298
--------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
--------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 4 | 12 | 2 (0)| 00:00:01 |
| 1 | INDEX FAST FULL SCAN| PK_DEPT | 4 | 12 | 2 (0)| 00:00:01 |
--------------------------------------------------------------------------------
......
scott@TEST>select deptno from dept;
4 rows selected.
Execution Plan
----------------------------------------------------------
Plan hash value: 3383998547
--------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
--------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 4 | 12 | 3 (0)| 00:00:01 |
| 1 | TABLE ACCESS FULL| DEPT | 4 | 12 | 3 (0)| 00:00:01 |
--------------------------------------------------------------------------
从上面的输出可以看出单独使用上面的两个Hint都能被Oracle生效,但如果这两个Hint合并到一起使用就不是那么回事了:
scott@TEST>select deptno from dept;
4 rows selected.
Execution Plan
----------------------------------------------------------
Plan hash value: 2913917002
----------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
----------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 4 | 12 | 1 (0)| 00:00:01 |
| 1 | INDEX FULL SCAN | PK_DEPT | 4 | 12 | 1 (0)| 00:00:01 |
----------------------------------------------------------------------------
从上面的输出可以看出执行计划没有走Hint中指定的执行计划,而是对主键索引PK_DEPT做的是INDEX FULL SCAN这说明Hint中的两个都失效了。
再来看下面的例子:
scott@TEST>select deptno from dept;
4 rows selected.
Execution Plan
----------------------------------------------------------
Plan hash value: 2913917002
----------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
----------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1000 | 3000 | 1 (0)| 00:00:01 |
| 1 | INDEX FULL SCAN | PK_DEPT | 1000 | 3000 | 1 (0)| 00:00:01 |
----------------------------------------------------------------------------
从上面的输出可以看出执行计划走的仍然是对主键索引PK_DEPT做的是INDEX FULL SCAN,但是做INDEX FULL SCAN反回结果集的cardinality从原来的4变为了1000,说明cardinality(dept 1000)生效了,也验证了如果使用的组合Hint中还有其他有效的Hint,则这些有效Hint不受影响。
4 使用的Hint受到了查询转换的干扰
有时候,查询转换也会导致相关的Hint失效,即Hint被Oracle忽略还可能是因为受到了查询转换的干扰。
下面来看一个因为使用了查询转换而导致相关Hint被Oracle忽略掉的实例。
创建一个测试表jobs
scott@TEST>create table jobs as select empno,job from emp;
Table created.
构造一个SQL
select
e.ename, j.job, e.sal, v.avg_sal
from emp e,
jobs j,
(select
e.deptno, avg(e.sal) avg_sal
from emp e, dept d
where d.loc = 'chicago'
and d.deptno = e.deptno
group by e.deptno) v
where e.empno = j.empno
and e.deptno = v.deptno
and e.sal > v.avg_sal
order by e.ename;
上面的SQL是两个表(EMP和JOBS)和内嵌视图V关联的SQL,其中内嵌视图V又是由表EMP和DEPT关联后得到的。在此SQL中使用了三个Hint,其中merge用于让内嵌视图V做视图合并,ordered表示上述SQL在执行时表EMP、JOBS和内嵌视图V的连接顺序应该和它们在该SQL的SQL文本中出现的顺序一致,即它们应该是按照从左至右的顺序依次做表连接。
如果上述三个Hint都生效的话,那目标SQL的执行计划中应该不会出现关键字“VIEW”(表示做了视图合并,体现了Merge Hint的作用),表EMP、JOBS和内嵌视图V的连接应该会变成表EMP、JOBS和内嵌视图V所对应的基表EMP和DEPT的连接,且连接的先后顺序应该是EMP->JOBS->内嵌视图V所对应的基表EMP和DEPT(体现了Ordered Hint的作用),外围查询中表EMP的扫描结果所对应的Cardinality的值应该是100(体现了Cardinality Hint的作用)。
现在看一下实际情况,执行上面的SQL:
scott@TEST>select
2 e.ename, j.job, e.sal, v.avg_sal
3 from emp e,
4 jobs j,
5 (select
6 e.deptno, avg(e.sal) avg_sal
7 from emp e, dept d
8 where d.loc = 'chicago'
9 and d.deptno = e.deptno
10 group by e.deptno) v
11 where e.empno = j.empno
12 and e.deptno = v.deptno
13 and e.sal > v.avg_sal
14 order by e.ename;
no rows selected
Execution Plan
----------------------------------------------------------
Plan hash value: 930847561
-------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
-------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 156 | 19656 | 15 (20)| 00:00:01 |
|* 1 | FILTER | | | | | |
| 2 | SORT GROUP BY | | 156 | 19656 | 15 (20)| 00:00:01 |
|* 3 | HASH JOIN | | 156 | 19656 | 14 (15)| 00:00:01 |
|* 4 | TABLE ACCESS FULL | DEPT | 1 | 11 | 3 (0)| 00:00:01 |
|* 5 | HASH JOIN | | 467 | 53705 | 10 (10)| 00:00:01 |
| 6 | TABLE ACCESS FULL | EMP | 14 | 364 | 3 (0)| 00:00:01 |
|* 7 | HASH JOIN | | 100 | 8900 | 7 (15)| 00:00:01 |
| 8 | TABLE ACCESS FULL| EMP | 100 | 5800 | 3 (0)| 00:00:01 |
| 9 | TABLE ACCESS FULL| JOBS | 14 | 434 | 3 (0)| 00:00:01 |
-------------------------------------------------------------------------------
从上面的执行计划可以看出,确实没有出现关键字“VIEW”,表EMP的扫描结果所对应的Cardinality的值确实是100,但连接顺序不是上面提到的顺序,而是先选择的表DEPT。这说明上述三个Hint中的Merge Hint和Cardinality Hint生效了,但Ordered Hint被Oracle忽略了。这是因为受到了查询转换的干扰(对内嵌视图V做视图合并是一种查询转换)。
为了证明上述SQL的Ordered Hint被Oracle忽略是因为受到了查询转换的干扰,现在将内嵌视图V中的merge替换为no_merge(不让内嵌视图做视图合并),再次执行该SQL:
scott@TEST>select
2 e.ename, j.job, e.sal, v.avg_sal
3 from emp e,
4 jobs j,
5 (select
6 e.deptno, avg(e.sal) avg_sal
7 from emp e, dept d
8 where d.loc = 'chicago'
9 and d.deptno = e.deptno
10 group by e.deptno) v
11 where e.empno = j.empno
12 and e.deptno = v.deptno
13 and e.sal > v.avg_sal
14 order by e.ename;
no rows selected
Execution Plan
----------------------------------------------------------
Plan hash value: 2898000699
--------------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
--------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 8 | 728 | 14 (22)| 00:00:01 |
| 1 | SORT ORDER BY | | 8 | 728 | 14 (22)| 00:00:01 |
|* 2 | HASH JOIN | | 8 | 728 | 13 (16)| 00:00:01 |
|* 3 | HASH JOIN | | 100 | 6500 | 7 (15)| 00:00:01 |
| 4 | TABLE ACCESS FULL | EMP | 100 | 4600 | 3 (0)| 00:00:01 |
| 5 | TABLE ACCESS FULL | JOBS | 14 | 266 | 3 (0)| 00:00:01 |
| 6 | VIEW | | 5 | 130 | 6 (17)| 00:00:01 |
| 7 | HASH GROUP BY | | 5 | 185 | 6 (17)| 00:00:01 |
| 8 | MERGE JOIN | | 5 | 185 | 6 (17)| 00:00:01 |
|* 9 | TABLE ACCESS BY INDEX ROWID| DEPT | 1 | 11 | 2 (0)| 00:00:01 |
| 10 | INDEX FULL SCAN | PK_DEPT | 4 | | 1 (0)| 00:00:01 |
|* 11 | SORT JOIN | | 14 | 364 | 4 (25)| 00:00:01 |
| 12 | TABLE ACCESS FULL | EMP | 14 | 364 | 3 (0)| 00:00:01 |
--------------------------------------------------------------------------------------------
从上面的执行计划中可以看出,出现了“VIEW”关键字,说明没有做视图合并,表EMP对就的Cardinality为100,连接顺序与前面预想的一致,这说明在禁掉了查询转换后之前被忽略的Ordered Hint又生效了。
5 使用的Hint受到了保留关键字的干扰
Oracle在解析Hint时,是按照从左到右的顺序进行的,如果遇到的词是Oracle的保留关键字,则Oracle将忽略这个词以及之后的所有词;如果遇到词既不是关键字也不是Hint,就忽略该词;如果遇到的词是有效的Hint,那么Oracle就会保留该Hing。
正是由于上述Oracle解析Hint的原则,保留关键字也可能导致相关的Hint失效。
Oracle的保留关键字可以从视图V$RESERVED_WORDS中查到,从下面的查询结果可以看到','、'COMMENT'、'IS'都是保留关键字,但“THIS”不是
scott@TEST>select keyword,length from v$reserved_words where keyword in (',','THIS','IS','COMMENT');
KEYWORD LENGTH
---------- ----------
, 1
COMMENT 7
IS 2
下面来看一个保留关键字导致Hint失效的实例,执行下面的SQL
scott@TEST>select 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 |
----------------------------------------------------------------------------------------
从执行计划上看走的是MERGE SORT JOIN,对SQL加入如下Hint并执行:
scott@TEST>select 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 |
----------------------------------------------------------------------------------------
从上面的执行计划中可以看出Hint中的两个都生效了,emp做HASH JOIN的被驱动表,对DEPT表做使用索引PK_DEPT。现在对Hint加入',',查看执行情况:
scott@TEST>select 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 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 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。
参考《基于Oracle的SQL优化》