_optimizer_null_aware_antijoin 参数是Oracle 11g引入的新参数，它用于解决在反连接（Anti-Join）时，关联列上存在空值（NULL）或关联列无非空约束的问题 
默认情况下，该参数的值为true，即开启状态。 
我们用一个例子简单看一下这个参数的效果 

设置关闭该参数： 
SQL> alter system set "_optimizer_null_aware_antijoin"=false; 
System altered. 

新建两张测试表： 
create table t1 as select * from dba_objects; 
create table t2 as select * from dba_tables; 

关闭情况下，使用not in子句查询时使用的filter 
SQL> select * from t1 where object_name not in (select table_name from t2); 

83339 rows selected. 

Execution Plan 
---------------------------------------------------------- 
Plan hash value: 895956251 


--------------------------------------------------------------------------- 
| Id  | Operation          | Name | Rows  | Bytes | Cost (%CPU)| Time     | 
--------------------------------------------------------------------------- 
|   0 | SELECT STATEMENT   |      | 87226 |    17M|   114K  (1)| 00:22:51 | 
|*  1 |  FILTER            |      |       |       |            |          | 
|   2 |   TABLE ACCESS FULL| T1   | 87258 |    17M|   345   (1)| 00:00:05 | 
|*  3 |   TABLE ACCESS FULL| T2   |  2453 | 41701 |     2   (0)| 00:00:01 | 
--------------------------------------------------------------------------- 

Predicate Information (identified by operation id): 
--------------------------------------------------- 

   1 - filter( NOT EXISTS (SELECT 0 FROM "T2" "T2" WHERE 
              LNNVL("TABLE_NAME"<>:B1))) 
   3 - filter(LNNVL("TABLE_NAME"<>:B1)) 


开启该参数 
SQL> alter system set "_optimizer_null_aware_antijoin"=true; 

System altered. 


可以看到查询效率得到很大的提升： 
SQL> select * from t1 where object_name not in (select table_name from t2); 

83339 rows selected. 

Execution Plan 
---------------------------------------------------------- 
Plan hash value: 1521920066 

--------------------------------------------------------------------------------- 
| Id  | Operation                | Name | Rows  | Bytes | Cost (%CPU)| Time     | 
--------------------------------------------------------------------------------- 
|   0 | SELECT STATEMENT         |      | 87258 |    18M|   377   (1)| 00:00:05 | 
|*  1 |  HASH JOIN RIGHT ANTI SNA|      | 87258 |    18M|   377   (1)| 00:00:05 | 
|   2 |   TABLE ACCESS FULL      | T2   |  2582 | 43894 |    31   (0)| 00:00:01 | 
|   3 |   TABLE ACCESS FULL      | T1   | 87258 |    17M|   345   (1)| 00:00:05 | 
--------------------------------------------------------------------------------- 

Predicate Information (identified by operation id): 
--------------------------------------------------- 

   1 - access("OBJECT_NAME"="TABLE_NAME") 


我们看看把not in换成not exists的SQL执行计划： 
SQL> select * from t1 where not exists (select 1 from t2 where t2.table_name=t1.object_name); 

83339 rows selected. 

Execution Plan 
---------------------------------------------------------- 
Plan hash value: 629543484 

----------------------------------------------------------------------------- 
| Id  | Operation            | Name | Rows  | Bytes | Cost (%CPU)| Time     | 
----------------------------------------------------------------------------- 
|   0 | SELECT STATEMENT     |      | 87258 |    18M|   377   (1)| 00:00:05 | 
|*  1 |  HASH JOIN RIGHT ANTI|      | 87258 |    18M|   377   (1)| 00:00:05 | 
|   2 |   TABLE ACCESS FULL  | T2   |  2582 | 43894 |    31   (0)| 00:00:01 | 
|   3 |   TABLE ACCESS FULL  | T1   | 87258 |    17M|   345   (1)| 00:00:05 | 
----------------------------------------------------------------------------- 

Predicate Information (identified by operation id): 
--------------------------------------------------- 

   1 - access("T2"."TABLE_NAME"="T1"."OBJECT_NAME") 

Note 
----- 
   - dynamic sampling used for this statement (level=2) 




_optimizer_null_aware_antijoin参数能优化查询效率，但是查看mos，开启该隐含参数会有很多的bug。可能存在不稳定，因此通常会建议把该参数关闭。 
alter system set "_optimizer_null_aware_antijoin"=false; 
该参数为动态参数。