子查询里有rownum函数时

select * from t1 where object_id in (select object_id from  t2 where  t1.object_id=t2.object_id  and rownum<10);

create table t1 as select * from dba_objects

create table t2 as select * from dba_objects

子查询返回1行还是100行,都没有意义

SQL> select * from t1 where object_id in (select object_id from  t2 where  t1.object_id=t2.object_id  and rownum<10);

已选择9行。


执行计划
----------------------------------------------------------
Plan hash value: 4165492305

-----------------------------------------------------------------------------
| Id  | Operation	     | Name | Rows  | Bytes | Cost (%CPU)| Time     |
-----------------------------------------------------------------------------
|   0 | SELECT STATEMENT     |	    |	 32 |  6624 |  4736   (1)| 00:00:57 |
|*  1 |  FILTER 	     |	    |	    |	    |		 |	    |
|   2 |   TABLE ACCESS FULL  | T1   | 72109 |	 14M|	228   (1)| 00:00:03 |
|*  3 |   FILTER	     |	    |	    |	    |		 |	    |
|*  4 |    COUNT STOPKEY     |	    |	    |	    |		 |	    |
|*  5 |     TABLE ACCESS FULL| T2   |	  1 |	 13 |	  2   (0)| 00:00:01 |
-----------------------------------------------------------------------------

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

   1 - filter( EXISTS (<not feasible>)
   3 - filter("OBJECT_ID"=:B1)
   4 - filter(ROWNUM<10)
   5 - filter("T2"."OBJECT_ID"=:B1)


这里T1表的OBJECT_ID 传入到子查询中,子查询里限制条数毫无意思

等价于
select * from t1 where object_id in (select object_id from  t2 where  t1.object_id=t2.object_id );

测试；
SQL> select count(t2.object_id) from  t2 ,t1 where  t1.object_id=t2.object_id and t2.object_id=3;

COUNT(T2.OBJECT_ID)
-------------------
		512


SQL>  select object_id from t1 where object_id=3;

 OBJECT_ID
----------
	 3

档T1把OBJECT_ID=3 传入到子查询时,那么子查询会返回512行

相当于:

select* from  t1 where  object_id in (select object_id from  t2 where object_id=3);

select * from t1 where object_id in (3,3,3,3,3,3,3,3,3................);512个

这个和

select * from t1 where object_id in (3）没有任何区别:

这个子查询加rownum 除了阻止子查询展开外,没有任何意思。