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_objectscreate 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)-------------------512SQL>  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。