收到开发这边的确认请求，如下三个语句，执行计划的COST均一样，但为什么实际执行效果相差很大（1)的查询速度很慢，2)和3)都正常）。

1) 
Select * 
From (Select * 
From V_Question_Head Vquest0_ 
Where 1 = 1  
Order By Vquest0_.State Asc, Vquest0_.Ask_Time Desc) Where Rownum <= 6; 
2) 
Select * 
From (Select * 
From V_Question_Head Vquest0_ 
Where 1 = 1 And Rownum <= 6 
Order By Vquest0_.State Asc, Vquest0_.Ask_Time Desc); 
3) 
Select * 
From (Select * 
From Question_Head Vquest0_ 
Where 1 = 1  
Order By Vquest0_.State Asc, Vquest0_.Ask_Time Desc) Where Rownum <= 6;

看如下执行计划，显然2速度快可以理解，因为行数只有6，而1和3为什么有那么大的速度差距，要命的是执行计划看起来没有差距。

SELECT STATEMENT, GOAL = ALL_ROWS                                          Cost=1710    Cardinality=6    Bytes=27912
COUNT STOPKEY                    
VIEW    Object owner=GAZA                                                Cost=1710    Cardinality=20237    Bytes=94142524
SORT ORDER BY STOPKEY                                                   Cost=1710    Cardinality=20237    Bytes=5949678
TABLE ACCESS FULL    Object owner=TEST    Object name=QUESTION_HEAD    Cost=421     Cardinality=20237    Bytes=5949678
SELECT STATEMENT, GOAL = ALL_ROWS                                          Cost=1710    Cardinality=6    Bytes=27912
VIEW                    Object owner=TEST                                 Cost=1710    Cardinality=6    Bytes=27912
SORT ORDER BY                                                            Cost=1710    Cardinality=6    Bytes=1764
COUNT STOPKEY                    
TABLE ACCESS FULL    Object owner=TEST    Object name=QUESTION_HEAD    Cost=421     Cardinality=20237    Bytes=5949678
SELECT STATEMENT, GOAL = ALL_ROWS                                          Cost=1710    Cardinality=6    Bytes=28590
COUNT STOPKEY                    
VIEW    Object owner=TEST                                                Cost=1710    Cardinality=20237    Bytes=96429305
SORT ORDER BY STOPKEY                                                   Cost=1710    Cardinality=20237    Bytes=5949678
TABLE ACCESS FULL    Object owner=TEST    Object name=QUESTION_HEAD    Cost=421     Cardinality=20237    Bytes=5949678

初步看看，无非1是查视图，3是直接查表的区别，但关键是去掉Order By Vquest0_.State Asc, Vquest0_.Ask_Time Desc后两者的执行时间差不多，而加上排序后时间就有数十倍的差距，再来看看视图V_Question_Head

create or replace view v_question_head as
Select t.Accept_Id, t.Industry_Id, t.Dept_Id, t.Title, t.Question, t.Requester, t.Requester_Name, t.Ask_Time, t.Answer,
t.Answer_Man, t.Answer_Time, t.Is_Hot, t.Check_Man, t.Check_Time, t.Satisfaction, t.Satis_Reason, t.Telphone,
t.Is_Display, t.Hot_Sort, t.Check_State, t.State, t.Bbs_Id, t.Acceptor, t.Accept_Time, t.Is_Anonymous,
t.Anonymous_Email, t.Ip_Address, t.Is_Requester_Read,
(Select Count(r1.Accept_Id) From Answer_Remind r1 Where t.Accept_Id = r1.Accept_Id) As Reminded_Count,
(Select (Case
When Count(r2.Accept_Id) > 0 Then
1
Else
0
End)
From Answer_Remind r2
Where t.Accept_Id = r2.Accept_Id) As Is_Reminded
From Question_Head t
Order By t.State Asc, t.Ask_Time Desc

通过如上视图，我们不难发现虽然执行计划中没有体现COUNT的代价，但其中有行级的COUNT，也就是主表有多少行就要COUNT多少次。