Oracle不合理的表設結構計導致執行緩慢

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; 

看如下執行計劃，顯然2速度快可以理解，因為行數只有6，而1和3為什麼有那麼大的速度差距，要命的是執行計劃看起來沒有差距。

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，無非1是查視圖，3是直接查表的區別，但關鍵是去掉Order By Vquest0_.State Asc, Vquest0_.Ask_Time Desc後兩者的執行時間差不多，而加上排序後時間就有數十倍的差距，再來看看視圖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 

通過如上視圖，我們不難發現雖然執行計劃中沒有體現COUNT的代價，但其中有行級的COUNT，也就是主表有多少行就要COUNT多少次，而這時一個很恐怖的數字。顯然這是表結構設計上的問題。而要優化就是要更改表結構，在主表中及上需要COUNT的字段。