函数使得索引列失效

在索引列上使用函数使得索引失效的是常见的索引失效原因之一,因此尽可能的避免在索引列上使用函数。尽管可以使用基于函数的索引来
解决索引失效的问题,但如此一来带来的比如磁盘空间的占用以及列上过多的索引导致DML性能的下降。本文描述的是一个索引列上使用函数使
其失效的案例。

一、数据版本与原始语句及相关信息
  1.版本信息    

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  1. SQL> select * from v$version;                                         
  2.                                                                       
  3. BANNER                                                                
  4. ----------------------------------------------------------------      
  5. Oracle Database 10g Release 10.2.0.3.0 - 64bit Production             
  6. PL/SQL Release 10.2.0.3.0 - Production                                
  7. CORE    10.2.0.3.0      Production                                    
  8. TNS for Linux: Version 10.2.0.3.0 - Production                        
  9. NLSRTL Version 10.2.0.3.0 - Production                                

  2.原始语句与其执行计划   

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  1. SQL> set autotrace traceonly exp;                                                                   
  2.                                                                                                     
  3. SELECT acc_num,                                                                                     
  4.        curr_cd,                                                                                       
  5.        DECODE('20110728',                                                                             
  6.               (SELECT TO_CHAR(LAST_DAY(TO_DATE('20110728', 'YYYYMMDD')),                                  
  7.                               'YYYYMMDD')                                                                         
  8.                FROM   DUAL),                                                                              
  9.               0,                                                                                          
  10.               adj_credit_int_lv1_amt + adj_credit_int_lv2_amt -                                           
  11.               adj_debit_int_lv1_amt - adj_debit_int_lv2_amt) AS interest                                  
  12. FROM   acc_pos_int_tbl ACC_POS_INT_TBL1                                                             
  13. WHERE  SUBSTR(business_date, 1, 6) = SUBSTR('20110728', 1, 6)                                       
  14.        AND business_date <= '20110728';                                                               
  15.                                                                                                     
  16. Execution Plan                                                                                      
  17. ----------------------------------------------------------                                          
  18. Plan hash value: 3114115399                                                                         
  19.                                                                                                     
  20. -------------------------------------------------------------------------------------               
  21. | Id  | Operation         | Name            | Rows  | Bytes | Cost (%CPU)| Time     |               
  22. -------------------------------------------------------------------------------------               
  23. |   0 | SELECT STATEMENT  |                 |   336K|    12M| 96399   (1)| 00:19:17 |               
  24. |   1 |  FAST DUAL        |                 |     1 |       |     2   (0)| 00:00:01 |               
  25. |*  2 |  TABLE ACCESS FULL| ACC_POS_INT_TBL |   336K|    12M| 96399   (1)| 00:19:17 |               
  26. -------------------------------------------------------------------------------------               
  27.                                                                                                     
  28. Predicate Information (identified by operation id):                                                 
  29. ---------------------------------------------------                                                 
  30.                                                                                                     
  31.    2 - filter(SUBSTR("BUSINESS_DATE",1,6)='201107' AND                                              
  32.               "BUSINESS_DATE"<='20110728')                                                          

    从执行计划可以看出,SQL语句使用了全表扫描,而where 子句中只有唯一的一列business_date

  3.表上的索引信息     

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  1. SQL> set autotrace off;                                                                                    
  2. SQL> set linesize 190                                                                                      
  3. SQL> @Idx_Info                                                                                             
  4. Enter value for owner: goex_admin                                                                          
  5. old  10:           AND owner = upper('&owner')                                                             
  6. new  10:           AND owner = upper('goex_admin')                                                         
  7. Enter value for table_name: ACC_POS_INT_TBL                                                                
  8. old  11:           AND a.table_name = upper('&table_name')                                                 
  9. new  11:           AND a.table_name = upper('ACC_POS_INT_TBL')                                             
  10.                                                                                                            
  11. TABLE_NAME         INDEX_NAME               COL_NAM              CL_POS STATUS   IDX_TYP         DSCD      
  12. ------------------ ------------------------ -------------------- ------ -------- --------------- ----      
  13. ACC_POS_INT_TBL    ACC_POS_INT_10DIG_IDX    SYS_NC00032$              1 VALID    FUNCTION-BASED  ASC       
  14.                                                                                  NORMAL                    
  15.                                                                                                            
  16. ACC_POS_INT_TBL    ACC_POS_INT_10DIG_IDX    BUSINESS_DATE             2 VALID    FUNCTION-BASED  ASC       
  17.                                                                                  NORMAL                    
  18.                                                                                                            
  19. ACC_POS_INT_TBL    ACC_POS_INT_10DIG_IDX    CURR_CD                   3 VALID    FUNCTION-BASED  ASC       
  20.                                                                                  NORMAL                    
  21.                                                                                                            
  22. ACC_POS_INT_TBL    PK_ACC_POS_INT_TBL       ACC_NUM                   1 VALID    NORMAL          ASC       
  23. ACC_POS_INT_TBL    PK_ACC_POS_INT_TBL       BUSINESS_DATE             2 VALID    NORMAL          ASC       

    从索引的情况上来看有一个基于主键的索引包含了BUSINESS_DATE列,而查询语句并没有走索引而是选择的全表扫描,而且预估所返回
    的行Rows与bytes也是大的惊人,cost的值96399,接近10W。

二、分析与改造SQL语句
  1.原始的SQL语句分析
       SQL语句中where子句的business_date列实现对记录过滤
       business_date <= '20110728'条件不会限制索引的使用
       SUBSTR(business_date, 1, 6) = SUBSTR('20110728', 1, 6)使用了SUBSTR函数,限制了优化器选择索引
       基于business_date列来建立索引函数,从已存在的索引来看,必要性不大
   
  2.改造SQL语句
    SUBSTR(business_date, 1, 6) = SUBSTR('20110728', 1, 6)的实质是等于当月,即限制返回的行为从2011.7.1日至2011.7.28
    因此其返回的记录大于等于2011.7.1,且小于2011.7.28
    做如下改造
     business_date >=to_char(last_day(add_months(to_date('20110728','yyyymmdd'),-1)) + 1,'yyyymmdd')
   
  3.改造后的SQL语句   

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  1. SELECT acc_num,                                                                  
  2.        curr_cd,                                                                    
  3.        DECODE('20110728',                                                          
  4.               (SELECT TO_CHAR(LAST_DAY(TO_DATE('20110728', 'YYYYMMDD')),               
  5.                               'YYYYMMDD')                                                      
  6.                FROM   DUAL),                                                           
  7.               0,                                                                       
  8.               adj_credit_int_lv1_amt + adj_credit_int_lv2_amt -                        
  9.               adj_debit_int_lv1_amt - adj_debit_int_lv2_amt) AS interest               
  10. FROM   acc_pos_int_tbl ACC_POS_INT_TBL1                                          
  11. WHERE  business_date >=                                                          
  12.        to_char(last_day(add_months(to_date('20110728', 'yyyymmdd'), -1)) + 1,      
  13.                'yyyymmdd')                                                             
  14.        AND business_date <= '20110728';                                               

   4.改造后的执行计划  

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  1. Execution Plan                                                                                                 
  2. ----------------------------------------------------------                                                     
  3. Plan hash value: 66267922                                                                                      
  4.                                                                                                                
  5. --------------------------------------------------------------------------------------------------             
  6. | Id  | Operation                   | Name               | Rows  | Bytes | Cost (%CPU)| Time     |             
  7. --------------------------------------------------------------------------------------------------             
  8. |   0 | SELECT STATEMENT            |                    |  1065K|    39M| 75043   (1)| 00:15:01 |             
  9. |   1 |  FAST DUAL                  |                    |     1 |       |     2   (0)| 00:00:01 |             
  10. |   2 |  TABLE ACCESS BY INDEX ROWID| ACC_POS_INT_TBL    |  1065K|    39M| 75043   (1)| 00:15:01 |             
  11. |*  3 |   INDEX SKIP SCAN           | PK_ACC_POS_INT_TBL | 33730 |       | 41180   (1)| 00:08:15 |             
  12. --------------------------------------------------------------------------------------------------             
  13.                                                                                                                
  14. Predicate Information (identified by operation id):                                                            
  15. ---------------------------------------------------                                                            
  16.                                                                                                                
  17.    3 - access("BUSINESS_DATE">='20110701' AND "BUSINESS_DATE"<='20110728')                                     
  18.        filter("BUSINESS_DATE">='20110701' AND "BUSINESS_DATE"<='20110728')                                     

    改造后可以看到SQL语句的执行计划已经由原来的全表扫描改为执行INDEX SKIP SCAN,但其cost也并没有降低多少

三、进一步分析
  1.表的相关信息  

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  1. SQL> @Tab_Stat                                                                                          
  2. Enter value for input_table_name: ACC_POS_INT_TBL                                                       
  3. old  11: WHERE  table_name = upper('&input_table_name')                                                 
  4. new  11: WHERE  table_name = upper('ACC_POS_INT_TBL')                                                   
  5. Enter value for input_owner: goex_admin                                                                 
  6. old  12:           AND owner = upper('&input_owner')                                                    
  7. new  12:           AND owner = upper('goex_admin')                                                      
  8.                                                                                                         
  9.   NUM_ROWS       BLKS    EM_BLKS  AVG_SPACE  CHAIN_CNT AVG_ROW_LEN AVG_ROWS_PER_BLOCK LST_ANLY  STA     
  10. ---------- ---------- ---------- ---------- ---------- ----------- ------------------ --------- ---     
  11.   33659947     437206       1322        855          0          99                 77 27-SEP-11 NO      

  2.索引的相关信息 

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  1. SQL> @Idx_Stat                                                                                                         
  2. Enter value for input_table_name: ACC_POS_INT_TBL                                                                      
  3. old  11: WHERE  table_name = upper('&input_table_name')                                                                
  4. new  11: WHERE  table_name = upper('ACC_POS_INT_TBL')                                                                  
  5. Enter value for input_owner: goex_admin                                                                                
  6. old  12:           AND owner = upper('&input_owner')                                                                   
  7. new  12:           AND owner = upper('goex_admin')                                                                     
  8.                                                                                                                        
  9. BLEV IDX_NAME                          LF_BLKS   DST_KEYS   NUM_ROWS LF_PER_KEY DAT_BLK_PER_KEY   CLUS_FCT LST_ANLY    
  10. ---- ------------------------------ ---------- ---------- ---------- ---------- --------------- ---------- ---------   
  11.    3 PK_ACC_POS_INT_TBL                 155658   33777720   33777720          1               1   33777447 27-SEP-11   
  12.    3 ACC_POS_INT_10DIG_IDX              160247   32850596   32850596          1               1   32763921 27-SEP-11   

  3.尝试在BUSINESS_DATE列上创建索引   

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  1. SQL> create index I_ACC_POS_INT_TBL_BS_DT on ACC_POS_INT_TBL(BUSINESS_DATE) tablespace tbs_tmp nologging;               
  2.                                                                                                                         
  3. Index created.                                                                                                          
  4.                                                                                                                         
  5. SQL> @Idx_Stat                                                                                                          
  6. Enter value for input_table_name: ACC_POS_INT_TBL                                                                       
  7. old  11: WHERE  table_name = upper('&input_table_name')                                                                 
  8. new  11: WHERE  table_name = upper('ACC_POS_INT_TBL')                                                                   
  9. Enter value for input_owner: goex_admin                                                                                 
  10. old  12:           AND owner = upper('&input_owner')                                                                    
  11. new  12:           AND owner = upper('goex_admin')                                                                      
  12.                                                                                                                         
  13. BLEV IDX_NAME                          LF_BLKS   DST_KEYS   NUM_ROWS LF_PER_KEY DAT_BLK_PER_KEY   CLUS_FCT LST_ANLY     
  14. ---- ------------------------------ ---------- ---------- ---------- ---------- --------------- ---------- ---------    
  15.    2 I_ACC_POS_INT_TBL_BS_DT             93761        908   33659855        103             506     460007 30-SEP-11    
  16.    3 PK_ACC_POS_INT_TBL                 155658   33777720   33777720          1               1   33777447 27-SEP-11    
  17.    3 ACC_POS_INT_10DIG_IDX              160247   32850596   32850596          1               1   32763921 27-SEP-11    

  建立索引后聚簇因子较小,差不多接近表上块的数量
  
  4.使用新创建索引后的执行计划   

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  1. Execution Plan                                                                                                 
  2. ----------------------------------------------------------                                                     
  3. Plan hash value: 2183566226                                                                                    
  4.                                                                                                                
  5. -------------------------------------------------------------------------------------------------------        
  6. | Id  | Operation                   | Name                    | Rows  | Bytes | Cost (%CPU)| Time     |        
  7. -------------------------------------------------------------------------------------------------------        
  8. |   0 | SELECT STATEMENT            |                         |  1065K|    39M| 17586   (1)| 00:03:32 |        
  9. |   1 |  FAST DUAL                  |                         |     1 |       |     2   (0)| 00:00:01 |        
  10. |   2 |  TABLE ACCESS BY INDEX ROWID| ACC_POS_INT_TBL         |  1065K|    39M| 17586   (1)| 00:03:32 |        
  11. |*  3 |   INDEX RANGE SCAN          | I_ACC_POS_INT_TBL_BS_DT |  1065K|       |  2984   (1)| 00:00:36 |        
  12. -------------------------------------------------------------------------------------------------------        
  13.                                                                                                                
  14. Predicate Information (identified by operation id):                                                            
  15. ---------------------------------------------------                                                            
  16.                                                                                                                
  17.    3 - access("BUSINESS_DATE">='20110701' AND "BUSINESS_DATE"<='20110728')                                     

  从上面的执行计划看出,SQL语句已经选择了新建的索引
  尽管返回的rows,bytes没有明显的变化,但cost已经少了近7倍。

posted @ 2016-05-13 11:15  AmilyAmily  阅读(473)  评论(0编辑  收藏  举报