查看SQL語句的真實執行計划

DBMS_XPLAN包中display_cursor函數不同於display函數，display_cursor用於顯示SQL語句的真實的執行計划，在大多數情況下，顯示真實
的執行計划有助於更好的分析SQL語句的全過程，尤其是運行此SQL語句實時的I/O開銷。通過對比預估的I/O與真實的I/O開銷來判斷SQL語句所存
在問題，如缺少統計信息，SQL語句執行的次數，根據實際中間結果集的大小來選擇合適的連接方式等。本文僅僅講述display_cursor函數的使
用。
 
     有關執行計划中各字段模塊的描述請參考： 執行計划中各字段各模塊描述
        有關由SQL語句來獲取執行計划請參考：    使用EXPLAIN PLAN獲取SQL語句執行計划
        有關使用autotrace來獲取執行計划請參考：啟用AUTOTRACE功能
        有關dbms_xplan之display函數請參考：   dbms_xplan之display函數的使用
一、display_cursor函數用法
1、display_cursor函數語法
 
DBMS_XPLAN.DISPLAY_CURSOR(                        
 sql_id        IN  VARCHAR2  DEFAULT  NULL,       
 child_number  IN  NUMBER    DEFAULT  NULL,       
 format        IN  VARCHAR2  DEFAULT  'TYPICAL'); 
2、display_cursor函數參數描述
        sql_id
                指定位於庫緩存執行計划中SQL語句的父游標。默認值為null。當使用默認值時當前會話的最后一條SQL語句的執行計划將被返回
                可以通過查詢V$SQL 或V$SQLAREA的SQL_ID列來獲得SQL語句的SQL_ID。
        child_number
                指定父游標下子游標的序號。即指定被返回執行計划的SQL語句的子游標。默認值為0。如果為null，則sql_id所指父游標下所有子游標
                的執行計划都將被返回。
        format
                控制SQL語句執行計划的輸出部分，即哪些可以顯示哪些不顯示。使用與display函數的format參數與修飾符在這里同樣適用。
                除此之外當在開啟statistics_level=all時或使用gather_plan_statistics提示可以獲得執行計划中實時的統計信息
                有關詳細的format格式描述請參考：dbms_xplan之display函數的使用 中format參數的描述
 
        下面給出啟用統計信息時format新增的修飾符
                iostats   控制I/O統計的顯示
                last      默認，顯示所有執行計算過的統計。如果指定該值，則只顯示最后一次執行的統計信息
                memstats  控制pga相關統計的顯示
                allstats  此為iostats memstats的快捷方式，即allstats包含了iostats和memstats
                run_stats_last 等同於iostats last。只能用於oracle 10g R1
                run_stats_tot  等同於iostats。只能用於oracle 10g R1                 
 
二、演示使用display_cursor函數獲取執行計划   
        1、當前數據庫版本以及加載執行計划到庫緩存               
 
SQL> select * from v$version where rownum<2;                       
                                                                   
BANNER                                                             
----------------------------------------------------------------   
Oracle Database 10g Release 10.2.0.3.0 - 64bit Production          
                                                                             
SQL> SELECT ename,dname,loc                                        
  2  FROM   emp e, dept d                                           
  3  WHERE  e.deptno = d.deptno                                    
  4  AND    e.empno  = 7788;                                       
                                                                   
ENAME      DNAME          LOC                                       
---------- -------------- -------------                            
SCOTT      RESEARCH       DALLAS                                   
        2、查看真實的執行計划             
 
/*----------------不傳遞任何參數給display_cursor函數，顯示當前會話最后一條SQL語句的執行計划-------------*/             
/**************************************************/                                                                   
/* Author: Robinson Cheng                         */                                                                   
/* Blog:   https://blog.csdn.net/robinson_0612     */                                                                   
/* MSN:    [e-mail]              */                                                                   
/* QQ:     645746311                              */                                                                   
/**************************************************/                                                                   
SQL> select * from table(dbms_xplan.display_cursor(null,null));                                                        
                                                                                                                        
PLAN_TABLE_OUTPUT                                                                                                      
------------------------------------------------------------------------------------------                             
SQL_ID  a67wqmkfb9j65, child number 0                                                                                  
-------------------------------------                                                                                   
SELECT ename,dname,loc FROM   emp e, dept d WHERE  e.deptno = d.deptno AND                                             
e.empno  = 7788                                                                                                         
                                                                                                                       
Plan hash value: 2385808155                                                                                             
                                                                                                                       
----------------------------------------------------------------------------------------                                
| Id  | Operation                    | Name    | Rows  | Bytes | Cost (%CPU)| Time     |                               
----------------------------------------------------------------------------------------                               
|   0 | SELECT STATEMENT             |         |       |       |     3 (100)|          |                               
|   1 |  NESTED LOOPS                |         |     1 |    63 |     3   (0)| 00:00:01 |                               
|   2 |   TABLE ACCESS BY INDEX ROWID| EMP     |     1 |    33 |     2   (0)| 00:00:01 |                               
|*  3 |    INDEX UNIQUE SCAN         | PK_EMP  |     1 |       |     1   (0)| 00:00:01 |                               
|   4 |   TABLE ACCESS BY INDEX ROWID| DEPT    |   409 | 12270 |     1   (0)| 00:00:01 |                               
|*  5 |    INDEX UNIQUE SCAN         | PK_DEPT |     1 |       |     0   (0)|          |                               
----------------------------------------------------------------------------------------                               
                                                                                                                       
Predicate Information (identified by operation id):                                                                     
---------------------------------------------------                                                                    
                                                                                                                       
   3 - access("E"."EMPNO"=7788)                                                                                        
   5 - access("E"."DEPTNO"="D"."DEPTNO")                                                                               
                                                                                                                       
/*------------------- 獲得SQL語句的SQL_ID，可以看出此SQL_ID與上面顯示的執行計划中的SQL_ID一致----------*/             
SQL> select sql_id,address,plan_hash_value,hash_value,child_number from v$sql                                          
  2  where sql_text like '%SELECT ename%' and sql_text not like '%from v$sql%';                                        
                                                                                                                       
SQL_ID        ADDRESS          PLAN_HASH_VALUE HASH_VALUE CHILD_NUMBER                                                  
------------- ---------------- --------------- ---------- ------------                                                 
a67wqmkfb9j65 0000000091DBFBC8      2385808155 2629092549            0                                                  
                                                                                                                       
/*-------------- 傳遞SQL_ID以及format參數，並配合修飾符控制執行計划的輸出------------------------*/                   
SQL> select * from table(dbms_xplan.display_cursor('a67wqmkfb9j65',null,'typical -predicate -rows'));                  
                                                                                                                       
PLAN_TABLE_OUTPUT                                                                                                       
------------------------------------------------------------------------------------                                   
SQL_ID  a67wqmkfb9j65, child number 0                                                                                   
-------------------------------------                                                                                  
SELECT ename,dname,loc FROM   emp e, dept d WHERE  e.deptno = d.deptno                                                 
AND    e.empno  = 7788                                                                                                 
                                                                                                                        
Plan hash value: 2385808155                                                                                            
                                                                                                                        
--------------------------------------------------------------------------------                                       
| Id  | Operation                    | Name    | Bytes | Cost (%CPU)| Time     |                                        
--------------------------------------------------------------------------------                                       
|   0 | SELECT STATEMENT             |         |       |     3 (100)|          |                                        
|   1 |  NESTED LOOPS                |         |    63 |     3   (0)| 00:00:01 |                                       
|   2 |   TABLE ACCESS BY INDEX ROWID| EMP     |    33 |     2   (0)| 00:00:01 |                                       
|   3 |    INDEX UNIQUE SCAN         | PK_EMP  |       |     1   (0)| 00:00:01 |                                       
|   4 |   TABLE ACCESS BY INDEX ROWID| DEPT    | 12270 |     1   (0)| 00:00:01 |                                       
|   5 |    INDEX UNIQUE SCAN         | PK_DEPT |       |     0   (0)|          |                                       
--------------------------------------------------------------------------------                                       
        3、查看真實執行計划並獲得統計信息
                前提條件
                        設置參數statistics_level為all,可以基於session級別以及實例級別
                        或者啟用gather_plan_statistics提示     
 
/*-------------查看實例參數statistics_level的值，並在會話級別將其設定為all  ---------*/                                 
SQL> show parameter statistics_le                                                                                       
                                                                                                                        
NAME                                 TYPE        VALUE                                                                  
------------------------------------ ----------- ------------------------------                                         
statistics_level                     string      ALL                                                                    
                                                                                                                        
SQL> alter session set statistics_level=all;                                                                            
                                                                                                                        
Session altered.                                                                                                         
                                                                                                                        
SQL> select e.ename,e.sal,s.grade                                                                                        
  2  from emp e                                                                                                         
  3  join salgrade s                                                                                                     
  4  on e.sal between losal and hisal                                                                                   
  5  and e.deptno = 20;                                                                                                  
                                                                                                                        
ENAME             SAL      GRADE                                                                                         
---------- ---------- ----------                                                                                        
SCOTT            3000          4                                                                                        
FORD             3000          4                                                                                        
JONES            2975          4                                                                                        
ADAMS            1100          1                                                                                        
SMITH             800          1                                                                                        
                                                                                                                        
/*------- 執行上述SQL語句后獲得其真實的執行計划，使用了iostats last -predicate -note 修飾符控制顯示輸出-----*/         
SQL> set pagesize 0                                                                                                     
SQL> select * from table(dbms_xplan.display_cursor(null,null,'iostats last -predicate -note'));                         
SQL_ID  243b0tpjxj6wv, child number 0                                                                                   
-------------------------------------                                                                                    
select e.ename,e.sal,s.grade from emp e join salgrade s on e.sal between losal and                                      
hisal and e.deptno = 20                                                                                                  
                                                                                                                        
Plan hash value: 4204027666                                                                                              
                                                                                                                        
-------------------------------------------------------------------------------------------                             
| Id  | Operation            | Name     | Starts | E-Rows | A-Rows |   A-Time   | Buffers |                             
-------------------------------------------------------------------------------------------                             
|   1 |  MERGE JOIN          |          |      1 |      1 |      5 |00:00:00.01 |      14 |                             
|   2 |   SORT JOIN          |          |      1 |      5 |      5 |00:00:00.01 |       7 |                             
|   3 |    TABLE ACCESS FULL | EMP      |      1 |      5 |      5 |00:00:00.01 |       7 |                             
|   4 |   FILTER             |          |      5 |        |      5 |00:00:00.01 |       7 |                             
|   5 |    SORT JOIN         |          |      5 |      5 |     14 |00:00:00.01 |       7 |                             
|   6 |     TABLE ACCESS FULL| SALGRADE |      1 |      5 |      5 |00:00:00.01 |       7 |                             
-------------------------------------------------------------------------------------------                             
                                                                                                                        
/*---------------- 修改會話級別的參數statistics_level為typical並驗證修改結果----------------*/                         
SQL> alter session set statistics_level=typical;                                                                        
                                                                                                                                
SQL> col name format a40                                                                                                
SQL> col value format a25                                                                                               
SQL> col display_value format a25                                                                                       
SQL> select name, value, display_value, isses_modifiable                                                                
  2  from v$parameter                                                                                                   
  3  where isses_modifiable = 'TRUE'                                                                                    
  4  and name like '%&input_name%';                                                                                     
Enter value for input_name: statistics_level                                                                            
old   4: and name like '%&input_name%'                                                                                  
new   4: and name like '%statistics_level%'                                                                              
                                                                                                                        
NAME                                     VALUE                     DISPLAY_VALUE             ISSES                      
---------------------------------------- ------------------------- ------------------------- -----                      
statistics_level                         TYPICAL                   TYPICAL                   TRUE                       
                                                                                                                        
/*-------- 使用提示gather_plan_statistics，並獲得其真實執行計划，使用了allstats -rows修飾符控制顯示輸出---*/            
SQL> set pagesize 180                                                                                                   
SQL> SELECT /*+ gather_plan_statistics */ ename,dname,loc                                                               
  2  FROM   emp e, dept d                                                                                               
  3  WHERE  e.deptno = d.deptno                                                                                         
  4  AND    d.deptno=20 ORDER BY 1,2,3;                                                                                 
                                                                                                                        
ENAME      DNAME          LOC                                                                                            
---------- -------------- -------------                                                                                 
ADAMS      RESEARCH       DALLAS                                                                                         
FORD       RESEARCH       DALLAS                                                                                        
JONES      RESEARCH       DALLAS                                                                                         
SCOTT      RESEARCH       DALLAS                                                                                        
SMITH      RESEARCH       DALLAS                                                                                        
                                                                                                                        
SQL> select * from table(dbms_xplan.display_cursor(null,null,'allstats -rows'));                                        
                                                                                                                        
PLAN_TABLE_OUTPUT                                                                                                        
-------------------------------------------------------------------------------------------------                       
SQL_ID  d2hh42yzqqjz7, child number 0                                                                                   
-------------------------------------                                                                                   
SELECT /*+ gather_plan_statistics */ ename,dname,loc FROM   emp e, dept d WHERE  e.deptno = d.deptno AND                
d.deptno=20 ORDER BY 1,2,3                                                                                              
                                                                                                                         
Plan hash value: 3339094711                                                                                             
                                                                                                                         
---------------------------------------------------------------------------------------------------------------------   
| Id  | Operation                     | Name    | Starts | A-Rows |   A-Time   | Buffers |  OMem |  1Mem |  O/1/M   |   
---------------------------------------------------------------------------------------------------------------------   
|   1 |  SORT ORDER BY                |         |      1 |      5 |00:00:00.01 |       9 |  2048 |  2048 |     1/0/0|   
|   2 |   NESTED LOOPS                |         |      1 |      5 |00:00:00.01 |       9 |       |       |          |   
|   3 |    TABLE ACCESS BY INDEX ROWID| DEPT    |      1 |      1 |00:00:00.01 |       2 |       |       |          |   
|*  4 |     INDEX UNIQUE SCAN         | PK_DEPT |      1 |      1 |00:00:00.01 |       1 |       |       |          |   
|*  5 |    TABLE ACCESS FULL          | EMP     |      1 |      5 |00:00:00.01 |       7 |       |       |          |    
---------------------------------------------------------------------------------------------------------------------   
                                                                                                                        
Predicate Information (identified by operation id):                                                                     
---------------------------------------------------                                                                     
                                                                                                                         
   4 - access("D"."DEPTNO"=20)                                                                                          
   5 - filter("E"."DEPTNO"=20)                                                                                           
                                                                                                                        
Note                                                                                                                    
-----                                                                                                                   
   - dynamic sampling used for this statement                                                                           
三、總結
        1、與display函數不同，display_cursor顯示的為真實的執行計划
        2、對於format參數，使用與display函數的各個值，同樣適用於display_cursor函數
        3、當statistics_level為all或使用gather_plan_statistics提示可以獲得執行時的統計信息
        4、根據真實與預估的統計信息可以初步判斷SQL效率低下的原因，如統計信息的准確性、主要的開銷位於那些步驟等