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游标脚本性能问题详解之游标分类特点篇

    【IT168 专稿】前文:游标脚本性能问题详解之案例实践片篇

  从上篇文章两个脚本执行情况的对比中可以看出,游标的选择对语句执行的性能具有一定的影响。

  在SQL Server联机丛书上列出了不止十种游标类型,但是所有游标都可以被划到两大类别:

  1. 通过从首次得到结果的临时拷贝映像静态进行

  2. 每次fetch都通过动态进行且真正查阅表

  STATIC、KEYSET、READ_ONLY和FAST_FORWARD属于第一大类,FORWARD_ONLY、DYNAMIC和OPTIMISTIC属于第二大类。

  下面我们来进行一定的比较分析,并学习如何使用各种游标。在进行这部分之前,我们要引入另一个set statistics的方法: set statistics profile on

  这个option会帮助我们打印出文本格式的执行计划和每一布的执行统计信息。这个部分的执行语句执行计划都是通过这个option打印的。

  1. 首先,我们把游标脚本中的SQL语句抽取出来直接运行而不使用游标:

SELECT       T1.*
FROM         dbo.S_AUDIT_ITEM T1            
LEFT OUTER JOIN dbo.S_USER T2
ON T1.USER_ID = T2.PAR_ROW_ID    
WHERE   T1.BC_BASE_TBL
= 'S_PARTY' AND   T1.RECORD_ID = '1-10350J'
ORDER BY       T1.OPERATION_DT DESC

  执行情况如下:逻辑读15次,使用的是索引查找(index seek)

Table 'S_USER'. Scan count 1, logical reads 260, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.
Table 'S_AUDIT_ITEM'. Scan count 1, logical reads 15, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.

  执行计划为:

Rows                 Executes             StmtText                                                                                                
-------------------- -------------------- ---------------------------------------------------------------------------------------------------------
4                    1                    SELECT  T1.*  FROM  dbo.S_AUDIT_ITEM T1                                                                                                                  
LEFT OUTER JOIN dbo.S_USER T2                                                                                                                      
ON T1.USER_ID = T2.PAR_ROW_ID                                                                                                                      
WHERE   T1.BC_BASE_TBL
= 'S_PARTY' AND   T1.RECORD_ID = '1-10350J'                                                                                 ORDER BY  T1.OPERATION_DT DESC            1    1    0      NULL         NULL                  
4                    1                      |--Sort(ORDER BY:([T1].[OPERATION_DT] DESC))                                                          
4                    1                           |--Nested Loops(Left Outer Join, WHERE:([testcursor].[dbo].[S_AUDIT_ITEM].[USER_ID] as [T1].[USER_
4                    1                                |--Nested Loops(Inner Join, OUTER REFERENCES:([Uniq1002], [T1].[ROW_ID]) OPTIMIZED)          
4                    1                                |  |--Index Seek(OBJECT:([testcursor].[dbo].[S_AUDIT_ITEM].[S_AUDIT_ITEM_M3] AS [T1]), SEEK
4                    4                                |  |--Clustered Index Seek(OBJECT:([testcursor].[dbo].[S_AUDIT_ITEM].[S_AUDIT_ITEM_P1] AS [
66908                4                                |--Table Scan(OBJECT:([testcursor].[dbo].[S_USER] AS [T2]))  

  2. 下面通过T-SQL语句打开一个游标。注意,这里创建的游标为dynamic类型,因为新声明的游标默认类型为dynamic。。本文开头使用的存储过程是调用API游标的写法,这里是用T-SQL语句打开游标,两种写法使用的游标类型和执行的语句是完全一样的。

declare @CONFLICT_ID int
declare curTest cursor

FOR
    
SELECT
       T1.CONFLICT_ID
    FROM         dbo.S_AUDIT_ITEM T1            
    
LEFT OUTER JOIN dbo.S_USER T2
    
ON T1.USER_ID = T2.PAR_ROW_ID    
    WHERE      T1.BC_BASE_TBL
= 'S_PARTY' AND   T1.RECORD_ID ='1-10350J'  
    ORDER BY       T1.OPERATION_DT  

OPEN curTest
FETCH
NEXT FROM curTest
INTO @CONFLICT_ID
CLOSE curTest

deallocate curTest

  执行情况为:逻辑读明显增多,使用索引扫描(index scan)

Table 'Worktable'. Scan count 0, logical reads 3, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.              
Table 'S_USER'. Scan count 1, logical reads 64, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.                
Table 'S_AUDIT_ITEM'. Scan count 1, logical reads 3026834, physical reads 1292, read-ahead reads 5574, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.

  执行计划如下:

Rows                 Executes             StmtText                                                                                                                        
-------------------- -------------------- --------------------------------------------------------------------------------------------------------------------------------
1                    1                    FETCH NEXT FROM curTest                                                                                                        
INTO @CONFLICT_ID                                                                                                                                                        
1                    1                      |--Clustered Index Insert(OBJECT:(CWT), SET:([CWT].[COLUMN0] = [testcursor].[dbo].[S_AUDIT_ITEM].[ROW_ID] as
1                    1                           |--Compute Scalar(DEFINE:([Expr1008]=CWT_ROWID()))                                                                      
1                1                                |--Nested Loops(Left Outer Join, WHERE:([testcursor].[dbo].[S_AUDIT_ITEM].[USER_ID] as
1                    1                                     |--Nested Loops(Inner Join, OUTER REFERENCES:([Uniq1004], [T1].[ROW_ID]))                                      
1007751              1                                     |    |--Index Scan(OBJECT:([testcursor].[dbo].[S_AUDIT_ITEM].[S_AUDIT_ITEM_M4] AS  
1                    1007751                               |    |--Clustered Index Seek(OBJECT:([testcursor].[dbo].[S_AUDIT_ITEM].[S_AUDIT_ITEM_P1] AS
16401                1                                     |--Table Scan(OBJECT:([testcursor].[dbo].[S_USER] AS [T2]))

  接下来,我们使用其他类型的游标进行测试,从它们的测试结果会发现:

  当使用STATIC、KEYSET、READ_ONLY、FAST_FORWARD类型的游标,可以得到理想的执行计划(索引S_AUDIT_ITEM_M3上使用索引查找)。

  但是,如果使用其他第二类游标类型,得到的执行计划就不甚理想了(索引S_AUDIT_ITEM_M4上使用索引扫描)。

  从上面的测试,我们知道STATIC、KEYSET、READ_ONLY及FAST_FORWARD游标可以带给我们同样的理想结果。那么,这些游标有什么共同点?

  我们可以分析一下两大游标类型执行计划的不同:

  1. STATIC、KEYSET、READ_ONLY、FAST_FORWARD类型游标的执行计划:

Executes             StmtText                                                                                                            
-------------------- --------------------------------------------------------------------------------------------------------------------
1                    OPEN curTest                                                                                                        
1                      |--Clustered Index Insert(OBJECT:(CWT), SET:([CWT].[COLUMN0] = [testcursor].[dbo].[S_AUDIT_ITEM].[CONFLICT_ID] as
1                           |--Sequence Project(DEFINE:([Expr1008]=i4_row_number))                                                      
1                                |--Segment                                                                                              
1                                     |--Sort(ORDER BY:([T1].[OPERATION_DT] ASC))                                                        
1                                          |--Nested Loops(Left Outer Join, WHERE:([testcursor].[dbo].[S_AUDIT_ITEM].[USER_ID] as [T1].[U
1                                               |--Nested Loops(Inner Join, OUTER REFERENCES:([Uniq1004], [T1].[ROW_ID]) OPTIMIZED)      
1                                               |    |--Index Seek(OBJECT:([testcursor].[dbo].[S_AUDIT_ITEM].[S_AUDIT_ITEM_M3] AS [T1]),
4                                               |    |--Clustered Index Seek(OBJECT:([testcursor].[dbo].[S_AUDIT_ITEM].[S_AUDIT_ITEM_P1]
4                                               |--Table Scan(OBJECT:([testcursor].[dbo].[S_USER] AS [T2]))                              
                                                                                                    
Executes             StmtText                                                                                        StmtId      NodeId  
-------------------- ----------------------------------------------------------------------------------------------- ----------- --------
1                    FETCH NEXT FROM curTest INTO @CONFLICT_ID                       2           1        
1                      |--Clustered Index Seek(OBJECT:(CWT), SEEK:([CWT].[ROWID]=FETCH_RANGE((0))) ORDERED FORWARD)  2           2      

  2. dynamic类型游标的执行计划

Executes   StmtText                                                                                                                    
---------------------------------------------------------------------------------------------------------------------------------------
1          FETCH NEXT FROM curTest                                                                                                    
                                                                                                                                      
1            |--Clustered Index Insert(OBJECT:(CWT), SET:([CWT].[COLUMN0] = [testcursor].[dbo].[S_AUDIT_ITEM].[ROW_ID] as [T1].[ROW_ID]
1                 |--Compute Scalar(DEFINE:([Expr1008]=CWT_ROWID()))                                                                  
1                      |--Nested Loops(Left Outer Join, WHERE:([testcursor].[dbo].[S_AUDIT_ITEM].[USER_ID] as [T1].[USER_ID]=[testcurso
1                           |--Nested Loops(Inner Join, OUTER REFERENCES:([Uniq1004], [T1].[ROW_ID]))                                  
1                           |    |--Index Scan(OBJECT:([testcursor].[dbo].[S_AUDIT_ITEM].[S_AUDIT_ITEM_M4] AS [T1]), ORDERED BACKWARD)
1007751                     |    |--Clustered Index Seek(OBJECT:([testcursor].[dbo].[S_AUDIT_ITEM].[S_AUDIT_ITEM_P1] AS [T1]), SEEK:([T
1                           |--Table Scan(OBJECT:([testcursor].[dbo].[S_USER] AS [T2]))

  比较一下两个执行计划的FETCH NEXT部分(SQL Server在游标打开阶段不会读取表):在第一个执行计划中,FETCH是直接从临时对象CWT中得到行,然后从CWT.ROWID中找到相应范围。而在第二个计划中,FETCH是动态的而且是真正对表进行了读取,从表中取得数据。

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