从上篇游标脚本性能问题详解之案例实践篇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，大家要引入另一个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，假如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。