如何诊断高扫描计数

Question

我很好奇如何在永久和临时表中诊断高扫描计数。我在运行存储过程时打开了统计信息，我试图弄清楚为什么扫描计数如此之高。我有实际的查询计划，SSMS已经建议添加一些丢失的索引，但此时没有别的。我可以研究哪些策略来诊断这些问题？

https://www.brentozar.com/pastetheplan/?id=HksFRMLw-

SQL Server Execution Times:
CPU time = 0 ms,  elapsed time = 0 ms.
Table 'disbursement_check'. Scan count 9, logical reads 2281, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.
Table 'cif'. Scan count 9, logical reads 490, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.
Table 'voucher'. Scan count 9, logical reads 3067, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.
Table 'cif_loan_relationship'. Scan count 9, logical reads 890, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.
Table 'loanacct'. Scan count 45, logical reads 44124, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.
Table 'Workfile'. Scan count 0, logical reads 0, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.
Table 'Worktable'. Scan count 0, logical reads 0, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.
Table 'loanacct_trans_history'. Scan count 1166870, logical reads 3654129, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.
Table '#B17AB456'. Scan count 8, logical reads 1180568, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.
Table '#BAD74620'. Scan count 2423794, logical reads 2486442, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.
Table '#BBCB6A59'. Scan count 62661, logical reads 125306, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.
Table 'loan_class'. Scan count 2, logical reads 4, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.
Table 'lookup_table'. Scan count 2, logical reads 6, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.
Table 'loanacct_detail'. Scan count 9, logical reads 1348, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.
Table 'Worktable'. Scan count 0, logical reads 0, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.

SQL Server Execution Times:
CPU time = 59160 ms,  elapsed time = 9877 ms.
Table '#TargetTransactions_________________________________________________________________________________________________0000000019E6'. Scan count 1, logical reads 1, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.

Answer 1

很棒的问题。

根据一些统计数据，我倾向于认为有一些索引或缺少索引，这些都是错误的。查询计划可能会有所启发，但截至发布时我还没有。

我认为假设loanacct_trans_history表是建议索引的表是安全的。此外，3个临时表似乎也做了相当多的工作......

以下链接有助于指导您充分了解索引以回答您的问题。

  

请参阅这个免费的90分钟网络研讨会，来自世界级的DBA，Brent Ozar：https://www.brentozar.com/archive/2016/10/think-like-engine-class-now-free-open-source/

Answer 2

  

我正在试图找出扫描计数如此之高的原因。

和

  

表'loanacct_trans_history'。扫描计数1166870，逻辑读取   3654129，物理读取0，预读读取0，lob逻辑读取0，   lob物理读取0，lob预读读取0.表'＃B17AB456'。扫描   计数8，逻辑读取1180568，物理读取0，预读读取0，   lob逻辑读取0，lob物理读取0，lob预读读取0。

因为我们没有（还有）实际执行计划，最佳假设是

[1]有一些Index Seek生成的大量范围扫描（很可能）具有大量Execution Count（可能被FORCESEEK表提示强制）

[2]和/或通过以下操作符组合生成的范围扫描或完整扫描

... - Nested Loops (Join) - Any Data Access Operator
           |
           ---------------- Index Seek / *Scan loanacct_trans_history 

在这种情况下，我假设 Estimated execution count是1，实际/真实Execution Count是高。

[3]来自执行计划的裁剪，演示了点[1]和[2]，并与表loanacct_trans_history上的高扫描计数相关：

  

表'loanacct_trans_history'。扫描计数1166870，...

以下语句似乎会产生这种高扫描次数：

INSERT INTO #TargetTransactions (Id, CreditServiceFeeTransactionId, CustomerCifNo, VoucherId)
 SELECT mt.transrefno, rt.CreditServiceFeeTransactionId, contact.cifno, rt.VoucherId
 FROM dbo.getMyTransactions
 (
 @AssignedDealerIds,
 @DealerIds
 ) mt
 JOIN loanacct_trans_history th ON th.transrefno = mt.transrefno
...

几个简单的解决方案：在本声明的最后，我会尝试添加以下查询提示：

INSERT #T...
SELECT ...
...
OPTION(HASH JOIN)
-- Or OPTION(MERGE JOIN)
-- Or OPTION(HASH JOIN, MERGE JOIN)