爲什麼選擇Top X Kill Performance

我有下面的T-SQL Query（由linq-to-sql生成，所以看起來有點奇怪）。當我刪除前10名時，它運行在1秒內。當我離開前10名時，需要68秒。爲什麼選擇Top X Kill Performance

如果沒有前10名，行的總數只有19

爲什麼排名前10位的性能消滅在這種情況下，這麼多呢？

SELECT TOP (10) [t3].[value] AS [StartDate], [t3].[AH_TimeStamp4] AS [EndDate], [t3].[value2] AS [RunTime], [t3].[AH_Idnr] AS [RunNumber], [t3].[AH_Status] AS [StatusCode], [t3].[STATUS_DESC] AS [StatusDesc], [t3].[STATUS_TYPE] AS [StatusType] 
FROM (
    SELECT COALESCE([t1].[AH_TimeStamp2],[t1].[AH_TimeStamp1]) AS [value], [t1].[AH_TimeStamp4], CONVERT(BigInt,(((CONVERT(BigInt,DATEDIFF(DAY, [t1].[AH_TimeStamp1], [t1].[AH_TimeStamp4]))) * 86400000) + DATEDIFF(MILLISECOND, DATEADD(DAY, DATEDIFF(DAY, [t1].[AH_TimeStamp1], [t1].[AH_TimeStamp4]), [t1].[AH_TimeStamp1]), [t1].[AH_TimeStamp4])) * 10000) AS [value2], [t1].[AH_Idnr], [t1].[AH_Status], [t2].[STATUS_DESC], [t2].[STATUS_TYPE], [t0].[OH_Name] 
    FROM [dbo].[OH] AS [t0] 
    INNER JOIN [dbo].[AH] AS [t1] ON ([t0].[OH_Idnr]) = [t1].[AH_OH_Idnr] 
    INNER JOIN [dbo].[CHK_JOB_STATUS_CODE] AS [t2] ON [t1].[AH_Status] = ([t2].[STATUS_CODE]) 
    ) AS [t3] 
WHERE [t3].[OH_Name] = @p0 
ORDER BY [t3].[AH_Idnr] DESC

編輯 - >的要求，這裏有沒有采取LINQ生成的查詢（10）

SELECT [t3].[value] AS [StartDate], [t3].[AH_TimeStamp4] AS [EndDate], [t3].[value2] AS [RunTime], [t3].[AH_Idnr] AS [RunNumber], [t3].[AH_Status] AS [StatusCode], [t3].[STATUS_DESC] AS [StatusDesc], [t3].[STATUS_TYPE] AS [StatusType] 
FROM (
    SELECT COALESCE([t1].[AH_TimeStamp2],[t1].[AH_TimeStamp1]) AS [value], [t1].[AH_TimeStamp4], CONVERT(BigInt,(((CONVERT(BigInt,DATEDIFF(DAY, [t1].[AH_TimeStamp1], [t1].[AH_TimeStamp4]))) * 86400000) + DATEDIFF(MILLISECOND, DATEADD(DAY, DATEDIFF(DAY, [t1].[AH_TimeStamp1], [t1].[AH_TimeStamp4]), [t1].[AH_TimeStamp1]), [t1].[AH_TimeStamp4])) * 10000) AS [value2], [t1].[AH_Idnr], [t1].[AH_Status], [t2].[STATUS_DESC], [t2].[STATUS_TYPE], [t0].[OH_Name] 
    FROM [dbo].[OH] AS [t0] 
    INNER JOIN [dbo].[AH] AS [t1] ON ([t0].[OH_Idnr]) = [t1].[AH_OH_Idnr] 
    INNER JOIN [dbo].[CHK_JOB_STATUS_CODE] AS [t2] ON [t1].[AH_Status] = ([t2].[STATUS_CODE]) 
    ) AS [t3] 
WHERE [t3].[OH_Name] = @p0 
ORDER BY [t3].[AH_Idnr] DESC

編輯2 - >這裏的LINQ查詢 - 沒有什麼特別的，只是3加入

List<UC4Status> statusList = uc4DB.OHs.Where(o => o.OH_Name == jobName).Join(uc4DB.AHs, oh => oh.OH_Idnr, ah => ah.AH_OH_Idnr, (oh, ah) => ah) 
         .Join(uc4DB.CHK_JOB_STATUS_CODEs, ah => ah.AH_Status, job => job.STATUS_CODE, (a, s) => 
          new UC4Status 
          { 
           StartDate = a.AH_TimeStamp2 ?? a.AH_TimeStamp1, 
           EndDate = a.AH_TimeStamp4, 
           RunTime = a.AH_TimeStamp4 - a.AH_TimeStamp1, 
           StatusType = s.STATUS_TYPE, 
           StatusDesc = s.STATUS_DESC, 
           StatusCode = a.AH_Status, 
           RunNumber = a.AH_Idnr 
          }).OrderByDescending(r => r.RunNumber).Take(maxResults).ToList();

來源

2011-01-07 Adam Rackis

您可以發佈生成的查詢沒有linq上的.Take（10）到sql好奇如果有關於查詢的任何其他改變（它不應該，但你永遠不知道） – Adrian 2011-01-07 23:12:05

此外，在這兩種情況下生成的查詢，你可以發佈執行計劃嗎？如果您安裝了SQL Management Studio，可以通過按下突出顯示查詢的Control-L來獲取它。 – SqlRyan 2011-01-07 23:15:10

哦，你還可以發佈生成這個sql的linq語句嗎？我們可以通過嘗試從linq聲明一方進行優化來獲得更多的信息。 – Adrian 2011-01-07 23:15:31

比返回的行數更重要的是每個表中的行數加入。如果沒有查詢計劃，很難說問題是什麼。但是，這個問題很可能是由於過時或缺少列統計信息以及缺少良好索引，導致查詢優化程序在使用top時做出非常糟糕的決定。例如，如果優化器錯誤地假設top語句僅對每個連接表中的少量行進行操作，則它可以選擇在每個連接的表上使用內部循環連接和表掃描 - 這可能是非常可怕的對大量記錄的緩慢操作（但只能處理少量記錄時可以非常有效地工作）。在沒有top的情況下，優化程序可以正確地假定它必須掃描連接表中的大量記錄，並使用更適合此類操作的方法，例如表掃描中的哈希匹配連接。

您可以嘗試更新統計信息並添加其他索引以查看是否有幫助。

來源

2011-01-07 23:50:58

沒有top 10的行數是多少？是否有任何指標實施？我猜子查詢T0，T1，T2和加入的是由於... 你可以嘗試類似，

 

select top 10 * from (
    select t3.value.... 
    from ( 
    select coalesce...) 
    where ... order by ... 
)

來源

2011-01-07 23:15:42 suhprano

爲什麼選擇Top X Kill Performance

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