EF6產生不需要的SQL查詢

我想知道爲什麼生成的SQL是檢查零位上的列，它是不可爲空（列Value這是一個非空浮）EF6產生不需要的SQL查詢

var query = _context.Events 
    .Select(e => new 
    { 
    eventId = e.EventId, 
    data = e.Data.Take(10).Select(x => new 
    { 
     name = x.Name, 
     value = Math.Round(x.Value,1), 
    }) 
    });

生成以下SQL代碼：

SELECT 
    [Project2].[EventId] AS [EventId], 
    [Project2].[DeviceId] AS [DeviceId], 
    [Project2].[TimeEnd] AS [TimeEnd], 
    [Project2].[C2] AS [C1], 
    [Project2].[EventId1] AS [EventId1], 
    [Project2].[Name] AS [Name], 
    [Project2].[C1] AS [C2] 
    FROM (SELECT 
     [Limit1].[EventId] AS [EventId], 
     [Limit1].[TimeEnd] AS [TimeEnd], 
     [Limit1].[DeviceId] AS [DeviceId], 
     [Extent2].[Name] AS [Name], 
     [Extent2].[EventId] AS [EventId1], 
     CASE WHEN ([Extent2].[Value] IS NULL) THEN CAST(NULL AS float) ELSE ROUND([Extent2].[Value], 1) END AS [C1], 
     CASE WHEN ([Extent2].[Value] IS NULL) THEN CAST(NULL AS int) ELSE 1 END AS [C2] 
     FROM (SELECT [Project1].[EventId] AS [EventId], [Project1].[TimeEnd] AS [TimeEnd], [Project1].[DeviceId] AS [DeviceId] 
      FROM (SELECT 
       [Extent1].[EventId] AS [EventId], 
       [Extent1].[TimeEnd] AS [TimeEnd], 
       [Extent1].[DeviceId] AS [DeviceId] 
       FROM [dbo].[Events] AS [Extent1] 
       WHERE ([Extent1].[DeviceId] = 1) 
      ) AS [Project1] 
      ORDER BY [Project1].[TimeEnd] DESC) AS [Limit1] 
     LEFT OUTER JOIN [dbo].[Octaves] AS [Extent2] ON [Limit1].[EventId] = [Extent2].[EventId] 
    ) AS [Project2] 
    ORDER BY [Project2].[TimeEnd] DESC, [Project2].[EventId] ASC, [Project2].[C2] ASC

如果我卸下兩個CASE WHEN ([Extent2].[Value] IS NULL)和剛剛離開ROUND([Limit2].[Value], 1) AS [C1]並刪除SQL Server管理的ORDER BY Project2].[C2] ASC查詢加快。

來源

2017-06-05 teocomi

在你的EF模型中，你應該檢查字段是否可以爲空。 –

@teocomi \t 如果按原樣運行，查詢需要多長時間？如果你然後刪除CASE？如果您隨後刪除CASE和訂單？ – mjwills

有一個LEFT OUTER JOIN，所以值（在查詢中）肯定可以是NULL（即使基礎表定義不可爲空）。

請參閱http://docs.oracle.com/javadb/10.8.3.0/ref/rrefsqlj18922.html（對於Oracle - 但所有主要SQL供應商都適用相同的基本模式）。

在這種情況下，由於ROUND（NULL，1）無論如何都會返回NULL（它在SQL Server上），所以它有點沒有意義。但是，由於無法控制其生成的SQL，因此您無法做到這一點。

來源

2017-06-05 11:25:27 mjwills

EF6產生不需要的SQL查詢

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