生成高效留下了COUNT在LINQ到實體

Question

JOIN我有這樣的模式：

Lists (ListId, Name, DateCreated, ...) 
ListItems(ListId, Text, Foo, Baz, Qux, ...) 

我有一個IQueryable<List>代表另一個Linq查詢返回一些List實體。

我想JOIN它與ListItems一些聚合數據，但這證明困難，因爲Linq生成低效的SQL - 但我也想使查詢組合。

這是類似於我想LINQ的生成SQL的東西：

 
SELECT 
    * 
FROM 
(
    -- This part represents the IQueryable: 

    SELECT 
     ListId, 
     Name, 
     ... 
    FROM 
     Lists 
    ORDER BY 
     Lists.DateCreated 
    OFFSET 
     0 ROWS FETCH NEXT 25 ROWS ONLY -- Linq's .Skip(0).Take(25) 
) AS ListsResult 
LEFT JOIN 
(
    -- This is the aggregate data query I want Linq to generate: 
    SELECT 
     ListId, 
     COUNT(1) AS [Count], 
     COUNT(CASE WHEN Foo = 'bar' THEN 1 ELSE NULL END) AS CountFooBar, 
     COUNT(CASE WHEN Baz > 5  THEN 1 ELSE NULL END) AS CountBaz5 
    FROM 
     ListItems 
    WHERE 
     Qux IS NOT NULL 
    GROUP BY 
     ListId 
) AS ItemsStats ON ListResults.ListId = ItemsStats.ListId 

這是LINQ的我 - 我喜歡的擴展方法的語法：

 
IQueryable lists = GetLists(0, 25); 

var stats = this.dbContext.ListItems 
    .Where((ListItem li) => li.Qux != null) 
    .GroupBy((ListItem li) => li.ListId) 
    .Select(grp => new 
    { 
     grp.Key, 
     Count  = grp.Count(), 
     CountFooBar = grp.Count((ListItem li) => li.Foo == "bar") 
     CountBaz5 = grp.Count((ListItem li) => li.Baz > 5) 
    }) 

return lists 
    .Join(
     inner: stats, 
     outerKeySelector: (List l) => l.ListId, 
     innerKeySelector: grp => grp.Key, 
     resultSelector: (list, itemStats) => new { list, itemsStats } 
    ) 

但是這會產生SQL看像這樣（這個查詢顯示了我真正的表和列名，這比我之前發佈的模式複雜一點:)

 
SELECT 
    [Project13].[C2] AS [C1], 
    [Project13].[ListId] AS [ListId], 
    [Project13].[C1] AS [C2], 
    [Project13].[C3] AS [C3], 
    [Project13].[C4] AS [C4], 
    [Project13].[C5] AS [C5], 
    [Project13].[C6] AS [C6], 
    [Project13].[C7] AS [C7] 
    FROM (SELECT 
     [Project11].[C1] AS [C1], 
     [Project11].[ListId] AS [ListId], 
     [Project11].[C2] AS [C2], 
     [Project11].[C3] AS [C3], 
     [Project11].[C4] AS [C4], 
     [Project11].[C5] AS [C5], 
     [Project11].[C6] AS [C6], 
     (SELECT 
      COUNT(1) AS [A1] 
      FROM (SELECT [Project12].[ListId] AS [ListId] 
       FROM (SELECT 
        [Extent11].[ListId] AS [ListId], 
        [Extent11].[Created] AS [Created] 
        FROM [dbo].[Lists] AS [Extent11] 
        WHERE ([Extent11].[TenantId] = 8) AND ([Extent11].[BlarghId] = 8) 
       ) AS [Project12] 
       ORDER BY [Project12].[Created] DESC 
       OFFSET 0 ROWS FETCH NEXT 25 ROWS ONLY ) AS [Limit6] 
      INNER JOIN [dbo].[ListItems] AS [Extent12] ON ([Limit6].[TenantId] = [Extent12].[TenantId]) AND ([Limit6].[BlarghId] = [Extent12].[BlarghId]) AND ([Limit6].[ListId] = [Extent12].[ListId]) 
      WHERE (([Extent12].[Baz] > 0) OR ((LEN([Extent12].[Notes])) > 0) OR ((LEN([Extent12].[Value])) > 0)) AND ([Project11].[TenantId] = [Extent12].[TenantId]) AND ([Project11].[BlarghId] = [Extent12].[BlarghId]) AND ([Project11].[ListId] = [Extent12].[ListId]) AND ([Extent12].[RecommendationRevision] IS NOT NULL)) AS [C7] 
     FROM (SELECT 
      [Project9].[C1] AS [C1], 
      [Project9].[TenantId] AS [TenantId], 
      [Project9].[BlarghId] AS [BlarghId], 
      [Project9].[ListId] AS [ListId], 
      [Project9].[C2] AS [C2], 
      [Project9].[C3] AS [C3], 
      [Project9].[C4] AS [C4], 
      [Project9].[C5] AS [C5], 
      (SELECT 
       COUNT(1) AS [A1] 
       FROM (SELECT [Project10].[TenantId] AS [TenantId], [Project10].[BlarghId] AS [BlarghId], [Project10].[ListId] AS [ListId] 
        FROM (SELECT 
         [Extent9].[TenantId] AS [TenantId], 
         [Extent9].[BlarghId] AS [BlarghId], 
         [Extent9].[ListId] AS [ListId], 
         [Extent9].[Created] AS [Created] 
         FROM [dbo].[Lists] AS [Extent9] 
         WHERE ([Extent9].[TenantId] = 8) AND ([Extent9].[BlarghId] = 8) 
        ) AS [Project10] 
        ORDER BY [Project10].[Created] DESC 
        OFFSET 0 ROWS FETCH NEXT 25 ROWS ONLY ) AS [Limit5] 
       INNER JOIN [dbo].[ListItems] AS [Extent10] ON ([Limit5].[TenantId] = [Extent10].[TenantId]) AND ([Limit5].[BlarghId] = [Extent10].[BlarghId]) AND ([Limit5].[ListId] = [Extent10].[ListId]) 
       WHERE (([Extent10].[Baz] > 0) OR ((LEN([Extent10].[Notes])) > 0) OR ((LEN([Extent10].[Value])) > 0)) AND ([Project9].[TenantId] = [Extent10].[TenantId]) AND ([Project9].[BlarghId] = [Extent10].[BlarghId]) AND ([Project9].[ListId] = [Extent10].[ListId]) AND (3 = [Extent10].[Baz])) AS [C6] 
      FROM (SELECT 
       [Project7].[C1] AS [C1], 
       [Project7].[TenantId] AS [TenantId], 
       [Project7].[BlarghId] AS [BlarghId], 
       [Project7].[ListId] AS [ListId], 
       [Project7].[C2] AS [C2], 
       [Project7].[C3] AS [C3], 
       [Project7].[C4] AS [C4], 
       (SELECT 
        COUNT(1) AS [A1] 
        FROM (SELECT [Project8].[TenantId] AS [TenantId], [Project8].[BlarghId] AS [BlarghId], [Project8].[ListId] AS [ListId] 
         FROM (SELECT 
          [Extent7].[TenantId] AS [TenantId], 
          [Extent7].[BlarghId] AS [BlarghId], 
          [Extent7].[ListId] AS [ListId], 
          [Extent7].[Created] AS [Created] 
          FROM [dbo].[Lists] AS [Extent7] 
          WHERE ([Extent7].[TenantId] = 8) AND ([Extent7].[BlarghId] = 8) 
         ) AS [Project8] 
         ORDER BY [Project8].[Created] DESC 
         OFFSET 0 ROWS FETCH NEXT 25 ROWS ONLY ) AS [Limit4] 
        INNER JOIN [dbo].[ListItems] AS [Extent8] ON ([Limit4].[TenantId] = [Extent8].[TenantId]) AND ([Limit4].[BlarghId] = [Extent8].[BlarghId]) AND ([Limit4].[ListId] = [Extent8].[ListId]) 
        WHERE (([Extent8].[Baz] > 0) OR ((LEN([Extent8].[Notes])) > 0) OR ((LEN([Extent8].[Value])) > 0)) AND ([Project7].[TenantId] = [Extent8].[TenantId]) AND ([Project7].[BlarghId] = [Extent8].[BlarghId]) AND ([Project7].[ListId] = [Extent8].[ListId]) AND (2 = [Extent8].[Baz])) AS [C5] 
       FROM (SELECT 
        [Project5].[C1] AS [C1], 
        [Project5].[TenantId] AS [TenantId], 
        [Project5].[BlarghId] AS [BlarghId], 
        [Project5].[ListId] AS [ListId], 
        [Project5].[C2] AS [C2], 
        [Project5].[C3] AS [C3], 
        (SELECT 
         COUNT(1) AS [A1] 
         FROM (SELECT [Project6].[TenantId] AS [TenantId], [Project6].[BlarghId] AS [BlarghId], [Project6].[ListId] AS [ListId] 
          FROM (SELECT 
           [Extent5].[TenantId] AS [TenantId], 
           [Extent5].[BlarghId] AS [BlarghId], 
           [Extent5].[ListId] AS [ListId], 
           [Extent5].[Created] AS [Created] 
           FROM [dbo].[Lists] AS [Extent5] 
           WHERE ([Extent5].[TenantId] = 8) AND ([Extent5].[BlarghId] = 8) 
          ) AS [Project6] 
          ORDER BY [Project6].[Created] DESC 
          OFFSET 0 ROWS FETCH NEXT 25 ROWS ONLY ) AS [Limit3] 
         INNER JOIN [dbo].[ListItems] AS [Extent6] ON ([Limit3].[TenantId] = [Extent6].[TenantId]) AND ([Limit3].[BlarghId] = [Extent6].[BlarghId]) AND ([Limit3].[ListId] = [Extent6].[ListId]) 
         WHERE (([Extent6].[Baz] > 0) OR ((LEN([Extent6].[Notes])) > 0) OR ((LEN([Extent6].[Value])) > 0)) AND ([Project5].[TenantId] = [Extent6].[TenantId]) AND ([Project5].[BlarghId] = [Extent6].[BlarghId]) AND ([Project5].[ListId] = [Extent6].[ListId]) AND (1 = [Extent6].[Baz])) AS [C4] 
        FROM (SELECT 
         [Project3].[C1] AS [C1], 
         [Project3].[TenantId] AS [TenantId], 
         [Project3].[BlarghId] AS [BlarghId], 
         [Project3].[ListId] AS [ListId], 
         [Project3].[C2] AS [C2], 
         (SELECT 
          COUNT(1) AS [A1] 
          FROM (SELECT [Project4].[TenantId] AS [TenantId], [Project4].[BlarghId] AS [BlarghId], [Project4].[ListId] AS [ListId] 
           FROM (SELECT 
            [Extent3].[TenantId] AS [TenantId], 
            [Extent3].[BlarghId] AS [BlarghId], 
            [Extent3].[ListId] AS [ListId], 
            [Extent3].[Created] AS [Created] 
            FROM [dbo].[Lists] AS [Extent3] 
            WHERE ([Extent3].[TenantId] = 8) AND ([Extent3].[BlarghId] = 8) 
           ) AS [Project4] 
           ORDER BY [Project4].[Created] DESC 
           OFFSET 0 ROWS FETCH NEXT 25 ROWS ONLY ) AS [Limit2] 
          INNER JOIN [dbo].[ListItems] AS [Extent4] ON ([Limit2].[TenantId] = [Extent4].[TenantId]) AND ([Limit2].[BlarghId] = [Extent4].[BlarghId]) AND ([Limit2].[ListId] = [Extent4].[ListId]) 
          WHERE (([Extent4].[Baz] > 0) OR ((LEN([Extent4].[Notes])) > 0) OR ((LEN([Extent4].[Value])) > 0)) AND ([Project3].[TenantId] = [Extent4].[TenantId]) AND ([Project3].[BlarghId] = [Extent4].[BlarghId]) AND ([Project3].[ListId] = [Extent4].[ListId]) AND ([Extent4].[Foo] = 1)) AS [C3] 
         FROM (SELECT 
          [GroupBy1].[A1] AS [C1], 
          [GroupBy1].[K1] AS [TenantId], 
          [GroupBy1].[K2] AS [BlarghId], 
          [GroupBy1].[K3] AS [ListId], 
          [GroupBy1].[K4] AS [C2] 
          FROM (SELECT 
           [Project2].[K1] AS [K1], 
           [Project2].[K2] AS [K2], 
           [Project2].[K3] AS [K3], 
           [Project2].[K4] AS [K4], 
           COUNT([Project2].[A1]) AS [A1] 
           FROM (SELECT 
            [Project2].[TenantId] AS [K1], 
            [Project2].[BlarghId] AS [K2], 
            [Project2].[ListId] AS [K3], 
            1 AS [K4], 
            1 AS [A1] 
            FROM (SELECT 
             [Extent2].[ListId] AS [ListId] 
             FROM (SELECT [Project1].[ListId] AS [ListId] 
              FROM (SELECT 
               [Extent1].[ListId] AS [ListId], 
               [Extent1].[Created] AS [Created] 
               FROM [dbo].[Lists] AS [Extent1] 
               WHERE ([Extent1].[TenantId] = 8) AND ([Extent1].[BlarghId] = 8) 
              ) AS [Project1] 
              ORDER BY [Project1].[Created] DESC 
              OFFSET 0 ROWS FETCH NEXT 25 ROWS ONLY ) AS [Limit1] 
             INNER JOIN [dbo].[ListItems] AS [Extent2] ON (([Limit1].[ListId] = [Extent2].[ListId]) 
             WHERE ([Extent2].[Baz] > 0) OR ((LEN([Extent2].[Notes])) > 0) OR ((LEN([Extent2].[Value])) > 0) 
            ) AS [Project2] 
           ) AS [Project2] 
           GROUP BY [K1], [K2], [K3], [K4] 
          ) AS [GroupBy1] 
         ) AS [Project3] 
        ) AS [Project5] 
       ) AS [Project7] 
      ) AS [Project9] 
     ) AS [Project11] 
    ) AS [Project13] 

它根本不會組合COUNT()聲明，並且它將COUNT謂詞移至單獨的WHERE子句。還要注意重複的分頁子查詢（其中使用OFFSET 0 ROW FETCH NEXT 25），而我的手寫查詢只執行一次。

Answer 1

這裏有一個半的解決方法，我有：

我認識的最好的短期解決方案是在數據庫的SQL（作爲UDF FUNCTION或VIEW），這將意味着，因此一些數據的代碼會必須在數據庫中（而不是將數據庫用作「啞巴商店」）。

我首先創建了一個表值UDF，它將接受一個表值參數，推理將允許組合，代價是需要生成輸入參數表（來自分頁查詢的ListId值數組） 。但是，在這樣做的時候，我意識到Linq-to-Entities（版本6）不支持Function Imports中的表值參數。

然後我推斷一個更好的辦法是到COUNT操作移動到VIEW（代表LEFT JOIN我在手寫查詢過的一半），然後我可以得到LINQ的加入，對現有的因此保留了組合性並生成了高效的運行時查詢（實際上，當我運行它時，查詢需要34ms根據SQL Server Profiler執行，而舊的Linq生成的低效查詢需要830ms）。

下面是我用什麼：

 
CREATE VIEW ListItemStatistics AS 

SELECT 
    ListId, 

    COUNT(*) AS [CountAll], 
    COUNT(CASE WHEN ...) AS Count... 

FROM 
    ListItems 
WHERE 
    Foo = 'bar' 
GROUP BY 
    ListId 

然後從內部的Linq：

 
IQueryable lists = GetListsQuery(0, 25); 

var listsWithItemsStats = lists. 
    .Join(
     inner: this.dbContext.ListItemStatistics, 
     outerKeySelector: list => list.ListId, 
     innerKeySelector: row => row.ListId, 
     resultSelector: (list,row) => new { list, row } 
    ); 

然而，因爲這確實使用數據庫方面的邏輯（在VIEW）這是不理想的。

Answer 2

根據我的經驗，當你在你的案例中使用像Count(predicate這樣的過濾的聚合函數時，EF會產生這樣的查詢。

var stats = db.ListItems 
    .Where(li => li.Qux != null) 
    .GroupBy(li => li.ListId) 
    .Select(grp => new 
    { 
     grp.Key, 
     Count = grp.Count(), 
     CountFooBar = grp.Sum(li => li.Foo == "bar" ? 1 : 0), 
     CountBaz5 = grp.Sum(li => li.Baz > 5 ? 1 : 0) 
    }); 

的其他部分保持不變：如果更換Count(condition)構建與條件的總和（Sum(condition ? 1 : 0)）這樣你會得到更好的SQL查詢。國防部之前，我是越來越類似於SQL的發佈，這裏就是我這個小MOD後得到：

SELECT 
    [Limit1].[ListId] AS [ListId], 
    [Limit1].[Name] AS [Name], 
    [Limit1].[DateCreated] AS [DateCreated], 
    [GroupBy1].[K1] AS [ListId1], 
    [GroupBy1].[A1] AS [C1], 
    [GroupBy1].[A2] AS [C2], 
    [GroupBy1].[A3] AS [C3] 
    FROM (SELECT [Extent1].[ListId] AS [ListId], [Extent1].[Name] AS [Name], [Extent1].[DateCreated] AS [DateCreated] 
     FROM [dbo].[List] AS [Extent1] 
     ORDER BY [Extent1].[DateCreated] ASC 
     OFFSET 0 ROWS FETCH NEXT 25 ROWS ONLY ) AS [Limit1] 
    INNER JOIN (SELECT 
     [Filter1].[K1] AS [K1], 
     COUNT([Filter1].[A1]) AS [A1], 
     SUM([Filter1].[A2]) AS [A2], 
     SUM([Filter1].[A3]) AS [A3] 
     FROM (SELECT 
      [Extent2].[ListId] AS [K1], 
      1 AS [A1], 
      CASE WHEN (N'bar' = [Extent2].[Foo]) THEN 1 ELSE 0 END AS [A2], 
      CASE WHEN ([Extent2].[Baz] > 5) THEN 1 ELSE 0 END AS [A3] 
      FROM [dbo].[ListItem] AS [Extent2] 
      WHERE [Extent2].[Qux] IS NOT NULL 
     ) AS [Filter1] 
     GROUP BY [K1]) AS [GroupBy1] ON [Limit1].[ListId] = [GroupBy1].[K1] 

UPDATE：以上只是LINQ相當於你手動SQL查詢（當您調整加入代碼以生成LEFT OUTER JOIN）。但是，考慮到主數據分頁，OUTER APPLY SQL查詢可能表現更好。您可以讓LINQ to Entities生成如下類型的查詢：

var listsWithItemsStats = lists 
    .SelectMany(l => db.ListItems 
     .Where(li => li.ListId == l.ListId && li.Qux != null) 
     .GroupBy(li => li.ListId) 
     .Select(grp => new 
     { 
      grp.Key, 
      Count = grp.Count(), 
      CountFooBar = grp.Sum(li => li.Foo == "bar" ? 1 : 0), 
      CountBaz5 = grp.Sum(li => li.Baz > 5 ? 1 : 0) 
     }) 
     .DefaultIfEmpty(), 
     (list, itemsStats) => new { list, itemsStats });