爲什麼Double GroupBy + ToList耗時過長？

Question

我有這些疑問：

var Data = (from ftr in db.TB_FTR 
         join mst in db.TB_MST on ftr.MST_ID equals mst.MST_ID 
         join trf in db.TB_TRF on mst.TRF_ID equals trf.ID 
         select new CityCountyType { City = ftr.CITY, County = ftr.COUNTY, Type = trf.TYPE } 
        ).OrderBy(i => i.City).ThenBy(i => i.County); 

var Data2 = 
    Data.GroupBy(i => new {i.City, i.County, i.Type}) 
     .Select(group => new {Name = group.Key, Count = group.Count()}) 
     .OrderBy(x => x.Name) 
     .ThenByDescending(x => x.Count) 
     .GroupBy(g => new {g.Name.City, g.Name.County}) 
     .Select(g => g.Select(g2 => 
      new {Name = new {g.Key.City, g.Key.County, g2.Name.Type}, g2.Count})).ToList(); 

我試圖讓對象，它們的縣市是相同的列表清單。但是第二個查詢花費太長時間才能得出結果。我等了大約30分鐘，但沒有回答，但是名單Data有大約5000條記錄。如何更改這些查詢以便我可以獲取我想要的列表清單？提前致謝。

例如該查詢返回這樣一個列表：

{ Name = {{ City = New York City, County = Bronx, Type = Type A }}, Count = 4 } 

{ Name = {{ City = New York City, County = Bronx, Type = Type B }}, Count = 8 } 

{ Name = {{ City = New York City, County = Bronx, Type = Type C }}, Count = 24 } 

{ Name = {{ City = New York City, County = Manhattan, Type = Type B }}, Count = 43 } 

{ Name = {{ City = New York City, County = Manhattan, Type = Type C }}, Count = 58 } 

{ Name = {{ City = Seattle, County = King County, Type = Type D }}, Count = 43 } 

{ Name = {{ City = Seattle, County = King County, Type = Type A }}, Count = 67 } 

{ Name = {{ City = Seattle, County = Snohomish County, Type = Type C }}, Count = 67 } 

我想使這個名單幾個列表如下：

表1：

{ Name = {{ City = New York City, County = Bronx, Type = Type A }}, Count = 4 } 

{ Name = {{ City = New York City, County = Bronx, Type = Type B }}, Count = 8 } 

{ Name = {{ City = New York City, County = Bronx, Type = Type C }}, Count = 24 } 

表2：

{ Name = {{ City = New York City, County = Manhattan, Type = Type B }}, Count = 43 } 

{ Name = {{ City = New York City, County = Manhattan, Type = Type C }}, Count = 58 } 

列表3：

{ Name = {{ City = Seattle, County = King County, Type = Type D }}, Count = 43 } 

{ Name = {{ City = Seattle, County = King County, Type = Type A }}, Count = 67 } 

表4：

{ Name = {{ City = Seattle, County = Snohomish County, Type = Type C }}, Count = 67 } 

Answer 1

可能性1：您的數據庫沒有被索引，以支持您的查詢（其中，並加入條款）。

爲了弄清楚，獲取生成的sql並查看執行計劃。如果該計劃顯示嵌套循環連接 - >聚集索引掃描，則發現問題。

可能性2：您發現了n + 1問題。

在Linq的GROUP BY中，一個組由組密鑰和組成員組成。但是在大多數SQL實現中，GROUP BY爲您提供組密鑰和聚合。爲了獲得一個組的成員，發出一個單獨的查詢。如果有n個組，則必須發出n個查詢（+1是原始查詢）。

爲了弄清楚，得到生成的sql。如果發佈了一堆額外的查詢，並且其中任何一個都說聚簇索引掃描，那麼你就發現了這個問題。

可能性3：您實際上正在發出n^2（〜5,000,000）個查詢。

那麼，你分組了兩次，所以它可能是一個雙重嵌套的循環。看看生成的SQL並找出答案。

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最簡單的解決方法就是在分組之前將5,000條記錄拉入內存。一個簡單的方法是在致電GroupBy之前致電ToList。