为什么双重GroupBy + ToList花费的时间太长？

Question

我有这些查询：

    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();

我正在尝试获取县和城市相同的对象列表。 但是第二个查询花费的时间太长，无法给出结果。 我等待了大约30分钟，但没有答案，但是列表Data有大约5000条记录。 如何更改这些查询，以便获得所需列表的列表？ 提前致谢。

例如，此查询返回这样的列表：

{ 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 }

我想将此列表添加到几个这样的列表中：

清单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 }

清单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 }

清单3：

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

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

清单4：

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

Answer 1

可能性1：您的数据库未编制索引以支持您的查询（where和join子句）。

要找出答案，请获取生成的sql并查看执行计划。 如果计划中显示嵌套循环联接->聚集索引扫描，则说明存在问题。

可能性2：您发现了n + 1问题。

在Linq的GROUP BY ，一个组由组键和组成员组成。 但是，在大多数SQL实现中， GROUP BY会为您提供组键和聚合。 为了获得组的成员，将发出一个单独的查询。 如果有n个组，则必须发出n个查询（+1是原始查询）。

要找出答案，请获取生成的sql。 如果发出了很多额外的查询，并且其中任何一个都显示了聚集索引扫描，那么您已经找到了问题。

情况3：您实际上是在发出n ^ 2（〜5,000,000）条查询。

好吧，您分组了两次，所以它可能是一个双重嵌套循环。 查看生成的sql并找出答案。

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最简单的解决方法是在分组之前将5,000条记录拉入内存。 一种简单的方法是在调用GroupBy之前先调用ToList 。