为什么双重 GroupBy + ToList 花费太长时间？

Question

我有这些疑问：

    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();

我正在尝试获取县市相同的对象列表。但是第二个查询需要很长时间才能给出结果。我等了大约 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，以便获得我想要的列表列表？提前致谢。

例如这个查询返回这样一个列表：

{ 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 }

我想把这个列表分成几个这样的列表：

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）。

要找出来，获取生成的sql，查看执行计划。如果计划说嵌套循环连接 -> 聚集索引扫描，那么你就发现了问题。

可能性 2：您已经找到了 n+1 问题。

在 Linq 的GROUP BY 中，组由组键和组成员组成。然而，在大多数 SQL 实现中，GROUP BY 为您提供组键和聚合。为了获取组的成员，发出单独的查询。如果有 n 个组，则必须发出 n 个查询（+1 是原始查询）。

要查找，获取生成的sql。如果发出了一堆额外的查询，其中任何一个都说聚集索引扫描，那么您就发现了问题。

可能性 3：您实际上发出了 n^2 (~5,000,000) 个查询。

嗯，你分组了两次，所以它可能是一个双重嵌套循环。查看生成的sql就知道了。

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解决所有这些问题的最简单方法是在分组之前将 5,000 条记录拉入内存。一个简单的方法是在调用GroupBy之前调用ToList。