为什么Double GroupBy + ToList耗时过长？

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(); 

我试图让对象，它们的县市是相同的列表清单。但是第二个查询花费太长时间才能得出结果。我等了大约30分钟，但没有回答，但是名单Data有大约5000条记录。如何更改这些查询以便我可以获取我想要的列表清单？提前致谢。

例如该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 } 

我想使这个名单几个列表如下：

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：您的数据库没有被索引，以支持您的查询（其中，并加入条款）。

为了弄清楚，获取生成的sql并查看执行计划。如果该计划显示嵌套循环连接 - >聚集索引扫描，则发现问题。

可能性2：您发现了n + 1问题。

在Linq的GROUP BY中，一个组由组密钥和组成员组成。但是在大多数SQL实现中，GROUP BY为您提供组密钥和聚合。为了获得一个组的成员，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）。

为了弄清楚，得到生成的sql。如果发布了一堆额外的查询，并且其中任何一个都说聚簇索引扫描，那么你就发现了这个问题。

可能性3：您实际上正在发出n^2（〜5,000,000）个查询。

那么，你分组了两次，所以它可能是一个双重嵌套的循环。看看生成的SQL并找出答案。

---

最简单的解决方法就是在分组之前将5,000条记录拉入内存。一个简单的方法是在致电GroupBy之前致电ToList。