Question

I have this query that I want translated pretty much 1:1 from Entity Framework to SQL:

SELECT GroupId, ItemId, count(*) as total
  FROM [TESTDB].[dbo].[TestTable]
  WHERE GroupId = '64025'
  GROUP BY GroupId, ItemId
  ORDER BY GroupId, total DESC

This SQL query should sort based on the number occurrence of the same ItemId (for that group).

I have this now:

from x in dataContext.TestTable.AsNoTracking()
where x.GroupId = 64025
group x by new {x.GroupId, x.ItemId}
into g
orderby g.Key.GroupId, g.Count() descending 
select new {g.Key.GroupId, g.Key.ItemId, Count = g.Count()};

But this generates the following SQL code:

SELECT 
  [GroupBy1].[K1] AS [GroupId], 
  [GroupBy1].[K2] AS [ItemId], 
  [GroupBy1].[A2] AS [C1]
FROM ( SELECT 
         [Extent1].[GroupId] AS [K1], 
         [Extent1].[ItemId] AS [K2], 
         COUNT(1) AS [A1], 
         COUNT(1) AS [A2]
       FROM [dbo].[TestTable] AS [Extent1]
       WHERE 64025 = [Extent1].[GroupId]
       GROUP BY [Extent1].[GroupId], [Extent1].[ItemId]
     )  AS [GroupBy1]
ORDER BY [GroupBy1].[K1] ASC, [GroupBy1].[A1] DESC

This also works but is a factor 2 slower than the SQL I created.

I’ve been fiddling around with the linq code for a while but I haven’t managed to create something similar to my query.

Execution plan (only the last two items, the first two are identical):

FIRST:   |--Stream Aggregate(GROUP BY:([Extent1].[ItemId]) DEFINE:([Expr1006]=Count(*), [Extent1].[GroupId]=ANY([TESTDB].[dbo].[TestTable].[GroupId] as [Extent1].[GroupId])))
           |--Index Seek(OBJECT:([TESTDB].[dbo].[TestTable].[IX_Group]), SEEK:([TESTDB].[dbo].[TestTable].[GroupId]=(64034)) ORDERED FORWARD)

SECOND:  |--Stream Aggregate(GROUP BY:([TESTDB].[dbo].[TestTable].[ItemId]) DEFINE:([Expr1007]=Count(*), [TESTDB].[dbo].[TestTable].[GroupId]=ANY([TESTDB].[dbo].[TestTable].[GroupId])))
           |--Index Seek(OBJECT:([TESTDB].[dbo].[TestTable].[IX_Group] AS [Extent1]), SEEK:([Extent1].[GroupId]=(64034)) ORDERED FORWARD)

Answer 1

The query that Entity Framework generates and your hand crafted query are semantically the same and will give the same plan.

The derived table definition is inlined during query optimisation so the only difference might be some extremely minor additional overhead during parsing and compilation.

The snippets of SHOWPLAN_TEXT you have posted are the same plan. The only difference is aliases. It looks as though your table definition is something like.

CREATE TABLE [dbo].[TestTable] 
(
[GroupId] INT,
[ItemId] INT
)

CREATE NONCLUSTERED INDEX IX_Group ON  [dbo].[TestTable] ([GroupId], [ItemId]) 

And you are getting a plan like this

To all intents and purposes the plans are the same. Your performance testing methodology is probably flawed. Maybe your first query brought pages into cache that then benefited the second query for example.