这些LINQ查询之间有什么区别

Question

I've been fooling around with some LINQ over Entities and I'm getting strange results and I would like to get an explanation...

Given the following LINQ query,

// Sample # 1
IEnumerable<GroupInformation> groupingInfo;
groupingInfo = from a in context.AccountingTransaction
               group a by a.Type into grp
               select new GroupInformation()
               {
                   GroupName = grp.Key,
                   GroupCount = grp.Count()
               };

I get the following SQL query (taken from SQL Profiler):

SELECT 
    1 AS [C1], 
    [GroupBy1].[K1] AS [Type], 
    [GroupBy1].[A1] AS [C2]
    FROM ( SELECT 
        [Extent1].[Type] AS [K1], 
        COUNT(1) AS [A1]
        FROM [dbo].[AccountingTransaction] AS [Extent1]
        GROUP BY [Extent1].[Type]
    )  AS [GroupBy1]

So far so good.

If I change my LINQ query to:

// Sample # 2
groupingInfo = context.AccountingTransaction.
                 GroupBy(a => a.Type).
                 Select(grp => new GroupInformation()
                               {
                                   GroupName = grp.Key,
                                   GroupCount = grp.Count()
                               });

it yields to the exact same SQL query. Makes sense to me.

Here comes the interesting part... If I change my LINQ query to:

// Sample # 3
IEnumerable<AccountingTransaction> accounts;
IEnumerable<IGrouping<object, AccountingTransaction>> groups;
IEnumerable<GroupInformation> groupingInfo;

accounts = context.AccountingTransaction;
groups = accounts.GroupBy(a => a.Type);
groupingInfo = groups.Select(grp => new GroupInformation()
                  {
                      GroupName = grp.Key,
                      GroupCount = grp.Count()
                  });

the following SQL is executed (I stripped a few of the fields from the actual query, but all the fields from the table (~ 15 fields) were included in the query, twice):

SELECT 
    [Project2].[C1] AS [C1], 
    [Project2].[Type] AS [Type], 
    [Project2].[C2] AS [C2], 
    [Project2].[Id] AS [Id], 
    [Project2].[TimeStamp] AS [TimeStamp], 
    -- <snip>
    FROM ( SELECT 
        [Distinct1].[Type] AS [Type], 
        1 AS [C1], 
        [Extent2].[Id] AS [Id], 
        [Extent2].[TimeStamp] AS [TimeStamp], 
        -- <snip>
        CASE WHEN ([Extent2].[Id] IS NULL) THEN CAST(NULL AS int) ELSE 1 END AS [C2]
        FROM   (SELECT DISTINCT 
            [Extent1].[Type] AS [Type]
            FROM [dbo].[AccountingTransaction] AS [Extent1] ) AS [Distinct1]
        LEFT OUTER JOIN [dbo].[AccountingTransaction] AS [Extent2] ON [Distinct1].[Type] = [Extent2].[Type]
    )  AS [Project2]
    ORDER BY [Project2].[Type] ASC, [Project2].[C2] ASC

Why are the SQLs generated are so different? After all, the exact same code is executed, it's just that sample # 3 is using intermediate variables to get the same job done!

Also, if I do:

Console.WriteLine(groupingInfo.ToString());

for sample # 1 and sample # 2, I get the exact same query that was captured by SQL Profiler, but for sample # 3, I get:

System.Linq.Enumerable+WhereSelectEnumerableIterator`2[System.Linq.IGrouping`2[System.Object,TestLinq.AccountingTransaction],TestLinq.GroupInformation]

What is the difference? Why can't I get the SQL Query generated by LINQ if I split the LINQ query in multiple instructions?

The ulitmate goal is to be able to add operators to the query (Where, OrderBy, etc.) at run-time.

BTW, I've seen this behavior in EF 4.0 and EF 6.0.

Thank you for your help.

Answer 1

The reason is because in your third attempt you're referring to accounts as IEnumerable<AccountingTransaction> which will cause the query to be invoked using Linq-To-Objects ( Enumerable.GroupBy and Enumerable.Select )

On the other hand, in your first and second attempts the reference to AccountingTransaction is preserved as IQueryable<AccountingTransaction> and the query will be executed using Linq-To-Entities which will then transform it to the appropriate SQL statement.