LINQ或Entity Framework创建无界的SQL语句

Question

I work on speed optimization of my application, and what I have found is that LINQ (or EF) is creating some strange SQL for me that works slow.

Here is some code :

SomeList.AddRange(_databaseView
                .Select(l=> new SomeViewModel
                                {
                                    Date = l.Date,
                                    Details = l.Details,
                                    Level = l.LevelName,
                                    Id = l.ViewID,
                                    Message = l.Message,
                                    ProjectName = l.projectName,
                                    StatusId = l.StatusID,
                                    StatusName = l.StatusName
                                })
                .Skip(50)
                .Take(25));

And in theory it should've created SQL statement that takes 25 record but profiler showes following SQL for it :

    SELECT [Extent1].[Date]  AS [Date],
       [Extent1].[ID]            AS [ID],
       [Extent1].[LevelID]       AS [LevelID],
       [Extent1].[StatusID]      AS [StatusID],
       [Extent1].[projectName]   AS [projectName],
       [Extent1].[LevelName]     AS [LevelName],
       [Extent1].[StatusName]    AS [StatusName],
       [Extent1].[Message]       AS [Message],
       [Extent1].[Details]       AS [Details],
       [Extent1].[LogViewID]     AS [LogViewID]
FROM   (SELECT [v_MyView].[Date]       AS [Date],
               [v_MyView].[ProjectID]     AS [ProjectID],
               [v_MyView].[LevelID]       AS [LevelID],
               [v_MyView].[StatusID]      AS [StatusID],
               [v_MyView].[projectName]   AS [projectName],
               [v_MyView].[LevelName]     AS [LevelName],
               [v_MyView].[StatusName]    AS [StatusName],
               [v_MyView].[Message]       AS [Message],
               [v_MyView].[Details]       AS [Details],
               [v_MyView].[ViewID]        AS [ID]
        FROM   [dbo].[v_MyView] AS [v_MyView]) AS [Extent1]

_databaseView is IQueryable object on which all my sorting and filtering logic is done.

Here is something I figured : if I don't do any filtering SQL is normal with SELECT TOP (25) on it. But whenever I do filtering something gets messed up. Here is the code to one of my filters:

if (Filters.ProjectName != null && Filters.ProjectName[0] != 0)    // check if "all" is not checked
    _databaseView = Filters.ProjectName
        .Join(_databaseView,  f => f, l => l.ProjectID,  (f,l) => new SomeViewModel
                                                                           {
                                                                               Date = l.Date,
                                                                               Details = l.Details,
                                                                               LevelName = l.LevelName,
                                                                               ViewID = l.ViewID,
                                                                               Message = l.Message,
                                                                               projectName = l.projectName,
                                                                               StatusID = l.StatusID,
                                                                               StatusName = l.StatusName
                                                                           })
    .AsQueryable();

And it is without any constraint. How do I make this LINQ-EF thing to produce some good SQL ?

Thx in advance!

Answer 1

You don't say what _DatabaseView is, but my wild guess based on your results is that it's not an ObjectQuery<T> . Which would explain your problem. ObjectQuery will convert to SQL; IEnumerable<T>.Skip() won't. Calling AsQueryable() on an enumerable is not enough to make this happen.

For example, this:

var foo = MyObjectContext.SomeEntitySet.AsEnumerable().AsQueryable().Take(10);

...won't put the TOP in the SQL.

But this:

var bar = MyObjectContext.SomeEntitySet.Take(10);

... will.

Once again: You haven't said what _DatabaseView is . Try this operation directly on your ObjectContext and you will see that it works. The bug is in the code you use to assign _DatabaseView , which you haven't shown us.

Answer 2

The only way you can actually change the SQL that is being used would be to write your own and use that instead of using the generated SQL.

You mean the Skip and Take portions of the LINQ not being converted to SQL. I think it is because of the way that you are doing the LINQ.

Try something like

(From l In DataBaseView Select new SomeViewModel
                                {
                                    Date = l.Date,
                                    Details = l.Details,
                                    Level = l.LevelName,
                                    Id = l.ViewID,
                                    Message = l.Message,
                                    ProjectName = l.projectName,
                                    StatusId = l.StatusID,
                                    StatusName = l.StatusName
                                }).Skip(50).Take(25)

Instead and see if it makes a difference in the generated code.

Edit Somehow I missed the portion where you said it should be SQL that takes 25 records.

Answer 3

LINQ Parser definitely consider the Skip and Take methods in your LINQ to Entities query and produces the correct expression tree and then Object Services converts the expression tree to a command tree which will be passed to the database provider for the specific SQL query generation.
In this case this 2 methods affecting the generated SQL with a WHERE [Extent1].[row_number] > 50 and SELECT TOP (25) for Skip and Take respectively.

Now, are you sure that you are looking at the right trace in the Profiler ? I suggest take a look at ObjectQuery.ToTraceString method by writing the below code before going to Profiler and then debug through your code and examine the value of sql variable:

var query = _DatabaseView.Select(l=> new SomeViewModel {
                                                     Date = l.Date,
                                                     Details = l.Details,
                                                     Level = l.LevelName,
                                                     Id = l.ViewID,
                                                     Message = l.Message,
                                                     ProjectName = l.projectName,
                                                     StatusId = l.StatusID,
                                                     StatusName = l.StatusName})
                         .Skip(50)
                         .Take(25));
string sql = (query as ObjectQuery).ToTraceString();

Answer 4

如果你无法使用适当的索引使SQL运行得足够好，那么你总是可以尝试编写一个存储过程并从LINQ调用它。

Answer 5

尝试在选择之前移动Skip and Take。