LINQ To SQL Very Slow Performance Without Compile (CompileQuery)


At my company, we have been running a workflow process that is taking 6 hours to complete.  This is a new project so we’ve had the good fortune to be able to use the latest Microsoft technologies.  We are a Microsoft shop, so that means Sql Server 2008, C#, ASP.NET, ExtJS, LINQ to SQL (hoping to move to Entity Framework someday) and others.  After spending all day running performance analysis tools and basically performing a full audit of the 6 hour process, we sadly concluded that our time was being eaten up by LINQ to SQL processing.  My experience has always been that anything you do on the compiled code side is usually overwhelmed by database access times, usually at least ten to one.  Well, I learned a lot yesterday.

The Problem

Since I already know the answer, I put together a very trivial problem to solve.  What I want to do is execute one simple SQL statement over and over.   I’m going to query just one table that actually has no records in it just to make sure the database really has nothing to do.  I’m going to make this connection on my blazingly fast Lenovo W500 with a SATA SSD running Sql Server 2008  locally.  I’m going to run that simple query first calling just ADO.NET (bare bones using best practices), Then using LINQ to SQL in the way I’ve always used it (and I would say 99% of the other developers out there), then I’m going to compile that LINQ to SQL and run it again.  Actually, I’m going to iterate 2000 times for each of the three conditions.

So, here is the simple SQL I’m executing:

  dbo.LinqTest.FileName = 'MyVal'

The Shocking Results


Or, for those graphically challenged (like search engines), Here is the actual data.

Test Description Seconds Execution for 2000 Iterations
ADO.NET 0.48
Linq2Sql Not Compiled 18.14
Linq2Sql Compiled 0.87

In English, this says that

  • LINQ to SQL is  37 times slower than running a raw ADO.NET Sql query
  • LINQ to SQL compiled 1.8 times slower than running a raw ADO.NET Sql query

I’ve read in several places that compiling your LINQ will help, but I have never heard anyone say how drastic the speed improvement can be.  For example, in one of my favorite books (LINQ in Action) by Fabrice Marguerie and others, he quotes on page 296  a blog post by Rico Mariani titled DLINQ (Linq to SQL Performance (Part 1) as saying using a compiled query offers nearly twice the performanced of a non-compiled query, and goes on to say that it brings the performance to within 93% of using a raw data reader.   Well, suffice it to say I never ran the test myself.  I could have lived with twice, but not 37 times.

Here is the Project File Source:

How to Compile you SQL (seems like a duh kind of thing)

It’s actually not very hard.  I’m attaching the Visual Studio 2008 project that I ran this test with to this post so you can run it for yourself as well as see an example of how to write the compiled LINQ code.  Below is the method that actually does the work.  I won’t include in this article the actual ADO.NET and LINQ to SQL not compiled code, but you can see those for yourself in the attached solution.

   1:  /// <summary>
   2:  /// This method compiles the LINQ to SQL query and 
   3:  /// then executes it the number of iterations passed
   4:  /// in.  
   5:  /// </summary>
   6:  /// <param name="iterations">Number of iterations</param>
   7:  /// <returns>time in seconds of execution</returns>
   8:  private static double 
   9:      TestDataAccessSpeedLinq2SqlCompiled(int iterations)
  10:  {
  11:      Func<DataClassesDataContext, string, IQueryable<LinqTest>> compiledQuery =
  12:          CompiledQuery.Compile((DataClassesDataContext meta,string fileNameForSearch) =>
  13:                                (from myData in meta.LinqTests
  14:                                 orderby myData.Id
  15:                                 where myData.FileName.Equals(fileNameForSearch)
  16:                                 select myData));
  18:      var metaNew = new DataClassesDataContext();
  19:      DateTime startTime = DateTime.Now;
  20:      for (int i = 0; i < iterations; i++)
  21:      {
  22:          IOrderedQueryable<LinqTest> query = 
  23:              (IOrderedQueryable<LinqTest>) 
  24:              compiledQuery(metaNew,string.Format("abcde{0}", i));
  25:          List<LinqTest> newList = query.ToList();
  26:      }
  27:      return 
  28:          DateTime.Now.Subtract(startTime).Duration().TotalSeconds;
  29:  }

Essentially, line 11 is compiling the code into an instance variable called compileQuery.  Using this instance variable, you can now execute the  LINQ query in a compiled form while still passing in variable data such as fileNameToSearch.  Again, the important thing to note is that one line 24 compiledQuery is already compiled so the IOrderedQueryable result is obtained without having to recompile the LINQ statement.


So, from this, it seems that you should always compile your LINQ to SQL queries.  Well, that’s not quite true.  What I’m recommending is that if you have a reason to execute the same query over and over you should strongly consider compiling.  If for example, you are just making a LINQ to SQL call once, there is no benefit because you have to compile it anyway.  Call it ten times?  Well, you will have to decide for yourself.

Forewarned is forearmed!  good luck and hope this helps.

About Peter Kellner

Peter is a software professional specializing in mobile and web technologies. He has also been a Microsoft MVP since 2007. In addition, he's a multi-course video author at Pluralsight. To read more about Peter Kellner and his experience click here. For information about how Peter Kellner might be able to help you with your project click here.

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  1. Your test is incorrect. In LINQ you select a list, in ADO you using a SqlDataReader. I used your test and my results:
    ADO SqlDataReader: 0.44
    ADO Type DataSet: 0.98
    LINQ TO SQL, to list: 1.09
    LINQ TO SQL, foreach: 1.03
    So, if you use DataSet or LINQ TO SQL in application, differences of the performance are minimal

  2. Ruwan Jayalath says:

    Thanks Peter, Today i got suffered severly with LINQ TO SQL. when i googling about it i found this .

  3. TATWORTH says:

    Thank you for publishing the execution times.

  4. I wrote something of the sort in my website too, please do check it out.

  5. I would like to be a volunteer writer in your website would that be okay?

  6. Very interesting input. 🙂

  7. was hoping to find this content here for a long time! cheers mate!

  8. Preetham K A says:

    Hi, i need a clarification, am using 5 table information in a single query, i have used 3 tier Architecture, in this 3 tierarchitecture i used to get the complete data from the each table in to a LIST, then i will write a LINQ Query by using these 5 LIST, assuming each table has 5 thousand Records. Will this Task reduce the performance of the application?, howmuch time will it require than normal ADO.Net.
    PlZ Reply…. i need it Urgently, Thanks in Adv…. Preetham K A.

  9. Preetham says:

    Hi, i need a clarification, am using 5 table information in a single query, i have used

  10. It seems to me no matter what method you use, LINQ will always be massively slower than using stored procedures to execute sql commands and then allowing to retrieve the results. Am I missing something?

  11. Everyone’s talking about Linq-To-Sql…
    what about Linq to object collections?
    with 0 optimization there… it suffers like HECk!

  12. Thanks Peter, We are are all learning a lot about Linq to Sql (even since writing this 2 days ago)

  13. Nice work, Peter. My “respect” meter just went up about 3 notches for you.


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