ChallengePremature optimization, and all of that…

time to read 6 min | 1174 words

Let us look at the following pieces of code:

public void Consume(MyBooksRequest message)
{
    var user = session.Get<User>(message.UserId);
    
    bus.Reply(new MyBooksResponse
    {
        UserId = message.UserId,
        Timestamp = DateTime.Now,
        Books = user.CurrentlyReading.ToBookDtoArray()
    });
}

public void Consume(MyQueueRequest message)
{
    var user = session.Get<User>(message.UserId);

    bus.Reply(new MyQueueResponse
    {
        UserId = message.UserId,
        Timestamp = DateTime.Now,
        Queue = user.Queue.ToBookDtoArray()
    });
}

public void Consume(MyRecommendationsRequest message)
{
    var user = session.Get<User>(message.UserId);

    bus.Reply(new MyRecommendationsResponse
    {
        UserId = message.UserId,
        Timestamp = DateTime.Now,
        Recommendations = user.Recommendations.ToBookDtoArray()
    });
}

Looking at this, I see that I have a requirement to getting my books, my queues and my recommendations. It appears that getting each datum is going to result in 2 queries, the first to load the User, and the second to lazy load the actual collection that we want to return.

An almost trivial optimization would be to eliminate the lazy loading, right? That would reduce the cost from 6 queries to just 3.

However, that assumption would be wrong. The following client code:

bus.Send(
    new MyBooksRequest
    {
        UserId = userId
    },
    new MyQueueRequest
    {
        UserId = userId
    },
    new MyRecommendationsRequest
    {
        UserId = userId
    });

Produces this SQL:

-- statement #1
enlisted session in distributed transaction with isolation level: Serializable

-- statement #2
SELECT user0_.Id          as Id2_0_,
       user0_.Name        as Name2_0_,
       user0_.Street      as Street2_0_,
       user0_.Country     as Country2_0_,
       user0_.City        as City2_0_,
       user0_.ZipCode     as ZipCode2_0_,
       user0_.HouseNumber as HouseNum7_2_0_
FROM   Users user0_
WHERE  user0_.Id = 1 /* @p0 */

-- statement #3
SELECT currentlyr0_.[User] as User1_1_,
       currentlyr0_.Book   as Book1_,
       book1_.Id           as Id0_0_,
       book1_.Name         as Name0_0_,
       book1_.ImageUrl     as ImageUrl0_0_,
       book1_.Image        as Image0_0_,
       book1_.Author       as Author0_0_
FROM   UsersReadingBooks currentlyr0_
       left outer join Books book1_
         on currentlyr0_.Book = book1_.Id
WHERE  currentlyr0_.[User] = 1 /* @p0 */

-- statement #4
SELECT queue0_.[User]  as User1_1_,
       queue0_.Book    as Book1_,
       queue0_.[Index] as Index3_1_,
       book1_.Id       as Id0_0_,
       book1_.Name     as Name0_0_,
       book1_.ImageUrl as ImageUrl0_0_,
       book1_.Image    as Image0_0_,
       book1_.Author   as Author0_0_
FROM   UsersWaitingBooks queue0_
       left outer join Books book1_
         on queue0_.Book = book1_.Id
WHERE  queue0_.[User] = 1 /* @p0 */

-- statement #5
SELECT recommenda0_.[User]  as User1_1_,
       recommenda0_.Book    as Book1_,
       recommenda0_.[Index] as Index3_1_,
       book1_.Id            as Id0_0_,
       book1_.Name          as Name0_0_,
       book1_.ImageUrl      as ImageUrl0_0_,
       book1_.Image         as Image0_0_,
       book1_.Author        as Author0_0_
FROM   UsersRecommendedBooks recommenda0_
       left outer join Books book1_
         on recommenda0_.Book = book1_.Id
WHERE  recommenda0_.[User] = 1 /* @p0 */


-- statement #7
commit transaction

That seems strange, can you figure out why?

Bonus points for figuring out whatever it would be worth it to do the eager load optimization or not.

More posts in "Challenge" series:

  1. (26 Jul 2016) The race condition in the TCP stack, answer
  2. (25 Jul 2016) The race condition in the TCP stack
  3. (28 Apr 2015) What is the meaning of this change?
  4. (26 Sep 2013) Spot the bug
  5. (27 May 2013) The problem of locking down tasks…
  6. (17 Oct 2011) Minimum number of round trips
  7. (23 Aug 2011) Recent Comments with Future Posts
  8. (02 Aug 2011) Modifying execution approaches
  9. (29 Apr 2011) Stop the leaks
  10. (23 Dec 2010) This code should never hit production
  11. (17 Dec 2010) Your own ThreadLocal
  12. (03 Dec 2010) Querying relative information with RavenDB
  13. (29 Jun 2010) Find the bug
  14. (23 Jun 2010) Dynamically dynamic
  15. (28 Apr 2010) What killed the application?
  16. (19 Mar 2010) What does this code do?
  17. (04 Mar 2010) Robust enumeration over external code
  18. (16 Feb 2010) Premature optimization, and all of that…
  19. (12 Feb 2010) Efficient querying
  20. (10 Feb 2010) Find the resource leak
  21. (21 Oct 2009) Can you spot the bug?
  22. (18 Oct 2009) Why is this wrong?
  23. (17 Oct 2009) Write the check in comment
  24. (15 Sep 2009) NH Prof Exporting Reports
  25. (02 Sep 2009) The lazy loaded inheritance many to one association OR/M conundrum
  26. (01 Sep 2009) Why isn’t select broken?
  27. (06 Aug 2009) Find the bug fixes
  28. (26 May 2009) Find the bug
  29. (14 May 2009) multi threaded test failure
  30. (11 May 2009) The regex that doesn’t match
  31. (24 Mar 2009) probability based selection
  32. (13 Mar 2009) C# Rewriting
  33. (18 Feb 2009) write a self extracting program
  34. (04 Sep 2008) Don't stop with the first DSL abstraction
  35. (02 Aug 2008) What is the problem?
  36. (28 Jul 2008) What does this code do?
  37. (26 Jul 2008) Find the bug fix
  38. (05 Jul 2008) Find the deadlock
  39. (03 Jul 2008) Find the bug
  40. (02 Jul 2008) What is wrong with this code
  41. (05 Jun 2008) why did the tests fail?
  42. (27 May 2008) Striving for better syntax
  43. (13 Apr 2008) calling generics without the generic type
  44. (12 Apr 2008) The directory tree
  45. (24 Mar 2008) Find the version
  46. (21 Jan 2008) Strongly typing weakly typed code
  47. (28 Jun 2007) Windsor Null Object Dependency Facility