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. (03 Jan 2020) Spot the bug in the stream–answer
  2. (15 Feb 2010) Where is the optimization?