Question

I am using EF code first. Recently I had to replace the following code:

User user = userRepository.GetByEmail("[e-mail]");

if (user == null)
{
    user = New User { Email = email, CreatedAt = DateTime.Now };

    userRepository.Add(user);
    unitOfWork.Commit();
}

with

Context.ExecuteSqlCommand("IF NOT EXISTS(SELECT 1 FROM Users WHERE Email = '{0}')
                           INSERT INTO Users(Email, CreatedAt)
                           VALUES ('email', GETDATE())");

The reason behind this is that it took EF a very long time to run the first piece of code when trying to add thousands of rows. By changing it to a ExecuteSqlCommand, the time to handle that many rows decreased by a multitude.

The problem I am seeing now (only occurred twice so far) is the following message from the database: Transaction (Process ID 52) was deadlocked on lock resources with another process and has been chosen as the deadlock victim. Rerun the transaction.

How would I go about resolving this? Most of my data access is done through EF with a few exceptions like the the one above. I have never seen a deadlock in my logs before so I assume this ha something to do with the query.

My questions are:

1. Is there a way to write the query using No LOCK? How would that
   query look?
2. Is there a way to tell EF to use NO LOCK for certain queries?

Answer 1

What you actually want is to lock the table earlier, not to prevent locking. Locking is necessary to ensure that between the two statements in your command, some other process hasn’t come along and inserted the same user. (Locking is always necessary when inserting data because the physical storage is being modified.)

Assuming that this is actually the command that is causing deadlock, the following should resolve it, because it only asks for an exclusive lock :

Context.ExecuteSqlCommand(“IF NOT EXISTS(SELECT 1 FROM Users WHERE Email = ‘{0}’)
INSERT INTO Users(Email, CreatedAt)
VALUES (’email’, GETDATE())”);