In SQL Server 2005 and 2008, how to tell I'm using pessimistic concurrency model or optimistic one?

Question

I know SQL Server 2000 has a pessimistic concurrency model. And the optimistic model was added in SQL Server 2005. So how do I tell whether I'm using the pessimistic concurrency model or the optimistic one in SQL Server 2005 and 2008?

Thanks.

Answer 1

SQL 2005 (and 2008) introduces SNAPSHOT issolation. This is the way to move to optimistic concurrency. Take a look to Transaction Isolation and the New Snapshot Isolation Level article:

Isolation level     Dirty Reads    Non-repeatable Phantom reads  Concurrency 
                                   reads                         control
READ UNCOMMITTED     Yes           Yes            Yes            Pessimistic
READ COMMITTED       No            Yes            Yes            Pessimistic
(with locking)    
READ COMMITTED       No            Yes            Yes            Optimistic
(with snapshot)
REPEATABLE READ      No            No             Yes            Pessimistic
SNAPSHOT             No            No             No             Optimistic
SERIALIZABLE         No            No             No             Pessimistic

Answer 2

After reading some articles and documents from Microsoft. I got the following conclusion.

On SQL Server 2005+

• If you are using read uncommitted, repeatable read or serializable isolation level, you are using pessimistic concurrency model.
• If you are using snapshot isolation level, you are using optimistic concurrency model.
• If you are using read committed isolation level and the database setting read_committed_snapshot is ON, then you are using optimistic concurrency model
• If you are using read committed isolation level and the database setting read_committed_snapshot is OFF, then you are using pessimistic concurrency model

However, I still need confirmation. Also, If there are some code to test the concurrency model that would be great.

Answer 3

Basically:

Pessimistic: you lock the record only for you until you have finished with it. So read committed transaction isolation level. (not uncommitted as you said)

Optimistic concurrency control works on the assumption that resource conflicts between multiple users are unlikely, and it permits transactions to execute without locking any resources. The resources are checked only when transactions are trying to change data. This determines whether any conflict has occurred (for example, by checking a version number). If a conflict occurs, the application must read the data and try the change again. Optimistic concurrency control is not provided with the product, but you can build it into your application manually by tracking database access. (Source)