Beginning Microsoft SQL Server 2008 ... - S3 Tech Training

Chapter 14: Transactions and Locks
Figure 14-3
Setting the Isolation Le vel
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We’ve seen that several different kinds of problems can be prevented by different locking strategies.
We’ve also seen what kinds of locks are available and how they have an impact on the availability of
resources. Now it’s time to take a closer look at how these process management pieces work together to
ensure overall data integrity — to make certain that you can get the results you expect.
The first thing to understand about the relationship between transactions and locks is that they are inextricably
linked with each other. By default, any lock that is data modification related will, once created,
be held for the duration of the transaction. If you have a long transaction, this means that your locks
may be preventing other processes from accessing the objects you have a lock on for a rather long time.
It probably goes without saying that this can be rather problematic.
However, that’s only the default. In fact, there are actually five different isolation levels that you can set:
❑ READ COMMITTED (the default)
❑ READ UNCOMMITTED
❑ REPEATABLE READ
❑ SERIALIZABLE
❑ SNAPSHOT