Optimistic transaction isolation levels were introduced in SQL Server 2005 as a new way to deal with blocking problems and address concurrency phenomena in a system. With optimistic transaction isolation levels, queries read “old” committed versions of rows while accessing data modified by the other sessions, rather than being blocked by the incompatibility of shared (S) and exclusive (X) locks.
This chapter will explain how optimistic isolation levels are implemented and how they affect the locking behavior of the system.
