The optimistic concurrency model assumes that several concurrent transactions can usually complete without interfering with each other, and therefore do not require draconian locking on the resources they access. SQL Server 2005, and later, implements a form of this model called row versioning concurrency. It works by remembering the value of the data at the start of the transaction and checking that no other transaction has modified it before committing. If this optimism is justified for the pattern of activity within a database, it can improve performance by greatly reducing blocking. Kalen Delaney explains how it works in SQL Server.
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