Under an alternative concurrency model, enabled via snapshot-based isolation, SQL Server can prevent some or all of these read phenomena, depending on the mode of snapshot-based isolation in use, without the need to acquire locks, therefore greatly reducing blocking in the database. However, the ANSI SQL definitions of each of the transaction isolation levels specify only which of the behaviors each level allows, not how to implement them. As a result, readers block writers and writers block readers. Row Versioning Concurrency in SQL Server - Simple Talkīy default, SQL Server adopts a pessimistic approach to concurrency, acquiring locks in order to avoid read phenomena such as dirty reads, non-repeatable reads and phantom reads, depending on the required ANSI isolation level.
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