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Microsoft® SQL Server 2012 Unleashed by Alex T. Silverstein, Chris Gallelli, Paul T. Bertucci, Ray Rankins

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Read Committed Isolation

The Read Committed mode is the default locking-isolation mode for SQL Server. With Read Committed as the transaction isolation level, read operations can read pages only for transactions that have already been committed. No “dirty reads” are allowed. Locks acquired by update transactions are held for the duration of the transaction. However, in this mode, read requests within the transaction release locks as soon as the query finishes reading the data. Although this improves concurrent access to the data for updates, it does not prevent nonrepeatable reads or phantom reads. For example, within a transaction, a process could read one set of rows early in the transaction and then, before reading the information again, another ...

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