![]() ![]() READ UNCOMMITTED is the least restrictive isolation level because it ignores locks placed by other transactions. Isolation levels or lock hints set in stored procedures or functions do not change the isolation level of the connection that calls them and are in effect only for the duration of the stored procedure or function call.įour isolation levels defined in the SQL-92 standard were supported in early versions of SQL Server: Individual queries issued within a connection can contain lock hints that modify the isolation for a single statement or transaction but do not affect the isolation level of the connection. Subsequent connections reusing a pooled connection use the isolation level that was in effect at the time the connection is pooled. When a connection is closed and returned to the pool, the isolation level from the last SET TRANSACTION ISOLATION LEVEL statement is retained. An isolation level has connection-wide scope, and once set for a connection with the SET TRANSACTION ISOLATION LEVEL statement, it remains in effect until the connection is closed or another isolation level is set. The isolation level under which a Transact-SQL statement executes determines its locking and row versioning behavior. ![]() Managing Concurrency with Isolation Levels If the READ_COMMITTED_SNAPSHOT option is set to OFF, you must explicitly set the Snapshot isolation level for each session in order to access versioned rows. Setting the READ_COMMITTED_SNAPSHOT ON option allows access to versioned rows under the default READ COMMITTED isolation level. The following statements activate snapshot isolation and replace the default READ COMMITTED behavior with SNAPSHOT: ALTER DATABASE MyDatabase In this respect, snapshot isolation differs from the traditional isolation levels of READ COMMITTED, REPEATABLE READ, SERIALIZABLE, and READ UNCOMMITTED, which require no configuration. You must enable snapshot isolation in each database that uses it with the Transact-SQL ALTER DATABASE statement. This activates the mechanism for storing row versions in the temporary database ( tempdb). Snapshot isolation must be enabled by setting the ALLOW_SNAPSHOT_ISOLATION ON database option before it is used in transactions. For more information, see Hints (Transact-SQL). You can avoid this by using UPDLOCK hints for SELECT statements that access data to be modified. If a snapshot transaction attempts to commit modifications to data that has changed since the transaction began, the transaction will roll back and an error will be raised. Snapshot isolation uses an optimistic concurrency model. This non-blocking behavior also significantly reduces the likelihood of deadlocks for complex transactions. Transactions that modify data do not block transactions that read data, and transactions that read data do not block transactions that write data, as they normally would under the default READ COMMITTED isolation level in SQL Server. No locks are acquired on the underlying data rows or data pages in a snapshot transaction, which permits other transactions to execute without being blocked by a prior uncompleted transaction. ![]() The term "snapshot" reflects the fact that all queries in the transaction see the same version, or snapshot, of the database, based on the state of the database at the moment in time when the transaction begins. Newer row versions created after the transaction has begun are ignored by the transaction. ![]() The transaction works with the most recent row versions having a sequence number before the sequence number of the transaction. A unique transaction sequence number identifies each transaction, and these unique numbers are recorded for each row version. If ADR is enabled, then all row versions, both related to snapshot isolation and ADR, are kept in ADR's Persistent Version Store (PVS), which is located in the user database in a filegroup which the user specifies. So, as of SQL Server 2019, if ADR is not enabled, row versions are kept in tempdb as always. SQL Server 2019 introduces a new feature, Accelerated Database Recovery (ADR) which requires its own set of row versions. Prior to SQL Server 2019, these versions were stored in tempdb. Once snapshot isolation is enabled, updated row versions for each transaction must be maintained. Understanding Snapshot Isolation and Row Versioning Snapshot isolation enhances concurrency for OLTP applications. ![]()
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