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Working with Snapshot Isolation

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Applies to: SQL Server Azure SQL Database Azure SQL Managed Instance Azure Synapse Analytics Analytics Platform System (PDW)

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SQL Server 2005 (9.x) introduced a new "snapshot" isolation level that is intended to enhance concurrency for online transaction processing (OLTP) applications. In earlier versions of SQL Server, concurrency was based solely on locking, which can cause blocking and deadlocking problems for some applications. Snapshot isolation depends on enhancements to row versioning and is intended to improve performance by avoiding reader-writer blocking scenarios.

Transactions that start under snapshot isolation read a database snapshot as of the time when the transaction starts. Keyset, dynamic and static server cursors, opened within a snapshot transaction context behave like much like static cursors that were opened within serializable transactions. However, when the cursors are opened under the snapshot isolation level locks are not taken. This fact can reduce blocking on the server.

OLE DB Driver for SQL Server

The OLE DB Driver for SQL Server has enhancements that take advantage of the snapshot isolation introduced in SQL Server 2005 (9.x). These enhancements include changes to the DBPROPSET_DATASOURCEINFO and DBPROPSET_SESSION property sets.

DBPROPSET_DATASOURCEINFO

The DBPROPSET_DATASOURCEINFO property set has been changed to indicate that the snapshot isolation level is supported by the addition of the DBPROPVAL_TI_SNAPSHOT value that is used in the DBPROP_SUPPORTEDTXNISOLEVELS property. This new value indicates that the snapshot isolation level is supported whether or not versioning has been enabled on the database. The following table lists the DBPROP_SUPPORTEDTXNISOLEVELS values:

Property ID Description
DBPROP_SUPPORTEDTXNISOLEVELS Type: VT_I4

R/W: Read only

Description: A bitmask specifying the supported transaction isolation levels. A combination of zero or more of the following:

DBPROPVAL_TI_CHAOS

DBPROPVAL_TI_READUNCOMMITTED

DBPROPVAL_TI_BROWSE

DBPROPVAL_TI_CURSORSTABILITY

DBPROPVAL_TI_READCOMMITTED

DBPROPVAL_TI_REPEATABLEREAD

DBPROPVAL_TI_SERIALIZABLE

DBPROPVAL_TI_ISOLATED

DBPROPVAL_TI_SNAPSHOT

DBPROPSET_SESSION

The DBPROPSET_SESSION property set has been changed to indicate that the snapshot isolation level is supported by the addition of the DBPROPVAL_TI_SNAPSHOT value that is used in the DBPROP_SESS_AUTOCOMMITISOLEVELS property. This new value indicates that the snapshot isolation level is supported whether or not versioning has been enabled on the database. The following table lists the DBPROP_SESS_AUTOCOMMITISOLEVELS values:

Property ID Description
DBPROP_SESS_AUTOCOMMITISOLEVELS Type: VT_I4

R/W: Read only

Description: Specifies a bitmask that indicates the transaction isolation level while in auto-commit mode. The values that can be set in this bitmask are the same as the values that can be set for DBPROP_SUPPORTEDTXNISOLEVELS.

Note

The errors DB_S_ERRORSOCCURRED or DB_E_ERRORSOCCURRED will occur if DBPROPVAL_TI_SNAPSHOT is set when using versions of SQL Server earlier than SQL Server 2005 (9.x).

For information about how snapshot isolation is supported in transactions, see Supporting Local Transactions.

See Also

OLE DB Driver for SQL Server Features
Rowset Properties and Behaviors