Working with Snapshot Isolation
SQL Server 2005 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. One result of this is that keyset, dynamic and static server cursors, when opened within a snapshot transaction context, behave much like static cursors opened within serializable transactions. However, when the cursors are opened under the snapshot isolation level locks are not taken, which can reduce blocking on the server.
For more information about SQL Server's support for snapshot isolation, see Understanding Row Versioning-Based Isolation Levels.
The SQL Server Native Client OLE DB provider has enhancements that take advantage of the snapshot isolation introduced in SQL Server 2005. These enhancements include changes to the DBPROPSET_DATASOURCEINFO and DBPROPSET_SESSION property sets.
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 is a list of the DBPROP_SUPPORTEDTXNISOLEVELS values:
R/W: Read only
Description: A bitmask specifying the supported transaction isolation levels. A combination of zero or more of the following:
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 is a list of the DBPROP_SESS_AUTOCOMMITISOLEVELS values:
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 those that can be set for DBPROP_SUPPORTEDTXNISOLEVELS.
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.
For information about how snapshot isolation is supported in transactions, see Supporting Local Transactions.
The SQLSetConnectAttr function now supports the use of the SQL_COPT_SS_TXN_ISOLATION attribute. Setting SQL_COPT_SS_TXN_ISOLATION to SQL_TXN_SS_SNAPSHOT indicates that the transaction will take place under the snapshot isolation level.