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SQL Anywhere 11.0.1 » SQL Anywhere Server - SQL Usage » Creating Databases » Using transactions and isolation levels » Isolation levels and consistency » Setting the isolation level from an ODBC-enabled application

 

Changing isolation levels within a transaction

Different isolation levels may be suitable for different parts of a single transaction. SQL Anywhere allows you to change the isolation level of your database in the middle of a transaction.

When you change the isolation_level option in the middle of a transaction, the new setting affects only the following:

  • Any cursors opened after the change

  • Any statements executed after the change

You may want to change the isolation level during a transaction to control the number of locks your transaction places. You may find a transaction needs to read a large table, but perform detailed work with only a few of the rows. If an inconsistency would not seriously affect your transaction, set the isolation to a low level while you scan the large table to avoid delaying the work of others.

You may also want to change the isolation level mid-transaction if, for example, just one table or group of tables requires serialized access.

For an example in which the isolation level is changed in the middle of a transaction, see Tutorial: Phantom rows.

Note

You can also set the isolation level (levels 0-3 only) using table hints, but this is an advanced feature that you should use only when needed. For more information, see the WITH table-hint section in FROM clause.

Changing isolation levels when using snapshot isolation

When using snapshot isolation, you can change the isolation level within a transaction. This can be done by changing the setting of the isolation_level option or by using table hints that affect the isolation level in a query. You can use statement-snapshot, readonly-statement-snapshot, and isolation levels 0-3 at any time. However, you cannot use the snapshot isolation level in a transaction if it began at an isolation level other than snapshot . A transaction is initiated by an update and continues until the next COMMIT or ROLLBACK. If the first update takes place at some isolation level other than snapshot, then any statement that tries to use the snapshot isolation level before the transaction commits or rolls back returns error -1065 NON_SNAPSHOT_TRANSACTION. For example:

SET OPTION PUBLIC.allow_snapshot_isolation = 'On';

BEGIN TRANSACTION
    SET OPTION isolation_level = 3;
    INSERT INTO Departments
        ( DepartmentID, DepartmentName, DepartmentHeadID )
        VALUES( 700, 'Foreign Sales', 129 );
    SET TEMPORARY OPTION isolation_level = 'snapshot';
    SELECT * FROM Departments;