2.3. Query Termination

When a query is canceled, processing will be stopped in the query engine and in all connectors involved in the query. The semantics of what a connector does in response to a cancellation command is dependent on the connector implementation. For example, JDBC connectors will asynchronously call cancel on the underlying JDBC driver, which may or may not actually support this method.

User query timeouts in Data Virtualization can be managed on the client-side or server-side. Timeouts are only relevant for the first record returned. If the first record has not been received by the client within the specified timeout period, a "cancel" command is issued to the server for the request and no results are returned to the client. The cancel command is issued asynchronously by the JDBC API without the client's intervention.

The JDBC API uses the query timeout set by the java.sql.Statement.setQueryTimeout method. You can also set a default statement timeout via the connection property QUERYTIMEOUT. ODBC clients may also use QUERYTIMEOUT as an execution property via a set statement to control the default timeout setting. See Red Hat JBoss Development Guide: Client Development for more on connection/execution properties and set statements.

Server-side timeouts start when the query is received by the engine. The timeout will be canceled if the first result is sent back before the timeout has ended. See Section 9.2, “VDB Definition: The VDB Element” for more on setting the query-timeout VDB property. See the Red Hat JBoss Administration Guide for more information on setting the default query timeout for all queries.