By default, when a query contains a class, all concrete, persistable subclasses (that is, all classes that have an associated table in the database) are queried. Use the boolean descendant query attribute to control this behavior. When this attribute is false, only the specified class in the query is queried in the database. Note that an exception is thrown if the class is not concrete and persistable. Use this feature only when it is necessary to perform a query against a single table.