1. A database system supporting persistent queries, the system comprising: a persistent query service stored and operating on a network-attached computer adapted to receive connections and requests from client software applications; and a plurality of network-attached data sources; wherein, on receiving a request to create a persistent query from a client software application, the persistent query service: creates a query virtual table corresponding to the persistent query; creates a tree structure representing a logical arrangement of a plurality of operators that yield results required by the persistent query; creates a plurality of intermediate virtual tables corresponding to the plurality of operators, wherein the step of creating an intermediate virtual table further comprises establishing listeners associated with the intermediate virtual table to receive data change notifications; establishes listeners for the query virtual table to receive data change notifications from a plurality of intermediate virtual tables; causes the plurality of data source virtual tables to retrieve initial data from the plurality of data sources; and propagates data via the plurality of intermediate virtual tables and their associated listeners to the persistent query virtual table; and wherein, on detection of a data change in a data source, a plurality of corresponding methods of listeners of a plurality of virtual intermediate tables are invoked, and the data change is propagated via the plurality of intermediate virtual tables and their associated listeners to the persistent query virtual table.