Optimization of Continual Queries

Abstract

Recent advances in the technologies have made it possible to access information from Internet-scale distributed data sources (Florescu, Levy & Mendelzon, 1998). However, finding the right information at the right time is difficult. Update monitoring (Seligman et al., 2000) is a technology that gathers relevant information and forwards it to users in a timely way. Continual queries (CQs) (Chen et al., 2000; Khan & Mott, 2002a; Liu, Pu & Tang, 1999, 2000) provide a significant toolkit for update monitoring. They are persistent queries that are issued once and then are run at regular intervals or when data change until a termination condition is satisfied. They are then removed from the system. They relieve users from having to revisit Web sites or other data sources and reissue their queries frequently to obtain new information that match their queries. A CQ is a typical SQL query having additional triggering and termination conditions. CQs are of two types: change-based and time-based. An example of a CQ is “notify me in the next six months whenever the Microsoft stock price drops by more than 5% from today level.”