The characterization of continuous queries

In a world where the amount of electronic information available is constant ly growing, techniques to select and filter information efficiently become increasingly important. Continuous queries are a tool that allows users to monitor one or more information sources, by giving the impression that the queries are being run continually over them. In this paper, we formalize th e notion of continuous queries for a wide spectrum of environments. We cons ider both append-only data sources and systems that allow more general data manipulation. We examine the case where the database management software m ay be modified as well as where we must treat it as a black box. We study t he classes of queries that can be supported in each case and present effici ent implementation techniques far them.