Multiple Query Optimization in Mediator Systems

With the increase in mediated applications, there are now mediated application servers that support concurrent client accesses. Multiple query optimization (MQO) is essential for efficient processing of concurrent read only queries. The MQO problem in mediated environments differs from that of ordinary relational MQO systems in two ways: first, in mediated environments, the MQO system must be capable of interacting with the "local" query optimizers of the various sources being accessed. Second, in mediated environments, the notionof what constitutes a "common" subexpression varies dramatically from one source to another, and the MQO system must be capable of working with these diverse notions of "common" subexpression. In this paper, we extend classical relational DBMS MQO techniques to the case of mediated applications. Our architecture has a compile-time and a runtime component. In the compile-time component, the developer describes common subexpressions via a set of structures calle..