Chapter 10: Join Methods and Query Optimization

Joins between data sources are an essential ingredient of multidomain queries, as they exploit connection patterns defined between service marts or between service interfaces. This chapter moves from the definition of a query language over service interfaces, sketching how queries can be directly expressed over service marts and how these can be translated over service interfaces. The fundamental operation discussed in this chapter is the binary join between two sources, which is influenced by the type (search vs. exact) of services and by the management (parallel vs. sequential) of service calls. Then, this chapter presents an optimization framework for queries over several service interfaces, which considers several cost metrics for mapping queries into query plans, consisting of specific operations over services, and includes a branch and bound approach to the exploration of the combinatorial search space of all possible query plans.