Analytical and Experimental Evaluation of Stream-based Join

Continuous queries over data streams have gained popularity as the breadth of possible applications, ranging from network monitoring to online pattern discovery, have increased. Joining of streams is a fundamental issue that must be resolved to enable complex queries over multiple streams. However, as streams can represent potentially infinite data, it is infeasible to have full join evaluations as is the case with traditional databases. Joins in a stream environment are thus evaluated not over entire streams, but on specific windows defined on the streams. In this paper, we present windowed implementations of the traditional nested loops and hash join algorithms. In our work we analytically and experimentally evaluate the performance of these algorithms for different parameters. We find that, in general, a hash join provides better performance. We also investigate invalidation strategies to remove stale data from the window buffers, and propose an optimal strategy that balances processing time versus buffer size.