Determining beneficial semijoins for a join sequence in distributed query procressing

The problem of combining join and semijoin reducers for distributed query processing is studied. An approach of interleaving a join sequence with beneficial semijoins is proposed. A join sequence is mapped into a join sequence tree that provides an efficient way to identify for each semijoin its correlated semijoins as well as its reducible relations under the join sequence. In light of these properties, an algorithm is developed to determine an effective sequence of join reducers. Examples are also given to illustrate the results, which show that the approach of interleaving a join sequence with beneficial semijoins is effective in reducing the amount of data transmission to process distributed queries.