An optimization of disjunctive queries: union-pushdown

Most previous works on query optimization techniques deal with conjunctive queries only because the queries with disjunctive predicates are complex to optimize. Hence, for disjunctive queries, query optimizers based on these techniques generate plans using rather simple methods such as CNF- and DNF-based optimization. However, the plans generated by these methods perform extremely poorly for certain types of queries. The authors propose new query optimization method, union-pushdown, for disjunctive queries. This method is composed of four phases, and each phase utilizes some advantageous techniques of CNF- and DNF-based methods. They analyze the performance of the union-pushdown plan against those of conventional plans and show that union-pushdown can be applied to various disjunctive query types without performance degradation.