Optimizing Multiset Relational Algebra Queries Using Weak-Equivalent Rewrite Rules

Abstract

Relational query languages rely heavily on costly join operations to combine tuples from multiple tables into a single resulting tuple. In many cases, the cost of query evaluation can be reduced by manually optimizing (parts of) queries to use cheaper semi-joins instead of joins. Unfortunately, existing database products can only apply such optimizations automatically in rather limited cases. To improve on this situation, we propose a framework for automatic query optimization via weak-equivalent rewrite rules for a multiset relational algebra (that serves as a faithful formalization of core SQL). The weak-equivalent rewrite rules we propose aim at replacing joins by semijoins. To further maximize their usability, these rewrite rules do so by only providing "weak guarantees" on the evaluation results of rewritten queries. We show that, in the context of certain operators, these weak-equivalent rewrite rules still provide strong guarantees on the final evaluation results of the rewritten queries.