From Binary Join to Free Join

Abstract

Over the last decade, worst-case optimal join (WCOJ) algorithms have emerged as a new paradigm for one of the most fundamental challenges in query processing: computing joins efficiently. Such an algorithm can be asymptotically faster than traditional binary joins, all the while remaining simple to understand and implement. However, they have been found to be less efficient than the old paradigm, traditional binary join plans, on the typical acyclic queries found in practice. In an effort to unify and generalize the two paradigms, we proposed a new framework, called Free Join, in our SIGMOD 2023 paper. Not only does Free Join unite the worlds of traditional and worst-case optimal join algorithms, it uncovers optimizations and evaluation strategies that outperform both. In this article, we approach Free Join from the traditional perspective of binary joins, and re-derive the more general framework via a series of gradual transformations. We hope this perspective from the past can help practitioners better understand the Free Join framework, and find ways to incorporate some of the ideas into their own systems.