Section: New Results

On the Optimization of Recursive Relational Queries

Graph databases have received a lot of attention recently as they are particularly useful in many applications such as social networks or for the semantic web. Various languages have emerged to query such graph databases. At the heart of many of those query languages, there is a construction to navigate through the graph which allows some form of recursion.The relational model has benefited from a huge body of research in the last half century and that is why many graph databases either rely on, or have adopted the techniques of, relational-based query engines. Since its introduction, the relational model has seen various attempts to extend it with recursion and it is now possible to use recursion in several SQL- or Datalog-based database systems. The optimization of recursive queries remains, however, a challenge. In this work, we introduce $\mu$-RA, a variation of the Relational Algebra that allows for the expression of relational queries with recursion. $\mu$-RA can express unions of conjunctive regular path queries as well as certain non-regular properties. We present its syntax, semantics and the rewriting rules we specifically devised to tackle the optimization of recursive queries. A prototype evaluator implementing these rewriting rules is shown to be more efficient than previous approaches.

These results were presented at the BDA 2018 conference [14].