# A Dichotomy for Evaluating Simple Regular Path Queries

# A Dichotomy for Evaluating Simple Regular Path Queries

*Talk by Wim Martens*

- Location: APB 3027
- Start: 5. July 2018 at 1:00 pm
- End: 5. July 2018 at 2:30 pm
- Research group: Knowledge-Based Systems
- Event series: KBS Seminar
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Whereas arbitrary and shortest paths can be enumerated in polynomial delay, the situation is much more intricate for simple paths. For instance, already the question if a given graph contains a simple path of a certain length has cases with highly non-trivial solutions and cases that are long-standing open problems. We study RPQ evaluation for simple paths from a parameterized complexity perspective and define a class of simple transitive expressions that is prominent in practice and for which we can prove a dichotomy for the evaluation problem. We observe that, even though simple path semantics is intractable for RPQs in general, it is feasible for the vast majority of RPQs that are used in practice. At the heart of our study on simple paths is a result of independent interest: the two disjoint paths problem in directed graphs is W[1]-hard if parameterized by the length of one of the two paths.

**Bio:**Wim Martens is a professor for Theoretical Computer Science at the University of Bayreuth. He is interested in theoretical aspects of data management, formal language theory, logic, and complexity. He was an invited speaker at STOC 2017 and his research received several awards, including the ACM SIGMOD research highlight award and the Dissertation Award for Computer Science, Belgium. Currently, he is on the editorial board of ACM TODS and he is chairing the ICDT Council. His talk reports about research for which he recently received the Best Paper Award of the International Conference on Database Theory 2018.