Wim Martens

Shqiponja Ahmetaj, Iovka Boneva, Jan Hidders et al.

As schema languages for RDF data become more mature, we are seeing efforts to extend them with recursive semantics, applying diverse ideas from logic programming and description logics. While ShEx has an official recursive semantics based on greatest fixpoints (GFP), the discussion for SHACL is ongoing and seems to be converging towards least fixpoints (LFP). A practical study we perform shows that, indeed, ShEx validators implement GFP, whereas SHACL validators are more heterogeneous. This situation creates tension between ShEx and SHACL, as their semantic commitments appear to diverge, potentially undermining interoperability and predictability. We aim to clarify this design space by comparing the main semantic options in a principled yet accessible way, hoping to engage both theoreticians and practitioners, especially those involved in developing tools and standards. We present a unifying formal semantics that treats LFP, GFP, and supported model semantics (SMS), clarifying their relationships and highlighting a duality between LFP and GFP on stratified fragments. Next, we investigate to which extent the directions taken by SHACL and ShEx are compatible. We show that, although ShEx and SHACL seem to be going in different directions, they include large fragments with identical expressive power. Moreover, there is a strong correspondence between these fragments through the aforementioned principle of duality. Finally, we present a complete picture of the data and combined complexity of ShEx and SHACL validation under LFP, GFP, and SMS, showing that SMS comes at a higher computational cost under standard complexity-theoretic assumptions.

Marcelo Arenas, Leonid Libkin, Wim Martens

A major shortcoming of the recently standardized graph query languages GQL and SQL/PGQ is their lack of compositionality. Given the importance of these languages in querying knowledge graphs, we address this shortcoming and propose both theoretical solutions and a path to adding them to the new standards. The highlight of the non-compositionality problem is that while both GQL and SQL/PGQ can express graph reachability and all first-order queries, they fall short of the problems in NLOGSPACE. In view of the completeness of reachability for NLOGSPACE under first-order reductions, this is extremely counterintuitive. The issue is well recognized by the standards committee that has been searching for language extensions to fill the gaps at the level of some specific inexpressible queries. We address the issue in a systematic way and propose a language that fills expressivity gaps by allowing full compositionality between graph patterns and relational queries. It does so by using two key components: a cleaned up definition of regular path queries with variables and data value comparisons, and a fully compositional graph-to-graph language #Datalog with complete support for constructing new graph elements from nodes, edges, lists of nodes and edges, and even entire paths. We show that the resulting language addresses the issues facing the standards committee, and propose a concrete addition to GQL and SQL/PGQ that incorporates its main features.

S. Ahmetaj, I. Boneva, J. Hidders et al.

Graphs have emerged as an important foundation for a variety of applications, including capturing and reasoning over factual knowledge, semantic data integration, social networks, and providing factual knowledge for machine learning algorithms. To formalise certain properties of the data and to ensure data quality, there is a need to describe the schema of such graphs. Because of the breadth of applications and availability of different data models, such as RDF and property graphs, both the Semantic Web and the database community have independently developed graph schema languages: SHACL, ShEx, and PG-Schema. Each language has its unique approach to defining constraints and validating graph data, leaving potential users in the dark about their commonalities and differences. In this paper, we provide formal, concise definitions of the core components of each of these schema languages. We employ a uniform framework to facilitate a comprehensive comparison between the languages and identify a common set of functionalities, shedding light on both overlapping and distinctive features of the three languages.

Angela Bonifati, Giovanna Guerrini, Carsten Lutz et al.

The joint EDBT/ICDT conference (International Conference on Extending Database Technology/International Conference on Database Theory) is a well established conference series on data management, with annual meetings in the second half of March that attract 250 to 300 delegates. Three weeks before EDBT/ICDT 2020 was planned to take place in Copenhagen, the rapidly developing Covid-19 pandemic led to the decision to cancel the face-to-face event. In the interest of the research community, it was decided to move the conference online while trying to preserve as much of the real-life experience as possible. As far as we know, we are one of the first conferences that moved to a fully synchronous online experience due to the COVID-19 outbreak. With fully synchronous, we mean that participants jointly listened to presentations, had live Q&A, and attended other live events associated with the conference. In this report, we share our decisions, experiences, and lessons learned.

Diego Figueira, Adwait Godbole, S. Krishna et al.

Testing containment of queries is a fundamental reasoning task in knowledge representation. We study here the containment problem for Conjunctive Regular Path Queries (CRPQs), a navigational query language extensively used in ontology and graph database querying. While it is known that containment of CRPQs is expspace-complete in general, we focus here on severely restricted fragments, which are known to be highly relevant in practice according to several recent studies. We obtain a detailed overview of the complexity of the containment problem, depending on the features used in the regular expressions of the queries, with completeness results for np, pitwo, pspace or expspace.