Expressive Path Queries on Graphs with Data

Abstract

Abstract. Graph data models have recently become popular owing to their applications, e.g., in social networks, semantic web. Typical navigational query languages over graph databases -such as Conjunctive Regular Path Queries (CRPQs) -cannot express relevant properties of the interaction between the underlying data and the topology. Two languages have been recently proposed to overcome this problem: walk logic (WL) and regular expressions with memory (REM). In this paper, we begin by investigating fundamental properties of WL and REM, i.e., complexity of evaluation problems and expressive power. We first show that the data complexity of WL is nonelementary, which rules out its practicality. On the other hand, while REM has low data complexity, we point out that many natural data/topology properties of graphs expressible in WL cannot be expressed in REM. To this end, we propose register logic, an extension of REM, which we show to be able to express many natural graph properties expressible in WL, while at the same time preserving the elementariness of data complexity of REMs. It is also incomparable in expressive power against WL.

Foundations of Modern Query Languages for Graph Databases

“…On the other hand, they did receive some attention in the theoretical literature. For instance, the base language for regular data path queries was introduced in [Libkin and Vrgoč 2012], and some further extensions allowing first order reasoning over paths [Hellings et al 2013], or unlimited use of variables [Libkin et al 2016;Barceló et al 2015] have also been considered. However, since there is still no clear consensus on the correct language for this task, this seems to be a promising area of future work, both with respect to the theoretical issues, and with respect to the correct techniques for implementing such queries in graph database systems.…”

“…On the other hand, extending such queries with more expressive features seems difficult, as evaluation quickly becomes intractable [Libkin et al 2016;Barceló et al 2015]. Furthermore, implementing these queries using the unwinding operator, as in Cypher, does also not seem to be the best solution, as the operator makes the evaluation NP-hard (see our online appendix for details).…”

Recursion in SPARQL

“…However, in terms of expressive power, several authors have noted that property paths fall short when trying to express a number of important properties related to navigating RDF documents (cf. [10,11,17,[20][21][22]40]), and that a more powerful form of recursion needs to be added to SPARQL to address this issue. Consequently, this realization has brought forward a good number of extensions of property paths that offer more expressive recursive functionalities (see e.g.…”

Context-free path querying with structural representation of result

“…Despite the fact that there is a set of path querying solutions [18,9,4,13], query result exploration is still a challenge [10], as also a simplification of complex query debugging. Structural representation of query result can be used to solve these problems, and classical parsing techniques provide such representation-derivation tree-which contains exhaustive information about parsed sentence structure in terms of specified grammar.…”