| Summary: | Graph data models have recently become popular owing to their applications,
e.g., in social networks and the 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 to WL in
terms of expressive power.
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