Executable First-Order Queries in the Logic of Information Flows

The logic of information flows (LIF) has recently been proposed as a general framework in the field of knowledge representation. In this framework, tasks of a procedural nature can still be modeled in a declarative, logic-based fashion. In this paper, we focus on the task of query processing under limited access patterns, a well-studied problem in the database literature. We show that LIF is well-suited for modeling this task. Toward this goal, we introduce a variant of LIF called "forward" LIF, in a first-order setting. We define FLIFio, a syntactical fragment of forward LIF, and show that it corresponds exactly to the "executable" fragment of first-order logic defined by Nash and Lud\"ascher. Moreover, we show that general FLIF expressions can also be put into io-disjoint form. The definition of FLIFio involves a classification of the free variables of an expression into "input" and "output" variables. Our result hinges on inertia and determinacy laws for forward LIF expressions, which are interesting in their own right. These laws are formulated in terms of the input and output variables.Comment: This paper is the extended version of the two papers presented at ICDT 2020 and ICDT 202

Constant-Delay Enumeration for Nondeterministic Document Spanners

We consider the information extraction framework known as document spanners, and study the problem of efficiently computing the results of the extraction from an input document, where the extraction task is described as a sequential variable-set automaton (VA). We pose this problem in the setting of enumeration algorithms, where we can first run a preprocessing phase and must then produce the results with a small delay between any two consecutive results. Our goal is to have an algorithm which is tractable in combined complexity, i.e., in the sizes of the input document and the VA; while ensuring the best possible data complexity bounds in the input document size, i.e., constant delay in the document size. Several recent works at PODS'18 proposed such algorithms but with linear delay in the document size or with an exponential dependency in size of the (generally nondeterministic) input VA. In particular, Florenzano et al. suggest that our desired runtime guarantees cannot be met for general sequential VAs. We refute this and show that, given a nondeterministic sequential VA and an input document, we can enumerate the mappings of the VA on the document with the following bounds: the preprocessing is linear in the document size and polynomial in the size of the VA, and the delay is independent of the document and polynomial in the size of the VA. The resulting algorithm thus achieves tractability in combined complexity and the best possible data complexity bounds. Moreover, it is rather easy to describe, in particular for the restricted case of so-called extended VAs. Finally, we evaluate our algorithm empirically using a prototype implementation.Comment: 29 pages. Extended version of arXiv:1807.09320. Integrates all corrections following reviewer feedback. Outside of some minor formatting differences and tweaks, this paper is the same as the paper to appear in the ACM TODS journa

Relative Expressive Power of Navigational Querying on Graphs

Motivated by both established and new applications, we study navigational query languages for graphs (binary relations). The simplest language has only the two operators union and composition, together with the identity relation. We make more powerful languages by adding any of the following operators: intersection; set difference; projection; coprojection; converse; and the diversity relation. All these operators map binary relations to binary relations. We compare the expressive power of all resulting languages. We do this not only for general path queries (queries where the result may be any binary relation) but also for boolean or yes/no queries (expressed by the nonemptiness of an expression). For both cases, we present the complete Hasse diagram of relative expressiveness. In particular the Hasse diagram for boolean queries contains some nontrivial separations and a few surprising collapses.Comment: An extended abstract announcing the results of this paper was presented at the 14th International Conference on Database Theory, Uppsala, Sweden, March 201

Relative Expressive Power of Navigational Querying on Graphs using Transitive Closure

Motivated by both established and new applications, we study navigational query languages for graphs (binary relations). The simplest language has only the two operators union and composition, together with the identity relation. We make more powerful languages by adding any of thefollowing operators: intersection; set difference; projection; coprojection; converse; transitive closure; and the diversity relation. All these operators map binary relations to binary relations. We compare the expressive power of all resulting languages, both for binary-relation queries as well as for boolean queries. In the absence of transitive closure, a complete Hasse diagram of relative expressiveness has already been established [8]. Moreover, it has already been shown that for boolean queries over a single edge label, transitive closure does not add any expressive power when only projection and diversity may be present [11]. In the present paper, we now complete the Hasse diagram in the presence of transitive closure, both for the case of a single edge label, as well as for the case of at least two edge labels. The main technical results are the following:1. In contrast to the above-stated result [11] transitive closure does add expressive power when coprojection is present.2. Transitive closure also adds expressive power as soon as converse is present.3. Conversely, converse adds expressive power in the presence of transitive closure. In particular, the converse elimination result from [8] no longer works in the presence of transitive closure.4. As a corollary, we show that the converse elimination result from [8] necessitates an exponential blow-up in the degree of the expressions.SCOPUS: ar.jinfo:eu-repo/semantics/publishe

AOD 2.0: A category based submission add-on for DSpace 4.0

Presentation at Open Repositories 2014, Helsinki, Finland, June 9-13, 2014DSpace Interest Group PresentationsThe Institute of Biology of the Southern Seas, Ukraine (IBSS) and Hasselt University (Belgium) are developing with the support of FAO of the United Nations a new version of AgriOcean DSpace named AOD 2.0, as an add-on for DSpace 4.0 and later. It works further on the specifications of AgriOcean DSpace 1.x including the type-based submission module, the use of controlled vocabularies (ASFA, AGROVOC) and an easy-to-install version for Windows. While AgriOcean DSpace 1.x was created as a fully customized version of DSpace 1.7.2, AOD 2.0 is developed with a modular approach. Different stand-alone modules are developed and integrated with DSpace 4.0: Ontology Plug-in, Category-based Submission, Batch Import for Submitter. They are made available as AOD 2.0, an easy-to-install package for Linux and Windows.Goovaerts, Marc (Hasselt University, Belgium)Slipetskyy, Denys (Institute of Biology of the Southern Seas)Subirats, Imma (Food and Agriculture Organizations of the United Nations)Keizer, Johannes (Food and Agriculture Organizations of the United Nations)Leinders, Dirk (Hasselt University, Belgium)Dupriez, Christophe (Destin)Surinx, Dimitri (Hasselt University, Belgium)Gybels, Michelle (Hasselt University, Belgium)Vaelen, Jeroen (Hasselt University, Belgium)Vandesbosch, Niki (Hasselt University, Belgium