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Dependencies for Graphs

This article proposes a class of dependencies for graphs, referred to as graph entity dependencies (GEDs). A GED is defined as a combination of a graph pattern and an attribute dependency. In a uniform format, GEDs can express graph functional dependencies with constant literals to catch inconsistencies, and keys carrying id literals to identify... (more)

NEWS

Updates to the TODS Editorial Board

As of January 1, 2018, five Associate Editors—Walid Aref, Graham Cormode, Gautam Das, Sabrina De Capitani di Vimercati, and Dirk Van Gucht—ended their terms, each having served on the editorial board for six years. Walid, Graham, Gautam, Sabrina, and Dirk have provided very substantial, high-caliber service to the journal and the database community. Also five new Associate Editors have joined the editorial board: Angela Bonifati, Université Claude Bernard Lyon 1, Wolfgang Lehner, TU Dresden, Dan Olteanu, University of Oxford, Evaggelia Pitoura, University of Ioannina, and Bernhard Seeger, University of Marburg. All five are highly regarded scholars in database systems.

Please read more here.

Updates to the TODS Editorial Board

As of January 1, 2017, three Associate Editors, Paolo Ciaccia, Divyakant Agrawal, and Sihem Amer-Yahia, ended their terms, each having served on the editorial board for some six years. In addition, they will stay on until they complete their current loads. We are fortunate that they have donated their time and world-class expertise during these years. Also three new Associate Editors have joined the editorial board: Feifei Li, University of Utah, Kian-Lee Tan, National University of Singapore, and Jeffrey Xu Yu, Chinese University of Hong Kong. All three are highly regarded scholars in database systems. 

Please read more here.

Forthcoming Articles
Verification of Hierarchical Artifact Systems

Data-driven workflows, of which IBM?s Business Artifacts are a prime exponent, have been successfully deployed in practice, adopted in industrial standards, and have spawned a rich body of research in academia, focused primarily on static analysis. The present work represents a significant advance on the problem of artifact verification, by considering a much richer and more realistic model than in previous work, incorporating core elements of IBM?s successful Guard-Stage-Milestone model. In particular, the model features task hierarchy, concurrency, and richer artifact data. It also allows database key and foreign key dependencies, as well as arithmetic constraints. The results show decidability of verification and establish its complexity, making use of novel techniques including a hierarchy of Vector Addition Systems and a variant of quantifier elimination tailored to our context.

Interactive Mapping Specification with Exemplar Tuples

While schema mapping specification is a cumbersome task for data curation specialists, it becomes unfeasible for non-expert users, who are unacquainted with the semantics and languages of the involved transformations. In this paper, we present an interactive framework for schema mapping specification suited for non-expert users. The underlying key intuition is to leverage a few exemplar tuples to infer the underlying mappings and iterate the inference process via simple user interactions under the form of boolean queries on the validity of the initial exemplar tuples. The approaches available so far are mainly assuming pairs of complete universal data examples, which can be solely provided by data curation experts, or are limited to poorly expressive mappings. We present a quasi-lattice based exploration of the space of all possible mappings that satisfy arbitrary user exemplar tuples. Along the exploration, we challenge the user to retain the mappings that fit the users requirements at best and to dynamically prune the exploration space, thus reducing the number of user interactions. We prove that after the refinement process, the obtained mappings are correct and complete. We present an extensive experimental analysis devoted to measure the feasibility of our interactive mapping strategies and the inherent quality of the obtained mappings.

Output-optimal Massively Parallel Algorithms for Similarity Joins

Parallel join algorithms have received much attention in recent years, due to the rapid development of massively parallel systems such as MapReduce and Spark. In the database theory community, most efforts have been focused on studying worst-optimal algorithms. However, the worst-case optimality of these join algorithms relies on the hard instances having very large output sizes. In the case of a two-relation join, the hard instance is just a Cartesian product, with an output size that is quadratic in the input size. In practice, however, the output size is usually much smaller. One recent parallel join algorithm by Beame et al. has achieved output-optimality, i.e., its cost is optimal in terms of both the input size and the output size, but their algorithm only works for a 2-relation equi-join, and has some imperfections. In this paper, we first improve their algorithm to true optimality. Then we design output-optimal algorithms for a large class of similarity joins. Finally, we present a lower bound, which essentially eliminates the possibility of having output-optimal algorithms for any join on more than two relations.

A Unified Framework for Frequent Sequence Mining with Subsequence Constraints

Frequent sequence mining methods often make use of constraints to control which subsequences should be mined. A variety of such subsequence constraints has been studied in the literature, including length, gap, span, regular-expression, and hierarchy constraints. In this paper, we show that many subsequence constraints---including and beyond those considered in the literature---can be unified in a single framework. A unified treatment allows researchers to study jointly many types of subsequence constraints (instead of each one individually) and helps to improve usability of pattern mining systems for practitioners. In more detail, we propose a set of simple and intuitive "pattern expressions" to describe subsequence constraints and explore algorithms for efficiently mining frequent subsequences under such general constraints. Our algorithms translate pattern expressions to succinct finite state transducers, which we use as computational model, and simulate these transducers in a way suitable for frequent sequence mining. Our experimental study on real-world datasets indicates that our algorithms---although more general---are efficient and, when used for sequence mining with prior constraints studied in literature, competitive to (and in some cases superior to) state-of-the-art specialized methods.

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