14.00, room 445, PCRI

Abstract
We present results of our study of pattern-based XML query languages. A variety of analogs of conjunctive queries over XML documents have been studied in the literature. We focus on languages based on tree patterns, which follow the structure of tree documents. They arise naturally in many problems related to integrating and exchanging data and to handling incomplete information. In this talk, we concentrate on the problems of finding certain answers to queries, and on static analysis, in particular, query containment.

The problem of computing certain answers arises when one queries incompletely specified databases, i.e., databases with missing information. As often happens, the complexity of the problem jumps when one moves from relations to XML. Nonetheless, we identified large relevant classes of queries for which efficient algorithms can be developed. Curiously, for some of these classes, no analogous results existed in the relational world. In fact, we uncovered a well-behaved class of relational queries which had been overlooked so far.

Testing for query containment is a fondamental task in query optimization. In the relational case, classical homomorphism based tools lead to reasonable complexity algorithms. In XML such techniques can be applied only for very simple queries. Beyond thoses classes of queries, they can either be adapted by using more sophisticated data structures, or – provably – they cannot be adapted at all. In addition, we look at the containment problem in the presence of extra features (such as schemas for documents).

The talk will be based on papers from ICDT’12 and ICDT’13.

Short bio
Amélie Gheerbrant did a PhD about fixed-point logics on trees in Amsterdam with Johan van Benthem and Balder ten Cate. She is currently doing a postdoc in Edinburgh with Leonid Libkin. Her main research interests are in the areas of data management and applications of logic in computer science.

Slides
Amelie Gheerbrant – Pattern-based XML queries

14.00, room 445, PCRI

Abstract
In this talk, I shall briefly present our recent research on two topics: (a) supporting diversity in query results and (b) managing historical queries on graphs. Diversity has recently attracted a lot of attention as a means of increasing user satisfaction in recommendation systems, information retrieval and database queries. Diversification takes many forms such as increasing novelty and covering different query aspects and user information needs. In our recent research, we have proposed efficient indexing techniques for supporting continuous diversification as well as a new definition of diversity based on graph theory that allows adapting the degree of diversification. In the second part of the talk, I shall touch upon our new work on supporting historical queries on large graphs (e.g., on social networks).

Short bio
Evaggelia Pitoura is an Associate Professor and Vice Chair of the Department of Computer Science, University of Ioannina, Greece where she also leads the Distributed Management of Data Lab. She holds a PhD and an MSc degree in Computer Science from Purdue University. Her research interests are in distributed data management, with a recent focus on social networks and ranking by using preferences and diversity. Her publications include more than 150 articles in international journals and conferences and a highly-cited book on mobile computing. She is the recipient of a Best Paper Award (ICDE 1999), a Marie Currie Fellowship (2009) and two ACM Recognition of Service Awards. She regularly serves on the PC of many highly ranked database conferences and currently as demo PC co-chair for VLDB 2012 and PC co-chair for ICDE 2012. Her research has been supported by national and European funding agencies.