Manolis Gergatsoulis

Multidimensional Semistructured Data: Representing Context-Dependent Information on the Web

In this paper, we address a problem common in the frame of WWW, namely, representing information that assumes different facets under different contexts (sets of worlds). For expressing context-dependent (or multidimensional) data, we introduce Multidimensional Semistructured Data, where context is defined through (dimension, value) pairs. An extension of OEM called Multidimensional Object Exchange Model (MOEM) is introduced, for representing multidimensional data. We discuss the properties of MOEM and define a transformation that, for any given world, reduces MOEM to a conventional OEM holding under that world. As a case study, we show how MOEM can be used to represent changes over time in an OEM database.

Answering Queries Using Materialized Views with Disjunctions

We consider the problem of answering datalog queries using materialized views. More specifically, queries are rewritten to refer to views instead of the base relations over which the queries were originally written. Much work has been done on program rewriting that produces an equivalent query. In the context of information integration, though, the importance of using views to infer as many answers as possible has been pointed out. Formally, the problem is: Given a datalog program P is there a datalog program Pv which uses only views as EDB predicates and (i) produces a subset of the answers that P produces and (ii) any other program P′v over the views with property (i) is contained in Pv? In this paper we investigate the problem in the case of disjunctive view definitions.

Representing Changes in XML Documents Using Dimensions

In this paper, we present a method for representing the history of XML documents using Multidimensional XML (MXML). We demonstrate how a set of basic change operations on XML documents can be represented in MXML, and show that temporal XML snapshots can be obtained from MXML representations of XML histories. We also argue that our model is capable to represent changes not only in an XML document but to the corresponding XML Schema document as well.

Ontology-based metadata integration in the cultural heritage domain

In this paper, we propose an ontology-based metadata integration methodology for the cultural heritage domain. The proposed real - world approach considers an integration architecture in which CIDOC/CRM ontology acts as a mediating scheme. In this context, we present a mapping methodology from Encoded Archival Description (EAD) and Dublin Core (DC) metadata to CIDOC/CRM, and discuss the faced difficulties.

Unfold/Fold Transformations For Definite Clause Programs

An unfold/fold program transformation system which extends the unfold/fold transformations of H. Tamaki and T. Sato is presented in this paper. The system consists of unfolding, simultaneous folding, and generalization + equality introduction rules. The simultaneous folding rule permits the folding of a set of folded clauses into a single clause, using a set of folding clauses, while the generalization + equality introduction rule facilitates the application of the simultaneous folding rule by performing appropriate abstractions. A proof of the correctness of the proposed transformations in the sense of the least Herbrand model semantics of the program is also presented.

On rewriting XPath queries using views

The problem of rewriting a query using a materialized view is studied for a well known fragment of XPath that includes the following three constructs: wildcards, descendant edges and branches. In earlier work, determining the existence of a rewriting was shown to be coNP-hard, but no tight complexity bound was given. While it was argued that Σ3p is an upper bound, the proof was based on results that have recently been refuted. Consequently, the exact complexity (and even decidability) of this basic problem has been unknown, and there have been no practical rewriting algorithms if the query and the view use all the three constructs mentioned above. It is shown that under fairly general conditions, there are only two candidates for rewriting and hence, the problem can be practically solved by two containment tests. In particular, under these conditions, determining the existence of a rewriting is coNP-complete. The proofs utilize various novel techniques for reasoning about XPath patterns. For the general case, the exact complexity remains unknown, but it is shown that the problem is decidable.

Incorporating Dimensions in XML and DTD

In this paper we investigate various aspects of representing multidimensional information in the frame of the WWW. Multidimensional XML (MXML) is an extension of XML suitable for representing data that assume different facets, having different value or different structure, under different contexts. In Multidimensional XML, elements and attributes may depend on a number of dimensions, that define worlds under which variants of those elements or attributes hold. Moreover, we propose an extension of DTD that takes dimensions into account and is suitable for describing the logical structure of MXML documents. We also present a graph data model for MXML, and show how MXML can be reduced to conventional XML for a given world.

Branching-time logic programming: the language Cactus and its applications

Temporal programming languages provide a powerful means for the description and implementation of dynamic systems. However, most temporal languages are based on linear time, a fact that renders them unsuitable for certain types of applications (such as expressing properties of non-deterministic programs). In this paper we introduce the new temporal logic programming language Cactus, which is based on a branching notion of time. In Cactus, the truth value of a predicate depends on a hidden time parameter which varies over a tree-like structure. As a result, Cactus can be used to express in a natural way non-deterministic computations or generally algorithms that involve the manipulation of tree data structures. Moreover, Cactus appears to be appropriate as the target language for compilers or program transformers. Cactus programs can be executed using BSLD-resolution, a proof procedure based on the notion of canonical temporal atoms/clauses.

Representing and querying histories of semistructured databases using multidimensional OEM

Multidimensional semistructured data (MSSD) are semistructured data that present different facets under different contexts. Context represents alternative worlds, and is expressed by assigning values to a set of user-defined variables called dimensions. The notion of context has been incorporated in the object exchange model (OEM), and the extended model is called Multidimensional OEM (MOEM), a graph model for MSSD. In this paper, we explain in detail how MOEM can represent the history of an OEM database. We discuss how MOEM properties are applied in the case of representing OEM histories, and show that temporal OEM snapshots can be obtained from MOEM. We present a system that implements the proposed ideas, and we use an example scenario to demonstrate how an underlying MOEM database accommodates changes in an OEM database. Furthermore, we show that MOEM is capable to model changes occurring not only in OEM databases, but in Multidimensional OEM databases as well. The use of multidimensional query language (MQL), a query language for MSSD, is proposed for querying the history of OEM databases and MOEM databases.

A Web-Based System for Handling Multidimensional Information through MXML

In this paper we address an issue common in the frame of WWW, namely information entities that present different facets under different contexts (or worlds). Handling such multifacet or multidimensional entities requires a multidimensional paradigm for Web data, which consists of representation, manipulation and presentation issues. For representing multidimensional data we employ Multidimensional XML, a markup language that incorporates dimensions in XML. We discuss the presentation of multidimensional data through multidimensional XSL stylesheets. We describe the design of a system that implements the basic functionality of the multidimensional paradigm, and demonstrates how a user can interact with a multidimensional document and view different variants of the document under different worlds.