Stavros Christodoulakis

On the propagation of errors in the size of join results

Query optimizers of current relational database systems use several statistics maintained by the system on the contents of the database to decide on the most efficient access plan for a given query. These statistics contain errors that transitively affect many estimates derived by the optimizer. We present a formal framework based on which the principles of this error propagation can be studied. Within this framework, we obtain several analytic results on how the error propagates in general, as well as in the extreme and average cases. We also provide results on guarantees that the database system can make based on the statistics that it maintains. Finally, we discuss some promising approaches to controlling the error propagation and derive several interesting properties of them.

Optimal histograms for limiting worst-case error propagation in the size of join results

Many current relational database systems use some form of histograms to approximate the frequency distribution of values in the attributes of relations and on this basis estimate query result sizes and access plan costs. The errors that exist in the histogram approximations directly or transitively affect many estimates derived by the database system. We identify the class of serial histograms and its subclass of end-btased histograms; the latter is of particular interest because such histograms are used in several database systems. We concentrate on equality join queries without function symbols where each relation is joined on the same attribute(s) for all joins in which it participates. Join queries of this restricted type are called t-cllque queries. We show that the optimal histogram for reducing the worst-case error in the result size of such a query is always serial. For queries with one join and no function symbols (all of which are vacuously t-clique queries), we present results on finding the optimal serial histogram and the optimal end-biased histogram based on the query characteristics and the frequency distributions of values in the join attributes of the query relations. Finally, we prove that for t-clique queries with a very large number of joins, h~gh-bzased h zstograms (which form a subclass of end-biased histograms) are always optimal. To construct a histogram for the join attribute(s) of a relation, the values in the attribute(s) must first be sorted based on their frequency and then assigned into buckets according to the optimality results above.

Coupling Ontologies with Graphics Content for Knowledge Driven Visualization

A great challenge in information visualization today is to provide models and software that effectively integrate the graphics content of scenes with domain-specific knowledge so that the users can effectively query, interpret, personalize and manipulate the visualized information [1]. Moreover, it is important that the intelligent visualization applications are interoperable in the semantic web environment and thus, require that the models and software supporting them integrate state-of-the-art international standards for knowledge representation, graphics and multimedia. In this paper, we present a model, a methodology and a software framework for the semantic web (Intelligent 3D Visualization Platform - I3DVP) for the development of interoperable intelligent visualization applications that support the coupling of graphics and virtual reality scenes with domain knowledge of different domains. The graphics content and the semantics of the scenes are married into a consistent and cohesive ontological model while at the same time knowledge- based techniques for the querying, manipulation, and semantic personalization of the scenes are introduced. We also provide methods for knowledge driven information visualization and visualization- aided decision making based on inference by reasoning.

Integration of OWL Ontologies in MPEG-7 and TV-Anytime Compliant Semantic Indexing

We describe a systematic methodology for extending the audiovisual content description standards (MPEG-7 and TV-Anytime) with domain-specific knowledge descriptions expressed in OWL. The domain-specific descriptions of the audiovisual content metadata are completely transparent to applications and tools that use MPEG-7 and TV-Anytime, allowing them to use the domain- specific ontologies without any software changes. We also present an interoperability mechanism between OWL and the audiovisual content description standards, which allows MPEG-7 and TV-Anytime descriptions and their domain-specific extensions to be described in OWL and vice versa. Thus, the methodology and the mechanisms presented here open up opportunities for reusing ontology tools and ontologies across a large number of applications (with or without audiovisual content) and across different professional and user communities. We present the details of the methodology and the implementation of the tools supporting it as well as its integration in a large framework for domain-specific indexing and retrieval of audiovisual content.

Interoperability Support for Ontology-Based Video Retrieval Applications

Domain ontologies are very useful for indexing, query specification, retrieval and filtering, user interfaces, even information extraction from audiovisual material. The dominant emerging language standard for the description of domain ontologies is OWL. We describe here a methodology and software that we have developed for the interoperability of OWL with the complete MPEG-7 MDS so that domain ontologies described in OWL can be transparently integrated with the MPEG-7 MDS metadata. This allows applications that recognize and use the MPEG-7 MDS constructs to make use of domain ontologies for applications like indexing, retrieval, filtering etc. resulting in more effective user retrieval and interaction with audiovisual material.

Ontology-Based Semantic Indexing for MPEG-7 and TV-Anytime Audiovisual Content

In this paper, we describe a framework that we have developed for the support of ontology-based semantic indexing and retrieval of audiovisual content following the MPEG-7 and TV-Anytime standard specifications for metadata descriptions. Our work aims to provide a methodology to enhance the retrieval effectiveness of audiovisual content, while maintaining compatibility with the international multimedia standards.In our framework, domain-specific ontologies guide the definition of both the application-specific metadata and the instance-description metadata that describe the contents of audiovisual programs and/or their segments. The ontologies are also used in order to provide compatible descriptions in both audiovisual content standards (MPEG-7 and TV-Anytime) for the same content. This approach allows indexing compatibility and interoperability of TV-Anytime and digital library applications.We describe the design and implementation of a system supporting this framework. The components of the system include a segmentation tool for segmenting audiovisual information, which also provides ontology-based semantic indexing capabilities, and an appropriate API for semantic query support. An application testbed for the domain of soccer games has been developed on top of this system. An ontology for soccer games has been defined and used for indexing and retrieval of soccer games that have been stored in the system database.The methodology we developed opens up a wide opportunity for the creation of MPEG-7 and TV-Anytime services offering structured domain-specific ontologies that can be integrated to these standards for enhancing audiovisual content retrieval performance.

An ontology-driven framework for the management of semantic metadata describing audiovisual information

In this paper, we describe a framework for the management of semantic metadata that describe audiovisual content. The main objectives of this work are to provide a framework allowing enhanced retrieval performance and better user interfaces for audiovisual data, based on extensions of the well accepted international standards for the digital media, digital broadcasting and TV-Anytime domains like TV-Anytime and MPEG-7, as well as to provide an interoperability framework between TV-Anytime and MPEG-7 for the rapidly expanding set of interoperating digital library and interactive TV applications. The framework was developed in the context of the UP-TV project, which investigates TV-Anytime architectures and services. Our approach is ontology-driven, in the sense that we provide ontology based domain-specific extensions of the standards for describing multimedia content. Several aspects of the framework are based on the existence of ontologies. Our framework allows the creation of more efficient user interfaces for accessing content, better retrieval performance and better support in the indexing phase. It also provides compatibility in indexing between the two standards and interoperability for TV-Anytime and digital library applications supported by TV-Anytime servers. We have developed a domain-specific ontology (football games) to test our framework ideas and implementation. The developed ontology uses as primitive modelling elements those provided by the MPEG-7 metadata model. The mapping of the ontology to TV-Anytime metadata primitives is straightforward. We show in detail the concrete implementation of the framework on top of relational databases and its interfaces with the other system components.

Ontology mapping and SPARQL rewriting for querying federated RDF data sources

The web of data consists of distributed, diverse (in terms of schema adopted), and large RDF datasets. In this paper we present a SPARQL query rewriting method which can be used to achieve interoperability in semantic information retrieval and/or knowledge discovery processes over interconnected RDF data sources. Formal mappings between different overlapping ontologies are exploited in order to rewrite initial user SPARQL queries, so that they can be evaluated over different RDF data sources on different sites. The proposed environment is utilized by an ontology-based mediator system, which we have developed in order to provide data integration within the Semantic Web environment.

A multimedia user preference model that supports semantics and its application to MPEG 7/21

Semantic interoperability is usually provided in open environments through standards and domain ontology. The dominant standards for multimedia content and service descriptions are MPEG-7 and MPEG-21. The MPEG-7 semantic DS has powerful semantic description capabilities and supports using semantic entities specified in domain ontology in multimedia content descriptions. However, the MPEG-7/21 usage environment allows neither the specification of semantic user preferences nor the exploitation of domain knowledge and MPEG-7 semantic metadata descriptions. In addition, the users cannot explicitly specify, in the hierarchical MPEG-7/21 filtering and search preferences, the Boolean operators that should be used during content filtering to combine the hierarchy components. We think these as serious limitations and we propose a hierarchical semantic user preference model that allows for the explicit specification of Boolean operators. Then, we present the application of the model in MPEG-7/21 and the model implementation within the DS-MIRF framework

SPARQL-RW: transparent query access over mapped RDF data sources

The Web of Data is an open environment consisting of very large, inter-linked RDF datasets from various domains (e.g., DBpedia, GeoNames, ACM, PubMed, etc.) accessed through SPARQL queries. Establishing interoperability in this environment has become a major research challenge. This paper presents Sparql--Rw (SPARQL--ReWriting), a framework which provides transparent query access over mapped RDF datasets. The Sparql--Rw provides a generic method for SPARQL query rewriting, with respect to a set of predefined mappings between ontology schemas. To this end, it supports a set of rich and flexible mapping types and it is proved to provide semantics preserving queries.