Gregory Karvounarakis

RQL: a declarative query language for RDF

Real-scale Semantic Web applications, such as Knowledge Portals and E-Marketplaces, require the management of large volumes of metadata, i.e., information describing the available Web content and services. Better knowledge about their meaning, usage, accessibility or quality will considerably facilitate an automated processing of Web resources. The Resource Description Framework (RDF) enables the creation and exchange of metadata as normal Web data. Although voluminous RDF descriptions are already appearing, sufficiently expressive declarative languages for querying both RDF descriptions and schemas are still missing. In this paper, we propose a new RDF query language called RQL. It is a typed functional language (a la OQL) and relies on a formal model for directed labeled graphs permitting the interpretation of superimposed resource descriptions by means of one or more RDF schemas. RQL adapts the functionality of semistructured/XML query languages to the peculiarities of RDF but, foremost, it enables to uniformly query both resource descriptions and schemas. We illustrate the RQL syntax, semantics and typing system by means of a set of example queries and report on the performance of our persistent RDF Store employed by the RQL interpreter.

Optimizing Taxonomic Semantic Web Queries using Labeling Schemes

Abstract This paper focuses on the optimization of the navigation through voluminous subsumption hierarchies of topics employed by portal catalogs like Netscape Open Directory (ODP). We advocate for the use of labeling schemes for modeling these hierarchies in order to efficiently answer queries such as subsumption check, descendants, ancestors or nearest common ancestor, which usually require costly transitive closure computations. We first give a qualitative comparison of three main families of schemes, namely bit vector, prefix and interval based schemes. We then show that two labeling schemes are good candidates for an efficient implementation of label querying using standard relational DBMS, namely the Dewey prefix scheme and an interval scheme by Agrawal et al. We compare their storage and query evaluation performance for the 16 ODP hierarchies using the PostgreSQL engine.

RQL: A Functional Query Language for RDF

Although real-scale Semantic Web applications, such as Knowledge Portals and E-Marketplaces, require the management of voluminous resource metadata, sufficiently expressive declarative languages for metadata created according to the W3C RDF/S standard1 are still missing. In answer to this need, we have designed a typed, functional query language, called RQL, whose novelty lies in its ability to smoothly combine schema and data querying. The purpose of this chapter is to present RQL’s formal data model and type system and illustrate its expressiveness by means of exemplary queries. RQL’s formal foundations capture the RDF/S modeling primitives and provide a well-founded semantics for a declarative query language involving recursion and functional composition over complex description graphs.

Managing RDF Metadata for Community Webs

The need for descriptive information, i.e., metadata, about Web resources has been recognized in several application contexts (e.g., digital libraries, portals). The Resource Description Framework (RDF) aims at facilitating the creation and exchange of metadata, as directed labeled graphs serialized in XML. In particular, the definition of schema vocabularies enables the interpretation of semistructured RDF descriptions using taxonomies of node and edge labels. In this paper, we propose (i) a formal model capturing RDF schema constructs; (ii) a declarative query language featuring generalized path expressions for taxonomies of labels (iii) a metadata management architecture for efficient storage and querying of RDF descriptions and schemas.

Submission and repository management of digital libraries, using WWW

Abstract Digital library objects can be quite complex, with a lot of metadata and data in different digital forms. Currently, most installations of digital library systems need personnel experienced with computers to administer and maintain them. We designed and implemented tools to simplify addition, modification and numerous other maintenance actions in a digital library, for use by content contributors (where appropriate) and librarians without special computer training. Our tools provide an easy way to input or modify the metadata describing a submission and also to upload the digital documents, as well as inspect, commit or modify new submissions (and old objects) to a set of collections, even with different metadata fields. The tools are easy to install, while at the same time they are powerful and configurable. The tools are modular and generic, and cover even rare requirements. The configuration choices allow the customization of the tools for use with different digital library systems using different metadata formats, fields, languages and many optional features, depending on the level of sophistication required for a particular environment.