George H. L. Fletcher

Learn More
Current approaches to RDF graph indexing suffer from weak data locality, i.e., information regarding a piece of data appears in multiple locations, spanning multiple data structures. Weak data locality negatively impacts storage and query processing costs. Towards stronger data locality, we propose a Three-way Triple Tree (TripleT) secondary memory indexing(More)
—We give a language-independent characterization of the expressive power of the relational algebra on finite sets of source-target relation instance pairs. The associated decision problem is shown to be co-graph-isomorphism hard and in coNP. The main result is also applied in providing a new characterization of the generic relational queries.
In this paper we introduce the first efficient external-memory algorithm to compute the bisimilarity equivalence classes of a directed acyclic graph (DAG). DAGs are commonly used to model data in a wide variety of practical applications, ranging from XML documents and data provenance models, to web taxonomies and scientific workflows. In the study of(More)
Motivated by both established and new applications, we study navigational query languages for graphs (binary relations). The simplest language has only the two operators union and composition, together with the identity relation. We make more powerful languages by adding any of the following operators: intersection; set difference; projection; coprojection;(More)
Performing efficient decentralized search is a fundamental problem in Peer-to-Peer (P2P) systems. There has been a significant amount of research recently on developing robust self-organizing P2P topologies that support efficient search. In this paper we discuss four structured and unstructured P2P models (CAN, Chord, PRU, and Hy-pergrid) and three(More)
Given a document <i>D</i> in the form of an unordered labeled tree, we study the expressibility on <i>D</i> of various fragments of XPath, the core navigational language on XML documents. We give characterizations, in terms of the structure of <i>D</i>, for when a binary relation on its nodes is definable by an XPath expression in these fragments. Since(More)
As an essential part of the W3C's semantic web stack and linked data initiative, RDF data management systems (also known as triplestores) have drawn a lot of research attention. The majority of these systems use value-based indexes (e.g., B +-trees) for physical storage , and ignore many of the structural aspects present in RDF graphs. Structural indexes,(More)
In this paper, we describe and situate the TUPELO system for data mapping in relational databases. Automating the discovery of mappings between structured data sources is a long standing and important problem in data management. Starting from user provided example instances of the source and target schemas, TUPELO approaches mapping discovery as search(More)