Juan Sequeda

Constitutional design and redesign is constant. Over the last 200 years, countries have replaced their constitutions an average of every 19 years and some have amended them almost yearly. A basic problem in the drafting of these documents is the search and analysis of model text deployed in other jurisdictions. Traditionally, this process has been ad hoc and the results suboptimal. As a result, drafters generally lack systematic information about the institutional options and choices available… Show more

Constitutional design and redesign is constant. Over the last 200 years, countries have replaced their constitutions an average of every 19 years and some have amended them almost yearly. A basic problem in the drafting of these documents is the search and analysis of model text deployed in other jurisdictions. Traditionally, this process has been ad hoc and the results suboptimal. As a result, drafters generally lack systematic information about the institutional options and choices available to them. In order to address this informational need, the investigators developed a web application, Constitute [online at http://www.constituteproject.org], with the use of semantic technologies. Constitute provides searchable access to the world’s constitutions using the conceptualization, texts, and data developed by the Comparative Constitutions Project. An OWL ontology represents 330 “topics”–e.g. right to health–with which the investigators have tagged relevant provisions of nearly all constitutions in force as of September of 2013. The tagged texts were then converted to an RDF representation using R2RML mappings and Capsenta’s Ultrawrap. The portal implements semantic search features to allow constitutional drafters to read, search, and compare the world’s constitutions. The goal of the project is to improve the efficiency and systemization of constitutional design and, thus, to support the independence and self-reliance of constitutional drafters. Show less

Given a source relational database, a target OWL ontology and a mapping from the source database to the target ontology, Ontology-Based Data Access (OBDA) concerns answering queries over the target ontology using these three components. This paper presents the development of UltrawrapOBDA, an OBDA system comprising bidirectional evaluation; that is, a hybridization of query rewriting and materialization. We observe that by compiling the ontological entailments as mappings, implementing the… Show more

Given a source relational database, a target OWL ontology and a mapping from the source database to the target ontology, Ontology-Based Data Access (OBDA) concerns answering queries over the target ontology using these three components. This paper presents the development of UltrawrapOBDA, an OBDA system comprising bidirectional evaluation; that is, a hybridization of query rewriting and materialization. We observe that by compiling the ontological entailments as mappings, implementing the mappings as SQL views and materializing a subset of the views, the underlying SQL optimizer is able to reduce the execution time of a SPARQL query by rewriting the query in terms of the views specified by the mappings. To the best of our knowledge, this is the first OBDA system supporting ontologies with transitivity by using SQL recursion. Our contributions include: (1) an efficient algorithm to compile ontological entailments as mappings; (2) a proof that every SPARQL query can be rewritten into a SQL query in the context of mappings; (3) a cost model to determine which views to materialize to attain the fastest execution time; and (4) an empirical evaluation comparing with a state-of-the-art OBDA system, which validates the cost model and demonstrates favorable execution times. Show less

The Semantic Web’s promise of web-wide data integration requires the inclusion of legacy relational databases, i.e. the execution of SPARQL queries on RDF representation of the legacy relational data. We explore a hypothesis: existing commercial relational databases already subsume the algorithms and optimizations needed to support effective SPARQL execution on existing relationally stored data. The experiment is embodied in a system, Ultrawrap, that encodes a logical representation of the… Show more

The Semantic Web’s promise of web-wide data integration requires the inclusion of legacy relational databases, i.e. the execution of SPARQL queries on RDF representation of the legacy relational data. We explore a hypothesis: existing commercial relational databases already subsume the algorithms and optimizations needed to support effective SPARQL execution on existing relationally stored data. The experiment is embodied in a system, Ultrawrap, that encodes a logical representation of the database as an RDF graph using SQL views and a simple syntactic translation of SPARQL queries to SQL queries on those views. Thus, in the course of executing a SPARQL query, the SQL optimizer uses the SQL views that represent a mapping of relational data to RDF, and optimizes its execution. In contrast, related research is predicated on incorporating optimizing transforms as part of the SPARQL to SQL translation, and/or executing some of the queries outside the underlying SQL environment.

Ultrawrap is evaluated using two existing benchmark suites that derive their RDF data from relational data through a Relational Database to RDF (RDB2RDF) Direct Mapping and repeated for each of the three major relational database management systems. Empirical analysis reveals two existing relational query optimizations that, if applied to the SQL produced from a simple syntactic translations of SPARQL queries (with bound predicate arguments) to SQL, consistently yield query execution time that is comparable to that of SQL queries written directly for the relational representation of the data. The analysis further reveals the two optimizations are not uniquely required to achieve a successful wrapper system. The evidence suggests effective wrappers will be those that are designed to complement the optimizer of the target database. Show less

Processing large volumes of RDF data requires sophisticated tools. In the recent years, much effort was spent on optimizing native RDF stores and on repurposing relational query engines for large-scale RDF processing. Concurrently, a number of new data management systems - regrouped under the NoSQL (for "not only SQL") umbrella - rapidly rose to prominence and represent today a popular alternative to classical databases. Though NoSQL systems are increasingly used to manage RDF data, it is still… Show more

Processing large volumes of RDF data requires sophisticated tools. In the recent years, much effort was spent on optimizing native RDF stores and on repurposing relational query engines for large-scale RDF processing. Concurrently, a number of new data management systems - regrouped under the NoSQL (for "not only SQL") umbrella - rapidly rose to prominence and represent today a popular alternative to classical databases. Though NoSQL systems are increasingly used to manage RDF data, it is still very difficult to grasp their key advantages and drawbacks in this context. The present works is, to the best of our knowledge, the first methodical attempt at characterizing and comparing NoSQL stores for RDF processing. In the following, we describe five different NoSQL stores and compare their key characteristics when running standard RDF benchmarks on a popular cloud infrastructure using both single-machine and distributed deployments. Show less

The Semantic Web is the initiative of the W3C to make information on the Web readable not only by humans but also by machines. RDF is the data model for Semantic Web data, and SPARQL is the standard query language for this data model. In recent years, we have witnessed a constant growth in the amount of RDF data available on the Web, which has motivated the theoretical study of fundamental aspects of RDF and SPARQL.

The goal of this paper is two-fold: to introduce SPARQL, which is a… Show more

The Semantic Web is the initiative of the W3C to make information on the Web readable not only by humans but also by machines. RDF is the data model for Semantic Web data, and SPARQL is the standard query language for this data model. In recent years, we have witnessed a constant growth in the amount of RDF data available on the Web, which has motivated the theoretical study of fundamental aspects of RDF and SPARQL.

The goal of this paper is two-fold: to introduce SPARQL, which is a fundamental technology for the development of the Semantic Web, and to present some interesting and non-trivial problems on RDF data management at a Web scale, that we think the database community should address. Show less

Mapping relational databases to RDF is a fundamental problem
for the development of the Semantic Web. We present a solution,
inspired by draft methods defined by the W3C where relational
databases are directly mapped to RDF and OWL. Given a relational
database schema and its integrity constraints, this direct mapping
produces an OWL ontology, which, provides the basis for generating RDF instances. The semantics of this mapping is defined using
Datalog. Two fundamental properties… Show more

Mapping relational databases to RDF is a fundamental problem
for the development of the Semantic Web. We present a solution,
inspired by draft methods defined by the W3C where relational
databases are directly mapped to RDF and OWL. Given a relational
database schema and its integrity constraints, this direct mapping
produces an OWL ontology, which, provides the basis for generating RDF instances. The semantics of this mapping is defined using
Datalog. Two fundamental properties are information preservation
and query preservation. We prove that our mapping satisfies both
conditions, even for relational databases that contain null values.
We also consider two desirable properties: monotonicity and semantics preservation. We prove that our mapping is monotone and
also prove that no monotone mapping, including ours, is semantic
preserving. We realize that monotonicity is an obstacle for semantic preservation and thus present a non-monotone direct mapping
that is semantics preserving. Show less