ClioPatria supports two query languages: SPARQL and [[SeRQL][http://www.openrdf.org]. For both languages we provide an interactive service that presents the results as a human-readable HTML table and an API that presents its result as RDF/XML or XML according to the HTTP protocol definition for the query language.

ClioPatria's SPARQL endpoint is intended to support clients that are designed for to work with a SPARQL endpoint. E.g., if an application requires a vocabulary browser and such a browser is available as a JavaScript library that formulates SPARQL queries, ClioPatria's SPARQL endpoints allow reuse of this component. In our experience, SPARQL often needs additional application logic that is located near the data to provide a task-specific API that drives the user interface. Locating this logic near the data is required to avoid protocol and latency overhead. RDF-based application logic is a perfect match for Prolog and the RDF data is much easier queried through the Prolog RDF libraries than through SPARQL.

For both SPARQL and SeRQL, queries are translated to a complex Prolog goal calling rdf/3 to resolve edges in the graph and calls to predicates from rdfql/rdfql_runtime.pl that realise constraints imposed by the SeRQL WHERE clause and SPARQL FILTER clauses.

SPARQL Support

SPARQL support is based on the SPARQL specification, versioned April 6, 2006. Status:

• No optimization of literal operations. Because many SPARQL operations require type-translations that depend on the literal found, many of these operations are slow. The engine does optimize graph expressions.
• Incomplete value-testing, notably on xsd:dateTime
• Incomplete ORDER BY support. Only ascending and all values are compared lexically.
• Passes current test-suite, except tests affected by the above or acknowledged as errornous.

SeRQL Support

SeRQL support and compatibility is based on development version 20040820, with additional support for the new 1.2 syntax and some of the built-in functions. Both SeRQL and the HTTP API are fully defined in the Sesame documentation.