An extension of SPARQL for expressing qualitative preferences

TL;DR

This paper introduces SPREFQL, an extension to SPARQL that allows expressing qualitative preferences separately, improving query clarity and optimization without increasing expressivity.

Contribution

The paper formally defines SPREFQL's syntax and semantics, demonstrating its advantages in clarity and efficiency over standard SPARQL.

Findings

Preferences can be expressed separately from query patterns

SPREFQL queries are equivalent to standard SPARQL queries

Optimizations specific to SPREFQL improve run-time efficiency

Abstract

In this paper we present SPREFQL, an extension of the SPARQL language that allows appending a PREFER clause that expresses "soft" preferences over the query results obtained by the main body of the query. The extension does not add expressivity and any SPREFQL query can be transformed to an equivalent standard SPARQL query. However, clearly separating preferences from the "hard" patterns and filters in the WHERE clause gives queries where the intention of the client is more cleanly expressed, an advantage for both human readability and machine optimization. In the paper we formally define the syntax and the semantics of the extension and we also provide empirical evidence that optimizations specific to SPREFQL improve run-time efficiency by comparison to the usually applied optimizations on the equivalent standard SPARQL query.