Reference conditions: relating mapping rules without joining
Keywords: R2RML, RML, relation, join, optimization
TL;DR: Short research paper proposing to add a reference condition for a referencing object map in (R2)RML, which allows expressing relations between mapping rules without imposing referential integrity.
Abstract: Existing knowledge graph construction mapping languages have a legacy of mapping over relational databases. A such, current mapping language join constructs conflate securing referential integrity with relating concepts across data sources. This leads to a significant amount of operations (resulting in performance bottlenecks), and loss of additional context of potential linking triples that are not generated due to a lack of referential integrity. We propose a reference condition next to the traditional join condition, allowing to express a relation between sources without imposing any referential integrity.
In this short research paper, we describe the concept, its applicability, how it could be integrated in existing and future mapping languages, and a proof-of-concept implementation. Our evaluation based on GTFS-Madrid-Bench confirms the assumption that removing these integrity checks leads to much faster generation times, but we also find that using reference conditions results in exactly the same graph output, i.e. these alternative semantics do not influence generation results for cases where referential integrity is assumed within the source system. Adding the reference conditions keeps the best of all worlds: generation time is shortened where possible, you have more context in the resulting RDF graph, and the mapping file still supplies relevant metadata about relations between triples maps. For future work, we will further research relations between triples maps, and expand our implementation to more complex reference conditions. This will allow us to investigate similar performance gains with other benchmarks and other mapping engines.
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