CODEWITHSUNDEEP
relational-databasesparqlrdfetlsemantic-web
rawan az
rawan az

Reputation: 101

Inference in RDB2RDF wrapper systems

In RDB2RDF wrapper systems where they build sql views based on relational data. I am wondering how inference is performed in these systems?

Upvotes: 2

Views: 97

Answers (1)

Juan Sequeda
Juan Sequeda

Reputation: 151

Inference in wrapper systems are usually done in a query rewriting (backward chaining) approach. This paradigm is also known as Ontology Based Data Access (OBDA).

Given a source relational database, a target OWL ontology and a mapping from the relational database to the ontology (for example in the form of an R2RML mapping), the goal is to answer queries (usually in SPARQL) in terms of the target ontology using the ontology, mapping and database. In the rewriting approach of OBDA, three steps are executed.

First, given a SPARQL query Q in terms of the target ontology O, a new query Qo is generated from the query Q and the ontology O. This is the rewriting of Q with respect to the target ontology O.

Second, the mapping M is used to compile Qo to a SQL query Qsql.

Finally, Qsql is evaluated on the source database, which gives us the answer to the initial query Q.

The expressivity of the ontology language considered in these approaches is the OWL-QL profile.

For more details take a look at "Ontology-Based Data Access with Databases: A Short Course" http://www.dcs.bbk.ac.uk/~roman/papers/RW-Chapter.pdf

Another approach is what the Ultrawrap system implements. In the Ultrawrap approach, instead of rewriting a Query Q in terms of the target ontology O, the mapping M is saturated with the entailments of the target ontology O, which generated a new mapping Ms. Subsequently, these mappings are implemented as SQL views. In order to optimize query performance, a subset of these mappings can be materialized. Furthermore, this approach takes advantage of the full potential of the SQL infrastructure, including recursion. Therefore this approach supports ontologies with transitivity (which is not included in the OWL-QL profile). For more information, take a look at "OBDA: Query Rewriting or Materialization? In Practice, Both!" https://www.cs.utexas.edu/~jsequeda/papers/ISWC2014_SequedaArenasMiranker_OBDA.pdf

(Disclaimer, I am the author of the Ultrawrap system)

Upvotes: 1

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