From-Enhancer

The From-Enhancer (FE) plugin features:

• Rewriting of FROM clauses as FILTER statements.
• Remapping graph names to custom filter expressions.

This plugin is designed for use with Jena assemblers.

Maven

<dependency>
    <groupId>org.aksw.jenax</groupId>
    <artifactId>jenax-arq-plugins-bundle</artifactId>
</dependency>>

Relevant Namespaces

PREFIX fe:        <http://jena.apache.org/from-enhancer#>
PREFIX xdt:       <http://jsa.aksw.org/dt/sparql/>

Note that the FE plugin loads jenax’s datatype extensions for jena. Most importantly this includes the xdt:expr datatype which is used to capture SPARQL expressions and get warnings about syntax errors (if any) during RDF parsing. The datatype is implemented in the class RDFDatatypeExpr.

Assembler Configuration

The type fe:DatasetFromAsFilter refers to the main dataset wrapper to rewrite FROM clauses as filter statements in. For example, given the initial query

SELECT * FROM <x> { ?s ?p ?o }

it will be rewritten to

SELECT * { GRAPH ?g { ?s ?p ?o } FILTER (?g = <x>) }

before being sent to the actual query evaluation engine.

It is possible to map specific graph IRIs to custom SPARQL expressions using fe:alias as demonstrated below.

PREFIX ja:        <http://jena.hpl.hp.com/2005/11/Assembler#>
PREFIX fe:        <http://jena.apache.org/from-enhancer#>
PREFIX xdt:       <http://jsa.aksw.org/dt/sparql/>

<urn:example:root> a fe:DatasetFromAsFilter
  ; ja:baseDataset <urn:example:base>

  # The boolean constants true / false can be used to match all / no graphs, respectively.
  ; fe:alias [ fe:graph <urn:example:all> ; fe:expr "true"^^xdt:expr ]

  # An IRI constant is used to remap a graph to the given one.
  ; fe:alias [ fe:graph <urn:example:dbpedia> ; fe:expr "<http://dbpedia.org/>"^^xdt:expr ]

  # Expressions can mention a single variable which during query rewriting will be bound to graph names
  ; fe:alias [ fe:graph <urn:exmaple:regex> ; fe:expr "regex(str(?g), '[0-9]+$')"^^xdt:expr ]
  .
  
<urn:example:base> a ja:MemoryDataset .

Implementation

• Assembler interpreter: org.aksw.jenax.arq.fromasfilter.assembler.DatasetAssemblerFromAsFilter
• DatasetWrapper: org.aksw.jenax.arq.fromasfilter.dataset.DatasetGraphFromAsFilter
• QueryEngineFactory: org.aksw.jenax.arq.fromasfilter.engine.QueryEngineFactoryFromAsFilter