7.6 Optimized Handling of SPARQL Queries

This section describes some performance-related features of the support for Apache Jena that can enhance SPARQL query processing. These features are performed automatically by default.

It assumes that you are familiar with SPARQL, including the CONSTRUCT feature and property paths.

• Compilation of SPARQL Queries to a Single SEM_MATCH Call
  
• Optimized Handling of Property Paths
  

7.6.1 Compilation of SPARQL Queries to a Single SEM_MATCH Call

SPARQL queries involving DISTINCT, OPTIONAL, FILTER, UNION, ORDER BY, and LIMIT are converted to a single Oracle SEM_MATCH table function. If a query cannot be converted directly to SEM_MATCH because it uses SPARQL features not supported by SEM_MATCH (for example, CONSTRUCT), the support for Apache Jena employs a hybrid approach and tries to execute the largest portion of the query using a single SEM_MATCH function while executing the rest using the Jena ARQ query engine.

For example, the following SPARQL query is directly translated to a single SEM_MATCH table function:

PREFIX dc:  <http://purl.org/dc/elements/1.1/> 
PREFIX rdf:  <http://www.w3.org/1999/02/22-rdf-syntax-ns#> 
PREFIX foaf: <http://xmlns.com/foaf/0.1/> 
SELECT ?person ?name 
  WHERE {
                 {?alice foaf:knows ?person . }
               UNION { 
                 ?person ?p ?name. OPTIONAL { ?person ?x ?name1 } 
                     }
        }

However, the following example query is not directly translatable to a single SEM_MATCH table function because of the CONSTRUCT keyword:

PREFIX vcard: <http://www.w3.org/2001/vcard-rdf/3.0#> 
CONSTRUCT  { <http://example.org/person#Alice> vcard:FN ?obj } 
  WHERE  { { ?x <http://pred/a> ?obj.}
         UNION
         { ?x <http://pred/b> ?obj.}  }

In this case, the support for Apache Jena converts the inner UNION query into a single SEM_MATCH table function, and then passes on the result set to the Jena ARQ query engine for further evaluation.

7.6.2 Optimized Handling of Property Paths

As defined in Jena, a property path is a possible route through an RDF graph between two graph nodes. Property paths are an extension of SPARQL and are more expressive than basic graph pattern queries, because regular expressions can be used over properties for pattern matching RDF graphs. For more information about property paths, see the documentation for the Jena ARQ query engine.

RDF graph support for Apache Jena supports all Jena property path types through the integration with the Jena ARQ query engine, but it converts some common path types directly to native SQL hierarchical queries (not based on SEM_MATCH) to improve performance. The following types of property paths are directly converted to SQL by the support for Apache Jena when dealing with triple data:

• Predicate alternatives: (p1 | p2 | … | pn) where pi is a property URI

• Predicate sequences: (p1 / p2 / … / pn) where pi is a property URI

• Reverse paths : ( ^ p ) where p is a predicate URI

• Complex paths: p+, p*, p{0, n} where p could be an alternative, sequence, reverse path, or property URI

Path expressions that cannot be captured in this grammar are not translated directly to SQL by the support for Apache Jena, and they are answered using the Jena query engine.

The following example contains a code snippet using a property path expression with path sequences:

String m = "PROP_PATH";
 
ModelOracleSem model = ModelOracleSem.createOracleSemModel(oracle, m);
 
GraphOracleSem graph = new GraphOracleSem(oracle, m);    
    
// populate the RDF Graph
    graph.add(Triple.create(Node.createURI("http://a"),
    Node.createURI("http://p1"),
    Node.createURI("http://b")));
 
graph.add(Triple.create(Node.createURI("http://b"),
 Node.createURI("http://p2"),
 Node.createURI("http://c")));
 
graph.add(Triple.create(Node.createURI("http://c"),
 Node.createURI("http://p5"),
 Node.createURI("http://d")));
 
String query =
" SELECT ?s  " +
" WHERE {?s (<http://p1>/<http://p2>/<http://p5>)+ <http://d>.}";
   
QueryExecution qexec = 
      QueryExecutionFactory.create(QueryFactory.create(query, 
 Syntax.syntaxARQ), model);
 
try {
  ResultSet results = qexec.execSelect();
  ResultSetFormatter.out(System.out, results);
}
finally {
  if (qexec != null)
    qexec.close();
}
     
OracleUtils.dropSemanticModel(oracle, m);
model.close();