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| <h1 class="title">ARQ - SPARQL Algebra</h1> |
| |
| <p>A SPARQL query in ARQ goes through several stages of processing:</p> |
| <ul> |
| <li>String to Query (parsing)</li> |
| <li>Translation from Query to a SPARQL algebra expression</li> |
| <li>Optimization of the algebra expression</li> |
| <li>Query plan determination and low-level optimization</li> |
| <li>Evaluation of the query plan</li> |
| </ul> |
| <p>This page describes how to access and use expressions in the SPARQL |
| algebra within ARQ. The definition of the SPARQL algebra is to be |
| found in the SPARQL specification in |
| <a href="http://www.w3.org/TR/sparql11-query/#sparqlDefinition">section 12</a>. |
| <a href="arq-query-eval.html">ARQ can be extended</a> to modify the evaluation |
| of the algebra form to access different graph storage |
| implementations.</p> |
| <p>The classes for the datastructures for the algebra resize in the |
| package <code>org.apache.jena.sparql.algebra</code> in the <code>op</code> subpackage. |
| All the classes are names “<code>Op...</code>”; the interface that they all |
| offer is “<code>Op</code>”.</p> |
| <h2 id="viewing-the-algebra-expression-for-a-query">Viewing the algebra expression for a Query</h2> |
| <p>The command line tool <a href="cmds.html#arq.qparse">arq.qparse</a> will print |
| the algebra form of a query:</p> |
| <pre><code>arq.qparse --print=op --query=Q.rq |
| arq.qparse --print=op 'SELECT * { ?s ?p ?o}' |
| </code></pre> |
| <p>The syntax of the output is <a href="../notes/sse.html">SSE</a>, a |
| simple format for writing data structures involving RDF terms. It |
| can be read back in again to produce the Java form of the algebra |
| expression.</p> |
| <h2 id="turning-a-query-into-an-algebra-expression">Turning a query into an algebra expression</h2> |
| <p>Getting the algebra expression for a Query is simply a matter of |
| passing the parsed Query object to the transaction function in the |
| <code>Algebra</code> class:</p> |
| <pre><code>Query query = QueryFactory.create(.....) ; |
| Op op = Algebra.compile(query) ; |
| </code></pre> |
| <p>And back again.</p> |
| <pre><code>Query query = OpAsQuery.asQuery(op) ; |
| System.out.println(query.serialize()) ; |
| </code></pre> |
| <p>This reverse translation can handle any algebra expression |
| originally from a SPARQL Query, but not any algebra expression. It |
| is possible to create programmatically useful algebra expressions |
| that can not be turned into a query, especially if they involve |
| algebra. Also, the query produced may not be exactly the same but |
| will yield the same results (for example, filters may be moved |
| because the SPARQL query algebra translation in the SPARQL |
| specification moves filter expressions around).</p> |
| <h2 id="directly-reading-and-writing-algebra-expression">Directly reading and writing algebra expression</h2> |
| <p>The SSE class is a collection of functions to parse SSE expressions |
| for the SPARQL algebra but also RDF terms, filter expressions and |
| even dataset and graphs.</p> |
| <pre><code>Op op = SSE.parseOp("(bgp (?s ?p ?o))") ; // Read a string |
| |
| Op op = SSE.readOp("filename.sse") ; // Read a file |
| </code></pre> |
| <p>The SSE class simply calls the appropriate builder operation from |
| the <code>org.apache.jena.sparql.sse.builder</code> package.</p> |
| <p>To go with this, there is a collection of writers for many of the |
| Java structures in ARQ. </p> |
| <pre><code>Op op = ... ; |
| SSE.write(op) ; // Write to stdout |
| </code></pre> |
| <p>Writers default to writing to <code>System.out</code> but support calls to any |
| output stream (it manages the conversion to UTF-8) and ARQ own |
| <code>IndentedWriter</code>s form for embedding in structured output. Again, |
| SSE is simply passing the calls to the writer operation from the |
| <code>org.apache.jena.sparql.sse.writer</code> package.</p> |
| <h2 id="creating-an-algebra-expression-programmatically">Creating an algebra expression programmatically</h2> |
| <p>See the example in |
| <a href="https://github.com/apache/jena/tree/main/jena-examples/src/main/java/arq/examples/algebra/AlgebraExec.java">AlgebraExec</a>.</p> |
| <p>To produce the complete javadoc for ARQ, download an ARQ |
| distribution and run the ant task ‘javadoc-all’.</p> |
| <h2 id="evaluating-a-algebra-expression">Evaluating a algebra expression</h2> |
| <pre><code>QueryIterator qIter = Algebra.exec(op,graph) ; |
| |
| QueryIterator qIter = Algebra.exec(op,datasetGraph) ; |
| </code></pre> |
| <p>Evaluating an algebra expression produces a iterator of query |
| solutions (called Bindings).</p> |
| <pre><code>for ( ; qIter.hasNext() ; ) |
| { |
| Binding b = qIter.nextBinding() ; |
| Node n = b.get(var_x) ; |
| System.out.println(var_x+" = "+FmtUtils.stringForNode(n)) ; |
| } |
| qIter.close() ; |
| </code></pre> |
| <p>Operations of <code>CONSTRUCT</code>, <code>DESCRIBE</code> and <code>ASK</code> are done on top |
| of algebra evaluation. Applications can access this functionality |
| by creating their own <code>QueryEngine</code> (see |
| <code>arq.examples.engine.MyQueryEngine</code>) and it’s factory. A query |
| engine is a one-time use object for each query execution.</p> |
| <p><a href="index.html">ARQ documentation index</a></p> |
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