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| <h1 class="title">Explaining ARQ queries</h1> |
| |
| |
| <main class="d-flex flex-xl-row flex-column"> |
| |
| <aside class="text-muted align-self-start mb-3 p-0 d-xl-none d-block"> |
| <h2 class="h6 sticky-top m-0 p-2 bg-body-tertiary">On this page</h2> |
| <nav id="TableOfContents"> |
| <ul> |
| <li><a href="#algebra-transformations">Algebra Transformations</a></li> |
| <li><a href="#execution-logging">Execution Logging</a></li> |
| </ul> |
| </nav> |
| </aside> |
| <article class="flex-column me-lg-4"> |
| <p>Optimization in ARQ proceeds on two levels. After the query is parsed, |
| the SPARQL algebra for the query is generated as described in the SPARQL |
| specification. High-level optimization occurs by rewriting the algebra |
| into new, equivalent algebra forms and introducing specialized algebra |
| operators. During query execution, the low-level, storage-specific |
| optimization occurs such as choosing the order of triple patterns within |
| basic graph patterns.</p> |
| <p>The effect of <a href="#algebra-transformations">high-level optimizations</a> can |
| be seen using <code>arq.qparse</code> and the low-level runtime optimizations can |
| be seen by <a href="#execution-logging">execution logging</a>.</p> |
| <h2 id="algebra-transformations">Algebra Transformations</h2> |
| <p>The preparation for a query for execution can be investigated with the |
| command <code>arq.qparse --explain --query QueryFile.rq</code>. Different storage |
| systems may perform different optimizations, usually chosen from the |
| standard set. <code>qparse</code> shows the action of the memory-storage optimizer |
| which applies all optimizations.</p> |
| <p>Other useful arguments are:</p> |
| <p><strong>qparse arguments</strong></p> |
| <table> |
| <thead> |
| <tr> |
| <th>Argument</th> |
| <th>Effect</th> |
| </tr> |
| </thead> |
| <tbody> |
| <tr> |
| <td><code>--print=query</code></td> |
| <td>Print the parsed query</td> |
| </tr> |
| <tr> |
| <td><code>--print=op</code></td> |
| <td>Print the SPARQL algebra for the query. This is exactly the algebra specified by the SPARQL standard.</td> |
| </tr> |
| <tr> |
| <td><code>--print=opt</code></td> |
| <td>Print the optimized algebra for the query.</td> |
| </tr> |
| <tr> |
| <td><code>--print=quad</code></td> |
| <td>Print the quad form algebra for the query.</td> |
| </tr> |
| <tr> |
| <td><code>--print=optquad</code></td> |
| <td>Print the quad-form optimized algebra for the query.</td> |
| </tr> |
| </tbody> |
| </table> |
| <p>The argument <code>--explain</code> is equivalent to <code>--print=query --print=opt</code></p> |
| <p>Examples:</p> |
| <div class="highlight"><pre tabindex="0" style="background-color:#f8f8f8;-moz-tab-size:4;-o-tab-size:4;tab-size:4;"><code class="language-bash" data-lang="bash"><span style="display:flex;"><span>arq.qparse --explain --query Q.rq |
| </span></span><span style="display:flex;"><span> |
| </span></span><span style="display:flex;"><span>arq.qparse --explain <span style="color:#b44">'SELECT * { ?s ?p ?o }'</span> |
| </span></span></code></pre></div><h2 id="execution-logging">Execution Logging</h2> |
| <p>ARQ can log query and update execution details globally or for an |
| individual operations. This adds another level of control on top of the |
| logger level controls.</p> |
| <p>From command line:</p> |
| <div class="highlight"><pre tabindex="0" style="background-color:#f8f8f8;-moz-tab-size:4;-o-tab-size:4;tab-size:4;"><code class="language-bash" data-lang="bash"><span style="display:flex;"><span>arq.sparql --explain --data ... --query ... |
| </span></span></code></pre></div><p>Explanatory messages are controlled by the <code>Explain.InfoLevel</code> level in |
| the execution context.</p> |
| <p>Execution logging at level <code>ALL</code> can cause a significant slowdown in |
| query execution speeds but the order of operations logged will be |
| correct.</p> |
| <p>The logger used is called <code>org.apache.jena.arq.exec</code>. Message are sent |
| at level “info”. So for log4j2, the following can be set in the |
| <code>log4j2.properties</code> file:</p> |
| <div class="highlight"><pre tabindex="0" style="background-color:#f8f8f8;-moz-tab-size:4;-o-tab-size:4;tab-size:4;"><code class="language-properties" data-lang="properties"><span style="display:flex;"><span><span style="color:#b44">logger.arq-exec.name</span> <span style="color:#666">=</span> <span style="color:#b44">org.apache.jena.arq.exec</span> |
| </span></span><span style="display:flex;"><span><span style="color:#b44">logger.arq-exec.level</span> <span style="color:#666">=</span> <span style="color:#b44">INFO</span> |
| </span></span></code></pre></div><p>The context setting is for key (Java constant) <code>ARQ.symLogExec</code>. To set |
| globally:</p> |
| <div class="highlight"><pre tabindex="0" style="background-color:#f8f8f8;-moz-tab-size:4;-o-tab-size:4;tab-size:4;"><code class="language-java" data-lang="java"><span style="display:flex;"><span>ARQ<span style="color:#666">.</span><span style="color:#b44">setExecutionLogging</span><span style="color:#666">(</span>Explain<span style="color:#666">.</span><span style="color:#b44">InfoLevel</span><span style="color:#666">.</span><span style="color:#b44">ALL</span><span style="color:#666">);</span> |
| </span></span></code></pre></div><p>and it may also be set on an individual query execution using its local |
| context.</p> |
| <div class="highlight"><pre tabindex="0" style="background-color:#f8f8f8;-moz-tab-size:4;-o-tab-size:4;tab-size:4;"><code class="language-java" data-lang="java"><span style="display:flex;"><span><span style="color:#a2f;font-weight:bold">try</span><span style="color:#666">(</span>QueryExecution qExec <span style="color:#666">=</span> QueryExecution<span style="color:#666">.</span><span style="color:#b44">create</span><span style="color:#666">()</span> <span style="color:#666">...</span> <span style="color:#666">.</span><span style="color:#b44">set</span><span style="color:#666">(</span>ARQ<span style="color:#666">.</span><span style="color:#b44">symLogExec</span><span style="color:#666">,</span> Explain<span style="color:#666">.</span><span style="color:#b44">InfoLevel</span><span style="color:#666">.</span><span style="color:#b44">ALL</span><span style="color:#666">).</span><span style="color:#b44">build</span><span style="color:#666">()</span> <span style="color:#666">)</span> <span style="color:#666">{</span> |
| </span></span><span style="display:flex;"><span> ResultSet rs <span style="color:#666">=</span> qExec<span style="color:#666">.</span><span style="color:#b44">execSelect</span><span style="color:#666">()</span> <span style="color:#666">;</span> |
| </span></span><span style="display:flex;"><span> <span style="color:#666">...</span> |
| </span></span><span style="display:flex;"><span><span style="color:#666">}</span> |
| </span></span></code></pre></div><p>On the command line:</p> |
| <div class="highlight"><pre tabindex="0" style="background-color:#f8f8f8;-moz-tab-size:4;-o-tab-size:4;tab-size:4;"><code class="language-bash" data-lang="bash"><span style="display:flex;"><span> arq.query --explain --data data file --query<span style="color:#666">=</span>queryfile |
| </span></span></code></pre></div><p>The command <code>tdbquery</code> takes the same –explain argument.</p> |
| <p>Logging information levels: see the <a href="logging.html">logging page</a></p> |
| <p>To get ARQ query explanation in Fuseki logs see <a href="/documentation/fuseki2/fuseki-logging.html">Fuseki logging</a></p> |
| |
| </article> |
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