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<section id="quickstart">
<h1>Quickstart<a class="headerlink" href="#quickstart" title="Link to this heading">#</a></h1>
<p>This page walks through a complete query end-to-end.</p>
<section id="the-full-example">
<h2>The full example<a class="headerlink" href="#the-full-example" title="Link to this heading">#</a></h2>
<div class="highlight-java notranslate"><div class="highlight"><pre><span></span><span class="kn">import</span><span class="w"> </span><span class="nn">org.apache.arrow.memory.RootAllocator</span><span class="p">;</span>
<span class="kn">import</span><span class="w"> </span><span class="nn">org.apache.arrow.vector.ipc.ArrowReader</span><span class="p">;</span>
<span class="kn">import</span><span class="w"> </span><span class="nn">org.apache.datafusion.DataFrame</span><span class="p">;</span>
<span class="kn">import</span><span class="w"> </span><span class="nn">org.apache.datafusion.SessionContext</span><span class="p">;</span>
<span class="k">try</span><span class="w"> </span><span class="p">(</span><span class="kd">var</span><span class="w"> </span><span class="n">allocator</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="k">new</span><span class="w"> </span><span class="n">RootAllocator</span><span class="p">();</span>
<span class="w"> </span><span class="kd">var</span><span class="w"> </span><span class="n">ctx</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="k">new</span><span class="w"> </span><span class="n">SessionContext</span><span class="p">())</span><span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="n">ctx</span><span class="p">.</span><span class="na">registerParquet</span><span class="p">(</span><span class="s">&quot;orders&quot;</span><span class="p">,</span><span class="w"> </span><span class="s">&quot;/path/to/orders.parquet&quot;</span><span class="p">);</span>
<span class="w"> </span><span class="k">try</span><span class="w"> </span><span class="p">(</span><span class="n">DataFrame</span><span class="w"> </span><span class="n">df</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">ctx</span><span class="p">.</span><span class="na">sql</span><span class="p">(</span>
<span class="w"> </span><span class="s">&quot;SELECT o_orderpriority, COUNT(*) AS n &quot;</span><span class="w"> </span><span class="o">+</span>
<span class="w"> </span><span class="s">&quot;FROM orders GROUP BY o_orderpriority&quot;</span><span class="p">);</span>
<span class="w"> </span><span class="n">ArrowReader</span><span class="w"> </span><span class="n">reader</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">df</span><span class="p">.</span><span class="na">collect</span><span class="p">(</span><span class="n">allocator</span><span class="p">))</span><span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="k">while</span><span class="w"> </span><span class="p">(</span><span class="n">reader</span><span class="p">.</span><span class="na">loadNextBatch</span><span class="p">())</span><span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="kd">var</span><span class="w"> </span><span class="n">batch</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">reader</span><span class="p">.</span><span class="na">getVectorSchemaRoot</span><span class="p">();</span>
<span class="w"> </span><span class="c1">// ...</span>
<span class="w"> </span><span class="p">}</span>
<span class="w"> </span><span class="p">}</span>
<span class="p">}</span>
</pre></div>
</div>
</section>
<section id="walkthrough">
<h2>Walkthrough<a class="headerlink" href="#walkthrough" title="Link to this heading">#</a></h2>
<p><strong>Allocator.</strong> <code class="docutils literal notranslate"><span class="pre">RootAllocator</span></code> is the Arrow off-heap memory allocator. Every
JVM-side Arrow buffer is tracked under an allocator; when the allocator is
closed, leaked buffers are reported. Use one allocator per query (or one
per application) and close it in a <code class="docutils literal notranslate"><span class="pre">try</span></code>-with-resources.</p>
<p><strong>Session context.</strong> <code class="docutils literal notranslate"><span class="pre">SessionContext</span></code> is the entry point into DataFusion. It
holds the catalog of registered tables and the query planner. It is
<code class="docutils literal notranslate"><span class="pre">AutoCloseable</span></code> and <strong>not thread-safe</strong> — use one per thread, or guard
access externally.</p>
<p><strong>Registering data.</strong> <code class="docutils literal notranslate"><span class="pre">registerParquet(name,</span> <span class="pre">path)</span></code> reads the file’s footer
on call and exposes it under the given table name. See
<a class="reference internal" href="parquet.html"><span class="std std-doc">Parquet</span></a> for the options form.</p>
<p><strong>SQL.</strong> <code class="docutils literal notranslate"><span class="pre">ctx.sql(&quot;...&quot;)</span></code> plans the query and returns a <code class="docutils literal notranslate"><span class="pre">DataFrame</span></code>. The
query is not executed until results are pulled.</p>
<p><strong>Collecting results.</strong> <code class="docutils literal notranslate"><span class="pre">df.collect(allocator)</span></code> starts native execution and
returns an <code class="docutils literal notranslate"><span class="pre">ArrowReader</span></code>. Each <code class="docutils literal notranslate"><span class="pre">loadNextBatch()</span></code> call pulls the next
<code class="docutils literal notranslate"><span class="pre">VectorSchemaRoot</span></code>; iterate until it returns <code class="docutils literal notranslate"><span class="pre">false</span></code>.</p>
<p><strong>Cleanup.</strong> Both <code class="docutils literal notranslate"><span class="pre">SessionContext</span></code> and <code class="docutils literal notranslate"><span class="pre">DataFrame</span></code> are <code class="docutils literal notranslate"><span class="pre">AutoCloseable</span></code>. Use
<code class="docutils literal notranslate"><span class="pre">try</span></code>-with-resources so native resources and Arrow buffers are released
even on exception.</p>
</section>
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