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| <section id="java-table-providers"> |
| <h1>Java table providers<a class="headerlink" href="#java-table-providers" title="Link to this heading">#</a></h1> |
| <p><code class="docutils literal notranslate"><span class="pre">SessionContext.registerTable(name,</span> <span class="pre">provider)</span></code> registers a Java-implemented |
| table. SQL queries that reference <code class="docutils literal notranslate"><span class="pre">name</span></code> call back into your <code class="docutils literal notranslate"><span class="pre">TableProvider</span></code> |
| to fetch batches. Data flows from Java to native code via the Arrow C Data |
| Interface, so there are no extra copies in the hot path. This is the Java |
| counterpart to DataFusion’s Rust <code class="docutils literal notranslate"><span class="pre">SessionContext::register_table</span></code>.</p> |
| <section id="implement"> |
| <h2>Implement<a class="headerlink" href="#implement" 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.BufferAllocator</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.arrow.vector.types.pojo.Schema</span><span class="p">;</span> |
| <span class="kn">import</span><span class="w"> </span><span class="nn">org.apache.datafusion.TableProvider</span><span class="p">;</span> |
| |
| <span class="kd">public</span><span class="w"> </span><span class="kd">final</span><span class="w"> </span><span class="kd">class</span> <span class="nc">MyTable</span><span class="w"> </span><span class="kd">implements</span><span class="w"> </span><span class="n">TableProvider</span><span class="w"> </span><span class="p">{</span> |
| <span class="w"> </span><span class="kd">private</span><span class="w"> </span><span class="kd">final</span><span class="w"> </span><span class="n">Schema</span><span class="w"> </span><span class="n">schema</span><span class="p">;</span> |
| |
| <span class="w"> </span><span class="kd">public</span><span class="w"> </span><span class="nf">MyTable</span><span class="p">(</span><span class="n">Schema</span><span class="w"> </span><span class="n">schema</span><span class="p">)</span><span class="w"> </span><span class="p">{</span> |
| <span class="w"> </span><span class="k">this</span><span class="p">.</span><span class="na">schema</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">schema</span><span class="p">;</span> |
| <span class="w"> </span><span class="p">}</span> |
| |
| <span class="w"> </span><span class="nd">@Override</span><span class="w"> </span><span class="kd">public</span><span class="w"> </span><span class="n">Schema</span><span class="w"> </span><span class="nf">schema</span><span class="p">()</span><span class="w"> </span><span class="p">{</span><span class="w"> </span><span class="k">return</span><span class="w"> </span><span class="n">schema</span><span class="p">;</span><span class="w"> </span><span class="p">}</span> |
| |
| <span class="w"> </span><span class="nd">@Override</span> |
| <span class="w"> </span><span class="kd">public</span><span class="w"> </span><span class="n">ArrowReader</span><span class="w"> </span><span class="nf">scan</span><span class="p">(</span><span class="n">BufferAllocator</span><span class="w"> </span><span class="n">allocator</span><span class="p">)</span><span class="w"> </span><span class="p">{</span> |
| <span class="w"> </span><span class="c1">// Return a fresh ArrowReader. The reader must allocate its buffers</span> |
| <span class="w"> </span><span class="c1">// from `allocator` (or a child of it) — the framework needs the</span> |
| <span class="w"> </span><span class="c1">// allocator hierarchy to share a root.</span> |
| <span class="w"> </span><span class="k">return</span><span class="w"> </span><span class="n">openMyReader</span><span class="p">(</span><span class="n">allocator</span><span class="p">);</span> |
| <span class="w"> </span><span class="p">}</span> |
| <span class="p">}</span> |
| </pre></div> |
| </div> |
| <p>For the common case of “I have a schema and a function that returns an |
| <code class="docutils literal notranslate"><span class="pre">ArrowReader</span></code>,” <code class="docutils literal notranslate"><span class="pre">SimpleTableProvider</span></code> packages those two into a ready-made |
| <code class="docutils literal notranslate"><span class="pre">TableProvider</span></code> without having to subclass:</p> |
| <div class="highlight-java notranslate"><div class="highlight"><pre><span></span><span class="n">TableProvider</span><span class="w"> </span><span class="n">t</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">SimpleTableProvider</span><span class="p">(</span><span class="n">mySchema</span><span class="p">(),</span><span class="w"> </span><span class="n">allocator</span><span class="w"> </span><span class="o">-></span><span class="w"> </span><span class="n">openMyReader</span><span class="p">(</span><span class="n">allocator</span><span class="p">));</span> |
| <span class="n">ctx</span><span class="p">.</span><span class="na">registerTable</span><span class="p">(</span><span class="s">"t"</span><span class="p">,</span><span class="w"> </span><span class="n">t</span><span class="p">);</span> |
| </pre></div> |
| </div> |
| </section> |
| <section id="register-and-query"> |
| <h2>Register and query<a class="headerlink" href="#register-and-query" title="Link to this heading">#</a></h2> |
| <div class="highlight-java notranslate"><div class="highlight"><pre><span></span><span class="k">try</span><span class="w"> </span><span class="p">(</span><span class="n">SessionContext</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="n">BufferAllocator</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="p">{</span> |
| <span class="w"> </span><span class="n">ctx</span><span class="p">.</span><span class="na">registerTable</span><span class="p">(</span><span class="s">"t"</span><span class="p">,</span><span class="w"> </span><span class="k">new</span><span class="w"> </span><span class="n">MyTable</span><span class="p">(</span><span class="n">mySchema</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="s">"SELECT * FROM t WHERE x > 10"</span><span class="p">);</span> |
| <span class="w"> </span><span class="n">ArrowReader</span><span class="w"> </span><span class="n">r</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">r</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="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="contract"> |
| <h2>Contract<a class="headerlink" href="#contract" title="Link to this heading">#</a></h2> |
| <ul class="simple"> |
| <li><p><code class="docutils literal notranslate"><span class="pre">schema()</span></code> is called exactly once, on the caller’s thread, at registration |
| time. Throwing from it aborts registration with the original exception.</p></li> |
| <li><p><code class="docutils literal notranslate"><span class="pre">scan(allocator)</span></code> is called once per SQL query that touches the table, on a |
| worker thread. It must return a fresh, independent <code class="docutils literal notranslate"><span class="pre">ArrowReader</span></code> on every |
| call — this is what makes self-joins and <code class="docutils literal notranslate"><span class="pre">UNION</span> <span class="pre">ALL</span></code> over the same table |
| work.</p></li> |
| <li><p>The reader returned by <code class="docutils literal notranslate"><span class="pre">scan</span></code> must allocate its buffers from the supplied |
| <code class="docutils literal notranslate"><span class="pre">allocator</span></code> (or a child of it). Arrow Java’s <code class="docutils literal notranslate"><span class="pre">Data.exportArrayStream</span></code> |
| requires the reader’s allocator and the export allocator to share a root.</p></li> |
| <li><p>The returned reader’s schema must equal the schema returned by <code class="docutils literal notranslate"><span class="pre">schema()</span></code>. |
| A mismatch fails the query.</p></li> |
| <li><p>You do not need to close the returned reader yourself. The framework |
| installs a release callback that closes it when the underlying FFI stream |
| is dropped.</p></li> |
| </ul> |
| </section> |
| <section id="errors"> |
| <h2>Errors<a class="headerlink" href="#errors" title="Link to this heading">#</a></h2> |
| <p>Exceptions thrown from <code class="docutils literal notranslate"><span class="pre">scan()</span></code> or from the returned reader surface in the |
| <code class="docutils literal notranslate"><span class="pre">RuntimeException</span></code> raised by <code class="docutils literal notranslate"><span class="pre">collect()</span></code>. The error message includes the Java |
| exception class and <code class="docutils literal notranslate"><span class="pre">getMessage()</span></code>, in the same format used for scalar UDF |
| errors.</p> |
| </section> |
| <section id="threading"> |
| <h2>Threading<a class="headerlink" href="#threading" title="Link to this heading">#</a></h2> |
| <p><code class="docutils literal notranslate"><span class="pre">SessionContext</span></code> is single-threaded, but <code class="docutils literal notranslate"><span class="pre">scan(allocator)</span></code> may be invoked from |
| any DataFusion worker thread. If your implementation maintains mutable state |
| across scans, synchronise it.</p> |
| </section> |
| <section id="limitations-v1"> |
| <h2>Limitations (v1)<a class="headerlink" href="#limitations-v1" title="Link to this heading">#</a></h2> |
| <ul class="simple"> |
| <li><p>Single-partition scans only. DataFusion sees the table as one partition; |
| multi-partition parallelism is a follow-up.</p></li> |
| <li><p>No projection or filter pushdown. DataFusion applies projection and |
| filters on top of the batches you return; the Java side always sees the |
| full schema. The interface is intentionally minimal so it can grow these |
| capabilities (as default methods) without breaking existing implementations.</p></li> |
| <li><p>No <code class="docutils literal notranslate"><span class="pre">deregisterTable</span></code>. Tables live until the <code class="docutils literal notranslate"><span class="pre">SessionContext</span></code> is closed.</p></li> |
| </ul> |
| </section> |
| </section> |
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