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| <section id="using-extension-libraries"> | |
| <span id="user-guide-extensions"></span><h1>Using extension libraries<a class="headerlink" href="#using-extension-libraries" title="Link to this heading">#</a></h1> | |
| <p>An extension library is a separate package that teaches a | |
| <code class="xref py py-class docutils literal notranslate"><span class="pre">SessionContext</span></code> something it does not know on its own — | |
| a new data source, extra functions, or a different way of executing your | |
| queries. You install it with <code class="docutils literal notranslate"><span class="pre">pip</span></code>, hand it to your session, and keep writing | |
| the same SQL and DataFrame code.</p> | |
| <p>Examples in the wild include <a class="reference external" href="https://delta-io.github.io/delta-rs/">delta-rs</a>, | |
| which exposes Delta Lake tables to DataFusion, and the two worked examples in | |
| this repository under | |
| <a class="reference external" href="https://github.com/apache/datafusion-python/tree/main/examples"><code class="docutils literal notranslate"><span class="pre">examples/</span></code></a>.</p> | |
| <section id="two-kinds-of-extension"> | |
| <h2>Two kinds of extension<a class="headerlink" href="#two-kinds-of-extension" title="Link to this heading">#</a></h2> | |
| <p>Which one you have determines how much setup you do.</p> | |
| <p><strong>Tables and functions register directly.</strong> If the library gives you a table | |
| or a function, register it the same way you would register a CSV file. No | |
| extra setup:</p> | |
| <div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="kn">from</span><span class="w"> </span><span class="nn">datafusion</span><span class="w"> </span><span class="kn">import</span> <span class="n">SessionContext</span> | |
| <span class="kn">import</span><span class="w"> </span><span class="nn">my_tables</span> | |
| <span class="n">ctx</span> <span class="o">=</span> <span class="n">SessionContext</span><span class="p">()</span> | |
| <span class="n">ctx</span><span class="o">.</span><span class="n">register_table</span><span class="p">(</span><span class="s2">"events"</span><span class="p">,</span> <span class="n">my_tables</span><span class="o">.</span><span class="n">TableProvider</span><span class="p">(</span><span class="s2">"s3://bucket/events"</span><span class="p">))</span> | |
| <span class="n">ctx</span><span class="o">.</span><span class="n">sql</span><span class="p">(</span><span class="s2">"SELECT count(*) FROM events"</span><span class="p">)</span><span class="o">.</span><span class="n">show</span><span class="p">()</span> | |
| </pre></div> | |
| </div> | |
| <p><strong>Libraries that change how queries run need to be installed on the session.</strong> | |
| A distributed engine, or anything that rewrites your query plan, has to be | |
| attached to the session before it can do its work. That is what | |
| <code class="xref py py-meth docutils literal notranslate"><span class="pre">with_extensions()</span></code> is for. The library | |
| documents an object — often called <code class="docutils literal notranslate"><span class="pre">Extension</span></code> — that you pass to it:</p> | |
| <div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="kn">from</span><span class="w"> </span><span class="nn">datafusion</span><span class="w"> </span><span class="kn">import</span> <span class="n">SessionContext</span> | |
| <span class="kn">import</span><span class="w"> </span><span class="nn">my_engine</span> | |
| <span class="n">ctx</span> <span class="o">=</span> <span class="n">SessionContext</span><span class="p">()</span><span class="o">.</span><span class="n">with_extensions</span><span class="p">(</span><span class="n">my_engine</span><span class="o">.</span><span class="n">Extension</span><span class="p">(</span><span class="s2">"scheduler:50050"</span><span class="p">))</span> | |
| <span class="n">ctx</span><span class="o">.</span><span class="n">register_table</span><span class="p">(</span><span class="s2">"events"</span><span class="p">,</span> <span class="n">my_engine</span><span class="o">.</span><span class="n">TableProvider</span><span class="p">(</span><span class="s2">"s3://bucket/events"</span><span class="p">))</span> | |
| <span class="n">ctx</span><span class="o">.</span><span class="n">sql</span><span class="p">(</span><span class="s2">"SELECT count(*) FROM events"</span><span class="p">)</span><span class="o">.</span><span class="n">show</span><span class="p">()</span> | |
| </pre></div> | |
| </div> | |
| <p><code class="docutils literal notranslate"><span class="pre">with_extensions</span></code> returns a context; use the returned one. It shares | |
| everything else with the context you called it on, so tables you registered | |
| before the call are still there.</p> | |
| </section> | |
| <section id="using-more-than-one-library"> | |
| <h2>Using more than one library<a class="headerlink" href="#using-more-than-one-library" title="Link to this heading">#</a></h2> | |
| <p>Pass them all to a single call:</p> | |
| <div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="n">ctx</span> <span class="o">=</span> <span class="n">SessionContext</span><span class="p">()</span><span class="o">.</span><span class="n">with_extensions</span><span class="p">(</span> | |
| <span class="n">my_tables</span><span class="o">.</span><span class="n">Extension</span><span class="p">(),</span> | |
| <span class="n">my_engine</span><span class="o">.</span><span class="n">Extension</span><span class="p">(</span><span class="s2">"scheduler:50050"</span><span class="p">),</span> | |
| <span class="p">)</span> | |
| </pre></div> | |
| </div> | |
| <p>One call rather than several is worth preferring: it lets the libraries see | |
| each other, which they cannot do if you install them one at a time. Order | |
| rarely matters. When a library needs a particular position — usually “list me | |
| last” for something that wraps the others — it says so in its own | |
| documentation.</p> | |
| </section> | |
| <section id="two-things-that-will-bite-you"> | |
| <h2>Two things that will bite you<a class="headerlink" href="#two-things-that-will-bite-you" title="Link to this heading">#</a></h2> | |
| <p><strong>Keep your context alive.</strong> A <code class="docutils literal notranslate"><span class="pre">DataFrame</span></code> or a plan does not keep its session | |
| alive on its own. If a context is garbage-collected while something built from | |
| it is still in use, the next query fails with:</p> | |
| <div class="highlight-text notranslate"><div class="highlight"><pre><span></span>TaskContextProvider went out of scope over FFI boundary | |
| </pre></div> | |
| </div> | |
| <p>Almost always this is a helper that built a context locally and returned a | |
| DataFrame:</p> | |
| <div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="c1"># Wrong — ctx is collected when the function returns.</span> | |
| <span class="k">def</span><span class="w"> </span><span class="nf">load</span><span class="p">():</span> | |
| <span class="n">ctx</span> <span class="o">=</span> <span class="n">SessionContext</span><span class="p">()</span><span class="o">.</span><span class="n">with_extensions</span><span class="p">(</span><span class="n">my_engine</span><span class="o">.</span><span class="n">Extension</span><span class="p">())</span> | |
| <span class="k">return</span> <span class="n">ctx</span><span class="o">.</span><span class="n">sql</span><span class="p">(</span><span class="s2">"SELECT * FROM events"</span><span class="p">)</span> | |
| <span class="c1"># Right — hand back the context too, or keep it on an object that lives</span> | |
| <span class="c1"># as long as the frames derived from it.</span> | |
| <span class="k">def</span><span class="w"> </span><span class="nf">load</span><span class="p">():</span> | |
| <span class="n">ctx</span> <span class="o">=</span> <span class="n">SessionContext</span><span class="p">()</span><span class="o">.</span><span class="n">with_extensions</span><span class="p">(</span><span class="n">my_engine</span><span class="o">.</span><span class="n">Extension</span><span class="p">())</span> | |
| <span class="k">return</span> <span class="n">ctx</span><span class="p">,</span> <span class="n">ctx</span><span class="o">.</span><span class="n">sql</span><span class="p">(</span><span class="s2">"SELECT * FROM events"</span><span class="p">)</span> | |
| </pre></div> | |
| </div> | |
| <p><strong>Versions have to match.</strong> An extension library is compiled against one | |
| DataFusion version. A mismatch raises an <code class="docutils literal notranslate"><span class="pre">ImportError</span></code> naming the version it | |
| found and the version expected, at the moment you register or install the | |
| library — not silently at query time. If you see one, upgrade or downgrade the | |
| extension library so its DataFusion version matches this package’s. See | |
| <a class="reference internal" href="upgrade-guides.html#extension-version-mismatch"><span class="std std-ref">Mismatched extension libraries now fail loudly</span></a>.</p> | |
| </section> | |
| <section id="checking-what-a-session-knows-about"> | |
| <h2>Checking what a session knows about<a class="headerlink" href="#checking-what-a-session-knows-about" title="Link to this heading">#</a></h2> | |
| <p><code class="xref py py-meth docutils literal notranslate"><span class="pre">logical_extension_codec_ids()</span></code> and | |
| <code class="xref py py-meth docutils literal notranslate"><span class="pre">physical_extension_codec_ids()</span></code> list which | |
| libraries a session has been taught about. Useful when a query fails and you | |
| want to confirm the library actually got installed:</p> | |
| <div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="n">ctx</span> <span class="o">=</span> <span class="n">SessionContext</span><span class="p">()</span><span class="o">.</span><span class="n">with_extensions</span><span class="p">(</span><span class="n">my_engine</span><span class="o">.</span><span class="n">Extension</span><span class="p">())</span> | |
| <span class="n">ctx</span><span class="o">.</span><span class="n">logical_extension_codec_ids</span><span class="p">()</span> | |
| <span class="c1"># ['my_engine.LogicalCodec']</span> | |
| </pre></div> | |
| </div> | |
| <p>An empty list means nothing extra is installed.</p> | |
| </section> | |
| <section id="next-steps"> | |
| <h2>Next steps<a class="headerlink" href="#next-steps" title="Link to this heading">#</a></h2> | |
| <ul class="simple"> | |
| <li><p><a class="reference internal" href="distributing-work/query-engines.html#distributed-query-engines"><span class="std std-ref">Distributed query engines</span></a> — running your queries across several | |
| machines with an engine library.</p></li> | |
| <li><p><a class="reference internal" href="distributing-work/expressions.html#distributed-expressions"><span class="std std-ref">Shipping expressions to workers</span></a> — the other road to parallelism, where you | |
| decide the partitioning and ship expressions to a worker pool.</p></li> | |
| <li><p><a class="reference internal" href="../extension-guide/index.html#ffi"><span class="std std-ref">Extension Guide</span></a> — writing an extension library of your own.</p></li> | |
| </ul> | |
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