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<span id="extension-planners"></span><h1>Query planners<a class="headerlink" href="#query-planners" title="Link to this heading">#</a></h1>
<p>A query planner turns a logical plan into a physical one. Contributing your own
is how a library changes the way queries execute rather than what data they can
reach — a distributed engine is the motivating case, and an optimizing rewriter
is another.</p>
<p>The session owns the codecs used for the exchange and supplies them to your
planner. That is what lets the planner decode provider-owned objects, and lets
datafusion-python decode the physical plan the planner returns. Your planner
uses <code class="docutils literal notranslate"><span class="pre">FFI_QueryPlanner::new_with_ffi_codecs</span></code> with the two codecs it takes off
the session, and never touches a task-context provider directly. That also
matches what installation does anyway:
<code class="xref py py-meth docutils literal notranslate"><span class="pre">set_query_planner()</span></code> builds the planner
against the codecs of the session that will run the query.</p>
<p><code class="docutils literal notranslate"><span class="pre">MyQueryPlanner</span></code> in <a class="reference external" href="https://github.com/apache/datafusion-python/tree/main/examples/datafusion-ffi-query-planner-example"><code class="docutils literal notranslate"><span class="pre">datafusion-ffi-query-planner-example</span></code></a> is the worked
implementation.</p>
<section id="one-planner-per-session">
<h2>One planner per session<a class="headerlink" href="#one-planner-per-session" title="Link to this heading">#</a></h2>
<p>A session holds exactly one query planner. Calling <code class="docutils literal notranslate"><span class="pre">set_query_planner</span></code> again
<strong>replaces</strong> the installed planner instead of layering another one.</p>
<p>To chain planners, have the new planner wrap the capsule returned by
<code class="xref py py-meth docutils literal notranslate"><span class="pre">SessionContext.__datafusion_query_planner__</span></code>,
captured before the new planner is installed, and delegate to it explicitly:</p>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="n">fallback</span> <span class="o">=</span> <span class="n">ctx</span><span class="o">.</span><span class="n">__datafusion_query_planner__</span><span class="p">()</span>
<span class="n">ctx</span><span class="o">.</span><span class="n">set_query_planner</span><span class="p">(</span><span class="n">MyPlanner</span><span class="p">(</span><span class="n">fallback</span><span class="o">=</span><span class="n">fallback</span><span class="p">))</span>
</pre></div>
</div>
<p><code class="docutils literal notranslate"><span class="pre">set_query_planner</span></code> returns nothing. The query planner lives in <code class="docutils literal notranslate"><span class="pre">SessionState</span></code>,
so it is a property of the session rather than of a handle on it, and
installing one is visible to every context sharing that session. See
<a class="reference internal" href="sessions.html#extension-sessions"><span class="std std-ref">Sessions, handles, and lifetimes</span></a>.</p>
<p>If the planners you are composing all ship as
<a class="reference internal" href="bundles.html#extension-bundles"><span class="std std-ref">bundles</span></a>, prefer <code class="docutils literal notranslate"><span class="pre">with_extensions</span></code> — it does the
nesting for you and cannot capture a partial codec chain.</p>
</section>
<section id="install-codecs-before-a-layered-planner">
<span id="planner-codec-rebinding"></span><h2>Install codecs before a layered planner<a class="headerlink" href="#install-codecs-before-a-layered-planner" title="Link to this heading">#</a></h2>
<p>Installing a codec on a session that already has a foreign planner rebuilds
that planner against the new chain: there is one planner, and it has to carry
the codecs currently in force. But that rebuild swaps the codecs on the
installed <code class="docutils literal notranslate"><span class="pre">ForeignQueryPlanner</span></code> handle, and <strong>only that handle</strong>.</p>
<p>A planner that wraps a fallback resolved that fallback when <em>it</em> was installed,
and holds the result inside its own library’s private data — behind a
<code class="docutils literal notranslate"><span class="pre">create_physical_plan</span></code> function pointer, with no Python-side handle. A codec
installed afterwards therefore reaches the outer planner and not the fallback,
which keeps whichever codecs were in force when it was imported.</p>
<p>The stale codecs stay usable rather than dangling — they hold weak handles to
the one <code class="docutils literal notranslate"><span class="pre">Arc&lt;SessionContext&gt;</span></code> the session keeps alive — so the effect is a
fallback hop serializing with an older codec, not a failure. Neither side can
repair it; the reasons are in <a class="reference internal" href="../contributor-guide/ffi-internals.html#ffi-internals-rebinding"><span class="std std-ref">Why planner codec rebinding is one level deep</span></a>, and a fix has to
come from upstream
(<a class="reference external" href="https://github.com/apache/datafusion/issues/24762">apache/datafusion#24762</a>).</p>
<p>So, three rules:</p>
<ul class="simple">
<li><p><strong>Install the codecs before a layered planner.</strong></p></li>
<li><p>If a codec has to go in afterwards, install the outer planner again <em>on the
handle that holds the new codec</em> — that re-runs its getter, which re-imports
the fallback against that handle’s codecs. Re-installing on the original
handle rebinds the session’s planner back to the original handle’s codecs
instead, which is the trap the query-planner example’s test suite pins.</p></li>
<li><p>Better, use <a class="reference internal" href="bundles.html#extension-bundles"><span class="std std-ref">Extension bundles</span></a>, where there is no “afterwards” within a
call.</p></li>
</ul>
<div class="admonition note">
<p class="admonition-title">Note</p>
<p>This is invisible in the examples here, which use one fallback in the same
cdylib as its wrapper; <code class="docutils literal notranslate"><span class="pre">datafusion-ffi</span></code> short-circuits a same-library hop
rather than serializing, so no codec runs. A fallback in a <em>different</em> library
would serialize, and would do it with the codecs it was imported with.</p>
</div>
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