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<section id="distributing-work">
<span id="id1"></span><h1>Distributing work<a class="headerlink" href="#distributing-work" title="Link to this heading">#</a></h1>
<p>A single <code class="xref py py-class docutils literal notranslate"><span class="pre">SessionContext</span></code> already uses every
core on the machine — DataFusion partitions and parallelizes within
a process without being asked. See
<a class="reference internal" href="../configuration.html"><span class="doc">Configuration</span></a> for tuning that.</p>
<p>This section is about the step after that: getting work onto more
than one process or more than one machine. There are two roads, and
they suit different problems.</p>
<section id="pick-your-road">
<h2>Pick your road<a class="headerlink" href="#pick-your-road" title="Link to this heading">#</a></h2>
<p><strong>You decide the partitioning → ship expressions.</strong></p>
<p>You already know how the data divides — one file per worker, one
customer per worker, one parameter setting per worker. You build an
<a class="reference internal" href="../../autoapi/datafusion/index.html#datafusion.Expr" title="datafusion.Expr"><code class="xref py py-class docutils literal notranslate"><span class="pre">Expr</span></code></a> in the driver and hand a copy to each
worker along with its slice. Standard Python <code class="docutils literal notranslate"><span class="pre">pickle</span></code> moves it, so
<code class="xref py py-mod docutils literal notranslate"><span class="pre">multiprocessing</span></code>, Ray, and anything else that ships function
arguments works with no extra machinery.</p>
<p>Best for embarrassingly-parallel work: parameter sweeps, per-file
transforms, scoring batches. Available today.</p>
<p>→ <a class="reference internal" href="expressions.html#distributed-expressions"><span class="std std-ref">Shipping expressions to workers</span></a></p>
<p><strong>A library decides the partitioning → install a query engine.</strong></p>
<p>You write one ordinary SQL or DataFrame query against a table too
large for one machine, and an engine library splits the plan into
stages, runs them on its workers, and reassembles the result. You do
not partition anything and your queries do not change.</p>
<p>Best for large-scale analytical queries — joins and aggregations over
data that does not fit on one node. Being built upstream; not yet
usable from datafusion-python.</p>
<p>→ <a class="reference internal" href="query-engines.html#distributed-query-engines"><span class="std std-ref">Distributed query engines</span></a></p>
</section>
<section id="choosing-between-them">
<h2>Choosing between them<a class="headerlink" href="#choosing-between-them" title="Link to this heading">#</a></h2>
<p>The two are not competing implementations of one feature. The
question is who owns the partitioning decision.</p>
<p>If you can state the partitioning in one line — “one worker per input
file” — the expression road is simpler, has no cluster to operate,
and works now. If stating it requires knowing how a join will
shuffle, that decision belongs to a query planner, which is what an
engine library provides.</p>
<p>They also compose. An engine handles the query; expressions handle
whatever you want to fan out around it.</p>
<div class="toctree-wrapper compound">
<ul>
<li class="toctree-l1"><a class="reference internal" href="expressions.html">Shipping expressions to workers</a><ul>
<li class="toctree-l2"><a class="reference internal" href="expressions.html#basic-worker-pool-example">Basic worker-pool example</a></li>
<li class="toctree-l2"><a class="reference internal" href="expressions.html#what-travels-with-the-expression">What travels with the expression</a></li>
<li class="toctree-l2"><a class="reference internal" href="expressions.html#portability-requirements-for-inline-python-udfs">Portability requirements for inline Python UDFs</a></li>
<li class="toctree-l2"><a class="reference internal" href="expressions.html#registering-shared-udfs-on-workers">Registering shared UDFs on workers</a></li>
<li class="toctree-l2"><a class="reference internal" href="expressions.html#python-3-14-default-change">Python 3.14 default change</a></li>
<li class="toctree-l2"><a class="reference internal" href="expressions.html#practical-considerations">Practical considerations</a></li>
<li class="toctree-l2"><a class="reference internal" href="expressions.html#disabling-python-udf-inlining">Disabling Python UDF inlining</a></li>
<li class="toctree-l2"><a class="reference internal" href="expressions.html#security">Security</a></li>
<li class="toctree-l2"><a class="reference internal" href="expressions.html#reference-session-context-slots">Reference: session context slots</a></li>
<li class="toctree-l2"><a class="reference internal" href="expressions.html#see-also">See also</a></li>
</ul>
</li>
<li class="toctree-l1"><a class="reference internal" href="query-engines.html">Distributed query engines</a><ul>
<li class="toctree-l2"><a class="reference internal" href="query-engines.html#how-an-engine-attaches-to-your-session">How an engine attaches to your session</a></li>
<li class="toctree-l2"><a class="reference internal" href="query-engines.html#available-engines">Available engines</a></li>
</ul>
</li>
</ul>
</div>
</section>
</section>
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