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<h1 class="title topictitle1" id="ariaid-title1">NUM_NODES Query Option</h1>
<div class="body conbody">
<p class="p">
Limit the number of nodes that process a query, typically during debugging.
</p>
<p class="p">
<strong class="ph b">Type:</strong> numeric
</p>
<p class="p">
<strong class="ph b">Allowed values:</strong> Only accepts the values 0
(meaning all nodes) or 1 (meaning all work is done on the coordinator node).
</p>
<p class="p">
<strong class="ph b">Default:</strong> 0
</p>
<p class="p">
<strong class="ph b">Usage notes:</strong>
</p>
<p class="p">
Setting <code class="ph codeph">NUM_NODES</code> to 1 disables multithreading, i.e. if
<code class="ph codeph">MT_DOP</code> is greater than 1, it is effectively reduced to 1.
</p>
<p class="p">
If you are diagnosing a problem that you suspect is due to a timing issue due to
distributed query processing, you can set <code class="ph codeph">NUM_NODES=1</code> to verify
if the problem still occurs when all the work is done on a single node.
</p>
<p class="p">
You might set the <code class="ph codeph">NUM_NODES</code> option to 1 briefly, during
<code class="ph codeph">INSERT</code> or <code class="ph codeph">CREATE TABLE AS SELECT</code> statements. Normally,
those statements produce one or more data files per data node. If the write operation
involves small amounts of data, a Parquet table, and/or a partitioned table, the default
behavior could produce many small files when intuitively you might expect only a single
output file. <code class="ph codeph">SET NUM_NODES=1</code> turns off the <span class="q">"distributed"</span> aspect of
the write operation, making it more likely to produce only one or a few data files.
</p>
<div class="note warning note_warning"><span class="note__title warningtitle">Warning:</span>
<p class="p">
Because this option results in increased resource utilization on a single host,
it could cause problems due to contention with other Impala statements or
high resource usage. Symptoms could include queries running slowly, exceeding the memory limit,
or appearing to hang. Use it only in a single-user development/test environment;
<strong class="ph b">do not</strong> use it in a production environment or in a cluster with a high-concurrency
or high-volume or performance-critical workload.
</p>
</div>
</div>
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