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<title>THREAD_RESERVATION_LIMIT Query Option (Impala 3.1 or higher only)</title>
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<h1 class="title topictitle1" id="ariaid-title1">THREAD_RESERVATION_LIMIT Query Option (<span class="keyword">Impala 3.1</span> or higher only)</h1>
<div class="body conbody">
<p class="p">
The <code class="ph codeph">THREAD_RESERVATION_LIMIT</code> query option limits the number of reserved
threads for a query on each node. The option is intended to prevent execution of complex
queries that can consume excessive CPU or operating system resources on a single node.
Queries that have more threads per node than this threshold are rejected by Impala’s
admission controller before they start executing. You can see the number of reserved
threads for a query in its
<a class="xref" href="impala_explain_plan.html#explain_plan">explain plan</a> in the
“Per-Host Resource Reservation" line.
</p>
<p class="p">
For example, an Impala administrator could set a default value of
<code class="ph codeph">THREAD_RESERVATION_LIMIT=100</code> for a resource pool where they expect only
relatively simple queries to run. This will reject queries that require more than 100
reserved threads on a node, for example, queries with more than 100 fragments.
</p>
<p class="p">
You can override the default value per-query or per-session, in the same way as other
query options, if you do not want the default <code class="ph codeph">THREAD_RESERVATION_LIMIT</code>
value to apply to a specific query or session.
</p>
<div class="p">
<div class="note note"><span class="notetitle">Note:</span>
The number of reserved threads on a node may be lower than the maximum value in the
explain plan if not all fragments of that query are scheduled on every node.
</div>
</div>
<p class="p">
<strong class="ph b">Syntax:</strong> <code class="ph codeph">SET THREAD_RESERVATION_LIMIT=number;</code>
</p>
<p class="p">
<strong class="ph b">Type:</strong> numeric
</p>
<p class="p">
<strong class="ph b">Default:</strong> 3000
</p>
<p class="p">
<strong class="ph b">Added in:</strong> <span class="keyword">Impala 3.1</span>
</p>
</div>
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<div class="parentlink"><strong>Parent topic:</strong> <a class="link" href="../topics/impala_query_options.html">Query Options for the SET Statement</a></div>
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