docs/topics/impala_num_nodes.xml

<?xml version="1.0" encoding="UTF-8"?>
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<!DOCTYPE concept PUBLIC "-//OASIS//DTD DITA Concept//EN" "concept.dtd">
<concept id="num_nodes">

  <title>NUM_NODES Query Option</title>
  <titlealts audience="PDF"><navtitle>NUM_NODES</navtitle></titlealts>
  <prolog>
    <metadata>
      <data name="Category" value="Impala"/>
      <data name="Category" value="Impala Query Options"/>
      <data name="Category" value="Developers"/>
      <data name="Category" value="Data Analysts"/>
      <data name="Category" value="Troubleshooting"/>
    </metadata>
  </prolog>

  <conbody>

    <p>
      <indexterm audience="hidden">NUM_NODES query option</indexterm>
      Limit the number of nodes that process a query, typically during debugging.
    </p>

    <p>
      <b>Type:</b> numeric
    </p>

<p>
      <b>Allowed values:</b> Only accepts the values 0
      (meaning all nodes) or 1 (meaning all work is done on the coordinator node).
</p>

    <p>
      <b>Default:</b> 0
    </p>

     <p conref="../shared/impala_common.xml#common/usage_notes_blurb"/>

     <p>
       If you are diagnosing a problem that you suspect is due to a timing issue due to
       distributed query processing, you can set <codeph>NUM_NODES=1</codeph> to verify
       if the problem still occurs when all the work is done on a single node.
     </p>

    <p conref="../shared/impala_common.xml#common/num_nodes_tip"/>

    <note type="warning" rev="DOCS-1161">
    <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;
      <b>do not</b> use it in a production environment or in a cluster with a high-concurrency
      or high-volume or performance-critical workload.
    </p>
    </note>


  </conbody>
</concept>