docs/topics/impala_replica_preference.xml

<?xml version="1.0" encoding="UTF-8"?>
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<!DOCTYPE concept PUBLIC "-//OASIS//DTD DITA Concept//EN" "concept.dtd">
<concept id="replica_preference" rev="2.7.0">

  <title>REPLICA_PREFERENCE Query Option (<keyword keyref="impala27"/> or higher only)</title>

  <titlealts audience="PDF">

    <navtitle>REPLICA_PREFERENCE</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"/>
    </metadata>
  </prolog>

  <conbody>

    <p rev="2.7.0">
      <indexterm audience="hidden">REPLICA_PREFERENCE query option</indexterm>
    </p>

    <p>
      The <codeph>REPLICA_PREFERENCE</codeph> query option lets you distribute the work more
      evenly if hotspots and bottlenecks persist. It causes the access cost of all replicas of a
      data block to be considered equal to or worse than the configured value. This allows
      Impala to schedule reads to suboptimal replicas (e.g. local in the presence of cached
      ones) in order to distribute the work across more executor nodes.
    </p>

    <p>
      Allowed values are: <codeph>CACHE_LOCAL</codeph> (<codeph>0</codeph>),
      <codeph>DISK_LOCAL</codeph> (<codeph>2</codeph>), <codeph>REMOTE</codeph>
      (<codeph>4</codeph>)
    </p>

    <p>
      <b>Type:</b> Enum
    </p>

    <p>
      <b>Default:</b> <codeph>CACHE_LOCAL (0)</codeph>
    </p>

    <p conref="../shared/impala_common.xml#common/added_in_270"/>

    <p>
      <b>Usage Notes:</b>
    </p>

    <p>
      By default Impala selects the best replica it can find in terms of access cost. The
      preferred order is cached, local, and remote. With <codeph>REPLICA_PREFERENCE</codeph>,
      the preference of all replicas are capped at the selected value. For example, when
      <codeph>REPLICA_PREFERENCE</codeph> is set to <codeph>DISK_LOCAL</codeph>, cached and
      local replicas are treated with the equal preference. When set to
      <codeph>REMOTE</codeph>, all three types of replicas, cached, local, remote, are treated
      with equal preference.
    </p>

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

    <p>
      <xref href="impala_perf_hdfs_caching.xml#hdfs_caching"/>,
      <xref href="impala_schedule_random_replica.xml#schedule_random_replica"/>
    </p>

  </conbody>

</concept>