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 spread the load more evenly if hotspots and bottlenecks persist, by allowing hosts to do local reads,
      or even remote reads, to retrieve the data for cached blocks if Impala can determine that it would be
      too expensive to do all such processing on a particular host.
    </p>

    <p>
      <b>Type:</b> numeric (0, 3, 5)
      or corresponding mnemonic strings (<codeph>CACHE_LOCAL</codeph>, <codeph>DISK_LOCAL</codeph>, <codeph>REMOTE</codeph>).
      The gaps in the numeric sequence are to accomodate other intermediate
      values that might be added in the future.
    </p>

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

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

    <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>