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<!DOCTYPE concept PUBLIC "-//OASIS//DTD DITA Concept//EN" "concept.dtd">
<concept id="schedule_random_replica" rev="2.5.0">

  <title>SCHEDULE_RANDOM_REPLICA Query Option (<keyword keyref="impala25"/> or higher only)</title>
  <titlealts audience="PDF"><navtitle>SCHEDULE_RANDOM_REPLICA</navtitle></titlealts>
  <prolog>
    <metadata>
      <data name="Category" value="Impala"/>
      <data name="Category" value="Impala Query Options"/>
      <data name="Category" value="Performance"/>
      <data name="Category" value="Developers"/>
      <data name="Category" value="Data Analysts"/>
    </metadata>
  </prolog>

  <conbody>

    <p rev="2.5.0">
      <indexterm audience="hidden">SCHEDULE_RANDOM_REPLICA query option</indexterm>
    </p>

    <p>
      The <codeph>SCHEDULE_RANDOM_REPLICA</codeph> query option fine-tunes the algorithm for deciding which host
      processes each HDFS data block. It only applies to tables and partitions that are not enabled
      for the HDFS caching feature.
    </p>

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

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

    <p conref="../shared/impala_common.xml#common/added_in_250"/>

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

    <p>
      In the presence of HDFS cached replicas, Impala randomizes
      which host processes each cached data block.
      To ensure that HDFS data blocks are cached on more
      than one host, use the <codeph>WITH REPLICATION</codeph> clause along with
      the <codeph>CACHED IN</codeph> clause in a
      <codeph>CREATE TABLE</codeph> or <codeph>ALTER TABLE</codeph> statement.
      Specify a replication value greater than or equal to the HDFS block replication factor.
    </p>

    <p>
      The <codeph>SCHEDULE_RANDOM_REPLICA</codeph> query option applies to tables and partitions
      that <i>do not</i> use HDFS caching.
      By default, Impala estimates how much work each host has done for
      the query, and selects the host that has the lowest workload.
      This algorithm is intended to reduce CPU hotspots arising when the
      same host is selected to process multiple data blocks, but hotspots
      might still arise for some combinations of queries and data layout.
      When the <codeph>SCHEDULE_RANDOM_REPLICA</codeph> option is enabled,
      Impala further randomizes the scheduling algorithm for non-HDFS cached blocks,
      which can further reduce the chance of CPU hotspots.
    </p>

    <p rev="IMPALA-2979">
      This query option works in conjunction with the work scheduling improvements
      in <keyword keyref="impala25_full"/> and higher. The scheduling improvements
      distribute the processing for cached HDFS data blocks to minimize hotspots:
      if a data block is cached on more than one host, Impala chooses which host
      to process each block based on which host has read the fewest bytes during
      the current query. Enable <codeph>SCHEDULE_RANDOM_REPLICA</codeph> setting if CPU hotspots
      still persist because of cases where hosts are <q>tied</q> in terms of
      the amount of work done; by default, Impala picks the first eligible host
      in this case.
    </p>

    <p conref="../shared/impala_common.xml#common/related_info"/>
    <p>
      <xref href="impala_perf_hdfs_caching.xml#hdfs_caching"/>,
      <xref href="impala_scalability.xml#scalability_hotspots"/>
      , <xref href="impala_replica_preference.xml#replica_preference"/>
    </p>

  </conbody>
</concept>