docs/topics/impala_scratch_limit.xml

<?xml version="1.0" encoding="UTF-8"?>
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<!DOCTYPE concept PUBLIC "-//OASIS//DTD DITA Concept//EN" "concept.dtd">
<concept rev="IMPALA-3671" id="scratch_limit">

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

  <conbody>

    <p>
      <indexterm audience="hidden">SCRATCH_LIMIT query option</indexterm>
      Specifies the maximum amount of disk storage, in bytes, that any Impala query can consume
      on any host using the <q>spill to disk</q> mechanism that handles queries that exceed
      the memory limit.
    </p>

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

    <p>
      Specify the size in bytes, or with a trailing <codeph>m</codeph> or <codeph>g</codeph> character to indicate
      megabytes or gigabytes. For example:
    </p>

<!-- Examples adapted from impala_parquet_file_size.xml.
     To do: demonstrate with queries that succeed / fail
     based on the query option setting, and interaction
     with MEM_LIMIT. -->
<codeblock>-- 128 megabytes.
set SCRATCH_LIMIT=134217728

-- 512 megabytes.
set SCRATCH_LIMIT=512m;

-- 1 gigabyte.
set SCRATCH_LIMIT=1g;
</codeblock>

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

    <p>
      A value of zero turns off the spill to disk feature for queries
      in the current session, causing them to fail immediately if they
      exceed the memory limit.
    </p>

    <p>
      The amount of memory used per host for a query is limited by the
      <codeph>MEM_LIMIT</codeph> query option.
    </p>

    <p>
      The more Impala daemon hosts in the cluster, the less memory is used on each host,
      and therefore also less scratch space is required for queries that
      exceed the memory limit.
    </p>

    <p>
      <b>Type:</b> numeric, with optional unit specifier
    </p>

    <p>
      <b>Default:</b> -1 (amount of spill space is unlimited)
    </p>

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

    <p>
      <xref href="impala_scalability.xml#spill_to_disk"/>,
      <xref href="impala_mem_limit.xml#mem_limit"/>
    </p>

  </conbody>
</concept>