docs/topics/impala_scan_node_codegen_threshold.xml

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<!DOCTYPE concept PUBLIC "-//OASIS//DTD DITA Concept//EN" "concept.dtd">
<concept id="scan_node_codegen_threshold" rev="2.5.0 IMPALA-1755">

  <title>SCAN_NODE_CODEGEN_THRESHOLD Query Option (<keyword keyref="impala25"/> or higher only)</title>
  <titlealts audience="PDF"><navtitle>SCAN_NODE_CODEGEN_THRESHOLD</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 IMPALA-1755">
      <indexterm audience="hidden">SCAN_NODE_CODEGEN_THRESHOLD query option</indexterm>
      The <codeph>SCAN_NODE_CODEGEN_THRESHOLD</codeph> query option
      adjusts the aggressiveness of the code generation optimization process
      when performing I/O read operations. It can help to work around performance problems
      for queries where the table is small and the <codeph>WHERE</codeph> clause is complicated.
    </p>

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

    <p>
      <b>Default:</b> 1800000 (1.8 million)
    </p>

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

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

    <p>
      This query option is intended mainly for the case where a query with a very complicated
      <codeph>WHERE</codeph> clause, such as an <codeph>IN</codeph> operator with thousands
      of entries, is run against a small table, especially a small table using Parquet format.
      The code generation phase can become the dominant factor in the query response time,
      making the query take several seconds even though there is relatively little work to do.
      In this case, increase the value of this option to a much larger amount, anything up to
      the maximum for a 32-bit integer.
    </p>

    <p>
      Because this option only affects the code generation phase for the portion of the
      query that performs I/O (the <term>scan nodes</term> within the query plan), it
      lets you continue to keep code generation enabled for other queries, and other parts
      of the same query, that can benefit from it. In contrast, the
      <codeph>IMPALA_DISABLE_CODEGEN</codeph> query option turns off code generation entirely.
    </p>

    <p>
      Because of the way the work for queries is divided internally, this option might not
      affect code generation for all kinds of queries. If a plan fragment contains a scan
      node and some other kind of plan node, code generation still occurs regardless of
      this option setting.
    </p>

    <p>
      To use this option effectively, you should be familiar with reading query profile output
      to determine the proportion of time spent in the code generation phase, and whether
      code generation is enabled or not for specific plan fragments.
    </p>

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  </conbody>
</concept>