docs/topics/impala_parquet_read_statistics.xml

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

  <title>PARQUET_READ_STATISTICS Query Option (<keyword keyref="impala29"/> or higher only)</title>

  <titlealts audience="PDF">

    <navtitle>PARQUET_READ_STATISTICS</navtitle>

  </titlealts>

  <prolog>
    <metadata>
      <data name="Category" value="Impala"/>
      <data name="Category" value="Impala Query Options"/>
      <data name="Category" value="Parquet"/>
      <data name="Category" value="Developers"/>
      <data name="Category" value="Data Analysts"/>
    </metadata>
  </prolog>

  <conbody>

    <p>
      The <codeph>PARQUET_READ_STATISTICS</codeph> query option controls whether to read
      statistics from Parquet files and use them during query processing.
    </p>

    <p>
      Parquet stores min/max stats which can be used to skip reading row groups if they don't
      qualify a certain predicate. When this query option is set to <codeph>true</codeph>,
      Impala reads the Parquet statistics and skips reading row groups that do not match the
      conditions in the <codeph>WHERE</codeph> clause.
    </p>

    <p>
      Impala supports filtering based on Parquet statistics:
    </p>

    <ul>
      <li>
        Of the numerical types for the old version of the statistics: Boolean, Integer, Float
      </li>

      <li>
        Of the types for the new version of the statistics (starting in IMPALA 2.8): Boolean,
        Integer, Float, Decimal, String, Timestamp
      </li>

      <li>
        For simple predicates of the forms: <codeph>&lt;slot> &lt;op> &lt;constant></codeph> or
        <codeph>&lt;constant> &lt;op> &lt;slot></codeph>, where <codeph>&lt;op></codeph> is LT,
        LE, GE, GT, and EQ
      </li>
    </ul>

    <p>
      The <codeph>PARQUET_READ_STATISTICS</codeph> option provides a workaround when dealing
      with files that have corrupt Parquet statistics and unknown errors.
    </p>

    <p>
      In the query runtime profile output for each Impalad instance, the
      <codeph>NumStatsFilteredRowGroups</codeph> field in the SCAN node section shows the number
      of row groups that were skipped based on Parquet statistics.
    </p>

    <p>
      The supported values for the query option are:
      <ul>
        <li>
          <codeph>true</codeph> (<codeph>1</codeph>): Read statistics from Parquet files and use
          them in query processing.
        </li>

        <li>
          <codeph>false</codeph> (<codeph>0</codeph>): Do not use Parquet read statistics.
        </li>

        <li>
          Any other values are treated as <codeph>false</codeph>.
        </li>
      </ul>
    </p>

    <p>
      <b>Type:</b> Boolean
    </p>

    <p>
      <b>Default:</b> <codeph>true</codeph>
    </p>

    <p conref="../shared/impala_common.xml#common/added_in_290"/>

  </conbody>

</concept>