docs/topics/impala_appx_count_distinct.xml

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<!DOCTYPE concept PUBLIC "-//OASIS//DTD DITA Concept//EN" "concept.dtd">
<concept rev="2.0.0" id="appx_count_distinct">

  <title>APPX_COUNT_DISTINCT Query Option (<keyword keyref="impala20"/> or higher only)</title>
  <titlealts audience="PDF"><navtitle>APPX_COUNT_DISTINCT</navtitle></titlealts>
  <prolog>
    <metadata>
      <data name="Category" value="Impala"/>
      <data name="Category" value="Impala Query Options"/>
      <data name="Category" value="Aggregate Functions"/>
      <data name="Category" value="Developers"/>
      <data name="Category" value="Data Analysts"/>
    </metadata>
  </prolog>

  <conbody>

    <p rev="2.0.0">
      <indexterm audience="hidden">APPX_COUNT_DISTINCT query option</indexterm>
      Allows multiple <codeph>COUNT(DISTINCT)</codeph> operations within a single query, by internally rewriting
      each <codeph>COUNT(DISTINCT)</codeph> to use the <codeph>NDV()</codeph> function. The resulting count is
      approximate rather than precise.
    </p>

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

    <p conref="../shared/impala_common.xml#common/default_false_0"/>

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

    <p>
      The following examples show how the <codeph>APPX_COUNT_DISTINCT</codeph> lets you work around the restriction
      where a query can only evaluate <codeph>COUNT(DISTINCT <varname>col_name</varname>)</codeph> for a single
      column. By default, you can count the distinct values of one column or another, but not both in a single
      query:
    </p>

<codeblock>[localhost:21000] &gt; select count(distinct x) from int_t;
+-------------------+
| count(distinct x) |
+-------------------+
| 10                |
+-------------------+
[localhost:21000] &gt; select count(distinct property) from int_t;
+--------------------------+
| count(distinct property) |
+--------------------------+
| 7                        |
+--------------------------+
[localhost:21000] &gt; select count(distinct x), count(distinct property) from int_t;
ERROR: AnalysisException: all DISTINCT aggregate functions need to have the same set of parameters
as count(DISTINCT x); deviating function: count(DISTINCT property)
</codeblock>

    <p>
      When you enable the <codeph>APPX_COUNT_DISTINCT</codeph> query option, now the query with multiple
      <codeph>COUNT(DISTINCT)</codeph> works. The reason this behavior requires a query option is that each
      <codeph>COUNT(DISTINCT)</codeph> is rewritten internally to use the <codeph>NDV()</codeph> function instead,
      which provides an approximate result rather than a precise count.
    </p>

<codeblock>[localhost:21000] &gt; set APPX_COUNT_DISTINCT=true;
[localhost:21000] &gt; select count(distinct x), count(distinct property) from int_t;
+-------------------+--------------------------+
| count(distinct x) | count(distinct property) |
+-------------------+--------------------------+
| 10                | 7                        |
+-------------------+--------------------------+
</codeblock>

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

    <p>
      <xref href="impala_count.xml#count"/>,
      <xref href="impala_distinct.xml#distinct"/>,
      <xref href="impala_ndv.xml#ndv"/>
    </p>

  </conbody>
</concept>