docs/topics/impala_appx_count_distinct.xml

<?xml version="1.0" encoding="UTF-8"?>
<!--
Licensed to the Apache Software Foundation (ASF) under one
or more contributor license agreements.  See the NOTICE file
distributed with this work for additional information
regarding copyright ownership.  The ASF licenses this file
to you under the Apache License, Version 2.0 (the
"License"); you may not use this file except in compliance
with the License.  You may obtain a copy of the License at

  http://www.apache.org/licenses/LICENSE-2.0

Unless required by applicable law or agreed to in writing,
software distributed under the License is distributed on an
"AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
KIND, either express or implied.  See the License for the
specific language governing permissions and limitations
under the License.
-->
<!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">
      When the <codeph>APPX_COUNT_DISTINCT</codeph> query option is set to
      <codeph>TRUE</codeph>, Impala implicitly converts <codeph>COUNT(DISTINCT)</codeph>
      operations to the <codeph>NDV()</codeph> function calls. The resulting count is
      approximate rather than precise. Enable the query option when a tolerable amount of error
      is acceptable in order to obtain faster query results than with a <codeph>COUNT
      (DISTINCT)</codeph> queries.
    </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/related_info"/>

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

  </conbody>

</concept>