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<?xml version="1.0" encoding="UTF-8"?>
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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">
<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"/>
<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 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"/>
<xref href="impala_count.xml#count"/>,
href="impala_distinct.xml#distinct"/>, <xref href="impala_ndv.xml#ndv"/>