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| <title>APPX_COUNT_DISTINCT Query Option (Impala 2.0 or higher only)</title> |
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| <h1 class="title topictitle1" id="ariaid-title1">APPX_COUNT_DISTINCT Query Option (<span class="keyword">Impala 2.0</span> or higher only)</h1> |
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| <p class="p"> |
| When the <code class="ph codeph">APPX_COUNT_DISTINCT</code> query option is set to |
| <code class="ph codeph">TRUE</code>, Impala implicitly converts <code class="ph codeph">COUNT(DISTINCT)</code> |
| operations to the <code class="ph codeph">NDV()</code> 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 <code class="ph codeph">COUNT |
| (DISTINCT)</code> queries. |
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| <p class="p"> |
| <strong class="ph b">Type:</strong> Boolean; recognized values are 1 and 0, or <code class="ph codeph">true</code> and |
| <code class="ph codeph">false</code>; any other value interpreted as <code class="ph codeph">false</code> |
| </p> |
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| <p class="p"> |
| <strong class="ph b">Default:</strong> <code class="ph codeph">false</code> (shown as 0 in output of <code class="ph codeph">SET</code> |
| statement) |
| </p> |
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| <p class="p"> |
| <strong class="ph b">Related information:</strong> |
| </p> |
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| <p class="p"> |
| <a class="xref" href="impala_count.html#count">COUNT Function</a>, |
| <a class="xref" href="impala_distinct.html#distinct">DISTINCT Operator</a>, <a class="xref" href="impala_ndv.html#ndv">NDV Function</a> |
| </p> |
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| <div class="parentlink"><strong>Parent topic:</strong> <a class="link" href="../topics/impala_query_options.html">Query Options for the SET Statement</a></div> |
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