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<h1 class="title topictitle1" id="ariaid-title1">PARQUET_DICTIONARY_FILTERING Query Option (<span class="keyword">Impala 2.9</span> or higher only)</h1>
<div class="body conbody">
<p class="p">
The <code class="ph codeph">PARQUET_DICTIONARY_FILTERING</code> query option controls whether Impala
uses dictionary filtering for Parquet files.
</p>
<p class="p">
To efficiently process a highly selective scan query, when this option is enabled, Impala
checks the values in the Parquet dictionary page and determines if the whole row group can
be thrown out.
</p>
<div class="p">
A column chunk is purely dictionary encoded and can be used by dictionary filtering if any
of the following conditions are met:
<ol class="ol">
<li class="li">
If the <code class="ph codeph">encoding_stats</code> is in the Parquet file, dictionary filtering
uses it to determine if there are only dictionary encoded pages (i.e. there are no
data pages with an encoding other than <code class="ph codeph">RLE_DICTIONARY</code> or
<code class="ph codeph">PLAIN_DICTIONARY</code>).
</li>
<li class="li">
If the encoding stats are not present, dictionary filtering looks at the encodings.
The column is purely dictionary encoded if both of the conditions satisfy:
<ul class="ul">
<li class="li">
<code class="ph codeph">PLAIN_DICTIONARY</code> or <code class="ph codeph">RLE_DICTIONARY</code> is present.
</li>
<li class="li">
Only <code class="ph codeph">PLAIN_DICTIONARY</code>, <code class="ph codeph">RLE_DICTIONARY</code>,
<code class="ph codeph">RLE</code>, or <code class="ph codeph">BIT_PACKED</code> encodings are listed.
</li>
</ul>
</li>
<li class="li">
Dictionary filtering works for the Parquet dictionaries with less than 40000 values if
the file was written by <span class="keyword"> or lower</span>.
</li>
</ol>
</div>
<p class="p">
In the query runtime profile output for each Impalad instance, the
<code class="ph codeph">NumDictFilteredRowGroups</code> field in the SCAN node section shows the number
of row groups that were skipped based on dictionary filtering.
</p>
<p class="p">
Note that row groups can be filtered out by Parquet statistics, and in such cases,
dictionary filtering will not be considered.
</p>
<div class="p">
The supported values for the query option are:
<ul class="ul">
<li class="li">
<code class="ph codeph">true</code> (<code class="ph codeph">1</code>): Use dictionary filtering.
</li>
<li class="li">
<code class="ph codeph">false</code> (<code class="ph codeph">0</code>): Do not use dictionary filtering
</li>
<li class="li">
Any other values are treated as <code class="ph codeph">false</code>.
</li>
</ul>
</div>
<p class="p">
<strong class="ph b">Type:</strong> Boolean
</p>
<p class="p">
<strong class="ph b">Default:</strong> <code class="ph codeph">true</code> (<code class="ph codeph">1</code>)
</p>
<p class="p">
<strong class="ph b">Added in:</strong> <span class="keyword">Impala 2.9.0</span>
</p>
</div>
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