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    <h1 class="heading"><a href="../index.html">
        <span>Presto 321 Documentation</span></a></h1>
    <h2 class="heading"><span>7.21. Quantile Digest Functions</span></h2>
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  <div class="section" id="quantile-digest-functions">
<h1>7.21. Quantile Digest Functions</h1>
<p>Presto implements the <code class="docutils literal"><span class="pre">approx_percentile</span></code>  function with the quantile digest
data structure.  The underlying data structure, <a class="reference internal" href="../language/types.html#qdigest-type"><span class="std std-ref">qdigest</span></a>,
is exposed as a data type in Presto, and can be created, queried and stored
separately from <code class="docutils literal"><span class="pre">approx_percentile</span></code>.</p>
<div class="section" id="data-structures">
<h2>Data Structures</h2>
<p>A quantile digest is a data sketch which stores approximate percentile
information.  The presto type for this data structure is called <code class="docutils literal"><span class="pre">qdigest</span></code>,
and it takes a parameter which must be one of <code class="docutils literal"><span class="pre">bigint</span></code>, <code class="docutils literal"><span class="pre">double</span></code> or
<code class="docutils literal"><span class="pre">real</span></code> which represent the set of numbers that may be ingested by the
<code class="docutils literal"><span class="pre">qdigest</span></code>.  They may be merged without losing precision, and for storage
and retrieval they may be cast to/from <code class="docutils literal"><span class="pre">VARBINARY</span></code>.</p>
</div>
<div class="section" id="functions">
<h2>Functions</h2>
<dl class="function">
<dt>
<code class="descname">merge</code><span class="sig-paren">(</span><em>qdigest</em><span class="sig-paren">)</span> &#x2192; qdigest</dt>
<dd><p>Merges all input <code class="docutils literal"><span class="pre">qdigest</span></code>s into a single <code class="docutils literal"><span class="pre">qdigest</span></code>.</p>
</dd></dl>

<dl class="function">
<dt id="value_at_quantile">
<code class="descname">value_at_quantile</code><span class="sig-paren">(</span><em>qdigest(T)</em>, <em>quantile</em><span class="sig-paren">)</span> &#x2192; T</dt>
<dd><p>Returns the approximate percentile values from the quantile digest given
the number <code class="docutils literal"><span class="pre">quantile</span></code> between 0 and 1.</p>
</dd></dl>

<dl class="function">
<dt id="values_at_quantiles">
<code class="descname">values_at_quantiles</code><span class="sig-paren">(</span><em>qdigest(T)</em>, <em>quantiles</em><span class="sig-paren">)</span> &#x2192; T</dt>
<dd><p>Returns the approximate percentile values as an array given the input
quantile digest and array of values between 0 and 1 which
represent the quantiles to return.</p>
</dd></dl>

<dl class="function">
<dt id="qdigest_agg">
<code class="descname">qdigest_agg</code><span class="sig-paren">(</span><em>x</em><span class="sig-paren">)</span> &#x2192; qdigest&lt;[same as x]&gt;</dt>
<dd><p>Returns the <code class="docutils literal"><span class="pre">qdigest</span></code> which is composed of  all input values of <code class="docutils literal"><span class="pre">x</span></code>.</p>
</dd></dl>

<dl class="function">
<dt>
<code class="descname">qdigest_agg</code><span class="sig-paren">(</span><em>x</em>, <em>w</em><span class="sig-paren">)</span> &#x2192; qdigest&lt;[same as x]&gt;</dt>
<dd><p>Returns the <code class="docutils literal"><span class="pre">qdigest</span></code> which is composed of  all input values of <code class="docutils literal"><span class="pre">x</span></code> using
the per-item weight <code class="docutils literal"><span class="pre">w</span></code>.</p>
</dd></dl>

<dl class="function">
<dt>
<code class="descname">qdigest_agg</code><span class="sig-paren">(</span><em>x</em>, <em>w</em>, <em>accuracy</em><span class="sig-paren">)</span> &#x2192; qdigest&lt;[same as x]&gt;</dt>
<dd><p>Returns the <code class="docutils literal"><span class="pre">qdigest</span></code> which is composed of  all input values of <code class="docutils literal"><span class="pre">x</span></code> using
the per-item weight <code class="docutils literal"><span class="pre">w</span></code> and maximum error of <code class="docutils literal"><span class="pre">accuracy</span></code>. <code class="docutils literal"><span class="pre">accuracy</span></code>
must be a value greater than zero and less than one, and it must be constant
for all input rows.</p>
</dd></dl>

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