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<h1 id="functions-qdigest--page-root">Quantile Digest Functions<a class="headerlink" href="#functions-qdigest--page-root" title="Permalink to this headline">#</a></h1>

<h2 id="data-structures">Data Structures<a class="headerlink" href="#data-structures" title="Permalink to this headline">#</a></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 notranslate"><span class="pre">qdigest</span></code>,
and it takes a parameter which must be one of <code class="docutils literal notranslate"><span class="pre">bigint</span></code>, <code class="docutils literal notranslate"><span class="pre">double</span></code> or
<code class="docutils literal notranslate"><span class="pre">real</span></code> which represent the set of numbers that may be ingested by the
<code class="docutils literal notranslate"><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 notranslate"><span class="pre">VARBINARY</span></code>.</p>


<h2 id="functions">Functions<a class="headerlink" href="#functions" title="Permalink to this headline">#</a></h2>
<dl class="function">
<dt>
<code class="sig-name descname">merge</code><span class="sig-paren">(</span><em class="sig-param">qdigest</em><span class="sig-paren">)</span> → qdigest</dt>
<dd><p>Merges all input <code class="docutils literal notranslate"><span class="pre">qdigest</span></code>s into a single <code class="docutils literal notranslate"><span class="pre">qdigest</span></code>.</p>
</dd></dl>
<dl class="function">
<dt id="value_at_quantile">
<code class="sig-name descname">value_at_quantile</code><span class="sig-paren">(</span><em class="sig-param">qdigest(T)</em>, <em class="sig-param">quantile</em><span class="sig-paren">)</span> → T<a class="headerlink" href="#value_at_quantile" title="Permalink to this definition">#</a></dt>
<dd><p>Returns the approximate percentile value from the quantile digest given
the number <code class="docutils literal notranslate"><span class="pre">quantile</span></code> between 0 and 1.</p>
</dd></dl>
<dl class="function">
<dt id="values_at_quantiles">
<code class="sig-name descname">values_at_quantiles</code><span class="sig-paren">(</span><em class="sig-param">qdigest(T)</em>, <em class="sig-param">quantiles) -&gt; array(T</em><span class="sig-paren">)</span><a class="headerlink" href="#values_at_quantiles" title="Permalink to this definition">#</a></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="sig-name descname">qdigest_agg</code><span class="sig-paren">(</span><em class="sig-param">x</em><span class="sig-paren">)</span> → qdigest&lt;[same as x]&gt;<a class="headerlink" href="#qdigest_agg" title="Permalink to this definition">#</a></dt>
<dd><p>Returns the <code class="docutils literal notranslate"><span class="pre">qdigest</span></code> which is composed of  all input values of <code class="docutils literal notranslate"><span class="pre">x</span></code>.</p>
</dd></dl>
<dl class="function">
<dt>
<code class="sig-name descname">qdigest_agg</code><span class="sig-paren">(</span><em class="sig-param">x</em>, <em class="sig-param">w</em><span class="sig-paren">)</span> → qdigest&lt;[same as x]&gt;</dt>
<dd><p>Returns the <code class="docutils literal notranslate"><span class="pre">qdigest</span></code> which is composed of  all input values of <code class="docutils literal notranslate"><span class="pre">x</span></code> using
the per-item weight <code class="docutils literal notranslate"><span class="pre">w</span></code>.</p>
</dd></dl>
<dl class="function">
<dt>
<code class="sig-name descname">qdigest_agg</code><span class="sig-paren">(</span><em class="sig-param">x</em>, <em class="sig-param">w</em>, <em class="sig-param">accuracy</em><span class="sig-paren">)</span> → qdigest&lt;[same as x]&gt;</dt>
<dd><p>Returns the <code class="docutils literal notranslate"><span class="pre">qdigest</span></code> which is composed of  all input values of <code class="docutils literal notranslate"><span class="pre">x</span></code> using
the per-item weight <code class="docutils literal notranslate"><span class="pre">w</span></code> and maximum error of <code class="docutils literal notranslate"><span class="pre">accuracy</span></code>. <code class="docutils literal notranslate"><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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