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<p>This module provides Apache Druid aggregators for distinct counting based on HLL sketch from <a href="https://datasketches.apache.org/">Apache DataSketches</a> library. At ingestion time, this aggregator creates the HLL sketch objects to be stored in Druid segments. At query time, sketches are read and merged together. In the end, by default, you receive the estimate of the number of distinct values presented to the sketch. Also, you can use post aggregator to produce a union of sketch columns in the same row.
You can use the HLL sketch aggregator on columns of any identifiers. It will return estimated cardinality of the column.</p>
<p>To use this aggregator, make sure you <a href="/docs/latest/development/extensions.html#loading-extensions">include</a> the extension in your config file:</p>
<pre><code class="hljs"><span class="hljs-attr">druid.extensions.loadList</span>=[<span class="hljs-string">"druid-datasketches"</span>]
</code></pre>
<h3><a class="anchor" aria-hidden="true" id="aggregators"></a><a href="#aggregators" aria-hidden="true" class="hash-link"><svg class="hash-link-icon" aria-hidden="true" height="16" version="1.1" viewBox="0 0 16 16" width="16"><path fill-rule="evenodd" d="M4 9h1v1H4c-1.5 0-3-1.69-3-3.5S2.55 3 4 3h4c1.45 0 3 1.69 3 3.5 0 1.41-.91 2.72-2 3.25V8.59c.58-.45 1-1.27 1-2.09C10 5.22 8.98 4 8 4H4c-.98 0-2 1.22-2 2.5S3 9 4 9zm9-3h-1v1h1c1 0 2 1.22 2 2.5S13.98 12 13 12H9c-.98 0-2-1.22-2-2.5 0-.83.42-1.64 1-2.09V6.25c-1.09.53-2 1.84-2 3.25C6 11.31 7.55 13 9 13h4c1.45 0 3-1.69 3-3.5S14.5 6 13 6z"></path></svg></a>Aggregators</h3>
<table>
<thead>
<tr><th>property</th><th>description</th><th>required?</th></tr>
</thead>
<tbody>
<tr><td><code>type</code></td><td>This String should be <a href="#hllsketchbuild-aggregator"><code>HLLSketchBuild</code></a> or <a href="#hllsketchmerge-aggregator"><code>HLLSketchMerge</code></a></td><td>yes</td></tr>
<tr><td><code>name</code></td><td>A String for the output (result) name of the calculation.</td><td>yes</td></tr>
<tr><td><code>fieldName</code></td><td>A String for the name of the input field.</td><td>yes</td></tr>
<tr><td><code>lgK</code></td><td>log2 of K that is the number of buckets in the sketch, parameter that controls the size and the accuracy. Must be between 4 and 21 inclusively.</td><td>no, defaults to <code>12</code></td></tr>
<tr><td><code>tgtHllType</code></td><td>The type of the target HLL sketch. Must be <code>HLL_4</code>, <code>HLL_6</code> or <code>HLL_8</code></td><td>no, defaults to <code>HLL_4</code></td></tr>
<tr><td><code>round</code></td><td>Round off values to whole numbers. Only affects query-time behavior and is ignored at ingestion-time.</td><td>no, defaults to <code>false</code></td></tr>
</tbody>
</table>
<blockquote>
<p>The default <code>lgK</code> value has proven to be sufficient for most use cases; expect only very negligible improvements in accuracy with <code>lgK</code> values over <code>16</code> in normal circumstances.</p>
</blockquote>
<h4><a class="anchor" aria-hidden="true" id="hllsketchbuild-aggregator"></a><a href="#hllsketchbuild-aggregator" aria-hidden="true" class="hash-link"><svg class="hash-link-icon" aria-hidden="true" height="16" version="1.1" viewBox="0 0 16 16" width="16"><path fill-rule="evenodd" d="M4 9h1v1H4c-1.5 0-3-1.69-3-3.5S2.55 3 4 3h4c1.45 0 3 1.69 3 3.5 0 1.41-.91 2.72-2 3.25V8.59c.58-.45 1-1.27 1-2.09C10 5.22 8.98 4 8 4H4c-.98 0-2 1.22-2 2.5S3 9 4 9zm9-3h-1v1h1c1 0 2 1.22 2 2.5S13.98 12 13 12H9c-.98 0-2-1.22-2-2.5 0-.83.42-1.64 1-2.09V6.25c-1.09.53-2 1.84-2 3.25C6 11.31 7.55 13 9 13h4c1.45 0 3-1.69 3-3.5S14.5 6 13 6z"></path></svg></a>HLLSketchBuild Aggregator</h4>
<pre><code class="hljs">{
  <span class="hljs-attr">"type"</span> : <span class="hljs-string">"HLLSketchBuild"</span>,
  <span class="hljs-attr">"name"</span> : &lt;output name&gt;,
  <span class="hljs-attr">"fieldName"</span> : &lt;metric name&gt;,
  <span class="hljs-attr">"lgK"</span> : &lt;size and accuracy parameter&gt;,
  <span class="hljs-attr">"tgtHllType"</span> : &lt;target HLL type&gt;,
  <span class="hljs-attr">"round"</span>: &lt;<span class="hljs-literal">false</span> | <span class="hljs-literal">true</span>&gt;
 }
</code></pre>
<blockquote>
<p>It is very common to use <code>HLLSketchBuild</code> in combination with <a href="/docs/latest/ingestion/rollup.html">rollup</a> to create a <a href="../../ingestion/ingestion-spec.html#metricsspec">metric</a> on high-cardinality columns.  In this example, a metric called <code>userid_hll</code> is included in the <code>metricsSpec</code>.  This will perform a HLL sketch on the <code>userid</code> field at ingestion time, allowing for highly-performant approximate <code>COUNT DISTINCT</code> query operations and improving roll-up ratios when <code>userid</code> is then left out of the <code>dimensionsSpec</code>.</p>
<pre><code class="hljs">:
<span class="hljs-string">"metricsSpec"</span>: [
 {
   <span class="hljs-string">"type"</span> : <span class="hljs-string">"HLLSketchBuild"</span>,
   <span class="hljs-string">"name"</span> : <span class="hljs-string">"userid_hll"</span>,
   <span class="hljs-string">"fieldName"</span> : <span class="hljs-string">"userid"</span>,
   <span class="hljs-string">"lgK"</span> : <span class="hljs-number">12</span>,
   <span class="hljs-string">"tgtHllType"</span> : <span class="hljs-string">"HLL_4"</span>
 }
]
:
</code></pre>
</blockquote>
<h4><a class="anchor" aria-hidden="true" id="hllsketchmerge-aggregator"></a><a href="#hllsketchmerge-aggregator" aria-hidden="true" class="hash-link"><svg class="hash-link-icon" aria-hidden="true" height="16" version="1.1" viewBox="0 0 16 16" width="16"><path fill-rule="evenodd" d="M4 9h1v1H4c-1.5 0-3-1.69-3-3.5S2.55 3 4 3h4c1.45 0 3 1.69 3 3.5 0 1.41-.91 2.72-2 3.25V8.59c.58-.45 1-1.27 1-2.09C10 5.22 8.98 4 8 4H4c-.98 0-2 1.22-2 2.5S3 9 4 9zm9-3h-1v1h1c1 0 2 1.22 2 2.5S13.98 12 13 12H9c-.98 0-2-1.22-2-2.5 0-.83.42-1.64 1-2.09V6.25c-1.09.53-2 1.84-2 3.25C6 11.31 7.55 13 9 13h4c1.45 0 3-1.69 3-3.5S14.5 6 13 6z"></path></svg></a>HLLSketchMerge Aggregator</h4>
<pre><code class="hljs">{
  <span class="hljs-attr">"type"</span> : <span class="hljs-string">"HLLSketchMerge"</span>,
  <span class="hljs-attr">"name"</span> : &lt;output name&gt;,
  <span class="hljs-attr">"fieldName"</span> : &lt;metric name&gt;,
  <span class="hljs-attr">"lgK"</span> : &lt;size and accuracy parameter&gt;,
  <span class="hljs-attr">"tgtHllType"</span> : &lt;target HLL type&gt;,
  <span class="hljs-attr">"round"</span>: &lt;<span class="hljs-literal">false</span> | <span class="hljs-literal">true</span>&gt;
 }
</code></pre>
<h3><a class="anchor" aria-hidden="true" id="post-aggregators"></a><a href="#post-aggregators" aria-hidden="true" class="hash-link"><svg class="hash-link-icon" aria-hidden="true" height="16" version="1.1" viewBox="0 0 16 16" width="16"><path fill-rule="evenodd" d="M4 9h1v1H4c-1.5 0-3-1.69-3-3.5S2.55 3 4 3h4c1.45 0 3 1.69 3 3.5 0 1.41-.91 2.72-2 3.25V8.59c.58-.45 1-1.27 1-2.09C10 5.22 8.98 4 8 4H4c-.98 0-2 1.22-2 2.5S3 9 4 9zm9-3h-1v1h1c1 0 2 1.22 2 2.5S13.98 12 13 12H9c-.98 0-2-1.22-2-2.5 0-.83.42-1.64 1-2.09V6.25c-1.09.53-2 1.84-2 3.25C6 11.31 7.55 13 9 13h4c1.45 0 3-1.69 3-3.5S14.5 6 13 6z"></path></svg></a>Post Aggregators</h3>
<h4><a class="anchor" aria-hidden="true" id="estimate"></a><a href="#estimate" aria-hidden="true" class="hash-link"><svg class="hash-link-icon" aria-hidden="true" height="16" version="1.1" viewBox="0 0 16 16" width="16"><path fill-rule="evenodd" d="M4 9h1v1H4c-1.5 0-3-1.69-3-3.5S2.55 3 4 3h4c1.45 0 3 1.69 3 3.5 0 1.41-.91 2.72-2 3.25V8.59c.58-.45 1-1.27 1-2.09C10 5.22 8.98 4 8 4H4c-.98 0-2 1.22-2 2.5S3 9 4 9zm9-3h-1v1h1c1 0 2 1.22 2 2.5S13.98 12 13 12H9c-.98 0-2-1.22-2-2.5 0-.83.42-1.64 1-2.09V6.25c-1.09.53-2 1.84-2 3.25C6 11.31 7.55 13 9 13h4c1.45 0 3-1.69 3-3.5S14.5 6 13 6z"></path></svg></a>Estimate</h4>
<p>Returns the distinct count estimate as a double.</p>
<pre><code class="hljs">{
  <span class="hljs-string">"type"</span>  : <span class="hljs-string">"HLLSketchEstimate"</span>,
  <span class="hljs-string">"name"</span>: &lt;output name&gt;,
  <span class="hljs-string">"field"</span>  : &lt;post aggregator that returns an HLL Sketch&gt;,
  <span class="hljs-string">"round"</span> : &lt;<span class="hljs-keyword">if</span> <span class="hljs-literal">true</span>, round the estimate.<span class="hljs-built_in"> Default </span>is <span class="hljs-literal">false</span>&gt;
}
</code></pre>
<h4><a class="anchor" aria-hidden="true" id="estimate-with-bounds"></a><a href="#estimate-with-bounds" aria-hidden="true" class="hash-link"><svg class="hash-link-icon" aria-hidden="true" height="16" version="1.1" viewBox="0 0 16 16" width="16"><path fill-rule="evenodd" d="M4 9h1v1H4c-1.5 0-3-1.69-3-3.5S2.55 3 4 3h4c1.45 0 3 1.69 3 3.5 0 1.41-.91 2.72-2 3.25V8.59c.58-.45 1-1.27 1-2.09C10 5.22 8.98 4 8 4H4c-.98 0-2 1.22-2 2.5S3 9 4 9zm9-3h-1v1h1c1 0 2 1.22 2 2.5S13.98 12 13 12H9c-.98 0-2-1.22-2-2.5 0-.83.42-1.64 1-2.09V6.25c-1.09.53-2 1.84-2 3.25C6 11.31 7.55 13 9 13h4c1.45 0 3-1.69 3-3.5S14.5 6 13 6z"></path></svg></a>Estimate with bounds</h4>
<p>Returns a distinct count estimate and error bounds from an HLL sketch.
The result will be an array containing three double values: estimate, lower bound and upper bound.
The bounds are provided at a given number of standard deviations (optional, defaults to 1).
This must be an integer value of 1, 2 or 3 corresponding to approximately 68.3%, 95.4% and 99.7% confidence intervals.</p>
<pre><code class="hljs">{
  <span class="hljs-string">"type"</span>  : <span class="hljs-string">"HLLSketchEstimateWithBounds"</span>,
  <span class="hljs-string">"name"</span>: &lt;output <span class="hljs-built_in">name</span>&gt;,
  <span class="hljs-string">"field"</span>  : &lt;post aggregator <span class="hljs-keyword">that</span> returns an HLL Sketch&gt;,
  <span class="hljs-string">"numStdDev"</span> : &lt;<span class="hljs-built_in">number</span> <span class="hljs-keyword">of</span> standard deviations: <span class="hljs-number">1</span> (default), <span class="hljs-number">2</span> <span class="hljs-keyword">or</span> <span class="hljs-number">3</span>&gt;
}
</code></pre>
<h4><a class="anchor" aria-hidden="true" id="union"></a><a href="#union" aria-hidden="true" class="hash-link"><svg class="hash-link-icon" aria-hidden="true" height="16" version="1.1" viewBox="0 0 16 16" width="16"><path fill-rule="evenodd" d="M4 9h1v1H4c-1.5 0-3-1.69-3-3.5S2.55 3 4 3h4c1.45 0 3 1.69 3 3.5 0 1.41-.91 2.72-2 3.25V8.59c.58-.45 1-1.27 1-2.09C10 5.22 8.98 4 8 4H4c-.98 0-2 1.22-2 2.5S3 9 4 9zm9-3h-1v1h1c1 0 2 1.22 2 2.5S13.98 12 13 12H9c-.98 0-2-1.22-2-2.5 0-.83.42-1.64 1-2.09V6.25c-1.09.53-2 1.84-2 3.25C6 11.31 7.55 13 9 13h4c1.45 0 3-1.69 3-3.5S14.5 6 13 6z"></path></svg></a>Union</h4>
<pre><code class="hljs">{
  <span class="hljs-attr">"type"</span>  : <span class="hljs-string">"HLLSketchUnion"</span>,
  <span class="hljs-attr">"name"</span>: &lt;output name&gt;,
  <span class="hljs-attr">"fields"</span>  : &lt;array of post aggregators that return HLL sketches&gt;,
  <span class="hljs-attr">"lgK"</span>: &lt;log2 of K for the target sketch&gt;,
  <span class="hljs-attr">"tgtHllType"</span> : &lt;target HLL type&gt;
}
</code></pre>
<h4><a class="anchor" aria-hidden="true" id="sketch-to-string"></a><a href="#sketch-to-string" aria-hidden="true" class="hash-link"><svg class="hash-link-icon" aria-hidden="true" height="16" version="1.1" viewBox="0 0 16 16" width="16"><path fill-rule="evenodd" d="M4 9h1v1H4c-1.5 0-3-1.69-3-3.5S2.55 3 4 3h4c1.45 0 3 1.69 3 3.5 0 1.41-.91 2.72-2 3.25V8.59c.58-.45 1-1.27 1-2.09C10 5.22 8.98 4 8 4H4c-.98 0-2 1.22-2 2.5S3 9 4 9zm9-3h-1v1h1c1 0 2 1.22 2 2.5S13.98 12 13 12H9c-.98 0-2-1.22-2-2.5 0-.83.42-1.64 1-2.09V6.25c-1.09.53-2 1.84-2 3.25C6 11.31 7.55 13 9 13h4c1.45 0 3-1.69 3-3.5S14.5 6 13 6z"></path></svg></a>Sketch to string</h4>
<p>Human-readable sketch summary for debugging.</p>
<pre><code class="hljs">{
  <span class="hljs-attr">"type"</span>  : <span class="hljs-string">"HLLSketchToString"</span>,
  <span class="hljs-attr">"name"</span>: &lt;output name&gt;,
  <span class="hljs-attr">"field"</span>  : &lt;post aggregator that returns an HLL Sketch&gt;
}
</code></pre>
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