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<blockquote>
<p>Apache Druid supports two query languages: <a href="/docs/latest/querying/sql.html">Druid SQL</a> and <a href="/docs/latest/querying/querying.html">native queries</a>.
This document describes the native
language. For information about functions available in SQL, refer to the
<a href="/docs/latest/querying/sql-aggregations.html">SQL documentation</a>.</p>
</blockquote>
<p>Post-aggregations are specifications of processing that should happen on aggregated values as they come out of Apache Druid. If you include a post aggregation as part of a query, make sure to include all aggregators the post-aggregator requires.</p>
<p>There are several post-aggregators available.</p>
<h3><a class="anchor" aria-hidden="true" id="arithmetic-post-aggregator"></a><a href="#arithmetic-post-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>Arithmetic post-aggregator</h3>
<p>The arithmetic post-aggregator applies the provided function to the given
fields from left to right. The fields can be aggregators or other post aggregators.</p>
<p>Supported functions are <code>+</code>, <code>-</code>, <code>*</code>, <code>/</code>, and <code>quotient</code>.</p>
<p><strong>Note</strong>:</p>
<ul>
<li><code>/</code> division always returns <code>0</code> if dividing by<code>0</code>, regardless of the numerator.</li>
<li><code>quotient</code> division behaves like regular floating point division</li>
<li>Arithmetic post-aggregators always use floating point arithmetic.</li>
</ul>
<p>Arithmetic post-aggregators may also specify an <code>ordering</code>, which defines the order
of resulting values when sorting results (this can be useful for topN queries for instance):</p>
<ul>
<li>If no ordering (or <code>null</code>) is specified, the default floating point ordering is used.</li>
<li><code>numericFirst</code> ordering always returns finite values first, followed by <code>NaN</code>, and infinite values last.</li>
</ul>
<p>The grammar for an arithmetic post aggregation is:</p>
<pre><code class="hljs css language-json">postAggregation : {
  "type"  : "arithmetic",
  "name"  : &lt;output_name&gt;,
  "fn"    : &lt;arithmetic_function&gt;,
  "fields": [&lt;post_aggregator&gt;, &lt;post_aggregator&gt;, ...],
  "ordering" : &lt;null (default), or "numericFirst"&gt;
}
</code></pre>
<h3><a class="anchor" aria-hidden="true" id="field-accessor-post-aggregators"></a><a href="#field-accessor-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>Field accessor post-aggregators</h3>
<p>These post-aggregators return the value produced by the specified <a href="/docs/latest/querying/aggregations.html">aggregator</a>.</p>
<p><code>fieldName</code> refers to the output name of the aggregator given in the <a href="/docs/latest/querying/aggregations.html">aggregations</a> portion of the query.
For complex aggregators, like &quot;cardinality&quot; and &quot;hyperUnique&quot;, the <code>type</code> of the post-aggregator determines what
the post-aggregator will return. Use type &quot;fieldAccess&quot; to return the raw aggregation object, or use type
&quot;finalizingFieldAccess&quot; to return a finalized value, such as an estimated cardinality.</p>
<pre><code class="hljs css language-json">{ <span class="hljs-attr">"type"</span> : <span class="hljs-string">"fieldAccess"</span>, <span class="hljs-attr">"name"</span>: &lt;output_name&gt;, <span class="hljs-attr">"fieldName"</span> : &lt;aggregator_name&gt; }
</code></pre>
<p>or</p>
<pre><code class="hljs css language-json">{ <span class="hljs-attr">"type"</span> : <span class="hljs-string">"finalizingFieldAccess"</span>, <span class="hljs-attr">"name"</span>: &lt;output_name&gt;, <span class="hljs-attr">"fieldName"</span> : &lt;aggregator_name&gt; }
</code></pre>
<h3><a class="anchor" aria-hidden="true" id="constant-post-aggregator"></a><a href="#constant-post-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>Constant post-aggregator</h3>
<p>The constant post-aggregator always returns the specified value.</p>
<pre><code class="hljs css language-json">{ <span class="hljs-attr">"type"</span>  : <span class="hljs-string">"constant"</span>, <span class="hljs-attr">"name"</span>  : &lt;output_name&gt;, <span class="hljs-attr">"value"</span> : &lt;numerical_value&gt; }
</code></pre>
<h3><a class="anchor" aria-hidden="true" id="expression-post-aggregator"></a><a href="#expression-post-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>Expression post-aggregator</h3>
<p>The expression post-aggregator is defined using a Druid <a href="/docs/latest/misc/math-expr.html">expression</a>.</p>
<pre><code class="hljs css language-json">{
  "type": "expression",
  "name": &lt;output_name&gt;,
  "expression": &lt;post-aggregation expression&gt;,
  "ordering" : &lt;null (default), or "numericFirst"&gt;
}
</code></pre>
<h3><a class="anchor" aria-hidden="true" id="greatest--least-post-aggregators"></a><a href="#greatest--least-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>Greatest / Least post-aggregators</h3>
<p><code>doubleGreatest</code> and <code>longGreatest</code> computes the maximum of all fields and Double.NEGATIVE_INFINITY.
<code>doubleLeast</code> and <code>longLeast</code> computes the minimum of all fields and Double.POSITIVE_INFINITY.</p>
<p>The difference between the <code>doubleMax</code> aggregator and the <code>doubleGreatest</code> post-aggregator is that <code>doubleMax</code> returns the highest value of
all rows for one specific column while <code>doubleGreatest</code> returns the highest value of multiple columns in one row. These are similar to the
SQL <code>MAX</code> and <code>GREATEST</code> functions.</p>
<p>Example:</p>
<pre><code class="hljs css language-json">{
  <span class="hljs-attr">"type"</span>  : <span class="hljs-string">"doubleGreatest"</span>,
  <span class="hljs-attr">"name"</span>  : &lt;output_name&gt;,
  <span class="hljs-attr">"fields"</span>: [&lt;post_aggregator&gt;, &lt;post_aggregator&gt;, ...]
}
</code></pre>
<h3><a class="anchor" aria-hidden="true" id="javascript-post-aggregator"></a><a href="#javascript-post-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>JavaScript post-aggregator</h3>
<p>Applies the provided JavaScript function to the given fields. Fields are passed as arguments to the JavaScript function in the given order.</p>
<pre><code class="hljs css language-json">postAggregation : {
  "type": "javascript",
  "name": &lt;output_name&gt;,
  "fieldNames" : [&lt;aggregator_name&gt;, &lt;aggregator_name&gt;, ...],
  "function": &lt;javascript function&gt;
}
</code></pre>
<p>Example JavaScript aggregator:</p>
<pre><code class="hljs css language-json">{
  <span class="hljs-attr">"type"</span>: <span class="hljs-string">"javascript"</span>,
  <span class="hljs-attr">"name"</span>: <span class="hljs-string">"absPercent"</span>,
  <span class="hljs-attr">"fieldNames"</span>: [<span class="hljs-string">"delta"</span>, <span class="hljs-string">"total"</span>],
  <span class="hljs-attr">"function"</span>: <span class="hljs-string">"function(delta, total) { return 100 * Math.abs(delta) / total; }"</span>
}
</code></pre>
<blockquote>
<p>JavaScript-based functionality is disabled by default. Please refer to the Druid <a href="/docs/latest/development/javascript.html">JavaScript programming guide</a> for guidelines about using Druid's JavaScript functionality, including instructions on how to enable it.</p>
</blockquote>
<h3><a class="anchor" aria-hidden="true" id="hyperunique-cardinality-post-aggregator"></a><a href="#hyperunique-cardinality-post-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>HyperUnique Cardinality post-aggregator</h3>
<p>The hyperUniqueCardinality post aggregator is used to wrap a hyperUnique object such that it can be used in post aggregations.</p>
<pre><code class="hljs css language-json">{
  <span class="hljs-attr">"type"</span>  : <span class="hljs-string">"hyperUniqueCardinality"</span>,
  <span class="hljs-attr">"name"</span>: &lt;output name&gt;,
  <span class="hljs-attr">"fieldName"</span>  : &lt;the name field value of the hyperUnique aggregator&gt;
}
</code></pre>
<p>It can be used in a sample calculation as so:</p>
<pre><code class="hljs css language-json">  "aggregations" : [{
    {"type" : "count", "name" : "rows"},
    {"type" : "hyperUnique", "name" : "unique_users", "fieldName" : "uniques"}
  }],
  "postAggregations" : [{
    "type"   : "arithmetic",
    "name"   : "average_users_per_row",
    "fn"     : "/",
    "fields" : [
      { "type" : "hyperUniqueCardinality", "fieldName" : "unique_users" },
      { "type" : "fieldAccess", "name" : "rows", "fieldName" : "rows" }
    ]
  }]
</code></pre>
<p>This post-aggregator will inherit the rounding behavior of the aggregator it references. Note that this inheritance
is only effective if you directly reference an aggregator. Going through another post-aggregator, for example, will
cause the user-specified rounding behavior to get lost and default to &quot;no rounding&quot;.</p>
<h2><a class="anchor" aria-hidden="true" id="example-usage"></a><a href="#example-usage" 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>Example Usage</h2>
<p>In this example, let’s calculate a simple percentage using post aggregators. Let’s imagine our data set has a metric called &quot;total&quot;.</p>
<p>The format of the query JSON is as follows:</p>
<pre><code class="hljs css language-json">{
  ...
  "aggregations" : [
    { "type" : "count", "name" : "rows" },
    { "type" : "doubleSum", "name" : "tot", "fieldName" : "total" }
  ],
  "postAggregations" : [{
    "type"   : "arithmetic",
    "name"   : "average",
    "fn"     : "/",
    "fields" : [
           { "type" : "fieldAccess", "name" : "tot", "fieldName" : "tot" },
           { "type" : "fieldAccess", "name" : "rows", "fieldName" : "rows" }
         ]
  }]
  ...
}
</code></pre>
<pre><code class="hljs css language-json">{
  ...
  "aggregations" : [
    { "type" : "doubleSum", "name" : "tot", "fieldName" : "total" },
    { "type" : "doubleSum", "name" : "part", "fieldName" : "part" }
  ],
  "postAggregations" : [{
    "type"   : "arithmetic",
    "name"   : "part_percentage",
    "fn"     : "*",
    "fields" : [
       { "type"   : "arithmetic",
         "name"   : "ratio",
         "fn"     : "/",
         "fields" : [
           { "type" : "fieldAccess", "name" : "part", "fieldName" : "part" },
           { "type" : "fieldAccess", "name" : "tot", "fieldName" : "tot" }
         ]
       },
       { "type" : "constant", "name": "const", "value" : 100 }
    ]
  }]
  ...
}
</code></pre>
<p>The same could be computed using an expression post-aggregator:</p>
<pre><code class="hljs css language-json">{
  ...
  "aggregations" : [
    { "type" : "doubleSum", "name" : "tot", "fieldName" : "total" },
    { "type" : "doubleSum", "name" : "part", "fieldName" : "part" }
  ],
  "postAggregations" : [{
    "type"       : "expression",
    "name"       : "part_percentage",
    "expression" : "100 * (part / tot)"
  }]
  ...
}
</code></pre>
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