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|  |  | 
|  | <h1 id="post-aggregations">Post-Aggregations</h1> | 
|  |  | 
|  | <p>Post-aggregations are specifications of processing that should happen on aggregated values as they come out of Apache Druid (incubating). 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 id="arithmetic-post-aggregator">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> | 
|  | </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> | 
|  | <div class="highlight"><pre><code class="language-json" data-lang="json"><span></span><span class="err">postAggregation</span> <span class="err">:</span> <span class="p">{</span> | 
|  | <span class="nt">"type"</span>  <span class="p">:</span> <span class="s2">"arithmetic"</span><span class="p">,</span> | 
|  | <span class="nt">"name"</span>  <span class="p">:</span> <span class="err"><output_name></span><span class="p">,</span> | 
|  | <span class="nt">"fn"</span>    <span class="p">:</span> <span class="err"><arithmetic_function></span><span class="p">,</span> | 
|  | <span class="nt">"fields"</span><span class="p">:</span> <span class="p">[</span><span class="err"><post_aggregator></span><span class="p">,</span> <span class="err"><post_aggregator></span><span class="p">,</span> <span class="err">...</span><span class="p">],</span> | 
|  | <span class="nt">"ordering"</span> <span class="p">:</span> <span class="err"><</span><span class="kc">null</span> <span class="err">(default)</span><span class="p">,</span> <span class="err">or</span> <span class="nt">"numericFirst"</span><span class="err">></span> | 
|  | <span class="p">}</span> | 
|  | </code></pre></div> | 
|  | <h3 id="field-accessor-post-aggregators">Field accessor post-aggregators</h3> | 
|  |  | 
|  | <p>These post-aggregators return the value produced by the specified <a href="../querying/aggregations.html">aggregator</a>.</p> | 
|  |  | 
|  | <p><code>fieldName</code> refers to the output name of the aggregator given in the <a href="../querying/aggregations.html">aggregations</a> portion of the query. | 
|  | For complex aggregators, like "cardinality" and "hyperUnique", the <code>type</code> of the post-aggregator determines what | 
|  | the post-aggregator will return. Use type "fieldAccess" to return the raw aggregation object, or use type | 
|  | "finalizingFieldAccess" to return a finalized value, such as an estimated cardinality.</p> | 
|  | <div class="highlight"><pre><code class="language-json" data-lang="json"><span></span><span class="p">{</span> <span class="nt">"type"</span> <span class="p">:</span> <span class="s2">"fieldAccess"</span><span class="p">,</span> <span class="nt">"name"</span><span class="p">:</span> <span class="err"><output_name></span><span class="p">,</span> <span class="nt">"fieldName"</span> <span class="p">:</span> <span class="err"><aggregator_name></span> <span class="p">}</span> | 
|  | </code></pre></div> | 
|  | <p>or</p> | 
|  | <div class="highlight"><pre><code class="language-json" data-lang="json"><span></span><span class="p">{</span> <span class="nt">"type"</span> <span class="p">:</span> <span class="s2">"finalizingFieldAccess"</span><span class="p">,</span> <span class="nt">"name"</span><span class="p">:</span> <span class="err"><output_name></span><span class="p">,</span> <span class="nt">"fieldName"</span> <span class="p">:</span> <span class="err"><aggregator_name></span> <span class="p">}</span> | 
|  | </code></pre></div> | 
|  | <h3 id="constant-post-aggregator">Constant post-aggregator</h3> | 
|  |  | 
|  | <p>The constant post-aggregator always returns the specified value.</p> | 
|  | <div class="highlight"><pre><code class="language-json" data-lang="json"><span></span><span class="p">{</span> <span class="nt">"type"</span>  <span class="p">:</span> <span class="s2">"constant"</span><span class="p">,</span> <span class="nt">"name"</span>  <span class="p">:</span> <span class="err"><output_name></span><span class="p">,</span> <span class="nt">"value"</span> <span class="p">:</span> <span class="err"><numerical_value></span> <span class="p">}</span> | 
|  | </code></pre></div> | 
|  | <h3 id="greatest-least-post-aggregators">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 <a href="https://dev.mysql.com/doc/refman/5.7/en/group-by-functions.html#function_max">MAX</a> and | 
|  | <a href="https://dev.mysql.com/doc/refman/5.7/en/comparison-operators.html#function_greatest">GREATEST</a> functions.</p> | 
|  |  | 
|  | <p>Example:</p> | 
|  | <div class="highlight"><pre><code class="language-json" data-lang="json"><span></span><span class="p">{</span> | 
|  | <span class="nt">"type"</span>  <span class="p">:</span> <span class="s2">"doubleGreatest"</span><span class="p">,</span> | 
|  | <span class="nt">"name"</span>  <span class="p">:</span> <span class="err"><output_name></span><span class="p">,</span> | 
|  | <span class="nt">"fields"</span><span class="p">:</span> <span class="p">[</span><span class="err"><post_aggregator></span><span class="p">,</span> <span class="err"><post_aggregator></span><span class="p">,</span> <span class="err">...</span><span class="p">]</span> | 
|  | <span class="p">}</span> | 
|  | </code></pre></div> | 
|  | <h3 id="javascript-post-aggregator">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> | 
|  | <div class="highlight"><pre><code class="language-json" data-lang="json"><span></span><span class="err">postAggregation</span> <span class="err">:</span> <span class="p">{</span> | 
|  | <span class="nt">"type"</span><span class="p">:</span> <span class="s2">"javascript"</span><span class="p">,</span> | 
|  | <span class="nt">"name"</span><span class="p">:</span> <span class="err"><output_name></span><span class="p">,</span> | 
|  | <span class="nt">"fieldNames"</span> <span class="p">:</span> <span class="p">[</span><span class="err"><aggregator_name></span><span class="p">,</span> <span class="err"><aggregator_name></span><span class="p">,</span> <span class="err">...</span><span class="p">],</span> | 
|  | <span class="nt">"function"</span><span class="p">:</span> <span class="err"><javascript</span> <span class="err">function></span> | 
|  | <span class="p">}</span> | 
|  | </code></pre></div> | 
|  | <p>Example JavaScript aggregator:</p> | 
|  | <div class="highlight"><pre><code class="language-json" data-lang="json"><span></span><span class="p">{</span> | 
|  | <span class="nt">"type"</span><span class="p">:</span> <span class="s2">"javascript"</span><span class="p">,</span> | 
|  | <span class="nt">"name"</span><span class="p">:</span> <span class="s2">"absPercent"</span><span class="p">,</span> | 
|  | <span class="nt">"fieldNames"</span><span class="p">:</span> <span class="p">[</span><span class="s2">"delta"</span><span class="p">,</span> <span class="s2">"total"</span><span class="p">],</span> | 
|  | <span class="nt">"function"</span><span class="p">:</span> <span class="s2">"function(delta, total) { return 100 * Math.abs(delta) / total; }"</span> | 
|  | <span class="p">}</span> | 
|  | </code></pre></div> | 
|  | <div class="note info"> | 
|  | JavaScript-based functionality is disabled by default. Please refer to the Druid <a href="../development/javascript.html">JavaScript programming guide</a> for guidelines about using Druid's JavaScript functionality, including instructions on how to enable it. | 
|  | </div> | 
|  |  | 
|  | <h3 id="hyperunique-cardinality-post-aggregator">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> | 
|  | <div class="highlight"><pre><code class="language-json" data-lang="json"><span></span><span class="p">{</span> | 
|  | <span class="nt">"type"</span>  <span class="p">:</span> <span class="s2">"hyperUniqueCardinality"</span><span class="p">,</span> | 
|  | <span class="nt">"name"</span><span class="p">:</span> <span class="err"><output</span> <span class="err">name></span><span class="p">,</span> | 
|  | <span class="nt">"fieldName"</span>  <span class="p">:</span> <span class="err"><the</span> <span class="err">name</span> <span class="err">field</span> <span class="err">value</span> <span class="err">of</span> <span class="err">the</span> <span class="err">hyperUnique</span> <span class="err">aggregator></span> | 
|  | <span class="p">}</span> | 
|  | </code></pre></div> | 
|  | <p>It can be used in a sample calculation as so:</p> | 
|  | <div class="highlight"><pre><code class="language-json" data-lang="json"><span></span>  <span class="s2">"aggregations"</span> <span class="err">:</span> <span class="p">[{</span> | 
|  | <span class="err">{</span><span class="nt">"type"</span> <span class="p">:</span> <span class="s2">"count"</span><span class="p">,</span> <span class="nt">"name"</span> <span class="p">:</span> <span class="s2">"rows"</span><span class="p">},</span> | 
|  | <span class="p">{</span><span class="nt">"type"</span> <span class="p">:</span> <span class="s2">"hyperUnique"</span><span class="p">,</span> <span class="nt">"name"</span> <span class="p">:</span> <span class="s2">"unique_users"</span><span class="p">,</span> <span class="nt">"fieldName"</span> <span class="p">:</span> <span class="s2">"uniques"</span><span class="p">}</span> | 
|  | <span class="err">}</span><span class="p">]</span><span class="err">,</span> | 
|  | <span class="s2">"postAggregations"</span> <span class="err">:</span> <span class="p">[{</span> | 
|  | <span class="nt">"type"</span>   <span class="p">:</span> <span class="s2">"arithmetic"</span><span class="p">,</span> | 
|  | <span class="nt">"name"</span>   <span class="p">:</span> <span class="s2">"average_users_per_row"</span><span class="p">,</span> | 
|  | <span class="nt">"fn"</span>     <span class="p">:</span> <span class="s2">"/"</span><span class="p">,</span> | 
|  | <span class="nt">"fields"</span> <span class="p">:</span> <span class="p">[</span> | 
|  | <span class="p">{</span> <span class="nt">"type"</span> <span class="p">:</span> <span class="s2">"hyperUniqueCardinality"</span><span class="p">,</span> <span class="nt">"fieldName"</span> <span class="p">:</span> <span class="s2">"unique_users"</span> <span class="p">},</span> | 
|  | <span class="p">{</span> <span class="nt">"type"</span> <span class="p">:</span> <span class="s2">"fieldAccess"</span><span class="p">,</span> <span class="nt">"name"</span> <span class="p">:</span> <span class="s2">"rows"</span><span class="p">,</span> <span class="nt">"fieldName"</span> <span class="p">:</span> <span class="s2">"rows"</span> <span class="p">}</span> | 
|  | <span class="p">]</span> | 
|  | <span class="p">}]</span> | 
|  | </code></pre></div> | 
|  | <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 "no rounding".</p> | 
|  |  | 
|  | <h2 id="example-usage">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 "total".</p> | 
|  |  | 
|  | <p>The format of the query JSON is as follows:</p> | 
|  | <div class="highlight"><pre><code class="language-json" data-lang="json"><span></span><span class="p">{</span> | 
|  | <span class="err">...</span> | 
|  | <span class="nt">"aggregations"</span> <span class="p">:</span> <span class="p">[</span> | 
|  | <span class="p">{</span> <span class="nt">"type"</span> <span class="p">:</span> <span class="s2">"count"</span><span class="p">,</span> <span class="nt">"name"</span> <span class="p">:</span> <span class="s2">"rows"</span> <span class="p">},</span> | 
|  | <span class="p">{</span> <span class="nt">"type"</span> <span class="p">:</span> <span class="s2">"doubleSum"</span><span class="p">,</span> <span class="nt">"name"</span> <span class="p">:</span> <span class="s2">"tot"</span><span class="p">,</span> <span class="nt">"fieldName"</span> <span class="p">:</span> <span class="s2">"total"</span> <span class="p">}</span> | 
|  | <span class="p">],</span> | 
|  | <span class="nt">"postAggregations"</span> <span class="p">:</span> <span class="p">[{</span> | 
|  | <span class="nt">"type"</span>   <span class="p">:</span> <span class="s2">"arithmetic"</span><span class="p">,</span> | 
|  | <span class="nt">"name"</span>   <span class="p">:</span> <span class="s2">"average"</span><span class="p">,</span> | 
|  | <span class="nt">"fn"</span>     <span class="p">:</span> <span class="s2">"/"</span><span class="p">,</span> | 
|  | <span class="nt">"fields"</span> <span class="p">:</span> <span class="p">[</span> | 
|  | <span class="p">{</span> <span class="nt">"type"</span> <span class="p">:</span> <span class="s2">"fieldAccess"</span><span class="p">,</span> <span class="nt">"name"</span> <span class="p">:</span> <span class="s2">"tot"</span><span class="p">,</span> <span class="nt">"fieldName"</span> <span class="p">:</span> <span class="s2">"tot"</span> <span class="p">},</span> | 
|  | <span class="p">{</span> <span class="nt">"type"</span> <span class="p">:</span> <span class="s2">"fieldAccess"</span><span class="p">,</span> <span class="nt">"name"</span> <span class="p">:</span> <span class="s2">"rows"</span><span class="p">,</span> <span class="nt">"fieldName"</span> <span class="p">:</span> <span class="s2">"rows"</span> <span class="p">}</span> | 
|  | <span class="p">]</span> | 
|  | <span class="p">}]</span> | 
|  | <span class="err">...</span> | 
|  | <span class="p">}</span> | 
|  | </code></pre></div><div class="highlight"><pre><code class="language-json" data-lang="json"><span></span><span class="p">{</span> | 
|  | <span class="err">...</span> | 
|  | <span class="nt">"aggregations"</span> <span class="p">:</span> <span class="p">[</span> | 
|  | <span class="p">{</span> <span class="nt">"type"</span> <span class="p">:</span> <span class="s2">"doubleSum"</span><span class="p">,</span> <span class="nt">"name"</span> <span class="p">:</span> <span class="s2">"tot"</span><span class="p">,</span> <span class="nt">"fieldName"</span> <span class="p">:</span> <span class="s2">"total"</span> <span class="p">},</span> | 
|  | <span class="p">{</span> <span class="nt">"type"</span> <span class="p">:</span> <span class="s2">"doubleSum"</span><span class="p">,</span> <span class="nt">"name"</span> <span class="p">:</span> <span class="s2">"part"</span><span class="p">,</span> <span class="nt">"fieldName"</span> <span class="p">:</span> <span class="s2">"part"</span> <span class="p">}</span> | 
|  | <span class="p">],</span> | 
|  | <span class="nt">"postAggregations"</span> <span class="p">:</span> <span class="p">[{</span> | 
|  | <span class="nt">"type"</span>   <span class="p">:</span> <span class="s2">"arithmetic"</span><span class="p">,</span> | 
|  | <span class="nt">"name"</span>   <span class="p">:</span> <span class="s2">"part_percentage"</span><span class="p">,</span> | 
|  | <span class="nt">"fn"</span>     <span class="p">:</span> <span class="s2">"*"</span><span class="p">,</span> | 
|  | <span class="nt">"fields"</span> <span class="p">:</span> <span class="p">[</span> | 
|  | <span class="p">{</span> <span class="nt">"type"</span>   <span class="p">:</span> <span class="s2">"arithmetic"</span><span class="p">,</span> | 
|  | <span class="nt">"name"</span>   <span class="p">:</span> <span class="s2">"ratio"</span><span class="p">,</span> | 
|  | <span class="nt">"fn"</span>     <span class="p">:</span> <span class="s2">"/"</span><span class="p">,</span> | 
|  | <span class="nt">"fields"</span> <span class="p">:</span> <span class="p">[</span> | 
|  | <span class="p">{</span> <span class="nt">"type"</span> <span class="p">:</span> <span class="s2">"fieldAccess"</span><span class="p">,</span> <span class="nt">"name"</span> <span class="p">:</span> <span class="s2">"part"</span><span class="p">,</span> <span class="nt">"fieldName"</span> <span class="p">:</span> <span class="s2">"part"</span> <span class="p">},</span> | 
|  | <span class="p">{</span> <span class="nt">"type"</span> <span class="p">:</span> <span class="s2">"fieldAccess"</span><span class="p">,</span> <span class="nt">"name"</span> <span class="p">:</span> <span class="s2">"tot"</span><span class="p">,</span> <span class="nt">"fieldName"</span> <span class="p">:</span> <span class="s2">"tot"</span> <span class="p">}</span> | 
|  | <span class="p">]</span> | 
|  | <span class="p">},</span> | 
|  | <span class="p">{</span> <span class="nt">"type"</span> <span class="p">:</span> <span class="s2">"constant"</span><span class="p">,</span> <span class="nt">"name"</span><span class="p">:</span> <span class="s2">"const"</span><span class="p">,</span> <span class="nt">"value"</span> <span class="p">:</span> <span class="mi">100</span> <span class="p">}</span> | 
|  | <span class="p">]</span> | 
|  | <span class="p">}]</span> | 
|  | <span class="err">...</span> | 
|  | <span class="p">}</span> | 
|  | </code></pre></div> | 
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