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<p>This Apache Druid extension adds the ability to both construct bloom filters from query results, and filter query results by testing
against a bloom filter. Make sure to <a href="/docs/latest/development/extensions.html#loading-extensions">include</a> <code>druid-bloom-filter</code> as an
extension.</p>
<p>A Bloom filter is a probabilistic data structure for performing a set membership check. A bloom filter is a good candidate
to use with Druid for cases where an explicit filter is impossible, e.g. filtering a query against a set of millions of
values.</p>
<p>Following are some characteristics of Bloom filters:</p>
<ul>
<li>Bloom filters are highly space efficient when compared to using a HashSet.</li>
<li>Because of the probabilistic nature of bloom filters, false positive results are possible (element was not actually
inserted into a bloom filter during construction, but <code>test()</code> says true)</li>
<li>False negatives are not possible (if element is present then <code>test()</code> will never say false).</li>
<li>The false positive probability of this implementation is currently fixed at 5%, but increasing the number of entries
that the filter can hold can decrease this false positive rate in exchange for overall size.</li>
<li>Bloom filters are sensitive to number of elements that will be inserted in the bloom filter. During the creation of bloom filter expected number of entries must be specified. If the number of insertions exceed
the specified initial number of entries then false positive probability will increase accordingly.</li>
</ul>
<p>This extension is currently based on <code>org.apache.hive.common.util.BloomKFilter</code> from <code>hive-storage-api</code>. Internally,
this implementation uses Murmur3 as the hash algorithm.</p>
<p>To construct a BloomKFilter externally with Java to use as a filter in a Druid query:</p>
<pre><code class="hljs css language-java">BloomKFilter bloomFilter = <span class="hljs-keyword">new</span> BloomKFilter(<span class="hljs-number">1500</span>);
bloomFilter.addString(<span class="hljs-string">"value 1"</span>);
bloomFilter.addString(<span class="hljs-string">"value 2"</span>);
bloomFilter.addString(<span class="hljs-string">"value 3"</span>);
ByteArrayOutputStream byteArrayOutputStream = <span class="hljs-keyword">new</span> ByteArrayOutputStream();
BloomKFilter.serialize(byteArrayOutputStream, bloomFilter);
String base64Serialized = Base64.encodeBase64String(byteArrayOutputStream.toByteArray());
</code></pre>
<p>This string can then be used in the native or SQL Druid query.</p>
<h2><a class="anchor" aria-hidden="true" id="filtering-queries-with-a-bloom-filter"></a><a href="#filtering-queries-with-a-bloom-filter" 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>Filtering queries with a Bloom Filter</h2>
<h3><a class="anchor" aria-hidden="true" id="json-specification-of-bloom-filter"></a><a href="#json-specification-of-bloom-filter" 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>JSON Specification of Bloom Filter</h3>
<pre><code class="hljs css language-json">{
  <span class="hljs-attr">"type"</span> : <span class="hljs-string">"bloom"</span>,
  <span class="hljs-attr">"dimension"</span> : &lt;dimension_name&gt;,
  <span class="hljs-attr">"bloomKFilter"</span> : &lt;serialized_bytes_for_BloomKFilter&gt;,
  <span class="hljs-attr">"extractionFn"</span> : &lt;extraction_fn&gt;
}
</code></pre>
<table>
<thead>
<tr><th>Property</th><th>Description</th><th>required?</th></tr>
</thead>
<tbody>
<tr><td><code>type</code></td><td>Filter Type. Should always be <code>bloom</code></td><td>yes</td></tr>
<tr><td><code>dimension</code></td><td>The dimension to filter over.</td><td>yes</td></tr>
<tr><td><code>bloomKFilter</code></td><td>Base64 encoded Binary representation of <code>org.apache.hive.common.util.BloomKFilter</code></td><td>yes</td></tr>
<tr><td><code>extractionFn</code></td><td><a href="/docs/latest/querying/dimensionspecs.html#extraction-functions">Extraction function</a> to apply to the dimension values</td><td>no</td></tr>
</tbody>
</table>
<h3><a class="anchor" aria-hidden="true" id="serialized-format-for-bloomkfilter"></a><a href="#serialized-format-for-bloomkfilter" 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>Serialized Format for BloomKFilter</h3>
<p>Serialized BloomKFilter format:</p>
<ul>
<li>1 byte for the number of hash functions.</li>
<li>1 big endian int(That is how OutputStream works) for the number of longs in the bitset</li>
<li>big endian longs in the BloomKFilter bitset</li>
</ul>
<p>Note: <code>org.apache.hive.common.util.BloomKFilter</code> provides a serialize method which can be used to serialize bloom filters to outputStream.</p>
<h3><a class="anchor" aria-hidden="true" id="filtering-sql-queries"></a><a href="#filtering-sql-queries" 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>Filtering SQL Queries</h3>
<p>Bloom filters can be used in SQL <code>WHERE</code> clauses via the <code>bloom_filter_test</code> operator:</p>
<pre><code class="hljs css language-sql"><span class="hljs-keyword">SELECT</span> <span class="hljs-keyword">COUNT</span>(*) <span class="hljs-keyword">FROM</span> druid.foo <span class="hljs-keyword">WHERE</span> bloom_filter_test(&lt;expr&gt;, <span class="hljs-string">'&lt;serialized_bytes_for_BloomKFilter&gt;'</span>)
</code></pre>
<h3><a class="anchor" aria-hidden="true" id="expression-and-virtual-column-support"></a><a href="#expression-and-virtual-column-support" 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 and Virtual Column Support</h3>
<p>The bloom filter extension also adds a bloom filter <a href="/docs/latest/misc/math-expr.html">Druid expression</a> which shares syntax
with the SQL operator.</p>
<pre><code class="hljs css language-sql">bloom_filter_test(&lt;expr&gt;, '&lt;serialized_bytes_for_BloomKFilter&gt;')
</code></pre>
<h2><a class="anchor" aria-hidden="true" id="bloom-filter-query-aggregator"></a><a href="#bloom-filter-query-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>Bloom Filter Query Aggregator</h2>
<p>Input for a <code>bloomKFilter</code> can also be created from a druid query with the <code>bloom</code> aggregator. Note that it is very
important to set a reasonable value for the <code>maxNumEntries</code> parameter, which is the maximum number of distinct entries
that the bloom filter can represent without increasing the false positive rate. It may be worth performing a query using
one of the unique count sketches to calculate the value for this parameter in order to build a bloom filter appropriate
for the query.</p>
<h3><a class="anchor" aria-hidden="true" id="json-specification-of-bloom-filter-aggregator"></a><a href="#json-specification-of-bloom-filter-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>JSON Specification of Bloom Filter Aggregator</h3>
<pre><code class="hljs css language-json">{
      <span class="hljs-attr">"type"</span>: <span class="hljs-string">"bloom"</span>,
      <span class="hljs-attr">"name"</span>: &lt;output_field_name&gt;,
      <span class="hljs-attr">"maxNumEntries"</span>: &lt;maximum_number_of_elements_for_BloomKFilter&gt;
      <span class="hljs-string">"field"</span>: &lt;dimension_spec&gt;
    }
</code></pre>
<table>
<thead>
<tr><th>Property</th><th>Description</th><th>required?</th></tr>
</thead>
<tbody>
<tr><td><code>type</code></td><td>Aggregator Type. Should always be <code>bloom</code></td><td>yes</td></tr>
<tr><td><code>name</code></td><td>Output field name</td><td>yes</td></tr>
<tr><td><code>field</code></td><td><a href="/docs/latest/querying/dimensionspecs.html">DimensionSpec</a> to add to <code>org.apache.hive.common.util.BloomKFilter</code></td><td>yes</td></tr>
<tr><td><code>maxNumEntries</code></td><td>Maximum number of distinct values supported by <code>org.apache.hive.common.util.BloomKFilter</code>, default <code>1500</code></td><td>no</td></tr>
</tbody>
</table>
<h3><a class="anchor" aria-hidden="true" id="example"></a><a href="#example" 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</h3>
<pre><code class="hljs css language-json">{
  <span class="hljs-attr">"queryType"</span>: <span class="hljs-string">"timeseries"</span>,
  <span class="hljs-attr">"dataSource"</span>: <span class="hljs-string">"wikiticker"</span>,
  <span class="hljs-attr">"intervals"</span>: [ <span class="hljs-string">"2015-09-12T00:00:00.000/2015-09-13T00:00:00.000"</span> ],
  <span class="hljs-attr">"granularity"</span>: <span class="hljs-string">"day"</span>,
  <span class="hljs-attr">"aggregations"</span>: [
    {
      <span class="hljs-attr">"type"</span>: <span class="hljs-string">"bloom"</span>,
      <span class="hljs-attr">"name"</span>: <span class="hljs-string">"userBloom"</span>,
      <span class="hljs-attr">"maxNumEntries"</span>: <span class="hljs-number">100000</span>,
      <span class="hljs-attr">"field"</span>: {
        <span class="hljs-attr">"type"</span>:<span class="hljs-string">"default"</span>,
        <span class="hljs-attr">"dimension"</span>:<span class="hljs-string">"user"</span>,
        <span class="hljs-attr">"outputType"</span>: <span class="hljs-string">"STRING"</span>
      }
    }
  ]
}
</code></pre>
<p>response</p>
<pre><code class="hljs css language-json">[{<span class="hljs-attr">"timestamp"</span>:<span class="hljs-string">"2015-09-12T00:00:00.000Z"</span>,<span class="hljs-attr">"result"</span>:{<span class="hljs-attr">"userBloom"</span>:<span class="hljs-string">"BAAAJhAAAA..."</span>}}]
</code></pre>
<p>These values can then be set in the filter specification described above.</p>
<p>Ordering results by a bloom filter aggregator, for example in a TopN query, will perform a comparatively expensive
linear scan <em>of the filter itself</em> to count the number of set bits as a means of approximating how many items have been
added to the set. As such, ordering by an alternate aggregation is recommended if possible.</p>
<h3><a class="anchor" aria-hidden="true" id="sql-bloom-filter-aggregator"></a><a href="#sql-bloom-filter-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>SQL Bloom Filter Aggregator</h3>
<p>Bloom filters can be computed in SQL expressions with the <code>bloom_filter</code> aggregator:</p>
<pre><code class="hljs css language-sql"><span class="hljs-keyword">SELECT</span> BLOOM_FILTER(&lt;expression&gt;, &lt;<span class="hljs-keyword">max</span> <span class="hljs-built_in">number</span> <span class="hljs-keyword">of</span> entries&gt;) <span class="hljs-keyword">FROM</span> druid.foo <span class="hljs-keyword">WHERE</span> dim2 = <span class="hljs-string">'abc'</span>
</code></pre>
<p>but requires the setting <code>druid.sql.planner.serializeComplexValues</code> to be set to <code>true</code>. Bloom filter results in a SQL
response are serialized into a base64 string, which can then be used in subsequent queries as a filter.</p>
</span></div></article></div><div class="docs-prevnext"><a class="docs-prev button" href="/docs/latest/development/extensions-core/azure.html"><span class="arrow-prev">← </span><span>Microsoft Azure</span></a><a class="docs-next button" href="/docs/latest/development/extensions-core/datasketches-extension.html"><span class="function-name-prevnext">DataSketches extension</span><span class="arrow-next"> →</span></a></div></div></div><nav class="onPageNav"><ul class="toc-headings"><li><a href="#filtering-queries-with-a-bloom-filter">Filtering queries with a Bloom Filter</a><ul class="toc-headings"><li><a href="#json-specification-of-bloom-filter">JSON Specification of Bloom Filter</a></li><li><a href="#serialized-format-for-bloomkfilter">Serialized Format for BloomKFilter</a></li><li><a href="#filtering-sql-queries">Filtering SQL Queries</a></li><li><a href="#expression-and-virtual-column-support">Expression and Virtual Column Support</a></li></ul></li><li><a href="#bloom-filter-query-aggregator">Bloom Filter Query Aggregator</a><ul class="toc-headings"><li><a href="#json-specification-of-bloom-filter-aggregator">JSON Specification of Bloom Filter Aggregator</a></li><li><a href="#example">Example</a></li><li><a href="#sql-bloom-filter-aggregator">SQL Bloom Filter Aggregator</a></li></ul></li></ul></nav></div><footer class="nav-footer druid-footer" id="footer"><div class="container"><div class="text-center"><p><a href="/technology">Technology</a> · <a href="/use-cases">Use Cases</a> · <a href="/druid-powered">Powered by Druid</a> · <a href="/docs/latest/latest">Docs</a> · <a href="/community/">Community</a> · <a href="/downloads.html">Download</a> · <a href="/faq">FAQ</a></p></div><div class="text-center"><a title="Join the user group" href="https://groups.google.com/forum/#!forum/druid-user" target="_blank"><span class="fa fa-comments"></span></a> · <a title="Follow Druid" href="https://twitter.com/druidio" target="_blank"><span class="fab fa-twitter"></span></a> · <a title="Download via Apache" href="https://www.apache.org/dyn/closer.cgi?path=/incubator/druid/{{ site.druid_versions[0].versions[0].version }}/apache-druid-{{ site.druid_versions[0].versions[0].version }}-bin.tar.gz" target="_blank"><span class="fas fa-feather"></span></a> · <a title="GitHub" href="https://github.com/apache/druid" target="_blank"><span class="fab fa-github"></span></a></div><div class="text-center license">Copyright © 2019 <a href="https://www.apache.org/" target="_blank">Apache Software Foundation</a>.<br/>Except where otherwise noted, licensed under <a rel="license" href="http://creativecommons.org/licenses/by-sa/4.0/">CC BY-SA 4.0</a>.<br/>Apache Druid, Druid, and the Druid logo are either registered trademarks or trademarks of The Apache Software Foundation in the United States and other countries.</div></div></footer></div><script type="text/javascript" src="https://cdn.jsdelivr.net/docsearch.js/1/docsearch.min.js"></script><script>
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