<!DOCTYPE html><html lang="en"><head><meta charSet="utf-8"/><meta http-equiv="X-UA-Compatible" content="IE=edge"/><title>DataSketches HLL Sketch module · Apache Druid</title><meta name="viewport" content="width=device-width"/><link rel="canonical" href="https://druid.apache.org/docs/24.0.0/development/extensions-core/datasketches-hll.html"/><meta name="generator" content="Docusaurus"/><meta name="description" content="&lt;!--"/><meta name="docsearch:language" content="en"/><meta name="docsearch:version" content="24.0.0" /><meta property="og:title" content="DataSketches HLL Sketch module · Apache Druid"/><meta property="og:type" content="website"/><meta property="og:url" content="https://druid.apache.org/index.html"/><meta property="og:description" content="&lt;!--"/><meta property="og:image" content="https://druid.apache.org/img/druid_nav.png"/><meta name="twitter:card" content="summary"/><meta name="twitter:image" content="https://druid.apache.org/img/druid_nav.png"/><link rel="shortcut icon" href="/img/favicon.png"/><link rel="stylesheet" href="https://cdn.jsdelivr.net/docsearch.js/1/docsearch.min.css"/><link rel="stylesheet" href="//cdnjs.cloudflare.com/ajax/libs/highlight.js/9.12.0/styles/default.min.css"/><script async="" src="https://www.googletagmanager.com/gtag/js?id=UA-131010415-1"></script><script>
              window.dataLayer = window.dataLayer || [];
              function gtag(){dataLayer.push(arguments); }
              gtag('js', new Date());
              gtag('config', 'UA-131010415-1');
            </script><link rel="stylesheet" href="https://use.fontawesome.com/releases/v5.7.2/css/all.css"/><link rel="stylesheet" href="/css/code-block-buttons.css"/><script type="text/javascript" src="https://cdnjs.cloudflare.com/ajax/libs/clipboard.js/2.0.4/clipboard.min.js"></script><script type="text/javascript" src="/js/code-block-buttons.js"></script><script src="/js/scrollSpy.js"></script><link rel="stylesheet" href="/css/main.css"/><script src="/js/codetabs.js"></script></head><body class="sideNavVisible separateOnPageNav"><div class="fixedHeaderContainer"><div class="headerWrapper wrapper"><header><a href="/"><img class="logo" src="/img/druid_nav.png" alt="Apache Druid"/></a><div class="navigationWrapper navigationSlider"><nav class="slidingNav"><ul class="nav-site nav-site-internal"><li class=""><a href="/technology" target="_self">Technology</a></li><li class=""><a href="/use-cases" target="_self">Use Cases</a></li><li class=""><a href="/druid-powered" target="_self">Powered By</a></li><li class="siteNavGroupActive"><a href="/docs/24.0.0/design/index.html" target="_self">Docs</a></li><li class=""><a href="/community/" target="_self">Community</a></li><li class=""><a href="https://www.apache.org" target="_self">Apache</a></li><li class=""><a href="/downloads.html" target="_self">Download</a></li><li class="navSearchWrapper reactNavSearchWrapper"><input type="text" id="search_input_react" placeholder="Search" title="Search"/></li></ul></nav></div></header></div></div><div class="navPusher"><div class="docMainWrapper wrapper"><div class="docsNavContainer" id="docsNav"><nav class="toc"><div class="toggleNav"><section class="navWrapper wrapper"><div class="navBreadcrumb wrapper"><div class="navToggle" id="navToggler"><div class="hamburger-menu"><div class="line1"></div><div class="line2"></div><div class="line3"></div></div></div><h2><i>›</i><span>Hidden</span></h2><div class="tocToggler" id="tocToggler"><i class="icon-toc"></i></div></div><div class="navGroups"><div class="navGroup"><h3 class="navGroupCategoryTitle collapsible">Getting started<span class="arrow"><svg width="24" height="24" viewBox="0 0 24 24"><path fill="#565656" d="M7.41 15.41L12 10.83l4.59 4.58L18 14l-6-6-6 6z"></path><path d="M0 0h24v24H0z" fill="none"></path></svg></span></h3><ul class="hide"><li class="navListItem"><a class="navItem" href="/docs/24.0.0/design/index.html">Introduction to Apache Druid</a></li><li class="navListItem"><a class="navItem" href="/docs/24.0.0/tutorials/index.html">Quickstart (local)</a></li><li class="navListItem"><a class="navItem" href="/docs/24.0.0/operations/single-server.html">Single server deployment</a></li><li class="navListItem"><a class="navItem" href="/docs/24.0.0/tutorials/cluster.html">Clustered deployment</a></li></ul></div><div class="navGroup"><h3 class="navGroupCategoryTitle collapsible">Tutorials<span class="arrow"><svg width="24" height="24" viewBox="0 0 24 24"><path fill="#565656" d="M7.41 15.41L12 10.83l4.59 4.58L18 14l-6-6-6 6z"></path><path d="M0 0h24v24H0z" fill="none"></path></svg></span></h3><ul class="hide"><li class="navListItem"><a class="navItem" href="/docs/24.0.0/tutorials/tutorial-batch.html">Load files natively</a></li><li class="navListItem"><a class="navItem" href="/docs/24.0.0/tutorials/tutorial-msq-extern.html">Load files using SQL 🆕</a></li><li class="navListItem"><a class="navItem" href="/docs/24.0.0/tutorials/tutorial-kafka.html">Load from Apache Kafka</a></li><li class="navListItem"><a class="navItem" href="/docs/24.0.0/tutorials/tutorial-batch-hadoop.html">Load from Apache Hadoop</a></li><li class="navListItem"><a class="navItem" href="/docs/24.0.0/tutorials/tutorial-query.html">Querying data</a></li><li class="navListItem"><a class="navItem" href="/docs/24.0.0/tutorials/tutorial-rollup.html">Roll-up</a></li><li class="navListItem"><a class="navItem" href="/docs/24.0.0/tutorials/tutorial-sketches-theta.html">Theta sketches</a></li><li class="navListItem"><a class="navItem" href="/docs/24.0.0/tutorials/tutorial-retention.html">Configuring data retention</a></li><li class="navListItem"><a class="navItem" href="/docs/24.0.0/tutorials/tutorial-update-data.html">Updating existing data</a></li><li class="navListItem"><a class="navItem" href="/docs/24.0.0/tutorials/tutorial-compaction.html">Compacting segments</a></li><li class="navListItem"><a class="navItem" href="/docs/24.0.0/tutorials/tutorial-delete-data.html">Deleting data</a></li><li class="navListItem"><a class="navItem" href="/docs/24.0.0/tutorials/tutorial-ingestion-spec.html">Writing an ingestion spec</a></li><li class="navListItem"><a class="navItem" href="/docs/24.0.0/tutorials/tutorial-transform-spec.html">Transforming input data</a></li><li class="navListItem"><a class="navItem" href="/docs/24.0.0/tutorials/docker.html">Tutorial: Run with Docker</a></li><li class="navListItem"><a class="navItem" href="/docs/24.0.0/tutorials/tutorial-kerberos-hadoop.html">Kerberized HDFS deep storage</a></li><li class="navListItem"><a class="navItem" href="/docs/24.0.0/tutorials/tutorial-msq-convert-spec.html">Convert ingestion spec to SQL</a></li></ul></div><div class="navGroup"><h3 class="navGroupCategoryTitle collapsible">Design<span class="arrow"><svg width="24" height="24" viewBox="0 0 24 24"><path fill="#565656" d="M7.41 15.41L12 10.83l4.59 4.58L18 14l-6-6-6 6z"></path><path d="M0 0h24v24H0z" fill="none"></path></svg></span></h3><ul class="hide"><li class="navListItem"><a class="navItem" href="/docs/24.0.0/design/architecture.html">Design</a></li><li class="navListItem"><a class="navItem" href="/docs/24.0.0/design/segments.html">Segments</a></li><li class="navListItem"><a class="navItem" href="/docs/24.0.0/design/processes.html">Processes and servers</a></li><li class="navListItem"><a class="navItem" href="/docs/24.0.0/dependencies/deep-storage.html">Deep storage</a></li><li class="navListItem"><a class="navItem" href="/docs/24.0.0/dependencies/metadata-storage.html">Metadata storage</a></li><li class="navListItem"><a class="navItem" href="/docs/24.0.0/dependencies/zookeeper.html">ZooKeeper</a></li></ul></div><div class="navGroup"><h3 class="navGroupCategoryTitle collapsible">Ingestion<span class="arrow"><svg width="24" height="24" viewBox="0 0 24 24"><path fill="#565656" d="M7.41 15.41L12 10.83l4.59 4.58L18 14l-6-6-6 6z"></path><path d="M0 0h24v24H0z" fill="none"></path></svg></span></h3><ul class="hide"><li class="navListItem"><a class="navItem" href="/docs/24.0.0/ingestion/index.html">Ingestion</a></li><li class="navListItem"><a class="navItem" href="/docs/24.0.0/ingestion/data-formats.html">Data formats</a></li><li class="navListItem"><a class="navItem" href="/docs/24.0.0/ingestion/data-model.html">Data model</a></li><li class="navListItem"><a class="navItem" href="/docs/24.0.0/ingestion/rollup.html">Data rollup</a></li><li class="navListItem"><a class="navItem" href="/docs/24.0.0/ingestion/partitioning.html">Partitioning</a></li><li class="navListItem"><a class="navItem" href="/docs/24.0.0/ingestion/ingestion-spec.html">Ingestion spec</a></li><li class="navListItem"><a class="navItem" href="/docs/24.0.0/ingestion/schema-design.html">Schema design tips</a></li><div class="navGroup subNavGroup"><h4 class="navGroupSubcategoryTitle">Stream ingestion</h4><ul><li class="navListItem"><a class="navItem" href="/docs/24.0.0/development/extensions-core/kafka-ingestion.html">Apache Kafka ingestion</a></li><li class="navListItem"><a class="navItem" href="/docs/24.0.0/development/extensions-core/kafka-supervisor-reference.html">Apache Kafka supervisor</a></li><li class="navListItem"><a class="navItem" href="/docs/24.0.0/development/extensions-core/kafka-supervisor-operations.html">Apache Kafka operations</a></li><li class="navListItem"><a class="navItem" href="/docs/24.0.0/development/extensions-core/kinesis-ingestion.html">Amazon Kinesis</a></li></ul></div><div class="navGroup subNavGroup"><h4 class="navGroupSubcategoryTitle">Batch ingestion</h4><ul><li class="navListItem"><a class="navItem" href="/docs/24.0.0/ingestion/native-batch.html">Native batch</a></li><li class="navListItem"><a class="navItem" href="/docs/24.0.0/ingestion/native-batch-input-sources.html">Native batch: input sources</a></li><li class="navListItem"><a class="navItem" href="/docs/24.0.0/ingestion/hadoop.html">Hadoop-based</a></li></ul></div><div class="navGroup subNavGroup"><h4 class="navGroupSubcategoryTitle">SQL-based ingestion 🆕</h4><ul><li class="navListItem"><a class="navItem" href="/docs/24.0.0/multi-stage-query/index.html">Overview</a></li><li class="navListItem"><a class="navItem" href="/docs/24.0.0/multi-stage-query/concepts.html">Key concepts</a></li><li class="navListItem"><a class="navItem" href="/docs/24.0.0/multi-stage-query/api.html">API</a></li><li class="navListItem"><a class="navItem" href="/docs/24.0.0/multi-stage-query/security.html">Security</a></li><li class="navListItem"><a class="navItem" href="/docs/24.0.0/multi-stage-query/examples.html">Examples</a></li><li class="navListItem"><a class="navItem" href="/docs/24.0.0/multi-stage-query/reference.html">Reference</a></li><li class="navListItem"><a class="navItem" href="/docs/24.0.0/multi-stage-query/known-issues.html">Known issues</a></li></ul></div><li class="navListItem"><a class="navItem" href="/docs/24.0.0/ingestion/tasks.html">Task reference</a></li><li class="navListItem"><a class="navItem" href="/docs/24.0.0/ingestion/faq.html">Troubleshooting FAQ</a></li></ul></div><div class="navGroup"><h3 class="navGroupCategoryTitle collapsible">Data management<span class="arrow"><svg width="24" height="24" viewBox="0 0 24 24"><path fill="#565656" d="M7.41 15.41L12 10.83l4.59 4.58L18 14l-6-6-6 6z"></path><path d="M0 0h24v24H0z" fill="none"></path></svg></span></h3><ul class="hide"><li class="navListItem"><a class="navItem" href="/docs/24.0.0/data-management/index.html">Overview</a></li><li class="navListItem"><a class="navItem" href="/docs/24.0.0/data-management/update.html">Data updates</a></li><li class="navListItem"><a class="navItem" href="/docs/24.0.0/data-management/delete.html">Data deletion</a></li><li class="navListItem"><a class="navItem" href="/docs/24.0.0/data-management/schema-changes.html">Schema changes</a></li><li class="navListItem"><a class="navItem" href="/docs/24.0.0/data-management/compaction.html">Compaction</a></li><li class="navListItem"><a class="navItem" href="/docs/24.0.0/data-management/automatic-compaction.html">Automatic compaction</a></li></ul></div><div class="navGroup"><h3 class="navGroupCategoryTitle collapsible">Querying<span class="arrow"><svg width="24" height="24" viewBox="0 0 24 24"><path fill="#565656" d="M7.41 15.41L12 10.83l4.59 4.58L18 14l-6-6-6 6z"></path><path d="M0 0h24v24H0z" fill="none"></path></svg></span></h3><ul class="hide"><div class="navGroup subNavGroup"><h4 class="navGroupSubcategoryTitle">Druid SQL</h4><ul><li class="navListItem"><a class="navItem" href="/docs/24.0.0/querying/sql.html">Overview and syntax</a></li><li class="navListItem"><a class="navItem" href="/docs/24.0.0/querying/sql-data-types.html">SQL data types</a></li><li class="navListItem"><a class="navItem" href="/docs/24.0.0/querying/sql-operators.html">Operators</a></li><li class="navListItem"><a class="navItem" href="/docs/24.0.0/querying/sql-scalar.html">Scalar functions</a></li><li class="navListItem"><a class="navItem" href="/docs/24.0.0/querying/sql-aggregations.html">Aggregation functions</a></li><li class="navListItem"><a class="navItem" href="/docs/24.0.0/querying/sql-multivalue-string-functions.html">Multi-value string functions</a></li><li class="navListItem"><a class="navItem" href="/docs/24.0.0/querying/sql-json-functions.html">JSON functions</a></li><li class="navListItem"><a class="navItem" href="/docs/24.0.0/querying/sql-functions.html">All functions</a></li><li class="navListItem"><a class="navItem" href="/docs/24.0.0/querying/sql-api.html">Druid SQL API</a></li><li class="navListItem"><a class="navItem" href="/docs/24.0.0/querying/sql-jdbc.html">JDBC driver API</a></li><li class="navListItem"><a class="navItem" href="/docs/24.0.0/querying/sql-query-context.html">SQL query context</a></li><li class="navListItem"><a class="navItem" href="/docs/24.0.0/querying/sql-metadata-tables.html">SQL metadata tables</a></li><li class="navListItem"><a class="navItem" href="/docs/24.0.0/querying/sql-translation.html">SQL query translation</a></li></ul></div><li class="navListItem"><a class="navItem" href="/docs/24.0.0/querying/querying.html">Native queries</a></li><li class="navListItem"><a class="navItem" href="/docs/24.0.0/querying/query-execution.html">Query execution</a></li><li class="navListItem"><a class="navItem" href="/docs/24.0.0/querying/troubleshooting.html">Troubleshooting</a></li><div class="navGroup subNavGroup"><h4 class="navGroupSubcategoryTitle">Concepts</h4><ul><li class="navListItem"><a class="navItem" href="/docs/24.0.0/querying/datasource.html">Datasources</a></li><li class="navListItem"><a class="navItem" href="/docs/24.0.0/querying/joins.html">Joins</a></li><li class="navListItem"><a class="navItem" href="/docs/24.0.0/querying/lookups.html">Lookups</a></li><li class="navListItem"><a class="navItem" href="/docs/24.0.0/querying/multi-value-dimensions.html">Multi-value dimensions</a></li><li class="navListItem"><a class="navItem" href="/docs/24.0.0/querying/nested-columns.html">Nested columns</a></li><li class="navListItem"><a class="navItem" href="/docs/24.0.0/querying/multitenancy.html">Multitenancy</a></li><li class="navListItem"><a class="navItem" href="/docs/24.0.0/querying/caching.html">Query caching</a></li><li class="navListItem"><a class="navItem" href="/docs/24.0.0/querying/using-caching.html">Using query caching</a></li><li class="navListItem"><a class="navItem" href="/docs/24.0.0/querying/query-context.html">Query context</a></li></ul></div><div class="navGroup subNavGroup"><h4 class="navGroupSubcategoryTitle">Native query types</h4><ul><li class="navListItem"><a class="navItem" href="/docs/24.0.0/querying/timeseriesquery.html">Timeseries</a></li><li class="navListItem"><a class="navItem" href="/docs/24.0.0/querying/topnquery.html">TopN</a></li><li class="navListItem"><a class="navItem" href="/docs/24.0.0/querying/groupbyquery.html">GroupBy</a></li><li class="navListItem"><a class="navItem" href="/docs/24.0.0/querying/scan-query.html">Scan</a></li><li class="navListItem"><a class="navItem" href="/docs/24.0.0/querying/searchquery.html">Search</a></li><li class="navListItem"><a class="navItem" href="/docs/24.0.0/querying/timeboundaryquery.html">TimeBoundary</a></li><li class="navListItem"><a class="navItem" href="/docs/24.0.0/querying/segmentmetadataquery.html">SegmentMetadata</a></li><li class="navListItem"><a class="navItem" href="/docs/24.0.0/querying/datasourcemetadataquery.html">DatasourceMetadata</a></li></ul></div><div class="navGroup subNavGroup"><h4 class="navGroupSubcategoryTitle">Native query components</h4><ul><li class="navListItem"><a class="navItem" href="/docs/24.0.0/querying/filters.html">Filters</a></li><li class="navListItem"><a class="navItem" href="/docs/24.0.0/querying/granularities.html">Granularities</a></li><li class="navListItem"><a class="navItem" href="/docs/24.0.0/querying/dimensionspecs.html">Dimensions</a></li><li class="navListItem"><a class="navItem" href="/docs/24.0.0/querying/aggregations.html">Aggregations</a></li><li class="navListItem"><a class="navItem" href="/docs/24.0.0/querying/post-aggregations.html">Post-aggregations</a></li><li class="navListItem"><a class="navItem" href="/docs/24.0.0/misc/math-expr.html">Expressions</a></li><li class="navListItem"><a class="navItem" href="/docs/24.0.0/querying/having.html">Having filters (groupBy)</a></li><li class="navListItem"><a class="navItem" href="/docs/24.0.0/querying/limitspec.html">Sorting and limiting (groupBy)</a></li><li class="navListItem"><a class="navItem" href="/docs/24.0.0/querying/topnmetricspec.html">Sorting (topN)</a></li><li class="navListItem"><a class="navItem" href="/docs/24.0.0/querying/sorting-orders.html">String comparators</a></li><li class="navListItem"><a class="navItem" href="/docs/24.0.0/querying/virtual-columns.html">Virtual columns</a></li><li class="navListItem"><a class="navItem" href="/docs/24.0.0/development/geo.html">Spatial filters</a></li></ul></div></ul></div><div class="navGroup"><h3 class="navGroupCategoryTitle collapsible">Configuration<span class="arrow"><svg width="24" height="24" viewBox="0 0 24 24"><path fill="#565656" d="M7.41 15.41L12 10.83l4.59 4.58L18 14l-6-6-6 6z"></path><path d="M0 0h24v24H0z" fill="none"></path></svg></span></h3><ul class="hide"><li class="navListItem"><a class="navItem" href="/docs/24.0.0/configuration/index.html">Configuration reference</a></li><li class="navListItem"><a class="navItem" href="/docs/24.0.0/development/extensions.html">Extensions</a></li><li class="navListItem"><a class="navItem" href="/docs/24.0.0/configuration/logging.html">Logging</a></li></ul></div><div class="navGroup"><h3 class="navGroupCategoryTitle collapsible">Operations<span class="arrow"><svg width="24" height="24" viewBox="0 0 24 24"><path fill="#565656" d="M7.41 15.41L12 10.83l4.59 4.58L18 14l-6-6-6 6z"></path><path d="M0 0h24v24H0z" fill="none"></path></svg></span></h3><ul class="hide"><li class="navListItem"><a class="navItem" href="/docs/24.0.0/operations/web-console.html">Web console</a></li><li class="navListItem"><a class="navItem" href="/docs/24.0.0/operations/java.html">Java runtime</a></li><div class="navGroup subNavGroup"><h4 class="navGroupSubcategoryTitle">Security</h4><ul><li class="navListItem"><a class="navItem" href="/docs/24.0.0/operations/security-overview.html">Security overview</a></li><li class="navListItem"><a class="navItem" href="/docs/24.0.0/operations/security-user-auth.html">User authentication and authorization</a></li><li class="navListItem"><a class="navItem" href="/docs/24.0.0/operations/auth-ldap.html">LDAP auth</a></li><li class="navListItem"><a class="navItem" href="/docs/24.0.0/operations/password-provider.html">Password providers</a></li><li class="navListItem"><a class="navItem" href="/docs/24.0.0/operations/dynamic-config-provider.html">Dynamic Config Providers</a></li><li class="navListItem"><a class="navItem" href="/docs/24.0.0/operations/tls-support.html">TLS support</a></li></ul></div><div class="navGroup subNavGroup"><h4 class="navGroupSubcategoryTitle">Performance tuning</h4><ul><li class="navListItem"><a class="navItem" href="/docs/24.0.0/operations/basic-cluster-tuning.html">Basic cluster tuning</a></li><li class="navListItem"><a class="navItem" href="/docs/24.0.0/operations/segment-optimization.html">Segment size optimization</a></li><li class="navListItem"><a class="navItem" href="/docs/24.0.0/operations/mixed-workloads.html">Mixed workloads</a></li><li class="navListItem"><a class="navItem" href="/docs/24.0.0/operations/http-compression.html">HTTP compression</a></li><li class="navListItem"><a class="navItem" href="/docs/24.0.0/operations/clean-metadata-store.html">Automated metadata cleanup</a></li></ul></div><div class="navGroup subNavGroup"><h4 class="navGroupSubcategoryTitle">Monitoring</h4><ul><li class="navListItem"><a class="navItem" href="/docs/24.0.0/operations/request-logging.html">Request logging</a></li><li class="navListItem"><a class="navItem" href="/docs/24.0.0/operations/metrics.html">Metrics</a></li><li class="navListItem"><a class="navItem" href="/docs/24.0.0/operations/alerts.html">Alerts</a></li></ul></div><li class="navListItem"><a class="navItem" href="/docs/24.0.0/operations/api-reference.html">API reference</a></li><li class="navListItem"><a class="navItem" href="/docs/24.0.0/operations/high-availability.html">High availability</a></li><li class="navListItem"><a class="navItem" href="/docs/24.0.0/operations/rolling-updates.html">Rolling updates</a></li><li class="navListItem"><a class="navItem" href="/docs/24.0.0/operations/rule-configuration.html">Retaining or automatically dropping data</a></li><li class="navListItem"><a class="navItem" href="/docs/24.0.0/operations/other-hadoop.html">Working with different versions of Apache Hadoop</a></li><div class="navGroup subNavGroup"><h4 class="navGroupSubcategoryTitle">Misc</h4><ul><li class="navListItem"><a class="navItem" href="/docs/24.0.0/operations/dump-segment.html">dump-segment tool</a></li><li class="navListItem"><a class="navItem" href="/docs/24.0.0/operations/reset-cluster.html">reset-cluster tool</a></li><li class="navListItem"><a class="navItem" href="/docs/24.0.0/operations/insert-segment-to-db.html">insert-segment-to-db tool</a></li><li class="navListItem"><a class="navItem" href="/docs/24.0.0/operations/pull-deps.html">pull-deps tool</a></li><li class="navListItem"><a class="navItem" href="/docs/24.0.0/operations/deep-storage-migration.html">Deep storage migration</a></li><li class="navListItem"><a class="navItem" href="/docs/24.0.0/operations/export-metadata.html">Export Metadata Tool</a></li><li class="navListItem"><a class="navItem" href="/docs/24.0.0/operations/metadata-migration.html">Metadata Migration</a></li><li class="navListItem"><a class="navItem" href="/docs/24.0.0/operations/use_sbt_to_build_fat_jar.html">Content for build.sbt</a></li></ul></div></ul></div><div class="navGroup"><h3 class="navGroupCategoryTitle collapsible">Development<span class="arrow"><svg width="24" height="24" viewBox="0 0 24 24"><path fill="#565656" d="M7.41 15.41L12 10.83l4.59 4.58L18 14l-6-6-6 6z"></path><path d="M0 0h24v24H0z" fill="none"></path></svg></span></h3><ul class="hide"><li class="navListItem"><a class="navItem" href="/docs/24.0.0/development/overview.html">Developing on Druid</a></li><li class="navListItem"><a class="navItem" href="/docs/24.0.0/development/modules.html">Creating extensions</a></li><li class="navListItem"><a class="navItem" href="/docs/24.0.0/development/javascript.html">JavaScript functionality</a></li><li class="navListItem"><a class="navItem" href="/docs/24.0.0/development/build.html">Build from source</a></li><li class="navListItem"><a class="navItem" href="/docs/24.0.0/development/versioning.html">Versioning</a></li><li class="navListItem"><a class="navItem" href="/docs/24.0.0/development/experimental.html">Experimental features</a></li></ul></div><div class="navGroup"><h3 class="navGroupCategoryTitle collapsible">Misc<span class="arrow"><svg width="24" height="24" viewBox="0 0 24 24"><path fill="#565656" d="M7.41 15.41L12 10.83l4.59 4.58L18 14l-6-6-6 6z"></path><path d="M0 0h24v24H0z" fill="none"></path></svg></span></h3><ul class="hide"><li class="navListItem"><a class="navItem" href="/docs/24.0.0/misc/papers-and-talks.html">Papers</a></li></ul></div><div class="navGroup"><h3 class="navGroupCategoryTitle collapsible">Hidden<span class="arrow"><svg width="24" height="24" viewBox="0 0 24 24"><path fill="#565656" d="M7.41 15.41L12 10.83l4.59 4.58L18 14l-6-6-6 6z"></path><path d="M0 0h24v24H0z" fill="none"></path></svg></span></h3><ul class="hide"><li class="navListItem"><a class="navItem" href="/docs/24.0.0/comparisons/druid-vs-elasticsearch.html">Apache Druid vs Elasticsearch</a></li><li class="navListItem"><a class="navItem" href="/docs/24.0.0/comparisons/druid-vs-key-value.html">Apache Druid vs. Key/Value Stores (HBase/Cassandra/OpenTSDB)</a></li><li class="navListItem"><a class="navItem" href="/docs/24.0.0/comparisons/druid-vs-kudu.html">Apache Druid vs Kudu</a></li><li class="navListItem"><a class="navItem" href="/docs/24.0.0/comparisons/druid-vs-redshift.html">Apache Druid vs Redshift</a></li><li class="navListItem"><a class="navItem" href="/docs/24.0.0/comparisons/druid-vs-spark.html">Apache Druid vs Spark</a></li><li class="navListItem"><a class="navItem" href="/docs/24.0.0/comparisons/druid-vs-sql-on-hadoop.html">Apache Druid vs SQL-on-Hadoop</a></li><li class="navListItem"><a class="navItem" href="/docs/24.0.0/design/auth.html">Authentication and Authorization</a></li><li class="navListItem"><a class="navItem" href="/docs/24.0.0/design/broker.html">Broker</a></li><li class="navListItem"><a class="navItem" href="/docs/24.0.0/design/coordinator.html">Coordinator Process</a></li><li class="navListItem"><a class="navItem" href="/docs/24.0.0/design/historical.html">Historical Process</a></li><li class="navListItem"><a class="navItem" href="/docs/24.0.0/design/indexer.html">Indexer Process</a></li><li class="navListItem"><a class="navItem" href="/docs/24.0.0/design/indexing-service.html">Indexing Service</a></li><li class="navListItem"><a class="navItem" href="/docs/24.0.0/design/middlemanager.html">MiddleManager Process</a></li><li class="navListItem"><a class="navItem" href="/docs/24.0.0/design/overlord.html">Overlord Process</a></li><li class="navListItem"><a class="navItem" href="/docs/24.0.0/design/router.html">Router Process</a></li><li class="navListItem"><a class="navItem" href="/docs/24.0.0/design/peons.html">Peons</a></li><li class="navListItem"><a class="navItem" href="/docs/24.0.0/development/extensions-core/approximate-histograms.html">Approximate Histogram aggregators</a></li><li class="navListItem"><a class="navItem" href="/docs/24.0.0/development/extensions-core/avro.html">Apache Avro</a></li><li class="navListItem"><a class="navItem" href="/docs/24.0.0/development/extensions-core/azure.html">Microsoft Azure</a></li><li class="navListItem"><a class="navItem" href="/docs/24.0.0/development/extensions-core/bloom-filter.html">Bloom Filter</a></li><li class="navListItem"><a class="navItem" href="/docs/24.0.0/development/extensions-core/datasketches-extension.html">DataSketches extension</a></li><li class="navListItem navListItemActive"><a class="navItem" href="/docs/24.0.0/development/extensions-core/datasketches-hll.html">DataSketches HLL Sketch module</a></li><li class="navListItem"><a class="navItem" href="/docs/24.0.0/development/extensions-core/datasketches-quantiles.html">DataSketches Quantiles Sketch module</a></li><li class="navListItem"><a class="navItem" href="/docs/24.0.0/development/extensions-core/datasketches-theta.html">DataSketches Theta Sketch module</a></li><li class="navListItem"><a class="navItem" href="/docs/24.0.0/development/extensions-core/datasketches-tuple.html">DataSketches Tuple Sketch module</a></li><li class="navListItem"><a class="navItem" href="/docs/24.0.0/development/extensions-core/druid-basic-security.html">Basic Security</a></li><li class="navListItem"><a class="navItem" href="/docs/24.0.0/development/extensions-core/druid-kerberos.html">Kerberos</a></li><li class="navListItem"><a class="navItem" href="/docs/24.0.0/development/extensions-core/druid-lookups.html">Cached Lookup Module</a></li><li class="navListItem"><a class="navItem" href="/docs/24.0.0/development/extensions-core/druid-ranger-security.html">Apache Ranger Security</a></li><li class="navListItem"><a class="navItem" href="/docs/24.0.0/development/extensions-core/google.html">Google Cloud Storage</a></li><li class="navListItem"><a class="navItem" href="/docs/24.0.0/development/extensions-core/hdfs.html">HDFS</a></li><li class="navListItem"><a class="navItem" href="/docs/24.0.0/development/extensions-core/kafka-extraction-namespace.html">Apache Kafka Lookups</a></li><li class="navListItem"><a class="navItem" href="/docs/24.0.0/development/extensions-core/lookups-cached-global.html">Globally Cached Lookups</a></li><li class="navListItem"><a class="navItem" href="/docs/24.0.0/development/extensions-core/mysql.html">MySQL Metadata Store</a></li><li class="navListItem"><a class="navItem" href="/docs/24.0.0/development/extensions-core/orc.html">ORC Extension</a></li><li class="navListItem"><a class="navItem" href="/docs/24.0.0/development/extensions-core/druid-pac4j.html">Druid pac4j based Security extension</a></li><li class="navListItem"><a class="navItem" href="/docs/24.0.0/development/extensions-core/parquet.html">Apache Parquet Extension</a></li><li class="navListItem"><a class="navItem" href="/docs/24.0.0/development/extensions-core/postgresql.html">PostgreSQL Metadata Store</a></li><li class="navListItem"><a class="navItem" href="/docs/24.0.0/development/extensions-core/protobuf.html">Protobuf</a></li><li class="navListItem"><a class="navItem" href="/docs/24.0.0/development/extensions-core/s3.html">S3-compatible</a></li><li class="navListItem"><a class="navItem" href="/docs/24.0.0/development/extensions-core/simple-client-sslcontext.html">Simple SSLContext Provider Module</a></li><li class="navListItem"><a class="navItem" href="/docs/24.0.0/development/extensions-core/stats.html">Stats aggregator</a></li><li class="navListItem"><a class="navItem" href="/docs/24.0.0/development/extensions-core/test-stats.html">Test Stats Aggregators</a></li><li class="navListItem"><a class="navItem" href="/docs/24.0.0/development/extensions-core/druid-aws-rds.html">Druid AWS RDS Module</a></li><li class="navListItem"><a class="navItem" href="/docs/24.0.0/development/extensions-core/kubernetes.html">Kubernetes</a></li><li class="navListItem"><a class="navItem" href="/docs/24.0.0/development/extensions-contrib/ambari-metrics-emitter.html">Ambari Metrics Emitter</a></li><li class="navListItem"><a class="navItem" href="/docs/24.0.0/development/extensions-contrib/cassandra.html">Apache Cassandra</a></li><li class="navListItem"><a class="navItem" href="/docs/24.0.0/development/extensions-contrib/cloudfiles.html">Rackspace Cloud Files</a></li><li class="navListItem"><a class="navItem" href="/docs/24.0.0/development/extensions-contrib/distinctcount.html">DistinctCount Aggregator</a></li><li class="navListItem"><a class="navItem" href="/docs/24.0.0/development/extensions-contrib/graphite.html">Graphite Emitter</a></li><li class="navListItem"><a class="navItem" href="/docs/24.0.0/development/extensions-contrib/influx.html">InfluxDB Line Protocol Parser</a></li><li class="navListItem"><a class="navItem" href="/docs/24.0.0/development/extensions-contrib/influxdb-emitter.html">InfluxDB Emitter</a></li><li class="navListItem"><a class="navItem" href="/docs/24.0.0/development/extensions-contrib/kafka-emitter.html">Kafka Emitter</a></li><li class="navListItem"><a class="navItem" href="/docs/24.0.0/development/extensions-contrib/materialized-view.html">Materialized View</a></li><li class="navListItem"><a class="navItem" href="/docs/24.0.0/development/extensions-contrib/momentsketch-quantiles.html">Moment Sketches for Approximate Quantiles module</a></li><li class="navListItem"><a class="navItem" href="/docs/24.0.0/development/extensions-contrib/moving-average-query.html">Moving Average Query</a></li><li class="navListItem"><a class="navItem" href="/docs/24.0.0/development/extensions-contrib/opentsdb-emitter.html">OpenTSDB Emitter</a></li><li class="navListItem"><a class="navItem" href="/docs/24.0.0/development/extensions-contrib/redis-cache.html">Druid Redis Cache</a></li><li class="navListItem"><a class="navItem" href="/docs/24.0.0/development/extensions-contrib/sqlserver.html">Microsoft SQLServer</a></li><li class="navListItem"><a class="navItem" href="/docs/24.0.0/development/extensions-contrib/statsd.html">StatsD Emitter</a></li><li class="navListItem"><a class="navItem" href="/docs/24.0.0/development/extensions-contrib/tdigestsketch-quantiles.html">T-Digest Quantiles Sketch module</a></li><li class="navListItem"><a class="navItem" href="/docs/24.0.0/development/extensions-contrib/thrift.html">Thrift</a></li><li class="navListItem"><a class="navItem" href="/docs/24.0.0/development/extensions-contrib/time-min-max.html">Timestamp Min/Max aggregators</a></li><li class="navListItem"><a class="navItem" href="/docs/24.0.0/development/extensions-contrib/gce-extensions.html">GCE Extensions</a></li><li class="navListItem"><a class="navItem" href="/docs/24.0.0/development/extensions-contrib/aliyun-oss.html">Aliyun OSS</a></li><li class="navListItem"><a class="navItem" href="/docs/24.0.0/development/extensions-contrib/prometheus.html">Prometheus Emitter</a></li><li class="navListItem"><a class="navItem" href="/docs/24.0.0/operations/kubernetes.html">kubernetes</a></li><li class="navListItem"><a class="navItem" href="/docs/24.0.0/querying/hll-old.html">Cardinality/HyperUnique aggregators</a></li><li class="navListItem"><a class="navItem" href="/docs/24.0.0/querying/select-query.html">Select</a></li><li class="navListItem"><a class="navItem" href="/docs/24.0.0/ingestion/native-batch-firehose.html">Firehose (deprecated)</a></li><li class="navListItem"><a class="navItem" href="/docs/24.0.0/ingestion/native-batch-simple-task.html">Native batch (simple)</a></li><li class="navListItem"><a class="navItem" href="/docs/24.0.0/ingestion/standalone-realtime.html">Realtime Process</a></li></ul></div></div></section></div><script>
            var coll = document.getElementsByClassName('collapsible');
            var checkActiveCategory = true;
            for (var i = 0; i < coll.length; i++) {
              var links = coll[i].nextElementSibling.getElementsByTagName('*');
              if (checkActiveCategory){
                for (var j = 0; j < links.length; j++) {
                  if (links[j].classList.contains('navListItemActive')){
                    coll[i].nextElementSibling.classList.toggle('hide');
                    coll[i].childNodes[1].classList.toggle('rotate');
                    checkActiveCategory = false;
                    break;
                  }
                }
              }

              coll[i].addEventListener('click', function() {
                var arrow = this.childNodes[1];
                arrow.classList.toggle('rotate');
                var content = this.nextElementSibling;
                content.classList.toggle('hide');
              });
            }

            document.addEventListener('DOMContentLoaded', function() {
              createToggler('#navToggler', '#docsNav', 'docsSliderActive');
              createToggler('#tocToggler', 'body', 'tocActive');

              var headings = document.querySelector('.toc-headings');
              headings && headings.addEventListener('click', function(event) {
                var el = event.target;
                while(el !== headings){
                  if (el.tagName === 'A') {
                    document.body.classList.remove('tocActive');
                    break;
                  } else{
                    el = el.parentNode;
                  }
                }
              }, false);

              function createToggler(togglerSelector, targetSelector, className) {
                var toggler = document.querySelector(togglerSelector);
                var target = document.querySelector(targetSelector);

                if (!toggler) {
                  return;
                }

                toggler.onclick = function(event) {
                  event.preventDefault();

                  target.classList.toggle(className);
                };
              }
            });
        </script></nav></div><div class="container mainContainer docsContainer"><div class="wrapper"><div class="post"><header class="postHeader"><a class="edit-page-link button" href="https://github.com/apache/druid/edit/master/docs/development/extensions-core/datasketches-hll.md" target="_blank" rel="noreferrer noopener">Edit</a><h1 id="__docusaurus" class="postHeaderTitle">DataSketches HLL Sketch module</h1></header><article><div><span><!--
  ~ Licensed to the Apache Software Foundation (ASF) under one
  ~ or more contributor license agreements.  See the NOTICE file
  ~ distributed with this work for additional information
  ~ regarding copyright ownership.  The ASF licenses this file
  ~ to you under the Apache License, Version 2.0 (the
  ~ "License"); you may not use this file except in compliance
  ~ with the License.  You may obtain a copy of the License at
  ~
  ~   http://www.apache.org/licenses/LICENSE-2.0
  ~
  ~ Unless required by applicable law or agreed to in writing,
  ~ software distributed under the License is distributed on an
  ~ "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
  ~ KIND, either express or implied.  See the License for the
  ~ specific language governing permissions and limitations
  ~ under the License.
  -->
<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/24.0.0/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/24.0.0/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>
</span></div></article></div><div class="docs-prevnext"><a class="docs-prev button" href="/docs/24.0.0/development/extensions-core/datasketches-extension.html"><span class="arrow-prev">← </span><span class="function-name-prevnext">DataSketches extension</span></a><a class="docs-next button" href="/docs/24.0.0/development/extensions-core/datasketches-quantiles.html"><span class="function-name-prevnext">DataSketches Quantiles Sketch module</span><span class="arrow-next"> →</span></a></div></div></div><nav class="onPageNav"></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/24.0.0/">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 © 2022 <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>
                document.addEventListener('keyup', function(e) {
                  if (e.target !== document.body) {
                    return;
                  }
                  // keyCode for '/' (slash)
                  if (e.keyCode === 191) {
                    const search = document.getElementById('search_input_react');
                    search && search.focus();
                  }
                });
              </script><script>
              var search = docsearch({
                
                apiKey: '2de99082a9f38e49dfaa059bbe4c901d',
                indexName: 'apache_druid',
                inputSelector: '#search_input_react',
                algoliaOptions: {"facetFilters":["language:en","version:24.0.0"]}
              });
            </script></body></html>