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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/latest/design/index.html">Introduction to Apache Druid</a></li><li class="navListItem"><a class="navItem" href="/docs/latest/tutorials/index.html">Quickstart</a></li><li class="navListItem"><a class="navItem" href="/docs/latest/tutorials/docker.html">Docker</a></li><li class="navListItem"><a class="navItem" href="/docs/latest/operations/single-server.html">Single server deployment</a></li><li class="navListItem"><a class="navItem" href="/docs/latest/tutorials/cluster.html">Clustered deployment</a></li></ul></div><div class="navGroup"><h3 class="navGroupCategoryTitle collapsible">Tutorials<span class="arrow"><svg 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| <blockquote> |
| <p>This document describes how Druid executes <a href="/docs/latest/querying/querying.html">native queries</a>, but since <a href="/docs/latest/querying/sql.html">Druid SQL</a> queries |
| are translated to native queries, this document applies to the SQL runtime as well. Refer to the SQL |
| <a href="/docs/latest/querying/sql.html#query-translation">Query translation</a> page for information about how SQL queries are translated to native |
| queries.</p> |
| </blockquote> |
| <p>Druid's approach to query execution varies depending on the kind of <a href="/docs/latest/querying/datasource.html">datasource</a> you are querying.</p> |
| <h2><a class="anchor" aria-hidden="true" id="datasource-type"></a><a href="#datasource-type" 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>Datasource type</h2> |
| <h3><a class="anchor" aria-hidden="true" id="table"></a><a href="#table" 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><code>table</code></h3> |
| <p>Queries that operate directly on <a href="/docs/latest/querying/datasource.html#table">table datasources</a> are executed using a scatter-gather approach |
| led by the Broker process. The process looks like this:</p> |
| <ol> |
| <li><p>The Broker identifies which <a href="/docs/latest/design/segments.html">segments</a> are relevant to the query based on the <code>"intervals"</code> |
| parameter. Segments are always partitioned by time, so any segment whose interval overlaps the query interval is |
| potentially relevant.</p></li> |
| <li><p>The Broker may additionally further prune the segment list based on the <code>"filter"</code>, if the input data was partitioned |
| by range using the <a href="/docs/latest/ingestion/native-batch.html#partitionsspec"><code>single_dim</code> partitionsSpec</a>, and if the filter matches |
| the dimension used for partitioning.</p></li> |
| <li><p>The Broker, having pruned the list of segments for the query, forwards the query to data servers (like Historicals |
| and tasks running on MiddleManagers) that are currently serving those segments.</p></li> |
| <li><p>For all query types except <a href="/docs/latest/querying/scan-query.html">Scan</a>, data servers process each segment in parallel and generate partial |
| results for each segment. The specific processing that is done depends on the query type. These partial results may be |
| cached if <a href="/docs/latest/querying/caching.html">query caching</a> is enabled. For Scan queries, segments are processed in order by a single thread, |
| and results are not cached.</p></li> |
| <li><p>The Broker receives partial results from each data server, merges them into the final result set, and returns them |
| to the caller. For Timeseries and Scan queries, and for GroupBy queries where there is no sorting, the Broker is able to |
| do this in a streaming fashion. Otherwise, the Broker fully computes the result set before returning anything.</p></li> |
| </ol> |
| <h3><a class="anchor" aria-hidden="true" id="lookup"></a><a href="#lookup" 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><code>lookup</code></h3> |
| <p>Queries that operate directly on <a href="/docs/latest/querying/datasource.html#lookup">lookup datasources</a> (without a join) are executed on the Broker |
| that received the query, using its local copy of the lookup. All registered lookup tables are preloaded in-memory on the |
| Broker. The query runs single-threaded.</p> |
| <p>Execution of queries that use lookups as right-hand inputs to a join are executed in a way that depends on their |
| "base" (bottom-leftmost) datasource, as described in the <a href="#join">join</a> section below.</p> |
| <h3><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><code>union</code></h3> |
| <p>Queries that operate directly on <a href="/docs/latest/querying/datasource.html#union">union datasources</a> are split up on the Broker into a separate |
| query for each table that is part of the union. Each of these queries runs separately, and the Broker merges their |
| results together.</p> |
| <h3><a class="anchor" aria-hidden="true" id="inline"></a><a href="#inline" 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><code>inline</code></h3> |
| <p>Queries that operate directly on <a href="/docs/latest/querying/datasource.html#inline">inline datasources</a> are executed on the Broker that received the |
| query. The query runs single-threaded.</p> |
| <p>Execution of queries that use inline datasources as right-hand inputs to a join are executed in a way that depends on |
| their "base" (bottom-leftmost) datasource, as described in the <a href="#join">join</a> section below.</p> |
| <h3><a class="anchor" aria-hidden="true" id="query"></a><a href="#query" 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><code>query</code></h3> |
| <p><a href="/docs/latest/querying/datasource.html#query">Query datasources</a> are subqueries. Each subquery is executed as if it was its own query and |
| the results are brought back to the Broker. Then, the Broker continues on with the rest of the query as if the subquery |
| was replaced with an inline datasource.</p> |
| <p>In most cases, subquery results are fully buffered in memory on the Broker before the rest of the query proceeds, |
| meaning subqueries execute sequentially. The total number of rows buffered across all subqueries of a given query |
| in this way cannot exceed the <a href="/docs/latest/configuration/index.html"><code>druid.server.http.maxSubqueryRows</code> property</a>.</p> |
| <p>There is one exception: if the outer query and all subqueries are the <a href="/docs/latest/querying/groupbyquery.html">groupBy</a> type, then subquery |
| results can be processed in a streaming fashion and the <code>druid.server.http.maxSubqueryRows</code> limit does not apply.</p> |
| <h3><a class="anchor" aria-hidden="true" id="join"></a><a href="#join" 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><code>join</code></h3> |
| <p><a href="/docs/latest/querying/datasource.html#join">Join datasources</a> are handled using a broadcast hash-join approach.</p> |
| <ol> |
| <li><p>The Broker executes any subqueries that are inputs the join, as described in the <a href="#query">query</a> section, and |
| replaces them with inline datasources.</p></li> |
| <li><p>The Broker flattens a join tree, if present, into a "base" datasource (the bottom-leftmost one) and other leaf |
| datasources (the rest).</p></li> |
| <li><p>Query execution proceeds using the same structure that the base datasource would use on its own. If the base |
| datasource is a <a href="#table">table</a>, segments are pruned based on <code>"intervals"</code> as usual, and the query is executed on the |
| cluster by forwarding it to all relevant data servers in parallel. If the base datasource is a <a href="#lookup">lookup</a> or |
| <a href="#inline">inline</a> datasource (including an inline datasource that was the result of inlining a subquery), the query is |
| executed on the Broker itself. The base query cannot be a union, because unions are not currently supported as inputs to |
| a join.</p></li> |
| <li><p>Before beginning to process the base datasource, the server(s) that will execute the query first inspect all the |
| non-base leaf datasources to determine if a new hash table needs to be built for the upcoming hash join. Currently, |
| lookups do not require new hash tables to be built (because they are preloaded), but inline datasources do.</p></li> |
| <li><p>Query execution proceeds again using the same structure that the base datasource would use on its own, with one |
| addition: while processing the base datasource, Druid servers will use the hash tables built from the other join inputs |
| to produce the join result row-by-row, and query engines will operate on the joined rows rather than the base rows.</p></li> |
| </ol> |
| </span></div></article></div><div class="docs-prevnext"><a class="docs-prev button" href="/docs/latest/querying/querying.html"><span class="arrow-prev">← </span><span>Native queries</span></a><a class="docs-next button" href="/docs/latest/querying/datasource.html"><span>Datasources</span><span class="arrow-next"> →</span></a></div></div></div><nav class="onPageNav"><ul class="toc-headings"><li><a href="#datasource-type">Datasource type</a><ul class="toc-headings"><li><a href="#table"><code>table</code></a></li><li><a href="#lookup"><code>lookup</code></a></li><li><a href="#union"><code>union</code></a></li><li><a href="#inline"><code>inline</code></a></li><li><a href="#query"><code>query</code></a></li><li><a href="#join"><code>join</code></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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