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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/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 width="24" height="24" viewBox="0 0 24 24"><path fill="#565656" d="M7.41 15.41L12 10.83l4.59 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| <p>Apache Druid has limited support for joins through <a href="/docs/latest/querying/lookups.html">query-time lookups</a>. The common use case of |
| query-time lookups is to replace one dimension value (e.g. a String ID) with another value (e.g. a human-readable String value). This is similar to a star-schema join.</p> |
| <p>Druid does not yet have full support for joins. Although Druid’s storage format would allow for the implementation |
| of joins (there is no loss of fidelity for columns included as dimensions), full support for joins have not yet been implemented yet |
| for the following reasons:</p> |
| <ol> |
| <li>Scaling join queries has been, in our professional experience, |
| a constant bottleneck of working with distributed databases.</li> |
| <li>The incremental gains in functionality are perceived to be |
| of less value than the anticipated problems with managing |
| highly concurrent, join-heavy workloads.</li> |
| </ol> |
| <p>A join query is essentially the merging of two or more streams of data based on a shared set of keys. The primary |
| high-level strategies for join queries we are aware of are a hash-based strategy or a |
| sorted-merge strategy. The hash-based strategy requires that all but |
| one data set be available as something that looks like a hash table, |
| a lookup operation is then performed on this hash table for every |
| row in the “primary” stream. The sorted-merge strategy assumes |
| that each stream is sorted by the join key and thus allows for the incremental |
| joining of the streams. Each of these strategies, however, |
| requires the materialization of some number of the streams either in |
| sorted order or in a hash table form.</p> |
| <p>When all sides of the join are significantly large tables (> 1 billion |
| records), materializing the pre-join streams requires complex |
| distributed memory management. The complexity of the memory |
| management is only amplified by the fact that we are targeting highly |
| concurrent, multi-tenant workloads.</p> |
| </span></div></article></div><div class="docs-prevnext"><a class="docs-prev button" href="/docs/latest/querying/lookups.html"><span class="arrow-prev">← </span><span>Lookups</span></a><a class="docs-next button" href="/docs/latest/querying/multitenancy.html"><span>Multitenancy considerations</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/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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