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aria-label="Home page" class="breadcrumbs__link" href="/"><svg viewBox="0 0 24 24" class="breadcrumbHomeIcon_YNFT"><path d="M10 19v-5h4v5c0 .55.45 1 1 1h3c.55 0 1-.45 1-1v-7h1.7c.46 0 .68-.57.33-.87L12.67 3.6c-.38-.34-.96-.34-1.34 0l-8.36 7.53c-.34.3-.13.87.33.87H5v7c0 .55.45 1 1 1h3c.55 0 1-.45 1-1z" fill="currentColor"></path></svg></a></li><li class="breadcrumbs__item"><span class="breadcrumbs__link">Querying</span><meta itemprop="position" content="1"></li><li class="breadcrumbs__item"><span class="breadcrumbs__link">Concepts</span><meta itemprop="position" content="2"></li><li itemscope="" itemprop="itemListElement" itemtype="https://schema.org/ListItem" class="breadcrumbs__item breadcrumbs__item--active"><span class="breadcrumbs__link" itemprop="name">Joins</span><meta itemprop="position" content="3"></li></ul></nav><div class="theme-doc-markdown markdown"><header><h1>Joins</h1></header><p>Apache Druid has two features related to joining of data:</p> |
| <ol> |
| <li><a href="/docs/latest/querying/datasource#join">Join</a> operators. These are available using a <a href="/docs/latest/querying/datasource#join">join datasource</a> in native |
| queries, or using the <a href="/docs/latest/querying/sql">JOIN operator</a> in Druid SQL. Refer to the |
| <a href="/docs/latest/querying/datasource#join">join datasource</a> documentation for information about how joins work in Druid native queries, |
| or the <a href="/docs/latest/multi-stage-query/reference#joins">multi-stage query join documentation</a> for information about how joins |
| work in multi-stage query tasks.</li> |
| <li><a href="/docs/latest/querying/lookups">Query-time lookups</a>, simple key-to-value mappings. These are preloaded on all servers that are involved |
| in queries and can be accessed with or without an explicit join operator. Refer to the <a href="/docs/latest/querying/lookups">lookups</a> |
| documentation for more details.</li> |
| </ol> |
| <p>Whenever possible, for best performance it is good to avoid joins at query time. Often this can be accomplished by |
| joining data before it is loaded into Druid. However, there are situations where joins or lookups are the best solution |
| available despite the performance overhead, including:</p> |
| <ul> |
| <li>The fact-to-dimension (star and snowflake schema) case: you need to change dimension values after initial ingestion, |
| and aren't able to reingest to do this. In this case, you can use lookups for your dimension tables.</li> |
| <li>Your workload requires joins or filters on subqueries.</li> |
| </ul></div></article><nav class="pagination-nav docusaurus-mt-lg" aria-label="Docs pages"><a class="pagination-nav__link pagination-nav__link--prev" href="/docs/latest/querying/datasource"><div class="pagination-nav__sublabel">Previous</div><div class="pagination-nav__label">Datasources</div></a><a class="pagination-nav__link pagination-nav__link--next" href="/docs/latest/querying/lookups"><div class="pagination-nav__sublabel">Next</div><div class="pagination-nav__label">Lookups</div></a></nav></div></div></div></div></main></div></div></div><footer class="footer"><div class="container container-fluid"><div class="footer__bottom text--center"><div class="margin-bottom--sm"><img src="/img/favicon.png" class="footer__logo themedComponent_mlkZ themedComponent--light_NVdE"><img src="/img/favicon.png" class="footer__logo themedComponent_mlkZ themedComponent--dark_xIcU"></div><div class="footer__copyright">Copyright © 2025 Apache Software Foundation. Except where otherwise noted, licensed under CC BY-SA 4.0. 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></div></footer></div> |
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