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class="tocCollapsible_ETCw theme-doc-toc-mobile tocMobile_ITEo"><button type="button" class="clean-btn tocCollapsibleButton_TO0P">On this page</button></div><div class="theme-doc-markdown markdown"><h2 class="anchor anchorWithStickyNavbar_LWe7" id="querying-across-databases">Querying across databases<a href="#querying-across-databases" class="hash-link" aria-label="Direct link to Querying across databases" title="Direct link to Querying across databases"></a></h2><p>Superset offers an experimental feature for querying across different databases. This is done via a special database called "Superset meta database" that uses the "superset://" SQLAlchemy URI. When using the database it's possible to query any table in any of the configured databases using the following syntax:</p><div class="language-sql codeBlockContainer_Ckt0 theme-code-block" style="--prism-color:#393A34;--prism-background-color:#f6f8fa"><div class="codeBlockContent_biex"><pre tabindex="0" class="prism-code language-sql codeBlock_bY9V thin-scrollbar"><code class="codeBlockLines_e6Vv"><span class="token-line" style="color:#393A34"><span class="token keyword" style="color:#00009f">SELECT</span><span class="token plain"> </span><span class="token operator" style="color:#393A34">*</span><span class="token plain"> </span><span class="token keyword" style="color:#00009f">FROM</span><span class="token plain"> </span><span class="token string" style="color:#e3116c">"database name.[[catalog.].schema].table name"</span><span class="token punctuation" style="color:#393A34">;</span><br></span></code></pre><div class="buttonGroup__atx"><button type="button" aria-label="Copy code to clipboard" title="Copy" class="clean-btn"><span class="copyButtonIcons_eSgA" aria-hidden="true"><svg viewBox="0 0 24 24" class="copyButtonIcon_y97N"><path fill="currentColor" d="M19,21H8V7H19M19,5H8A2,2 0 0,0 6,7V21A2,2 0 0,0 8,23H19A2,2 0 0,0 21,21V7A2,2 0 0,0 19,5M16,1H4A2,2 0 0,0 2,3V17H4V3H16V1Z"></path></svg><svg viewBox="0 0 24 24" class="copyButtonSuccessIcon_LjdS"><path fill="currentColor" d="M21,7L9,19L3.5,13.5L4.91,12.09L9,16.17L19.59,5.59L21,7Z"></path></svg></span></button></div></div></div><p>For example:</p><div class="language-sql codeBlockContainer_Ckt0 theme-code-block" style="--prism-color:#393A34;--prism-background-color:#f6f8fa"><div class="codeBlockContent_biex"><pre tabindex="0" class="prism-code language-sql codeBlock_bY9V thin-scrollbar"><code class="codeBlockLines_e6Vv"><span class="token-line" style="color:#393A34"><span class="token keyword" style="color:#00009f">SELECT</span><span class="token plain"> </span><span class="token operator" style="color:#393A34">*</span><span class="token plain"> </span><span class="token keyword" style="color:#00009f">FROM</span><span class="token plain"> </span><span class="token string" style="color:#e3116c">"examples.birth_names"</span><span class="token punctuation" style="color:#393A34">;</span><br></span></code></pre><div class="buttonGroup__atx"><button type="button" aria-label="Copy code to clipboard" title="Copy" class="clean-btn"><span class="copyButtonIcons_eSgA" aria-hidden="true"><svg viewBox="0 0 24 24" class="copyButtonIcon_y97N"><path fill="currentColor" d="M19,21H8V7H19M19,5H8A2,2 0 0,0 6,7V21A2,2 0 0,0 8,23H19A2,2 0 0,0 21,21V7A2,2 0 0,0 19,5M16,1H4A2,2 0 0,0 2,3V17H4V3H16V1Z"></path></svg><svg viewBox="0 0 24 24" class="copyButtonSuccessIcon_LjdS"><path fill="currentColor" d="M21,7L9,19L3.5,13.5L4.91,12.09L9,16.17L19.59,5.59L21,7Z"></path></svg></span></button></div></div></div><p>Spaces are allowed, but periods in the names must be replaced by <code>%2E</code>. Eg:</p><div class="language-sql codeBlockContainer_Ckt0 theme-code-block" style="--prism-color:#393A34;--prism-background-color:#f6f8fa"><div class="codeBlockContent_biex"><pre tabindex="0" class="prism-code language-sql codeBlock_bY9V thin-scrollbar"><code class="codeBlockLines_e6Vv"><span class="token-line" style="color:#393A34"><span class="token keyword" style="color:#00009f">SELECT</span><span class="token plain"> </span><span class="token operator" style="color:#393A34">*</span><span class="token plain"> </span><span class="token keyword" style="color:#00009f">FROM</span><span class="token plain"> </span><span class="token string" style="color:#e3116c">"Superset meta database.examples%2Ebirth_names"</span><span class="token punctuation" style="color:#393A34">;</span><br></span></code></pre><div class="buttonGroup__atx"><button type="button" aria-label="Copy code to clipboard" title="Copy" class="clean-btn"><span class="copyButtonIcons_eSgA" aria-hidden="true"><svg viewBox="0 0 24 24" class="copyButtonIcon_y97N"><path fill="currentColor" d="M19,21H8V7H19M19,5H8A2,2 0 0,0 6,7V21A2,2 0 0,0 8,23H19A2,2 0 0,0 21,21V7A2,2 0 0,0 19,5M16,1H4A2,2 0 0,0 2,3V17H4V3H16V1Z"></path></svg><svg viewBox="0 0 24 24" class="copyButtonSuccessIcon_LjdS"><path fill="currentColor" d="M21,7L9,19L3.5,13.5L4.91,12.09L9,16.17L19.59,5.59L21,7Z"></path></svg></span></button></div></div></div><p>The query above returns the same rows as <code>SELECT * FROM "examples.birth_names"</code>, and also shows that the meta database can query tables from any table — even itself!</p><h2 class="anchor anchorWithStickyNavbar_LWe7" id="considerations">Considerations<a href="#considerations" class="hash-link" aria-label="Direct link to Considerations" title="Direct link to Considerations"></a></h2><p>Before enabling this feature, there are a few considerations that you should have in mind. First, the meta database enforces permissions on the queried tables, so users should only have access via the database to tables that they originally have access to. Nevertheless, the meta database is a new surface for potential attacks, and bugs could allow users to see data they should not.</p><p>Second, there are performance considerations. The meta database will push any filtering, sorting, and limiting to the underlying databases, but any aggregations and joins will happen in memory in the process running the query. Because of this, it's recommended to run the database in async mode, so queries are executed in Celery workers, instead of the web workers. Additionally, it's possible to specify a hard limit on how many rows are returned from the underlying databases.</p><h2 class="anchor anchorWithStickyNavbar_LWe7" id="enabling-the-meta-database">Enabling the meta database<a href="#enabling-the-meta-database" class="hash-link" aria-label="Direct link to Enabling the meta database" title="Direct link to Enabling the meta database"></a></h2><p>To enable the Superset meta database, first you need to set the <code>ENABLE_SUPERSET_META_DB</code> feature flag to true. Then, add a new database of type "Superset meta database" with the SQLAlchemy URI "superset://".</p><p>If you enable DML in the meta database users will be able to run DML queries on underlying databases <strong>as long as DML is also enabled in them</strong>. This allows users to run queries that move data across databases.</p><p>Second, you might want to change the value of <code>SUPERSET_META_DB_LIMIT</code>. The default value is 1000, and defines how many are read from each database before any aggregations and joins are executed. You can also set this value <code>None</code> if you only have small tables.</p><p>Additionally, you might want to restrict the databases to with the meta database has access to. This can be done in the database configuration, under "Advanced" -> "Other" -> "ENGINE PARAMETERS" and adding:</p><div class="language-json codeBlockContainer_Ckt0 theme-code-block" style="--prism-color:#393A34;--prism-background-color:#f6f8fa"><div class="codeBlockContent_biex"><pre tabindex="0" class="prism-code language-json codeBlock_bY9V thin-scrollbar"><code class="codeBlockLines_e6Vv"><span class="token-line" style="color:#393A34"><span class="token punctuation" style="color:#393A34">{</span><span class="token property" style="color:#36acaa">"allowed_dbs"</span><span class="token operator" style="color:#393A34">:</span><span class="token punctuation" style="color:#393A34">[</span><span class="token string" style="color:#e3116c">"Google Sheets"</span><span class="token punctuation" style="color:#393A34">,</span><span class="token string" style="color:#e3116c">"examples"</span><span class="token punctuation" style="color:#393A34">]</span><span class="token punctuation" style="color:#393A34">}</span><br></span></code></pre><div class="buttonGroup__atx"><button type="button" aria-label="Copy code to clipboard" title="Copy" class="clean-btn"><span class="copyButtonIcons_eSgA" aria-hidden="true"><svg viewBox="0 0 24 24" class="copyButtonIcon_y97N"><path fill="currentColor" d="M19,21H8V7H19M19,5H8A2,2 0 0,0 6,7V21A2,2 0 0,0 8,23H19A2,2 0 0,0 21,21V7A2,2 0 0,0 19,5M16,1H4A2,2 0 0,0 2,3V17H4V3H16V1Z"></path></svg><svg viewBox="0 0 24 24" class="copyButtonSuccessIcon_LjdS"><path fill="currentColor" d="M21,7L9,19L3.5,13.5L4.91,12.09L9,16.17L19.59,5.59L21,7Z"></path></svg></span></button></div></div></div></div><footer class="theme-doc-footer docusaurus-mt-lg"><div class="theme-doc-footer-edit-meta-row row"><div class="col"><a href="https://github.com/apache/superset/tree/master/docs/docs/databases/meta-database.mdx" target="_blank" rel="noreferrer noopener" class="theme-edit-this-page"><svg fill="currentColor" height="20" width="20" viewBox="0 0 40 40" class="iconEdit_Z9Sw" aria-hidden="true"><g><path d="m34.5 11.7l-3 3.1-6.3-6.3 3.1-3q0.5-0.5 1.2-0.5t1.1 0.5l3.9 3.9q0.5 0.4 0.5 1.1t-0.5 1.2z m-29.5 17.1l18.4-18.5 6.3 6.3-18.4 18.4h-6.3v-6.2z"></path></g></svg>Edit this page</a></div><div class="col lastUpdated_vwxv"></div></div></footer></article><nav class="pagination-nav docusaurus-mt-lg" aria-label="Docs pages"><a class="pagination-nav__link pagination-nav__link--prev" href="/docs/databases/kusto"><div class="pagination-nav__sublabel">Previous</div><div class="pagination-nav__label">kusto</div></a><a class="pagination-nav__link pagination-nav__link--next" href="/docs/miscellaneous/country-map-tools"><div class="pagination-nav__sublabel">Next</div><div class="pagination-nav__label">Country Map Tools</div></a></nav></div></div><div class="col col--3"><div class="tableOfContents_bqdL thin-scrollbar theme-doc-toc-desktop"><ul class="table-of-contents table-of-contents__left-border"><li><a href="#querying-across-databases" class="table-of-contents__link toc-highlight">Querying across databases</a></li><li><a href="#considerations" class="table-of-contents__link toc-highlight">Considerations</a></li><li><a href="#enabling-the-meta-database" class="table-of-contents__link toc-highlight">Enabling the meta database</a></li></ul></div></div></div></div></main></div></div><footer class="footer"><div class="container container-fluid"><div class="footer__bottom text--center"><div class="footer__copyright"> |
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