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docs(meta-db): warn that SUPERSET_META_DB_LIMIT truncates tables before joins

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Addresses #36304. The Superset meta database applies SUPERSET_META_DB_LIMIT
(default 1000) to each underlying table before the in-memory join runs, so a
join over tables larger than the limit can silently return incomplete or empty
results. Document this caveat near the existing SUPERSET_META_DB_LIMIT prose.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
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Evan
2026-06-22 17:46:21 -07:00
parent 3b46a5f121
commit 2c63a83829
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docs/user_docs_versioned_docs/version-6.0.0/configuration/databases.mdx
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@@ -1808,6 +1808,10 @@ If you enable DML in the meta database users will be able to run DML queries on
Second, you might want to change the value of `SUPERSET_META_DB_LIMIT`. 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 `None` if you only have small tables.
:::warning
`SUPERSET_META_DB_LIMIT` is applied to **each** underlying table *before* the in-memory join runs, not to the final result. If any table involved in a join has more rows than the limit, the meta database will read only the first `SUPERSET_META_DB_LIMIT` rows of that table, which means matching rows can be silently dropped and the join can return **incomplete or even empty** results with no error. If you join tables larger than the limit, raise `SUPERSET_META_DB_LIMIT` to comfortably exceed your largest joined table, or set it to `None` when working only with small tables, to get correct results.
:::
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:
```json