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skills: add TTL and partition key alignment guidance (#21)

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* skills: add TTL and partition key alignment guidance
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Paco González López
2026-07-29 13:00:01 +02:00
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skills/tinybird-best-practices/rules/datasource-files.md
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@@ -69,6 +69,23 @@ FORWARD_QUERY >
Once the deploy applies the `FORWARD_QUERY` and the schema change is live, the `FORWARD_QUERY` has done its job. You can remove it from the datafile in a subsequent deploy if no further schema changes are pending. Keeping stale `FORWARD_QUERY` blocks around adds unnecessary complexity.
## TTL and Partition Key Alignment
Apply when a datasource sets both `ENGINE_TTL` and `ENGINE_PARTITION_KEY`: use a partition granularity equal to or finer than the TTL window (e.g. daily partitions for a TTL in days), so partitions expire and drop as whole units instead of ClickHouse rewriting them. A partition coarser than the TTL window (e.g. yearly partitions with a 65-day TTL) never fully expires, forcing constant rewrites instead of cheap drops.
```
# Bad: yearly partition, 65-day TTL — partition never fully expires
ENGINE_PARTITION_KEY "toYYYY(timestamp)"
ENGINE_TTL "toDateTime(timestamp) + toIntervalDay(65)"
# Good: daily partition matches the TTL window — old partitions drop whole
ENGINE_PARTITION_KEY "toDate(timestamp)"
ENGINE_TTL "toDate(timestamp) + toIntervalDay(65)"
ENGINE_SETTINGS "ttl_only_drop_parts=1"
```
When possible, set `ttl_only_drop_parts=1` in `ENGINE_SETTINGS` — it makes ClickHouse only drop whole expired parts instead of rewriting partially-expired ones, which is much cheaper.
## Sharing Datasources
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