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Docs: Add info on lightweight_deletes_sync

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Christoph Wurm 2024-09-05 13:42:09 +01:00
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docs/en/sql-reference/statements/delete.md
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@ -24,9 +24,15 @@ DELETE FROM hits WHERE Title LIKE '%hello%';
## Lightweight `DELETE` does not delete data immediately
Lightweight `DELETE` is implemented as a [mutation](/en/sql-reference/statements/alter#mutations), which is executed asynchronously in the background by default. The statement is going to return almost immediately, but the data can still be visible to queries until the mutation is finished.
Lightweight `DELETE` is implemented as a [mutation](/en/sql-reference/statements/alter#mutations) that marks rows as deleted but does not immediately physically delete them.
The mutation marks rows as deleted, and at that point, they will no longer show up in query results. It does not physically delete the data, this will happen during the next merge. As a result, it is possible that for an unspecified period, data is not actually deleted from storage and is only marked as deleted.
By default, `DELETE` statements wait until marking the rows as deleted is completed before returning. This can take a long time if the amount of data is large. Alternatively, you can run it asynchronously in the background using the setting [`lightweight_deletes_sync`](/en/operations/settings/settings#lightweight_deletes_sync). If disabled, the `DELETE` statement is going to return immediately, but the data can still be visible to queries until the background mutation is finished.
:::note
Before version 24.4, lightweight deletes were asynchronous by default.
:::
The mutation does not physically delete the rows that have been marked as deleted, this will only happen during the next merge. As a result, it is possible that for an unspecified period, data is not actually deleted from storage and is only marked as deleted.
If you need to guarantee that your data is deleted from storage in a predictable time, consider using the table setting [`min_age_to_force_merge_seconds`](https://clickhouse.com/docs/en/operations/settings/merge-tree-settings#min_age_to_force_merge_seconds). Or you can use the [ALTER TABLE ... DELETE](/en/sql-reference/statements/alter/delete) command. Note that deleting data using `ALTER TABLE ... DELETE` may consume significant resources as it recreates all affected parts.