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    When Is Ttl Applied

    When are TTL rules applied, and do we have control over it?

    TTL rules in ClickHouse are eventually applied, and you can control when they are executed using the `merge_with_ttl_timeout` setting. Learn how to force TTL application and manage background threads for TTL execution.

    TTL Rules and Control​

    TTL is going to be eventually applied. What does that mean? The MergeTree table setting merge_with_ttl_timeout sets the minimum delay in seconds before repeating a merge with delete TTL. The default value is 14400 seconds (4 hours). But that is just the minimum delay, it can take longer until a merge for delete TTL is triggered.

    You can view all of your current TTL settings (like merge_with_ttl_timeout) with this query:

    The response looks like:

    You can use SHOW CREATE TABLE to check if your table contains TTL rules, as well as if any of the table SETTINGS modified the values of the settings above:

    Force a TTL rule to be applied​

    This is not the most elegant solution, but you can explicitly call MATERIALIZE TTL, which forces all the TTL rules of a table to be materialized:

    Background threads affecting TTL​

    It is possible that your TTL rules are not being applied because there are not enough working threads in the background pool. For example, if you insert data intensively, then the whole background pool might be utilized for normal merges. However, you can increase the background pool size.

    You can check your current background pool size with this query:

    The response looks like:

    Check the docs for how to modify the background_pool_size setting, which is configured as:

    You can check the current background pool activity with this query:

    · 3 min read