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I'd like to understand if the data removal mechanism used by the Clickhouse TTL feature is the same as for the ALTER DELETE mutation. In particular, is the following statement (documentation) referring the ALTER DELETE mutation also valid for the TTL feature:

For *MergeTree tables mutations execute by rewriting whole data parts. There is no atomicity - parts are substituted for mutated parts as soon as they are ready and a SELECT query that started executing during a mutation will see data from parts that have already been mutated along with data from parts that have not been mutated yet.

In other words is my understanding correct that TTL feature is simply an automatically scheduled ALTER DELETE mutation?

makasprzak
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  • "TTL deletes cause an expensive mutation operation" - look at this article [Putting Things Where They Belong Using New TTL Moves](https://www.altinity.com/blog/2020/3/23/putting-things-where-they-belong-using-new-ttl-moves) for details – vladimir Apr 02 '20 at 23:52

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