v.20.3Improvement

Implement ALTER MODIFY/DROP Queries for ReplicatedMergeTree Engines

Official CH changelog text
Implement ALTER MODIFY/DROP queries on top of mutations for ReplicatedMergeTree* engines family. Now ALTERS blocks only at the metadata update stage, and don't block after that. #8701 (alesapin)
Implement ALTER MODIFY and ALTER DROP queries on top of mutations for the ReplicatedMergeTree* family of engines, reducing the duration of ALTER query blocking.

Why it matters

Previously, ALTER queries would block not only during the metadata update stage but also afterwards, which could impact performance and concurrency. This feature ensures that ALTER queries only block during the metadata update stage and do not block after that by leveraging mutations, improving system responsiveness and reducing interference with data operations.

How to use it

Users can run ALTER MODIFY and ALTER DROP queries as usual on tables with ReplicatedMergeTree* engines. The underlying implementation automatically uses mutations, so no additional user action is needed to enable or configure this behavior.

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