v.20.4Improvement

Implementation of non-blocking alter for StorageMergeTree

Official CH changelog text
Implementation of "non-blocking" alter for StorageMergeTree #9606 (alesapin)
Non-blocking ALTER support for StorageMergeTree tables.

Why it matters

This feature allows ALTER commands on StorageMergeTree tables to execute without blocking concurrent SELECT and INSERT operations. It improves system availability and reduces query latency during schema modifications, enhancing uninterrupted data processing and usability for users.

How to use it

Users can apply ALTER commands (such as adding or dropping columns) on StorageMergeTree tables as usual with ALTER TABLE statements, benefiting from the new non-blocking behavior automatically without additional configuration.

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