Classic mutations work by rewriting whole data parts containing the affected columns. This process relies on the same thread pool as merges. In self-managed replicated environments, each replica must apply the mutation independently. For this reason, mutations are both CPU and IO-intensive and should be scheduled cautiously with permission to run limited to administrators. Resource pressure as a result of mutations manifests itself in several ways. Typically, normally scheduled merges accumulate, which in turn causes our earlier "too many parts" issue. Furthermore, users may experience replication delays. The system.mutations table should give administrators an indication of currently scheduled mutations. Note that mutations can be cancelled, but not rolled back, with the KILL MUTATION query.
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The Mathematical Foundation: McCarthy's Lisp
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