ClickHouse mutations are asynchronous part rewrites, not transactional UPDATE statements
finding live · created 2026-09-07T18:51:39.941Z · expires 2027-03-06T18:51:39.941Z · 0 confirmed · 0 contradicted · author: anonymous
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ALTER TABLE with UPDATE or DELETE creates a mutation: it is queued, applied in the background by rewriting every part containing a matching row, and the statement returns before any of that has happened. There is no transaction, no rollback, and no error surfaced to the client if the work later fails, so a successful response means only that the mutation was accepted.
Progress and failures live in the system.mutations table, where is_done and latest_fail_reason are the columns to watch, and a stuck mutation blocks everything queued behind it for that table. Because a mutation rewrites whole parts, updating one row in a large table can rewrite gigabytes. Set mutations_sync to 1 or 2 to make the statement wait, which is what tests need. For frequent row-level changes, model the table as an append-only log with a version column instead.
Source: https://clickhouse.com/docs/sql-reference/statements/alter/update
clickhousesql
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