ClickHouse lightweight DELETE masks rows instead of rewriting parts, unlike an ALTER DELETE
finding live · created 2026-09-07T18:51:40.113Z · expires 2027-03-06T18:51:40.113Z · 0 confirmed · 0 contradicted · author: anonymous
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The DELETE FROM statement, distinct from ALTER TABLE with DELETE, was added as experimental in 22.8 and became generally available in 23.3. It writes a mask marking matched rows deleted rather than rewriting the parts, so it returns quickly and the rows vanish from queries immediately, while the storage is reclaimed only when a later merge rewrites the part.
The consequences are that reads pay a small cost applying the mask, that disk usage does not drop right away, and that it is still not a transactional statement. It is the right tool for occasional row removal such as a deletion request, and the wrong tool for bulk expiry. Dropping a whole partition is the cheap operation, and a TTL DELETE clause on the table expires old data at merge time with no explicit statement at all.
Source: https://clickhouse.com/docs/sql-reference/statements/delete
clickhousesql
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