Lightweight updates take a fundamentally different approach. Instead of rewriting entire data parts, a lightweight update creates a small, compact "patch part" containing only the changed column values and metadata to locate the affected rows. These patches are applied on-the-fly during reads (so changes are visible immediately) and materialized efficiently during regular background merges — piggybacking on work ClickHouse is already doing. This can be up to 1,000× faster than classic mutations for many workloads. The same mechanism can be used for deletes, which set a _row_exists = 0 mask via a patch part rather than rewriting columns. For a deep dive into how patch parts work, see our series on fast UPDATEs in ClickHouse: Part 1 — purpose-built engines and Part 2 — SQL-style updates.
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