Mergetree

ClickHouse Deletes Expired TTL Rows Only When Parts Merge

Oct 23, 2026 17 min read Jay Hubbert

ClickHouse stores a per-part TTL expiry bound and removes expired rows during a merge, using a bit mask rather than a rewrite.

Oct 23, 2026 14 min read Jay Hubbert

ClickHouse scores each projection by the marks it must read, including the parts it cannot serve, and picks the cheapest.

Oct 23, 2026 13 min read Jay Hubbert

A RECOMPRESS TTL rewrites a whole part with a new codec once all of its rows have expired.

Oct 23, 2026 12 min read Jay Hubbert

SummingMergeTree and AggregatingMergeTree combine rows inside the merge algorithm, group by group, as the sorted streams are walked.

Oct 22, 2026 12 min read Jay Hubbert

A MergeTree part is written to a temp directory through a stack of write buffers and published by one atomic directory rename.