How ClickHouse Recompresses a Part When Its TTL Expires
A RECOMPRESS TTL rewrites a whole part with a new codec once all of its rows have expired.
Mergetree
How SummingMergeTree and AggregatingMergeTree Merge Rows
SummingMergeTree and AggregatingMergeTree combine rows inside the merge algorithm, group by group, as the sorted streams are walked.
ClickHouse Makes a MergeTree Part Visible with One Directory Rename
A MergeTree part is written to a temp directory through a stack of write buffers and published by one atomic directory rename.
How ClickHouse Spreads Parts Across Disks: the Storage Policy
ClickHouse places parts with a three-level reserve ladder: policy → volume → disk, with per-volume part-size limits and TTL-driven moves.
How ReplacingMergeTree Collapses Duplicates and What SELECT FINAL Really Does
ReplacingMergeTree collapses rows by sorting key and version; SELECT FINAL recomputes that collapse at query time.