How ClickHouse Picks a Projection to Read
ClickHouse scores each projection by the marks it must read, including the parts it cannot serve, and picks the cheapest.
Internals
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.
How ClickHouse Splits a Session Context from a Query Context
ClickHouse splits session state from per-query state, and gates every setting change through a constraint checker.
How ClickHouse Turns Row-Level Security into a Filter Expression
ClickHouse mixes row policies into one boolean expression and injects it into the query plan as a filter on the table read.
How SummingMergeTree and AggregatingMergeTree Merge Rows
SummingMergeTree and AggregatingMergeTree combine rows inside the merge algorithm, group by group, as the sorted streams are walked.