Clickhouse

ClickHouse Recycles Small Objects Through Per-Size Free Lists

Oct 23, 2026 18 min read Jay Hubbert

ClickHouse’s ArenaWithFreeLists keeps one free list per power-of-two size class and threads the free-list link through the freed block itself.

Oct 23, 2026 19 min read Jay Hubbert

ClickHouse inlines aliases, runs 47 ordered tree passes, prunes columns and rewrites count(*) into a metadata read before planning.

Oct 23, 2026 16 min read Jay Hubbert

Distributed DDL is a ZooKeeper queue of sequential entries plus a per-host finished directory that the initiator lists to collect results.

Oct 23, 2026 21 min read Jay Hubbert

ClickHouse defers heavy and/or/if arguments into a thunk and runs them only on the rows a UInt8 mask leaves undecided.

Oct 23, 2026 20 min read Jay Hubbert

ClickHouse’s LRU cache is a hash map of cells pointing into a linked list, so a lookup is an O(1) splice and eviction is a pop from the front.