ClickHouse's ArenaWithFreeLists Recycles by Size Class: 14 Lists From 8 to 65536 Bytes
ClickHouse’s ArenaWithFreeLists keeps one free list per power-of-two size class and threads the free-list link through the freed block itself.
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ClickHouse's LRU Cache Evicts by Bytes, Not Entries: A Hit Is One List Splice
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.
ClickHouse's SLRU Cache Promotes an Entry on Its Second Touch: A Scan Can't Flush It
ClickHouse’s default cache policy is SLRU, not LRU: entries hit twice are promoted to a protected queue that a sequential scan cannot flush.
ClickHouse Short-Circuits AND and OR: A Thunk and a UInt8 Mask of Undecided Rows
ClickHouse defers heavy and/or/if arguments into a thunk and runs them only on the rows a UInt8 mask leaves undecided.
ClickHouse Runs 47 Rewrite Passes Before Planning: count(*) Becomes a Metadata Read
ClickHouse inlines aliases, runs 47 ordered tree passes, prunes columns and rewrites count(*) into a metadata read before planning.