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.
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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.
ClickHouse Flattens an Expression DAG into Fixed Slots: Replayed Once Per 65 409-Row Block
ClickHouse compiles a SELECT into an ActionsDAG and reuses the sorted plan for every block.
ClickHouse Reads MergeTree Parts in Key Order: Constant Key Columns Count Toward the Prefix
ClickHouse matches ORDER BY against the sorting key and reads MergeTree parts in key order instead of sorting everything.