ClickHouse Marks Empty Hash Table Cells With Zeroed Memory
ClickHouse’s hash table has no occupancy bit, no tombstone, and no sentinel pointer.
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ClickHouse Merges Columns One at a Time: The Vertical Merge Algorithm
ClickHouse’s vertical merge splits a merge into two stages: merge the key and record where each output row came from (one byte per row), then gather each payload column…
ClickHouse Packs Short Strings Into Hash Table Keys: No Allocation Per Key
ClickHouse never stores a std::string in a hash table cell. It splits keys into size classes and packs strings of up to 24 bytes into plain integers, so your GROUP BY…
ClickHouse Plans Distributed Queries with Cascades: A Real Cost-Based Optimizer
ClickHouse’s new Cascades optimizer treats a distributed query as a search problem.
ClickHouse Runs a Query Pipeline on N Threads: Task Queues, Work Stealing and a Pool That Grows on Demand
ClickHouse doesn’t hand you max_threads threads. It starts with one, measures how much parallel work the pipeline actually creates, and grows the pool only when idle…