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…
Performance
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 Skips Data: Granules, Marks, and Bit-Packed Indexes
A MergeTree scan isn’t a full scan. Parts are cut into granules, each granule gets a bit-packed mark, and the primary key index stores one key value per granule — so…
ClickHouse Sorts in Parallel: Partial Sort, Merge Sorted Streams and External Sort
ClickHouse never sorts the whole dataset at once. It sorts each chunk, throws away rows that can’t make the top-N, merges the sorted streams, and spills runs to disk when…