ClickHouse Estimates Distinct Counts by Thinning a Hash Table
uniq() is a hash table that deliberately throws data away. When it fills up, it halves its sample rate, keeps the survivors, and multiplies the count back up by the…
Hash table
ClickHouse Evaluates IN with Hash Sets
IN is not a comparison, it is a hash set. ClickHouse picks one of thirteen specialisations based on the shape of the right-hand side, packs integer keys into the hash…
ClickHouse Marks Empty Hash Table Cells With Zeroed Memory
ClickHouse’s hash table has no occupancy bit, no tombstone, and no sentinel pointer.
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 Splits GROUP BY into 256 Buckets for Parallel Merges
A GROUP BY needs a serial merge at the end — and that’s where the memory high-water mark lives.