ClickHouse Aggregates In Order and Stops Reading Early
If your table is sorted by the GROUP BY key, ClickHouse can skip the hash table entirely: it counts each contiguous run of equal keys and emits it.
Aggregation
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…
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.