How ClickHouse Stores Dictionaries in Sharded Hash Tables
The hashed dictionary packs keys compactly, shards the table for parallel loading, and picks its hash table by measured memory footprint.
Performance
How ClickHouse Writes the Native Format, Column at a Time
The Native format writes a row count, then for each column its name, type, serialization kind and bulk data — never a row.
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.
ClickHouse Decides Which Parts to Merge: A Cost Model for Write Amplification
ClickHouse picks merge candidates with a tiny, explicit cost model: score each contiguous range as sum_size / (count - 1.9), pick the cheapest admissible one, and only ever…
ClickHouse Evaluates IN with Hash Sets: Build Once, Probe Once Per Row
ClickHouse picks one of thirteen hash-table shapes for IN based on the key type, then spills the set to disk behind per-block Bloom filters when it outgrows RAM.