Internals

How ClickHouse Compresses Columns: Delta, Gorilla and the CODEC Pipeline

Oct 22, 2026 21 min read Jay Hubbert

ClickHouse’s column codecs are bit-level transforms chained under a generic compressor: Delta, DoubleDelta and Gorilla, nested by CODEC(…).

Oct 22, 2026 18 min read Jay Hubbert

ClickHouse hashes each inserted block into a UInt128 dedup token and keeps a bounded window of recent tokens to detect retries.

Oct 22, 2026 17 min read Jay Hubbert

ClickHouse splits CSV into row-aligned chunks with a quote-aware scanner, parses them on separate threads, and reorders them with a ticket ring.

Oct 22, 2026 16 min read Jay Hubbert

Replication is a ZooKeeper log plus a content-hash deduplication check that rides along with the block-number allocation.

Oct 22, 2026 30 min read Jay Hubbert

ClickHouse’s function factory: static registration, alias maps, and a declarative signature DSL for overload resolution.