Compression modes

ClickHouse column compression modes

ClickHouse protocol supports data blocks compression with checksums. Use LZ4 if not sure what mode to pick. “

Learn more about the column compression codecs available and specify them when creating your tables, or afterward.

Modes

value	name	description
0x02	None	No compression, only checksums
0x82	LZ4	Extremely fast, good compression
0x90	ZSTD	Zstandard, pretty fast, best compression

Both LZ4 and ZSTD are made by same author, but with different tradeoffs. From Facebook benchmarks:

name	ratio	encoding	decoding
zstd 1.4.5 -1	2.8	500 MB/s	1660 MB/s
lz4 1.9.2	2.1	740 MB/s	4530 MB/s

Block

field	type	description
checksum	uint128	Hash of (header + compressed data)
raw_size	uint32	Raw size without header
data_size	uint32	Uncompressed data size
mode	byte	Compression mode
compressed_data	binary	Block of compressed data

Header is (raw_size + data_size + mode), raw size consists of len(header + compressed_data).

Checksum is hash(header + compressed_data), using ClickHouse CityHash.

None mode

If None mode is used, compressed_data is equal to original data. No compression mode is useful to ensure additional data integrity with checksums, because hashing overhead is negligible.