CityHash
ClickHouse uses one of previous versions of CityHash from Google.
CityHash has changed the algorithm after we have added it into ClickHouse.
CityHash documentation specifically notes that the user should not rely to specific hash values and should not save it anywhere or use it as sharding key.
But as we exposed this function to the user, we had to fix the version of CityHash (to 1.0.2). And now we guarantee that the behaviour of CityHash functions available in SQL will not change.
— Alexey Milovidov
Current version of Google's CityHash differs from ClickHouse cityHash64 variant.
Don't use farmHash64 to get Google's CityHash value! FarmHash is a successor to CityHash, but they are not fully compatible.
| String | ClickHouse64 | CityHash64 | FarmHash64 |
|---|---|---|---|
Moscow | 12507901496292878638 | 5992710078453357409 | 5992710078453357409 |
How can you write a big system without C++? -Paul Glick | 6237945311650045625 | 749291162957442504 | 11716470977470720228 |
Also see Introducing CityHash for description and reasoning behind creation. TL;DR non-cryptographic hash that is faster than MurmurHash, but more complex.
Implementations
Go
You can use go-faster/city Go package that implements both variants.