mirror of
https://github.com/ClickHouse/ClickHouse.git
synced 2024-11-23 16:12:01 +00:00
1.8 KiB
1.8 KiB
| toc_priority |
|---|
| 194 |
uniqHLL12
Calculates the approximate number of different argument values, using the HyperLogLog algorithm.
uniqHLL12(x[, ...])
Arguments
The function takes a variable number of parameters. Parameters can be Tuple, Array, Date, DateTime, String, or numeric types.
Returned value
- A UInt64-type number.
Implementation details
Function:
-
Calculates a hash for all parameters in the aggregate, then uses it in calculations.
-
Uses the HyperLogLog algorithm to approximate the number of different argument values.
212 5-bit cells are used. The size of the state is slightly more than 2.5 KB. The result is not very accurate (up to ~10% error) for small data sets (<10K elements). However, the result is fairly accurate for high-cardinality data sets (10K-100M), with a maximum error of ~1.6%. Starting from 100M, the estimation error increases, and the function will return very inaccurate results for data sets with extremely high cardinality (1B+ elements). -
Provides the determinate result (it doesn’t depend on the query processing order).
We don’t recommend using this function. In most cases, use the uniq or uniqCombined function.
See Also