ClickHouse/docs/en/sql-reference/aggregate-functions/reference/uniqhll12.md

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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.

  2^12 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 does not depend on the query processing order).

We do not recommend using this function. In most cases, use the uniq or uniqCombined function.

See Also

• uniq
• uniqCombined
• uniqExact
• uniqTheta