ClickHouse/docs/zh/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[, ...])

Parameters

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

• uniq
• uniqCombined
• uniqExact