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40 lines
1.8 KiB
Markdown
40 lines
1.8 KiB
Markdown
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---
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toc_priority: 194
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---
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# uniqHLL12 {#agg_function-uniqhll12}
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Calculates the approximate number of different argument values, using the [HyperLogLog](https://en.wikipedia.org/wiki/HyperLogLog) algorithm.
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``` sql
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uniqHLL12(x[, ...])
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```
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**Parameters**
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The function takes a variable number of parameters. Parameters can be `Tuple`, `Array`, `Date`, `DateTime`, `String`, or numeric types.
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**Returned value**
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- A [UInt64](../../../sql-reference/data-types/int-uint.md)-type number.
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**Implementation details**
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Function:
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- Calculates a hash for all parameters in the aggregate, then uses it in calculations.
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- Uses the HyperLogLog algorithm to approximate the number of different argument values.
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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).
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- Provides the determinate result (it doesn’t depend on the query processing order).
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We don’t recommend using this function. In most cases, use the [uniq](../../../sql-reference/aggregate-functions/reference/uniq.md#agg_function-uniq) or [uniqCombined](../../../sql-reference/aggregate-functions/reference/uniqcombined.md#agg_function-uniqcombined) function.
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**See Also**
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- [uniq](../../../sql-reference/aggregate-functions/reference/uniq.md#agg_function-uniq)
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- [uniqCombined](../../../sql-reference/aggregate-functions/reference/uniqcombined.md#agg_function-uniqcombined)
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- [uniqExact](../../../sql-reference/aggregate-functions/reference/uniqexact.md#agg_function-uniqexact)
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