mirror of
https://github.com/ClickHouse/ClickHouse.git
synced 2024-09-24 10:40:49 +00:00
258d2fd499
* normalize
* split & adjust links
* re-normalize
* adjust ru links
* adjust ja/tr links
* partially apply e0d19d2aea
* reset contribs
40 lines
1.8 KiB
Markdown
40 lines
1.8 KiB
Markdown
---
|
||
toc_priority: 194
|
||
---
|
||
|
||
# uniqHLL12 {#agg_function-uniqhll12}
|
||
|
||
Calculates the approximate number of different argument values, using the [HyperLogLog](https://en.wikipedia.org/wiki/HyperLogLog) algorithm.
|
||
|
||
``` sql
|
||
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](../../../sql-reference/data-types/int-uint.md)-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](../../../sql-reference/aggregate-functions/reference/uniq.md#agg_function-uniq) or [uniqCombined](../../../sql-reference/aggregate-functions/reference/uniqcombined.md#agg_function-uniqcombined) function.
|
||
|
||
**See Also**
|
||
|
||
- [uniq](../../../sql-reference/aggregate-functions/reference/uniq.md#agg_function-uniq)
|
||
- [uniqCombined](../../../sql-reference/aggregate-functions/reference/uniqcombined.md#agg_function-uniqcombined)
|
||
- [uniqExact](../../../sql-reference/aggregate-functions/reference/uniqexact.md#agg_function-uniqexact)
|