mirror of
https://github.com/ClickHouse/ClickHouse.git
synced 2024-11-28 02:21:59 +00:00
53 lines
2.8 KiB
Markdown
53 lines
2.8 KiB
Markdown
---
|
||
toc_priority: 192
|
||
---
|
||
|
||
# uniqCombined {#agg_function-uniqcombined}
|
||
|
||
Calculates the approximate number of different argument values.
|
||
|
||
``` sql
|
||
uniqCombined(HLL_precision)(x[, ...])
|
||
```
|
||
|
||
The `uniqCombined` function is a good choice for calculating the number of different values.
|
||
|
||
**Arguments**
|
||
|
||
The function takes a variable number of parameters. Parameters can be `Tuple`, `Array`, `Date`, `DateTime`, `String`, or numeric types.
|
||
|
||
`HLL_precision` is the base-2 logarithm of the number of cells in [HyperLogLog](https://en.wikipedia.org/wiki/HyperLogLog). Optional, you can use the function as `uniqCombined(x[, ...])`. The default value for `HLL_precision` is 17, which is effectively 96 KiB of space (2^17 cells, 6 bits each).
|
||
|
||
**Returned value**
|
||
|
||
- A number [UInt64](../../../sql-reference/data-types/int-uint.md)-type number.
|
||
|
||
**Implementation details**
|
||
|
||
Function:
|
||
|
||
- Calculates a hash (64-bit hash for `String` and 32-bit otherwise) for all parameters in the aggregate, then uses it in calculations.
|
||
|
||
- Uses a combination of three algorithms: array, hash table, and HyperLogLog with an error correction table.
|
||
|
||
For a small number of distinct elements, an array is used. When the set size is larger, a hash table is used. For a larger number of elements, HyperLogLog is used, which will occupy a fixed amount of memory.
|
||
|
||
- Provides the result deterministically (it doesn’t depend on the query processing order).
|
||
|
||
!!! note "Note"
|
||
Since it uses 32-bit hash for non-`String` type, the result will have very high error for cardinalities significantly larger than `UINT_MAX` (error will raise quickly after a few tens of billions of distinct values), hence in this case you should use [uniqCombined64](../../../sql-reference/aggregate-functions/reference/uniqcombined64.md#agg_function-uniqcombined64)
|
||
|
||
Compared to the [uniq](../../../sql-reference/aggregate-functions/reference/uniq.md#agg_function-uniq) function, the `uniqCombined`:
|
||
|
||
- Consumes several times less memory.
|
||
- Calculates with several times higher accuracy.
|
||
- Usually has slightly lower performance. In some scenarios, `uniqCombined` can perform better than `uniq`, for example, with distributed queries that transmit a large number of aggregation states over the network.
|
||
|
||
**See Also**
|
||
|
||
- [uniq](../../../sql-reference/aggregate-functions/reference/uniq.md#agg_function-uniq)
|
||
- [uniqCombined64](../../../sql-reference/aggregate-functions/reference/uniqcombined64.md#agg_function-uniqcombined64)
|
||
- [uniqHLL12](../../../sql-reference/aggregate-functions/reference/uniqhll12.md#agg_function-uniqhll12)
|
||
- [uniqExact](../../../sql-reference/aggregate-functions/reference/uniqexact.md#agg_function-uniqexact)
|
||
- [uniqTheta](../../../sql-reference/aggregate-functions/reference/uniqthetasketch.md#agg_function-uniqthetasketch)
|