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83 lines
3.3 KiB
Markdown
83 lines
3.3 KiB
Markdown
---
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slug: /en/sql-reference/aggregate-functions/reference/uniqcombined64
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sidebar_position: 193
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---
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# uniqCombined64
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Calculates the approximate number of different argument values. It is the same as [uniqCombined](../../../sql-reference/aggregate-functions/reference/uniqcombined.md#agg_function-uniqcombined), but uses a 64-bit hash for all data types rather than just for the String data type.
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``` sql
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uniqCombined64(HLL_precision)(x[, ...])
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```
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**Parameters**
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- `HLL_precision`: The base-2 logarithm of the number of cells in [HyperLogLog](https://en.wikipedia.org/wiki/HyperLogLog). Optionally, you can use the function as `uniqCombined64(x[, ...])`. The default value for `HLL_precision` is 17, which is effectively 96 KiB of space (2^17 cells, 6 bits each).
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- `X`: 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 number [UInt64](../../../sql-reference/data-types/int-uint.md)-type number.
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**Implementation details**
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The `uniqCombined64` function:
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- Calculates a hash (64-bit hash for all data types) for all parameters in the aggregate, then uses it in calculations.
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- Uses a combination of three algorithms: array, hash table, and HyperLogLog with an error correction table.
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- For a small number of distinct elements, an array is used.
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- When the set size is larger, a hash table is used.
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- For a larger number of elements, HyperLogLog is used, which will occupy a fixed amount of memory.
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- Provides the result deterministically (it does not depend on the query processing order).
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:::note
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Since it uses 64-bit hash for all types, the result does not suffer from very high error for cardinalities significantly larger than `UINT_MAX` like [uniqCombined](../../../sql-reference/aggregate-functions/reference/uniqcombined.md) does, which uses a 32-bit hash for non-`String` types.
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:::
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Compared to the [uniq](../../../sql-reference/aggregate-functions/reference/uniq.md#agg_function-uniq) function, the `uniqCombined64` function:
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- Consumes several times less memory.
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- Calculates with several times higher accuracy.
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**Example**
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In the example below `uniqCombined64` is run on `1e10` different numbers returning a very close approximation of the number of different argument values.
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Query:
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```sql
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SELECT uniqCombined64(number) FROM numbers(1e10);
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```
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Result:
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```response
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┌─uniqCombined64(number)─┐
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│ 9998568925 │ -- 10.00 billion
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└────────────────────────┘
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```
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By comparison the `uniqCombined` function returns a rather poor approximation for an input this size.
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Query:
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```sql
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SELECT uniqCombined(number) FROM numbers(1e10);
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```
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Result:
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```response
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┌─uniqCombined(number)─┐
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│ 5545308725 │ -- 5.55 billion
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└──────────────────────┘
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```
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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)
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- [uniqHLL12](../../../sql-reference/aggregate-functions/reference/uniqhll12.md#agg_function-uniqhll12)
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- [uniqExact](../../../sql-reference/aggregate-functions/reference/uniqexact.md#agg_function-uniqexact)
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- [uniqTheta](../../../sql-reference/aggregate-functions/reference/uniqthetasketch.md#agg_function-uniqthetasketch)
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