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52 lines
2.6 KiB
Markdown
52 lines
2.6 KiB
Markdown
---
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toc_priority: 192
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---
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# uniqCombined {#agg_function-uniqcombined}
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Calculates the approximate number of different argument values.
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``` sql
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uniqCombined(HLL_precision)(x[, ...])
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```
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The `uniqCombined` function is a good choice for calculating the number of different values.
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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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`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).
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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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Function:
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- Calculates a hash (64-bit hash for `String` and 32-bit otherwise) 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. 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.
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- Provides the result deterministically (it doesn’t depend on the query processing order).
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!!! note "Note"
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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)
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Compared to the [uniq](../../../sql-reference/aggregate-functions/reference/uniq.md#agg_function-uniq) function, the `uniqCombined`:
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- Consumes several times less memory.
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- Calculates with several times higher accuracy.
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- 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.
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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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- [uniqCombined64](../../../sql-reference/aggregate-functions/reference/uniqcombined64.md#agg_function-uniqcombined64)
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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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