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uniqCombined

Calculates the approximate number of different argument values.

uniqCombined(HLL_precision)(x[, ...])

The uniqCombined function is a good choice for calculating the number of different values.

Arguments

  • HLL_precision: The base-2 logarithm of the number of cells in 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).
  • X: A variable number of parameters. Parameters can be Tuple, Array, Date, DateTime, String, or numeric types.

Returned value

Implementation details

The uniqCombined 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 does not depend on the query processing order).
Note

Since it uses a 32-bit hash for non-String types, 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.

Compared to the uniq function, the uniqCombined function:

  • 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.

Example

Query:

SELECT uniqCombined(number) FROM numbers(1e6);

Result:

┌─uniqCombined(number)─┐
│              1001148 │ -- 1.00 million
└──────────────────────┘

See the example section of uniqCombined64 for an example of the difference between uniqCombined and uniqCombined64 for much larger inputs.

See Also