uniqCombined64

Introduced in: v20.1.0

Calculates the approximate number of different argument values. It is the same as uniqCombined, but uses a 64-bit hash for all data types rather than just for the String data type.

This function provides the result deterministically (it does not depend on the query processing order).

Compared to the uniq function, the uniqCombined64 function:

Consumes several times less memory
Calculates with several times higher accuracy

Implementation details

This function calculates a 64-bit hash for all data types for all parameters in the aggregate, then uses it in calculations. It 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

Syntax

uniqCombined64(HLL_precision)(x[, ...])
uniqCombined64(x[, ...])

Parameters

HLL_precision — Optional. The base-2 logarithm of the number of cells in HyperLogLog. The default value is 17, which is effectively 96 KiB of space (2^17 cells, 6 bits each). Range: [12, 20]. UInt8

Arguments

x — A variable number of parameters. Tuple(T) or Array(T) or Date or DateTime or String or (U)Int* or Float* or Decimal

Returned value

Returns a UInt64-type number representing the approximate number of different argument values. UInt64

Examples

Large dataset example

SELECT uniqCombined64(number) FROM numbers(1e10);

┌─uniqCombined64(number)─┐
│             9998568925 │
└────────────────────────┘

Comparison with uniqCombined

-- uniqCombined64 with large dataset
SELECT uniqCombined64(number) FROM numbers(1e10);

-- uniqCombined with same dataset shows poor approximation
SELECT uniqCombined(number) FROM numbers(1e10);

┌─uniqCombined64(number)─┐
│             9998568925 │
└────────────────────────┘
┌─uniqCombined(number)─┐
│           5545308725 │
└──────────────────────┘

See Also