uniqCombined

Introduced in: v1.1.0

Calculates the approximate number of different argument values. It provides the result deterministically (it does not depend on the query processing order).

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

Implementation details

This function calculates a hash (64-bit hash for String and 32-bit otherwise) 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

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

Basic usage

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

┌─uniqCombined(number)─┐
│              1001148 │
└──────────────────────┘

With custom precision

SELECT uniqCombined(15)(number) FROM numbers(1e5);

┌─uniqCombined(15)(number)─┐
│                   100768 │
└──────────────────────────┘

See Also