2.8 KiB
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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
The function takes a variable number of parameters. Parameters can be Tuple, Array, Date, DateTime, String, or numeric types.
HLL_precision is 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).
Returned value
- A number UInt64-type number.
Implementation details
Function:
-
Calculates a hash (64-bit hash for
Stringand 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 "Note"
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
Compared to the uniq function, the uniqCombined:
- Consumes several times less memory.
- Calculates with several times higher accuracy.
- Usually has slightly lower performance. In some scenarios,
uniqCombinedcan perform better thanuniq, for example, with distributed queries that transmit a large number of aggregation states over the network.
See Also