hll() (aggregation function)

Learn how to use the hll() function to calculate the results of the dcount() function.

The hll() function is a way to estimate the number of unique values in a set of values. It does so by calculating intermediate results for aggregation within the summarize operator for a group of data using the dcount function.

Read about the underlying algorithm (HyperLogLog) and the estimation accuracy.

Syntax

hll (expr [, accuracy])

Parameters

NameTypeRequiredDescription
exprstring✔️The expression used for the aggregation calculation.
accuracyintThe value that controls the balance between speed and accuracy. If unspecified, the default value is 1. For supported values, see Estimation accuracy.

Returns

Returns the intermediate results of distinct count of expr across the group.

Example

In the following example, the hll() function is used to estimate the number of unique values of the DamageProperty column within each 10-minute time bin of the StartTime column.

StormEvents
| summarize hll(DamageProperty) by bin(StartTime,10m)

Output

The results table shown includes only the first 10 rows.

StartTimehll_DamageProperty
2007-01-01T00:20:00Z[[1024,14],[“3803688792395291579”],[]]
2007-01-01T01:00:00Z[[1024,14],[“7755241107725382121”,"-5665157283053373866",“3803688792395291579”,"-1003235211361077779"],[]]
2007-01-01T02:00:00Z[[1024,14],["-1003235211361077779","-5665157283053373866",“7755241107725382121”],[]]
2007-01-01T02:20:00Z[[1024,14],[“7755241107725382121”],[]]
2007-01-01T03:30:00Z[[1024,14],[“3803688792395291579”],[]]
2007-01-01T03:40:00Z[[1024,14],["-5665157283053373866"],[]]
2007-01-01T04:30:00Z[[1024,14],[“3803688792395291579”],[]]
2007-01-01T05:30:00Z[[1024,14],[“3803688792395291579”],[]]
2007-01-01T06:30:00Z[[1024,14],[“1589522558235929902”],[]]

Estimation accuracy

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