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hll() (aggregation function)

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  • قراءة خلال 2 دقائق

Calculates the Intermediate results of dcount across the group, only in context of aggregation inside summarize.

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

Syntax

hll (Expr [, Accuracy])

Arguments

  • Expr: Expression that will be used for aggregation calculation.

  • Accuracy, if specified, controls the balance between speed and accuracy.

    Accuracy Value Accuracy Speed Error
    0 lowest fastest 1.6%
    1 default balanced 0.8%
    2 high slow 0.4%
    3 high slow 0.28%
    4 extra high slowest 0.2%

Returns

The Intermediate results of distinct count of Expr across the group.

Tips

  1. You may use the aggregation function hll_merge to merge more than one hll intermediate results (it works on hll output only).

  2. You may use the function dcount_hll, which will calculate the dcount from hll / hll_merge aggregation functions.

Examples

StormEvents
| summarize hll(DamageProperty) by bin(StartTime,10m)
StartTime hll_DamageProperty
2007-09-18 20:00:00.0000000 [[1024,14],[-5473486921211236216,-6230876016761372746,3953448761157777955,4246796580750024372],[]]
2007-09-20 21:50:00.0000000 [[1024,14],[4835649640695509390],[]]
2007-09-29 08:10:00.0000000 [[1024,14],[4246796580750024372],[]]
2007-12-30 16:00:00.0000000 [[1024,14],[4246796580750024372,-8936707700542868125],[]]