druid/docs/development/extensions-core/datasketches-hll.md

id	title
datasketches-hll	DataSketches HLL Sketch module

This module provides Apache Druid (incubating) aggregators for distinct counting based on HLL sketch from datasketches library. At ingestion time, this aggregator creates the HLL sketch objects to be stored in Druid segments. At query time, sketches are read and merged together. In the end, by default, you receive the estimate of the number of distinct values presented to the sketch. Also, you can use post aggregator to produce a union of sketch columns in the same row. You can use the HLL sketch aggregator on columns of any identifiers. It will return estimated cardinality of the column.

To use this aggregator, make sure you include the extension in your config file:

druid.extensions.loadList=["druid-datasketches"]

Aggregators

{
  "type" : "HLLSketchBuild",
  "name" : <output name>,
  "fieldName" : <metric name>,
  "lgK" : <size and accuracy parameter>,
  "tgtHllType" : <target HLL type>,
  "round": <false | true>
 }

{
  "type" : "HLLSketchMerge",
  "name" : <output name>,
  "fieldName" : <metric name>,
  "lgK" : <size and accuracy parameter>,
  "tgtHllType" : <target HLL type>,
  "round": <false | true>
 }

property	description	required?
type	This String should be "HLLSketchBuild" or "HLLSketchMerge"	yes
name	A String for the output (result) name of the calculation.	yes
fieldName	A String for the name of the input field.	yes
lgK	log2 of K that is the number of buckets in the sketch, parameter that controls the size and the accuracy. Must be a power of 2 from 4 to 21 inclusively.	no, defaults to 12
tgtHllType	The type of the target HLL sketch. Must be "HLL_4", "HLL_6" or "HLL_8"	no, defaults to "HLL_4"
round	Round off values to whole numbers. Only affects query-time behavior and is ignored at ingestion-time.	no, defaults to false

Post Aggregators

Estimate with bounds

{
  "type"  : "HLLSketchEstimateWithBounds",
  "name": <output name>,
  "field"  : <post aggregator that returns an HLL Sketch>,
  "numStdDev" : <number of standard deviations: 1 (default), 2 or 3>
}

Union

{
  "type"  : "HLLSketchUnion",
  "name": <output name>,
  "fields"  : <array of post aggregators that return HLL sketches>,
  "lgK": <log2 of K for the target sketch>,
  "tgtHllType" : <target HLL type>
}

Sketch to string

Human-readable sketch summary for debugging

{
  "type"  : "HLLSketchToString",
  "name": <output name>,
  "field"  : <post aggregator that returns an HLL Sketch>
}