druid/docs/querying/aggregations.md

id	title
aggregations	Aggregations

Apache Druid supports two query languages: Druid SQL and native queries. This document describes the native language. For information about aggregators available in SQL, refer to the SQL documentation.

You can use aggregations:

• in the ingestion spec during ingestion to summarize data before it enters Apache Druid.
• at query time to summarize result data.

The following sections list the available aggregate functions. Unless otherwise noted, aggregations are available at both ingestion and query time.

Exact aggregations

Count aggregator

count computes the count of Druid rows that match the filters.

Property	Description	Required
type	Must be "count".	Yes
name	Output name of the aggregator	Yes

Example:

{ "type" : "count", "name" : "count" }

The count aggregator counts the number of Druid rows, which does not always reflect the number of raw events ingested. This is because Druid can be configured to roll up data at ingestion time. To count the number of ingested rows of data, include a count aggregator at ingestion time and a longSum aggregator at query time.

Sum aggregators

Property	Description	Required
type	Must be "longSum", "doubleSum", or "floatSum".	Yes
name	Output name for the summed value.	Yes
fieldName	Name of the input column to sum over.	No. You must specify fieldName or expression.
expression	You can specify an inline expression as an alternative to fieldName.	No. You must specify fieldName or expression.

longSum aggregator

Computes the sum of values as a 64-bit, signed integer.

Example:

{ "type" : "longSum", "name" : "sumLong", "fieldName" : "aLong" }

doubleSum aggregator

Computes and stores the sum of values as a 64-bit floating point value. Similar to longSum.

Example:

{ "type" : "doubleSum", "name" : "sumDouble", "fieldName" : "aDouble" }

floatSum aggregator

Computes and stores the sum of values as a 32-bit floating point value. Similar to longSum and doubleSum.

Example:

{ "type" : "floatSum", "name" : "sumFloat", "fieldName" : "aFloat" }

Min and max aggregators

Property	Description	Required
type	Must be "doubleMin", "doubleMax", "floatMin", "floatMax", "longMin", or "longMax".	Yes
name	Output name for the min or max value.	Yes
fieldName	Name of the input column to compute the minimum or maximum value over.	No. You must specify fieldName or expression.
expression	You can specify an inline expression as an alternative to fieldName.	No. You must specify fieldName or expression.

doubleMin aggregator

doubleMin computes the minimum of all input values and null if druid.generic.useDefaultValueForNull is false or Double.POSITIVE_INFINITY if true.

Example:

{ "type" : "doubleMin", "name" : "maxDouble", "fieldName" : "aDouble" }

doubleMax aggregator

doubleMax computes the maximum of all input values and null if druid.generic.useDefaultValueForNull is false or Double.NEGATIVE_INFINITY if true.

Example:

{ "type" : "doubleMax", "name" : "minDouble", "fieldName" : "aDouble" }

floatMin aggregator

floatMin computes the minimum of all input values and null if druid.generic.useDefaultValueForNull is false or Float.POSITIVE_INFINITY if true.

Example:

{ "type" : "floatMin", "name" : "minFloat", "fieldName" : "aFloat" }

floatMax aggregator

floatMax computes the maximum of all input values and null if druid.generic.useDefaultValueForNull is false or Float.NEGATIVE_INFINITY if true.

Example:

{ "type" : "floatMax", "name" : "maxFloat", "fieldName" : "aFloat" }

longMin aggregator

longMin computes the minimum of all input values and null if druid.generic.useDefaultValueForNull is false or Long.MAX_VALUE if true.

Example:

{ "type" : "longMin", "name" : "minLong", "fieldName" : "aLong" }

longMax aggregator

longMax computes the maximum of all metric values and null if druid.generic.useDefaultValueForNull is false or Long.MIN_VALUE if true.

Example:

{ "type" : "longMax", "name" : "maxLong", "fieldName" : "aLong" }

doubleMean aggregator

Computes and returns the arithmetic mean of a column's values as a 64-bit floating point value.

Property	Description	Required
type	Must be "doubleMean".	Yes
name	Output name for the mean value.	Yes
fieldName	Name of the input column to compute the arithmetic mean value over.	Yes

Example:

{ "type" : "doubleMean", "name" : "aMean", "fieldName" : "aDouble" }

doubleMean is a query time aggregator only. It is not available for indexing. To accomplish mean aggregation on ingestion, refer to the Quantiles aggregator from the DataSketches extension.

First and last aggregators

The first and last aggregators determine the metric values that respectively correspond to the earliest and latest values of a time column.

Do not use first and last aggregators for the double, float, and long types in an ingestion spec. They are only supported for queries. The string-typed aggregators, stringFirst and stringLast, are supported for both ingestion and querying.

Queries with first or last aggregators on a segment created with rollup return the rolled up value, not the first or last value from the raw ingested data.

Numeric first and last aggregators

Property	Description	Required
type	Must be "doubleFirst", "doubleLast", "floatFirst", "floatLast", "longFirst", "longLast".	Yes
name	Output name for the first or last value.	Yes
fieldName	Name of the input column to compute the first or last value over.	Yes
timeColumn	Name of the input column to use for time values. Must be a LONG typed column.	No. Defaults to __time.

doubleFirst aggregator

doubleFirst computes the input value with the minimum value for time column or 0 in default mode, or null in SQL-compatible mode if no row exists.

Example:

{
  "type" : "doubleFirst",
  "name" : "firstDouble",
  "fieldName" : "aDouble"
}

doubleLast aggregator

doubleLast computes the input value with the maximum value for time column or 0 in default mode, or null in SQL-compatible mode if no row exists.

Example:

{
  "type" : "doubleLast",
  "name" : "lastDouble",
  "fieldName" : "aDouble",
  "timeColumn" : "longTime"
}

floatFirst aggregator

floatFirst computes the input value with the minimum value for time column or 0 in default mode, or null in SQL-compatible mode if no row exists.

Example:

{
  "type" : "floatFirst",
  "name" : "firstFloat",
  "fieldName" : "aFloat"
}

floatLast aggregator

floatLast computes the metric value with the maximum value for time column or 0 in default mode, or null in SQL-compatible mode if no row exists.

Example:

{
  "type" : "floatLast",
  "name" : "lastFloat",
  "fieldName" : "aFloat"
}

longFirst aggregator

longFirst computes the metric value with the minimum value for time column or 0 in default mode, or null in SQL-compatible mode if no row exists.

Example:

{
  "type" : "longFirst",
  "name" : "firstLong",
  "fieldName" : "aLong"
}

longLast aggregator

longLast computes the metric value with the maximum value for time column or 0 in default mode, or null in SQL-compatible mode if no row exists.

Example:

{
  "type" : "longLast",
  "name" : "lastLong",
  "fieldName" : "aLong",
  "timeColumn" : "longTime"
}

String first and last aggregators

Property	Description	Required
type	Must be "stringFirst", "stringLast".	Yes
name	Output name for the first or last value.	Yes
fieldName	Name of the input column to compute the first or last value over.	Yes
timeColumn	Name of the input column to use for time values. Must be a LONG typed column.	No. Defaults to __time.
maxStringBytes	Maximum size of string values to accumulate when computing the first or last value per group. Values longer than this will be truncated.	No. Defaults to 1024.

stringFirst aggregator

stringFirst computes the metric value with the minimum value for time column or null if no row exists.

Example:

{
  "type" : "stringFirst",
  "name" : "firstString",
  "fieldName" : "aString",
  "maxStringBytes" : 2048,
  "timeColumn" : "longTime"
}

stringLast aggregator

stringLast computes the metric value with the maximum value for time column or null if no row exists.

Example:

{
  "type" : "stringLast",
  "name" : "lastString",
  "fieldName" : "aString"
}

ANY aggregators

(Double/Float/Long/String) ANY aggregator cannot be used in ingestion spec, and should only be specified as part of queries.

Returns any value including null. This aggregator can simplify and optimize the performance by returning the first encountered value (including null)

Numeric any aggregators

Property	Description	Required
type	Must be "doubleAny", "floatAny", or "longAny".	Yes
name	Output name for the value.	Yes
fieldName	Name of the input column to compute the value over.	Yes

doubleAny aggregator

doubleAny returns any double metric value.

Example:

{
  "type" : "doubleAny",
  "name" : "anyDouble",
  "fieldName" : "aDouble"
}

floatAny aggregator

floatAny returns any float metric value.

Example:

{
  "type" : "floatAny",
  "name" : "anyFloat",
  "fieldName" : "aFloat"
}

longAny aggregator

longAny returns any long metric value.

Example:

{
  "type" : "longAny",
  "name" : "anyLong",
  "fieldName" : "aLong"
}

stringAny aggregator

stringAny returns any string value present in the input.

Property	Description	Required
type	Must be "stringAny".	Yes
name	Output name for the value.	Yes
fieldName	Name of the input column to compute the value over.	Yes
maxStringBytes	Maximum size of string values to accumulate when computing the first or last value per group. Values longer than this will be truncated.	No. Defaults to 1024.

Example:

{
  "type" : "stringAny",
  "name" : "anyString",
  "fieldName" : "aString",
  "maxStringBytes" : 2048
}

Approximate aggregations

Count distinct

Apache DataSketches Theta Sketch

The DataSketches Theta Sketch extension-provided aggregator gives distinct count estimates with support for set union, intersection, and difference post-aggregators, using Theta sketches from the Apache DataSketches library.

Apache DataSketches HLL Sketch

The DataSketches HLL Sketch extension-provided aggregator gives distinct count estimates using the HyperLogLog algorithm.

Compared to the Theta sketch, the HLL sketch does not support set operations and has slightly slower update and merge speed, but requires significantly less space.

Cardinality, hyperUnique

For new use cases, we recommend evaluating DataSketches Theta Sketch or DataSketches HLL Sketch instead. The DataSketches aggregators are generally able to offer more flexibility and better accuracy than the classic Druid cardinality and hyperUnique aggregators.

The Cardinality and HyperUnique aggregators are older aggregator implementations available by default in Druid that also provide distinct count estimates using the HyperLogLog algorithm. The newer DataSketches Theta and HLL extension-provided aggregators described above have superior accuracy and performance and are recommended instead.

The DataSketches team has published a comparison study between Druid's original HLL algorithm and the DataSketches HLL algorithm. Based on the demonstrated advantages of the DataSketches implementation, we are recommending using them in preference to Druid's original HLL-based aggregators. However, to ensure backwards compatibility, we will continue to support the classic aggregators.

Please note that hyperUnique aggregators are not mutually compatible with Datasketches HLL or Theta sketches.

Multi-column handling

Note the DataSketches Theta and HLL aggregators currently only support single-column inputs. If you were previously using the Cardinality aggregator with multiple-column inputs, equivalent operations using Theta or HLL sketches are described below:

• Multi-column byValue Cardinality can be replaced with a union of Theta sketches on the individual input columns
• Multi-column byRow Cardinality can be replaced with a Theta or HLL sketch on a single virtual column that combines the individual input columns.

Histograms and quantiles

DataSketches Quantiles Sketch

The DataSketches Quantiles Sketch extension-provided aggregator provides quantile estimates and histogram approximations using the numeric quantiles DoublesSketch from the datasketches library.

We recommend this aggregator in general for quantiles/histogram use cases, as it provides formal error bounds and has distribution-independent accuracy.

Moments Sketch (Experimental)

The Moments Sketch extension-provided aggregator is an experimental aggregator that provides quantile estimates using the Moments Sketch.

The Moments Sketch aggregator is provided as an experimental option. It is optimized for merging speed and it can have higher aggregation performance compared to the DataSketches quantiles aggregator. However, the accuracy of the Moments Sketch is distribution-dependent, so users will need to empirically verify that the aggregator is suitable for their input data.

As a general guideline for experimentation, the Moments Sketch paper points out that this algorithm works better on inputs with high entropy. In particular, the algorithm is not a good fit when the input data consists of a small number of clustered discrete values.

Fixed Buckets Histogram

Druid also provides a simple histogram implementation that uses a fixed range and fixed number of buckets with support for quantile estimation, backed by an array of bucket count values.

The fixed buckets histogram can perform well when the distribution of the input data allows a small number of buckets to be used.

We do not recommend the fixed buckets histogram for general use, as its usefulness is extremely data dependent. However, it is made available for users that have already identified use cases where a fixed buckets histogram is suitable.

Approximate Histogram (deprecated)

The Approximate Histogram aggregator is deprecated. There are a number of other quantile estimation algorithms that offer better performance, accuracy, and memory footprint. We recommend using DataSketches Quantiles instead.

The Approximate Histogram extension-provided aggregator also provides quantile estimates and histogram approximations, based on http://jmlr.org/papers/volume11/ben-haim10a/ben-haim10a.pdf.

The algorithm used by this deprecated aggregator is highly distribution-dependent and its output is subject to serious distortions when the input does not fit within the algorithm's limitations.

A study published by the DataSketches team demonstrates some of the known failure modes of this algorithm:

• The algorithm's quantile calculations can fail to provide results for a large range of rank values (all ranks less than 0.89 in the example used in the study), returning all zeroes instead.
• The algorithm can completely fail to record spikes in the tail ends of the distribution
• In general, the histogram produced by the algorithm can deviate significantly from the true histogram, with no bounds on the errors.

It is not possible to determine a priori how well this aggregator will behave for a given input stream, nor does the aggregator provide any indication that serious distortions are present in the output.

For these reasons, we have deprecated this aggregator and recommend using the DataSketches Quantiles aggregator instead for new and existing use cases, although we will continue to support Approximate Histogram for backwards compatibility.

Expression aggregations

Expression aggregator

Aggregator applicable only at query time. Aggregates results using Druid expressions functions to facilitate building custom functions.

Property	Description	Required
type	Must be "expression".	Yes
name	The aggregator output name.	Yes
fields	The list of aggregator input columns.	Yes
accumulatorIdentifier	The variable which identifies the accumulator value in the fold and combine expressions.	No. Default __acc.
fold	The expression to accumulate values from fields. The result of the expression is stored in accumulatorIdentifier and available to the next computation.	Yes
combine	The expression to combine the results of various fold expressions of each segment when merging results. The input is available to the expression as a variable identified by the name.	No. Default to fold expression if the expression has a single input in fields.
compare	The comparator expression which can only refer to two input variables, o1 and o2, where o1 and o2 are the output of fold or combine expressions, and must adhere to the Java comparator contract. If not set, the aggregator will try to fall back to an output type appropriate comparator.	No
finalize	The finalize expression which can only refer to a single input variable, o. This expression is used to perform any final transformation of the output of the fold or combine expressions. If not set, then the value is not transformed.	No
initialValue	The initial value of the accumulator for the fold (and combine, if InitialCombineValue is null) expression.	Yes
initialCombineValue	The initial value of the accumulator for the combine expression.	No. Default initialValue.
isNullUnlessAggregated	Indicates that the default output value should be null if the aggregator does not process any rows. If true, the value is null, if false, the result of running the expressions with initial values is used instead.	No. Defaults to the value of druid.generic.useDefaultValueForNull.
shouldAggregateNullInputs	Indicates if the fold expression should operate on any null input values.	No. Defaults to true.
shouldCombineAggregateNullInputs	Indicates if the combine expression should operate on any null input values.	No. Defaults to the value of shouldAggregateNullInputs.
maxSizeBytes	Maximum size in bytes that variably sized aggregator output types such as strings and arrays are allowed to grow to before the aggregation fails.	No. Default is 8192 bytes.

Example: a "count" aggregator

The initial value is 0. fold adds 1 for each row processed.

{
  "type": "expression",
  "name": "expression_count",
  "fields": [],
  "initialValue": "0",
  "fold": "__acc + 1",
  "combine": "__acc + expression_count"
}

Example: a "sum" aggregator

The initial value is 0. fold adds the numeric value column_a for each row processed.

{
  "type": "expression",
  "name": "expression_sum",
  "fields": ["column_a"],
  "initialValue": "0",
  "fold": "__acc + column_a"
}

Example: a "distinct array element" aggregator, sorted by array_length

The initial value is an empty array. fold adds the elements of column_a to the accumulator using set semantics, combine merges the sets, and compare orders the values by array_length.

{
  "type": "expression",
  "name": "expression_array_agg_distinct",
  "fields": ["column_a"],
  "initialValue": "[]",
  "fold": "array_set_add(__acc, column_a)",
  "combine": "array_set_add_all(__acc, expression_array_agg_distinct)",
  "compare": "if(array_length(o1) > array_length(o2), 1, if (array_length(o1) == array_length(o2), 0, -1))"
}

Example: an "approximate count" aggregator using the built-in hyper-unique

Similar to the cardinality aggregator, the default value is an empty hyper-unique sketch, fold adds the value of column_a to the sketch, combine merges the sketches, and finalize gets the estimated count from the accumulated sketch.

{
  "type": "expression",
  "name": "expression_cardinality",
  "fields": ["column_a"],
  "initialValue": "hyper_unique()",
  "fold": "hyper_unique_add(column_a, __acc)",
  "combine": "hyper_unique_add(expression_cardinality, __acc)",
  "finalize": "hyper_unique_estimate(o)"
}

JavaScript aggregator

Computes an arbitrary JavaScript function over a set of columns (both metrics and dimensions are allowed). Your JavaScript functions are expected to return floating-point values.

Property	Description	Required
type	Must be "javascript".	Yes
name	The aggregator output name.	Yes
fieldNames	The list of aggregator input columns.	Yes
fnAggregate	JavaScript function that updates partial aggregate based on the current row values, and returns the updated partial aggregate.	Yes
fnCombine	JavaScript function to combine partial aggregates and return the combined result.	Yes
fnReset	JavaScript function that returns the 'initial' value.	Yes

Example

{
  "type": "javascript",
  "name": "sum(log(x)*y) + 10",
  "fieldNames": ["x", "y"],
  "fnAggregate" : "function(current, a, b)      { return current + (Math.log(a) * b); }",
  "fnCombine"   : "function(partialA, partialB) { return partialA + partialB; }",
  "fnReset"     : "function()                   { return 10; }"
}

JavaScript functionality is disabled by default. Refer to the Druid JavaScript programming guide for guidelines about using Druid's JavaScript functionality, including instructions on how to enable it.

Miscellaneous aggregations

Filtered aggregator

A filtered aggregator wraps any given aggregator, but only aggregates the values for which the given dimension filter matches.

This makes it possible to compute the results of a filtered and an unfiltered aggregation simultaneously, without having to issue multiple queries, and use both results as part of post-aggregations.

If only the filtered results are required, consider putting the filter on the query itself. This will be much faster since it does not require scanning all the data.

Property	Description	Required
type	Must be "filtered".	Yes
name	The aggregator output name.	No
aggregator	Inline aggregator specification.	Yes
filter	Inline filter specification.	Yes

Example:

{
  "type": "filtered",
  "name": "filteredSumLong",
  "filter": {
    "type" : "selector",
    "dimension" : "someColumn",
    "value" : "abcdef"
  },
  "aggregator": {
    "type": "longSum",
    "name": "sumLong",
    "fieldName": "aLong"
  }
}

Grouping aggregator

A grouping aggregator can only be used as part of GroupBy queries which have a subtotal spec. It returns a number for each output row that lets you infer whether a particular dimension is included in the sub-grouping used for that row. You can pass a non-empty list of dimensions to this aggregator which must be a subset of dimensions that you are grouping on.

Property	Description	Required
type	Must be "grouping".	Yes
name	The aggregator output name.	Yes
groupings	The list of columns to use in the grouping set.	Yes

For example, the following aggregator has ["dim1", "dim2"] as input dimensions:

{ "type" : "grouping", "name" : "someGrouping", "groupings" : ["dim1", "dim2"] }

and used in a grouping query with [["dim1", "dim2"], ["dim1"], ["dim2"], []] as subtotals, the possible output of the aggregator is:

subtotal used in query	Output	(bits representation)
["dim1", "dim2"]	0	(00)
["dim1"]	1	(01)
["dim2"]	2	(10)
[]	3	(11)

As the example illustrates, you can think of the output number as an unsigned n bit number where n is the number of dimensions passed to the aggregator. Druid sets the bit at position X for the number to 0 if the sub-grouping includes a dimension at position X in the aggregator input. Otherwise, Druid sets this bit to 1.