Aggregator — Contract for User-Defined Typed Aggregate Functions (UDAFs)

Aggregator is the contract for user-defined typed aggregate functions (aka user-defined typed aggregations or UDAFs in short).

package org.apache.spark.sql.expressions

abstract class Aggregator[-IN, BUF, OUT] extends Serializable {
  // only required methods that have no implementation
  def bufferEncoder: Encoder[BUF]
  def finish(reduction: BUF): OUT
  def merge(b1: BUF, b2: BUF): BUF
  def outputEncoder: Encoder[OUT]
  def reduce(b: BUF, a: IN): BUF
  def zero: BUF
}

After you create a custom Aggregator, you should use toColumn method to convert it to a TypedColumn that can be used with Dataset.select and KeyValueGroupedDataset.agg typed operators.

// From Spark MLlib's org.apache.spark.ml.recommendation.ALSModel
// Step 1. Create Aggregator
val topKAggregator: Aggregator[Int, Int, Float] = ???
val recs = ratings
  .as[(Int, Int, Float)]
  .groupByKey(_._1)
  .agg(topKAggregator.toColumn) // <-- use the custom Aggregator
  .toDF("id", "recommendations")

Note

Use org.apache.spark.sql.expressions.scalalang.typed object to access the type-safe aggregate functions, i.e. avg, count, sum and sumLong.

import org.apache.spark.sql.expressions.scalalang.typed

// Example 1
ds.groupByKey(_._1).agg(typed.sum(_._2))

// Example 2
ds.select(typed.sum((i: Int) => i))

Note	`Aggregator` is an `Experimental` and `Evolving` contract that is evolving towards becoming a stable API, but is not a stable API yet and can change from one feature release to another release. In other words, using the contract is as treading on thin ice.

Aggregator is used when:

SimpleTypedAggregateExpression and ComplexTypedAggregateExpression are created
TypedAggregateExpression is requested for the aggregator

Table 1. Aggregator Contract
Method	Description
`bufferEncoder`	Used when…FIXME
`finish`	Used when…FIXME
`merge`	Used when…FIXME
`outputEncoder`	Used when…FIXME
`reduce`	Used when…FIXME
`zero`	Used when…FIXME

Table 2. Aggregators
Aggregator	Description
`ParameterizedTypeSum`
`ReduceAggregator`
`TopByKeyAggregator`	Used exclusively in Spark MLlib
`TypedAverage`
`TypedCount`
`TypedSumDouble`
`TypedSumLong`

Converting Aggregator to TypedColumn — `toColumn` Method

toColumn: TypedColumn[IN, OUT]

toColumn…FIXME

Note	`toColumn` is used when…FIXME

Aggregator — Contract for User-Defined Typed Aggregate Functions (UDAFs)