如何定义自定义聚合函数来对一列向量求和? [英] How to define a custom aggregation function to sum a column of Vectors?

问题描述

我有一个两列的数据帧，Int 类型的 ID 和 Vector 类型的 Vec(<代码>org.apache.spark.mllib.linalg.Vector).

I have a DataFrame of two columns, ID of type Int and Vec of type Vector (org.apache.spark.mllib.linalg.Vector).

DataFrame 如下所示:

The DataFrame looks like follow:

ID,Vec
1,[0,0,5]
1,[4,0,1]
1,[1,2,1]
2,[7,5,0]
2,[3,3,4]
3,[0,8,1]
3,[0,0,1]
3,[7,7,7]
....

我想做一个 groupBy($"ID") 然后通过对向量求和对每个组内的行应用聚合.

I would like to do a groupBy($"ID") then apply an aggregation on the rows inside each group by summing the vectors.

上面例子的期望输出是:

The desired output of the above example would be:

ID,SumOfVectors
1,[5,2,7]
2,[10,8,4]
3,[7,15,9]
...

可用的聚合函数将不起作用，例如df.groupBy($"ID").agg(sum($"Vec") 将导致 ClassCastException.

The available aggregation functions will not work, e.g. df.groupBy($"ID").agg(sum($"Vec") will lead to an ClassCastException.

如何实现自定义聚合函数，允许我对向量或数组求和或任何其他自定义操作?

How to implement a custom aggregation function that allows me to do the sum of vectors or arrays or any other custom operation?

推荐答案

Spark >= 3.0

您可以将 Summarizer 与 sum

import org.apache.spark.ml.stat.Summarizer

df
  .groupBy($"id")
  .agg(Summarizer.sum($"vec").alias("vec"))

火花 <= 3.0

就我个人而言，我不会打扰 UDAF.不仅仅是冗长而且不是很快(Spark UDAF 与 ArrayType 作为 bufferSchema 性能问题)相反，我会简单地使用 reduceByKey/foldByKey:

Personally I wouldn't bother with UDAFs. There are more than verbose and not exactly fast (Spark UDAF with ArrayType as bufferSchema performance issues) Instead I would simply use reduceByKey / foldByKey:

import org.apache.spark.sql.Row
import breeze.linalg.{DenseVector => BDV}
import org.apache.spark.ml.linalg.{Vector, Vectors}

def dv(values: Double*): Vector = Vectors.dense(values.toArray)

val df = spark.createDataFrame(Seq(
    (1, dv(0,0,5)), (1, dv(4,0,1)), (1, dv(1,2,1)),
    (2, dv(7,5,0)), (2, dv(3,3,4)), 
    (3, dv(0,8,1)), (3, dv(0,0,1)), (3, dv(7,7,7)))
  ).toDF("id", "vec")

val aggregated = df
  .rdd
  .map{ case Row(k: Int, v: Vector) => (k, BDV(v.toDense.values)) }
  .foldByKey(BDV.zeros[Double](3))(_ += _)
  .mapValues(v => Vectors.dense(v.toArray))
  .toDF("id", "vec")

aggregated.show

// +---+--------------+
// | id|           vec|
// +---+--------------+
// |  1| [5.0,2.0,7.0]|
// |  2|[10.0,8.0,4.0]|
// |  3|[7.0,15.0,9.0]|
// +---+--------------+

只是为了比较一个简单的"UDAF.所需的导入:

And just for comparison a "simple" UDAF. Required imports:

import org.apache.spark.sql.expressions.{MutableAggregationBuffer,
  UserDefinedAggregateFunction}
import org.apache.spark.ml.linalg.{Vector, Vectors, SQLDataTypes}
import org.apache.spark.sql.types.{StructType, ArrayType, DoubleType}
import org.apache.spark.sql.Row
import scala.collection.mutable.WrappedArray

类定义:

class VectorSum (n: Int) extends UserDefinedAggregateFunction {
    def inputSchema = new StructType().add("v", SQLDataTypes.VectorType)
    def bufferSchema = new StructType().add("buff", ArrayType(DoubleType))
    def dataType = SQLDataTypes.VectorType
    def deterministic = true 

    def initialize(buffer: MutableAggregationBuffer) = {
      buffer.update(0, Array.fill(n)(0.0))
    }

    def update(buffer: MutableAggregationBuffer, input: Row) = {
      if (!input.isNullAt(0)) {
        val buff = buffer.getAs[WrappedArray[Double]](0) 
        val v = input.getAs[Vector](0).toSparse
        for (i <- v.indices) {
          buff(i) += v(i)
        }
        buffer.update(0, buff)
      }
    }

    def merge(buffer1: MutableAggregationBuffer, buffer2: Row) = {
      val buff1 = buffer1.getAs[WrappedArray[Double]](0) 
      val buff2 = buffer2.getAs[WrappedArray[Double]](0) 
      for ((x, i) <- buff2.zipWithIndex) {
        buff1(i) += x
      }
      buffer1.update(0, buff1)
    }

    def evaluate(buffer: Row) =  Vectors.dense(
      buffer.getAs[Seq[Double]](0).toArray)
} 

还有一个用法示例:

df.groupBy($"id").agg(new VectorSum(3)($"vec") alias "vec").show

// +---+--------------+
// | id|           vec|
// +---+--------------+
// |  1| [5.0,2.0,7.0]|
// |  2|[10.0,8.0,4.0]|
// |  3|[7.0,15.0,9.0]|
// +---+--------------+

另见:如何在 Spark SQL 中找到分组向量列的平均值?.

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