Github user twalthr commented on a diff in the pull request:

    https://github.com/apache/flink/pull/2562#discussion_r82177163
  
    --- Diff: 
flink-libraries/flink-table/src/main/scala/org/apache/flink/api/table/runtime/aggregate/AggregateReduceCombineFunction.scala
 ---
    @@ -0,0 +1,132 @@
    +/*
    + * Licensed to the Apache Software Foundation (ASF) under one
    + * or more contributor license agreements.  See the NOTICE file
    + * distributed with this work for additional information
    + * regarding copyright ownership.  The ASF licenses this file
    + * to you under the Apache License, Version 2.0 (the
    + * "License"); you may not use this file except in compliance
    + * with the License.  You may obtain a copy of the License at
    + *
    + *     http://www.apache.org/licenses/LICENSE-2.0
    + *
    + * Unless required by applicable law or agreed to in writing, software
    + * distributed under the License is distributed on an "AS IS" BASIS,
    + * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
    + * See the License for the specific language governing permissions and
    + * limitations under the License.
    + */
    +
    +package org.apache.flink.api.table.runtime.aggregate
    +
    +import java.lang.Iterable
    +
    +import org.apache.flink.api.common.functions.{CombineFunction, 
RichGroupReduceFunction}
    +import org.apache.flink.api.table.Row
    +import org.apache.flink.configuration.Configuration
    +import org.apache.flink.streaming.api.windowing.windows.Window
    +import org.apache.flink.util.{Collector, Preconditions}
    +
    +import scala.collection.JavaConversions._
    +
    +
    +/**
    + * It wraps the aggregate logic inside of
    + * [[org.apache.flink.api.java.operators.GroupReduceOperator]] and
    + * [[org.apache.flink.api.java.operators.GroupCombineOperator]]
    + *
    + * @param aggregates   The aggregate functions.
    + * @param groupKeysMapping The index mapping of group keys between 
intermediate aggregate Row
    + *                         and output Row.
    + * @param aggregateMapping The index mapping between aggregate function 
list and aggregated value
    + *                         index in output Row.
    + */
    +class AggregateReduceCombineFunction(
    +    private val aggregates: Array[Aggregate[_ <: Any]],
    +    private val groupKeysMapping: Array[(Int, Int)],
    +    private val aggregateMapping: Array[(Int, Int)],
    +    private val intermediateRowArity: Int)
    +    extends RichGroupReduceFunction[Row, Row] with CombineFunction[Row, 
Row] {
    +
    +  private var aggregateBuffer: Row = _
    +  private var output: Row = _
    +  private var aggContext: AggContext = _
    +
    +  /**
    +    * Sets a new aggregation context used for [[Aggregate.evaluate()]].
    +    */
    +  def setAggContext(aggContext: AggContext): Unit = {
    +    this.aggContext = aggContext
    +  }
    +
    +  override def open(config: Configuration): Unit = {
    +    Preconditions.checkNotNull(aggregates)
    +    Preconditions.checkNotNull(groupKeysMapping)
    +    val finalRowLength: Int = groupKeysMapping.length + 
aggregateMapping.length
    +    aggregateBuffer = new Row(intermediateRowArity)
    +    output = new Row(finalRowLength)
    +    aggContext = new AggContext
    +  }
    +
    +  /**
    +   * For grouped intermediate aggregate Rows, merge all of them into 
aggregate buffer,
    +   * calculate aggregated values output by aggregate buffer, and set them 
into output
    +   * Row based on the mapping relation between intermediate aggregate Row 
and output Row.
    +   *
    +   * @param records  Grouped intermediate aggregate Rows iterator.
    +   * @param out The collector to hand results to.
    +   *
    +   */
    +  override def reduce(records: Iterable[Row], out: Collector[Row]): Unit = 
{
    +
    +    // Initiate intermediate aggregate value.
    +    aggregates.foreach(_.initiate(aggregateBuffer))
    +
    +    // Merge intermediate aggregate value to buffer.
    +    var last: Row = null
    +    records.foreach((record) => {
    +      aggregates.foreach(_.merge(record, aggregateBuffer))
    +      last = record
    +    })
    +
    +    // Set group keys value to final output.
    +    groupKeysMapping.map {
    +      case (after, previous) =>
    +        output.setField(after, last.productElement(previous))
    +    }
    +
    +    // Evaluate final aggregate value and set to output.
    +    aggregateMapping.map {
    --- End diff --
    
    Done.


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