[jira] [Commented] (FLINK-3477) Add hash-based combine strategy for ReduceFunction

["ASF GitHub Bot (JIRA)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/FLINK-3477?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15368461#comment-15368461 ] 

ASF GitHub Bot commented on FLINK-3477:
---------------------------------------

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

    https://github.com/apache/flink/pull/1517#discussion_r70139954
  
    --- Diff: flink-scala/src/main/scala/org/apache/flink/api/scala/GroupedDataSet.scala ---
    @@ -282,27 +283,57 @@ class GroupedDataSet[T: ClassTag](
       }
     
       /**
    -   * Creates a new [[DataSet]] by merging the elements of each group (elements with the same key)
    -   * using an associative reduce function.
    -   */
    +    * Creates a new [[DataSet]] by merging the elements of each group (elements with the same key)
    +    * using an associative reduce function.
    +    */
       def reduce(fun: (T, T) => T): DataSet[T] = {
    +    reduce(getCallLocationName(), fun, CombineHint.OPTIMIZER_CHOOSES)
    +  }
    +
    +  /**
    +   * Special [[reduce]] operation for explicitly telling the system what strategy to use for the
    +   * combine phase.
    +   * If null is given as the strategy, then the optimizer will pick the strategy.
    +   */
    +  def reduce(fun: (T, T) => T, strategy: CombineHint): DataSet[T] = {
    --- End diff --
    
    @fhueske or @StephanEwen since we cannot break the scala DataSet API by creating and returning a `ReduceOperator`, do you agree with Gábor's recommendation to overload `DataSet.reduce` with `CombineHint`?


> Add hash-based combine strategy for ReduceFunction
> --------------------------------------------------
>
>                 Key: FLINK-3477
>                 URL: https://issues.apache.org/jira/browse/FLINK-3477
>             Project: Flink
>          Issue Type: Sub-task
>          Components: Local Runtime
>            Reporter: Fabian Hueske
>            Assignee: Gabor Gevay
>
> This issue is about adding a hash-based combine strategy for ReduceFunctions.
> The interface of the {{reduce()}} method is as follows:
> {code}
> public T reduce(T v1, T v2)
> {code}
> Input type and output type are identical and the function returns only a single value. A Reduce function is incrementally applied to compute a final aggregated value. This allows to hold the preaggregated value in a hash-table and update it with each function call. 
> The hash-based strategy requires special implementation of an in-memory hash table. The hash table should support in place updates of elements (if the updated value has the same size as the new value) but also appending updates with invalidation of the old value (if the binary length of the new value differs). The hash table needs to be able to evict and emit all elements if it runs out-of-memory.
> We should also add {{HASH}} and {{SORT}} compiler hints to {{DataSet.reduce()}} and {{Grouping.reduce()}} to allow users to pick the execution strategy.



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