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https://issues.apache.org/jira/browse/FLINK-3179?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15150391#comment-15150391
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ASF GitHub Bot commented on FLINK-3179:
---------------------------------------
Github user fhueske commented on the pull request:
https://github.com/apache/flink/pull/1553#issuecomment-185170325
Hi @ramkrish86, thanks for the update.
In addition to my comments inline we also need to extend the `ReduceITCase`.
Also we must take care of the case where the result of the partition
operator goes into more than one function. Consider the following case:
```
/--fwd--> [Reduce]
[Input] --shuffle--> [Partitioner] -<
\--fwd--> [Map]
```
which should be translated to:
```
/--fwd--> [Combine] --shuffle--> [Partitioner] --fwd--> [Reduce]
[Input] --<
\--shuffle--> [Partitioner] --fwd--> [Map]
```
Both translation tests need to be extended to cover this case.
Thanks, Fabian
> Combiner is not injected if Reduce or GroupReduce input is explicitly
> partitioned
> ---------------------------------------------------------------------------------
>
> Key: FLINK-3179
> URL: https://issues.apache.org/jira/browse/FLINK-3179
> Project: Flink
> Issue Type: Bug
> Components: Optimizer
> Affects Versions: 0.10.1
> Reporter: Fabian Hueske
> Assignee: ramkrishna.s.vasudevan
> Priority: Critical
> Fix For: 1.0.0, 0.10.2
>
>
> The optimizer does not inject a combiner if the input of a Reducer or
> GroupReducer is explicitly partitioned as in the following example
> {code}
> DataSet<Tuple2<String,Integer>> words = ...
> DataSet<Tuple2<String,Integer>> counts = words
> .partitionByHash(0)
> .groupBy(0)
> .sum(1);
> {code}
> Explicit partitioning can be useful to enforce partitioning on a subset of
> keys or to use a different partitioning method (custom or range partitioning).
> This issue should be fixed by changing the {{instantiate()}} methods of the
> {{ReduceProperties}} and {{GroupReduceWithCombineProperties}} classes such
> that a combine is injected in front of a {{PartitionPlanNode}} if it is the
> input of a Reduce or GroupReduce operator. This should only happen, if the
> Reducer is the only successor of the Partition operator.
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