Github user fhueske commented on the pull request:
https://github.com/apache/flink/pull/466#issuecomment-78091524
I have to correct myself. A combiner should of course be called on groups
of records. Therefore, calling it on a Grouping makes absolute sense. However,
the semantics of the Grouping are different for Combine and Reduce. For Reduce
a full Grouping is established with repartitioning and full sort, whereas for
Combine only a partial local grouping should be created.
---
If your project is set up for it, you can reply to this email and have your
reply appear on GitHub as well. If your project does not have this feature
enabled and wishes so, or if the feature is enabled but not working, please
contact infrastructure at infrastruct...@apache.org or file a JIRA ticket
with INFRA.
---
