Github user StephanEwen commented on the pull request:
https://github.com/apache/flink/pull/408#issuecomment-77774282
The vertex-centric iterations use semantic annotations a lot to save
sorting/partitioning where possible. This results in a plan for the
vertex-centric iterations that partitions one and sorts one, despite the fact
that it uses wither two coGroups or 2 joins plus a reduce. I think this would
help big time here as well. In general, plan optimization may be something that
Gelly could take a look at.
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