Github user mengxr commented on the pull request:
https://github.com/apache/spark/pull/1698#issuecomment-50899568
What if you have 10M columns? I agree that not sending data to the driver
is a good practice. But the current operations `reduceByKey` and `cartesian`
are not optimized for very big data. Please test it on a cluster with many
partitions and you should see the bottleneck.
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