Github user jkbradley commented on the pull request:
https://github.com/apache/spark/pull/3702#issuecomment-67223829
Yep, that's what I meant. I think it would be extra code, but I don't
think it would affect the runtime that much. (One pass to collect the number
of elements in each partition should not take that long relative to the
sortByKey.) Still, unless you're interested, I don't see a pressing need to
make it exact. The simplistic approach should never be off by more than
numPartitions.
One intermediate option would be to oversample by a factor of 2 or 3 (up to
the dataset size) and then choose the correct number of bins on the driver.
The bins would be shifted by a tiny bit, but that sounds OK to me.
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