Github user marmbrus commented on the pull request:
https://github.com/apache/spark/pull/8592#issuecomment-154247089
Thanks for working on this! I would like to include this, but I'm worried
about the performance. Unfortunately, I think that in order to get something
that is not n^2 we are going to have to make major changes to aggregation (and
its after code freeze already).
Fortunately, we already have a hive UDF that does this in `O(n)`. I'd
propose that we just add a wrapper so that people can find this in the
functions list for DataFrames. You can just make the function call:
`callUDF("collect_set", ...)`, etc.
If you can make a PR in the next day or two we can still include it in
Spark 1.6.
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