Github user mengxr commented on the pull request:
https://github.com/apache/spark/pull/7987#issuecomment-133085190
@dputler Under distributed setting, we need to make at least one pass to
collect all categories. The ordering is not alphabetical but by frequency
(https://github.com/apache/spark/blob/master/mllib/src/main/scala/org/apache/spark/ml/feature/StringIndexer.scala#L86).
The most frequent category gets index `0`, and one-hot encoder by default
drops the last category, i.e., the least frequent one.
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