Github user cloud-fan commented on the issue:
https://github.com/apache/spark/pull/23131
shall we say `union` is an alias of `unionAll` instead of `unionAll` is an
alias of `Union`? According to the SQL spec, `unionAll` is implemented
correctly that it keeps duplicated rows, while `union` does not follow SQL
spec, as it's too widely used and it's too late to change behavior.
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