Github user scwf commented on the issue:
https://github.com/apache/spark/pull/15297
@YuhuWang2002
We should limit the use case for outer join:
For left outer join, such as A left join B, this implementation now can not
handle the case of skew of table B. That's because the result of join
depends on the all data of the same reduce data of B, you can not split it to
multi-tasks.
Similarly, for right outer join, such as A right join B, this
implementation now can not handle the case of skew of table A. And for full
outer join, we can not use the optimization.
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