Github user cloud-fan commented on the pull request:
https://github.com/apache/spark/pull/13322
@zhzhan , for outer join, if it's a typed join, then both of the join side
will include this hidden column, as their columns are produced by serializing
custom object. If we need to return null for one join side, we will null out
all columns of that side, including the hidden column. Then when we deserialize
it, we can know that this join side is null and we can build a null object for
it.
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