Github user marmbrus commented on the pull request:
https://github.com/apache/spark/pull/10566#issuecomment-188920585
> The first way is based on the constraints. If the parent join is full
outer, the parent join will not have any IsNotNull constraint. In the current
constraint propagation, its constraints is Set.empty[Expression]. Thus,
$"c.int" === $"a.int" is not eligible for using the first way.
Why isn't the constraint present? We should fix that instead of inventing
another unrelated way to reason about nullability.
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