Github user viirya commented on the pull request:
https://github.com/apache/spark/pull/12138#issuecomment-208158392
@cloud-fan Thanks for comment.
The problem is we re-use child's output as `Expand`s output. We will
create new attributes in `Expand` because `Expand` actually performs multiple
projections. However, we let the projections in `Expand` as `Expression`
instead of `NamedExpression` and re-use child output attributes. Thus there is
a inconsistency between `Expand`'s output attributes and projected values.
The obvious example for this inconsistency is constraints. Previously
`Expand` inherits child's constraints. As we will change child's output values
by projections (e.g., set it as null), these constraints bound on child's
attributes are not valid.
In previous PR we just set `Expand`s `validConstraints` to empty to avoid
such inconsistency. But as the result, we don't have reliable constraints after
`Expand` operator.
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