Github user marmbrus commented on the pull request:
https://github.com/apache/spark/pull/2109#issuecomment-53467276
> What if we just override equals in Attribute?
This is actually how things were implemented originally. However, its
really weird when `AttributeReference("a"...) == AttrributeReference("b",
...)`. It breaks tests, and also makes doing transformations hard (we always
try keep older trees instead of new ones when the transformation was a no-op).
> Does AttributeSet provide anything that Scala's Set doesn't provide?
No, it is actually a subset, this class is just encapsulating a very common
pattern where you create sets that contain expression ids. Furthermore, since
an AttributeSet is not a Set[Attribute] you get nice type errors in many cases
where you are trying to do semantically invalid things.
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