Github user liancheng commented on the pull request:
https://github.com/apache/spark/pull/11822#issuecomment-198318534
Personally, I had once been quite confused by the fact that
`NamedExpression.qualifiers` is a `Seq[String]` and thought that attributes can
be qualified with multiple qualifiers like `db.table.column`. That's why
current version of `AttributeReference.sql` joins all qualifiers using `.`
rather than picking the first one.
I believe it's safe and good to enforce the at-most-one-qualifier
constraint at type level unless there do exist valid cases where using multiple
qualifiers makes sense but haven't been implemented in Spark SQL yet.
cc @marmbrus @rxin @yhuai
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