[GitHub] spark pull request: [SPARK-14004][SQL] NamedExpressions should hav...

[liancheng <gi...@git.apache.org>]

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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