Github user maropu commented on a diff in the pull request:

    https://github.com/apache/spark/pull/13290#discussion_r64533746
  
    --- Diff: 
sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/analysis/Analyzer.scala
 ---
    @@ -1448,6 +1450,37 @@ class Analyzer(
       }
     
       /**
    +   * Fixes nullability of Attributes in a resolved LogicalPlan by using 
the nullability of
    +   * corresponding Attributes of its children output Attributes. This step 
is needed because
    +   * users can use a resolved AttributeReference in the Dataset API and 
outer joins
    +   * can change the nullability of an AttribtueReference. Without the fix, 
a nullable column's
    +   * nullable field can be actually set as non-nullable, which cause 
illegal optimization
    +   * (e.g., NULL propagation) and wrong answers.
    +   * See SPARK-13484 and SPARK-13801 for the concrete queries of this case.
    +   */
    +  object FixNullability extends Rule[LogicalPlan] {
    +
    +    def apply(plan: LogicalPlan): LogicalPlan = plan transformUp {
    +      case q: LogicalPlan if q.resolved =>
    +        val childrenOutput = q.children.flatMap(c => 
c.output).groupBy(_.exprId).flatMap {
    --- End diff --
    
    I think the bug occurs in case of filters and projects just after 
outer-joins.
    Seems this fix could add more overheads since the rule overwrites 
nullability in every operator nodes.
    Or, can we ignore the overheads?


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