Github user dongjoon-hyun commented on a diff in the pull request:
https://github.com/apache/spark/pull/17251#discussion_r105528166
--- Diff:
sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/analysis/TypeCoercion.scala
---
@@ -590,6 +591,22 @@ object TypeCoercion {
}
/**
+ * Coerces NullTypes of a Stack function to the corresponding column
types.
+ */
+ object StackCoercion extends Rule[LogicalPlan] {
+ def apply(plan: LogicalPlan): LogicalPlan = plan resolveExpressions {
+ case s @ Stack(children) if s.childrenResolved =>
+ val schema = s.elementSchema
--- End diff --
Oops. This breaks
[GeneratorFunctionSuite](https://github.com/apache/spark/blob/master/sql/core/src/test/scala/org/apache/spark/sql/GeneratorFunctionSuite.scala#L30).
For `StackCoercision`, we need the schema which is based on the number of
rows (and derived columns) from the **first argument**. Actually, the content
of the first argument. The validate of the value is done by
`Stack.checkInputDataTypes`. It seems we cannot add `StackCoercision` in this
manner. How do you think about this?
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