mihailoale-db commented on code in PR #59271:
URL: https://github.com/apache/spark/pull/59271#discussion_r4260509198


##########
sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/plans/logical/basicLogicalOperators.scala:
##########
@@ -1759,15 +1759,26 @@ case class Unpivot(
 
   def canBeCoercioned: Boolean = values.exists(_.nonEmpty) &&
     values.exists(_.forall(_.forall(_.resolved))) &&
+    // values are coerced by position, so every value group must have one 
value per value column,
+    // otherwise CheckAnalysis reports UNPIVOT_VALUE_SIZE_MISMATCH
+    values.exists(_.forall(_.length == valueColumnNames.length)) &&
     // when no ids are given, values must be Attributes (column names) to 
allow detecting ids
     // coercion will add aliases, would disallow detecting ids, so defer 
coercion after id detection
     ids.exists(_.forall(_.resolved))
 
-  def valuesTypeCoercioned: Boolean = canBeCoercioned &&
-    // all inner values at position idx must have the same data type
+  def valuesTypeCoercioned: Boolean = canBeCoercioned && {
+    // all inner values at position idx must have exactly the same data type, 
including nested
+    // nullability, because the value column takes the data type of the first 
value
+    val sameType: (DataType, DataType) => Boolean =
+      if 
(conf.getConf(SQLConf.LEGACY_UNPIVOT_USE_FIRST_VALUE_NESTED_NULLABILITY)) {
+        DataTypeUtils.sameType
+      } else {
+        _ == _

Review Comment:
   The same rework addresses this. Values that differ only in nested 
nullability no longer get a `Cast`: the merged type is set only on the `Expand` 
output attribute, as `Union` does, so `Cast` itself is unchanged. The new test 
asserts that the analyzed plan has no
     `Cast`, and the `unpivot.sql` analyzer results no longer show one.



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