cloud-fan commented on code in PR #50593:
URL: https://github.com/apache/spark/pull/50593#discussion_r2046322982


##########
sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/plans/logical/ColumnDefinition.scala:
##########
@@ -194,17 +194,32 @@ object ColumnDefinition {
 /**
  * A fake expression to hold the column/variable default value expression and 
its original SQL text.
  */
-case class DefaultValueExpression(child: Expression, originalSQL: String)
+case class DefaultValueExpression(
+    child: Expression,
+    originalSQL: String,
+    exposeCurrentDefaultAsExprV2: Boolean)
   extends UnaryExpression with Unevaluable {
   override def dataType: DataType = child.dataType
+  override def stringArgs: Iterator[Any] = Iterator(child, originalSQL)
   override protected def withNewChildInternal(newChild: Expression): 
Expression =
     copy(child = newChild)
 
   // Convert the default expression to ColumnDefaultValue, which is required 
by DS v2 APIs.
   def toV2(statement: String, colName: String): ColumnDefaultValue = child 
match {
     case Literal(value, dataType) =>
-      new ColumnDefaultValue(originalSQL, LiteralValue(value, dataType))
+      val literalV2 = LiteralValue(value, dataType)
+      val exprV2 = if (exposeCurrentDefaultAsExprV2) literalV2 else null
+      new ColumnDefaultValue(originalSQL, exprV2, literalV2)
     case _ =>
       throw QueryCompilationErrors.defaultValueNotConstantError(statement, 
colName, originalSQL)
   }
 }
+
+object DefaultValueExpression {
+  def apply(expr: Expression, originalSQL: String): DefaultValueExpression = {
+    // only expose V2 expressions as current defaults if all Catalyst 
expressions are evaluable
+    // Catalyst expressions like CURRENT_USER or CURRENT_DATE will require 
special handling
+    val exposeCurrentDefaultAsExprV2 = 
!expr.exists(_.isInstanceOf[FoldableUnevaluable])

Review Comment:
   shall we allow literals for now to be safe? I don't quite get what 
`!expr.exists(_.isInstanceOf[FoldableUnevaluable])` means under this context.



##########
sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/plans/logical/ColumnDefinition.scala:
##########
@@ -194,17 +194,32 @@ object ColumnDefinition {
 /**
  * A fake expression to hold the column/variable default value expression and 
its original SQL text.
  */
-case class DefaultValueExpression(child: Expression, originalSQL: String)
+case class DefaultValueExpression(
+    child: Expression,
+    originalSQL: String,
+    exposeCurrentDefaultAsExprV2: Boolean)
   extends UnaryExpression with Unevaluable {
   override def dataType: DataType = child.dataType
+  override def stringArgs: Iterator[Any] = Iterator(child, originalSQL)
   override protected def withNewChildInternal(newChild: Expression): 
Expression =
     copy(child = newChild)
 
   // Convert the default expression to ColumnDefaultValue, which is required 
by DS v2 APIs.
   def toV2(statement: String, colName: String): ColumnDefaultValue = child 
match {
     case Literal(value, dataType) =>
-      new ColumnDefaultValue(originalSQL, LiteralValue(value, dataType))
+      val literalV2 = LiteralValue(value, dataType)
+      val exprV2 = if (exposeCurrentDefaultAsExprV2) literalV2 else null
+      new ColumnDefaultValue(originalSQL, exprV2, literalV2)
     case _ =>
       throw QueryCompilationErrors.defaultValueNotConstantError(statement, 
colName, originalSQL)
   }
 }
+
+object DefaultValueExpression {
+  def apply(expr: Expression, originalSQL: String): DefaultValueExpression = {
+    // only expose V2 expressions as current defaults if all Catalyst 
expressions are evaluable
+    // Catalyst expressions like CURRENT_USER or CURRENT_DATE will require 
special handling
+    val exposeCurrentDefaultAsExprV2 = 
!expr.exists(_.isInstanceOf[FoldableUnevaluable])

Review Comment:
   shall we only allow literals for now to be safe? I don't quite get what 
`!expr.exists(_.isInstanceOf[FoldableUnevaluable])` means under this context.



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