mihailotim-db commented on code in PR #53508:
URL: https://github.com/apache/spark/pull/53508#discussion_r2633849094


##########
sql/core/src/main/scala/org/apache/spark/sql/execution/datasources/rules.scala:
##########
@@ -695,19 +695,43 @@ object ViewSyncSchemaToMetaStore extends (LogicalPlan => 
Unit) {
           (field.dataType != planField.dataType ||
             field.nullable != planField.nullable ||
             (viewSchemaMode == SchemaEvolution && (
-              field.getComment() != planField.getComment() ||
-              field.name != planField.name)))
+              field.name != planField.name ||
+                // Only trigger redo on comment changes if preserve flag is 
disabled.
+                
(!session.sessionState.conf.viewSchemaEvolutionPreserveUserComments &&
+                  field.getComment() != planField.getComment()))))
         }
 
+        lazy val viewFieldsByName = viewFields.map(f => f.name -> f).toMap
+
         if (redo) {
           val newSchema = if (viewSchemaMode == SchemaTypeEvolution) {
             val newFields = viewQuery.schema.map {
               case StructField(name, dataType, nullable, _) =>
                 StructField(name, dataType, nullable,
-                  viewFields.find(_.name == name).get.metadata)
+                  viewFieldsByName(name).metadata)
+            }
+            StructType(newFields)
+          } else if 
(session.sessionState.conf.viewSchemaEvolutionPreserveUserComments) {
+            // Adopt types/nullable/names from query, but preserve view 
comments.
+            val newFields = viewQuery.schema.map { planField =>
+              val newMetadata = viewFieldsByName.get(planField.name) match {
+                case Some(viewField) =>
+                  // Use table metadata but override with view comment
+                  val builder = new 
MetadataBuilder().withMetadata(planField.metadata)
+                  viewField.getComment() match {
+                    case Some(comment) => builder.putString("comment", comment)
+                    case None => builder.remove("comment")

Review Comment:
   By doing this, do we lose table commnets where a view body is a simple 
`select * from table`, if the table has comments and the view didn't change 
anything? Is that the intended behavior?



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