singhpk234 commented on code in PR #51002:
URL: https://github.com/apache/spark/pull/51002#discussion_r2114168866


##########
sql/catalyst/src/main/scala/org/apache/spark/sql/connector/catalog/CatalogV2Util.scala:
##########
@@ -597,11 +597,31 @@ private[sql] object CatalogV2Util {
       // Note: the back-fill here is a logical concept. The data source can 
keep the existing
       //       data unchanged and let the data reader to return "exist 
default" for missing
       //       columns.
-      val existingDefault = Literal(default.getValue.value(), 
default.getValue.dataType()).sql
-      
f.withExistenceDefaultValue(existingDefault).withCurrentDefaultValue(default.getSql)
+      val existsDefault = extractExistsDefault(default)
+      val (sql, expr) = extractCurrentDefault(default)
+      val newMetadata = new MetadataBuilder()
+        .withMetadata(f.metadata)
+        .putString(EXISTS_DEFAULT_COLUMN_METADATA_KEY, existsDefault)
+        .putExpression(CURRENT_DEFAULT_COLUMN_METADATA_KEY, sql, expr)
+        .build()
+      f.copy(metadata = newMetadata)
     }.getOrElse(f)
   }
 
+  private def extractExistsDefault(default: ColumnDefaultValue): String = {
+    Literal(default.getValue.value(), default.getValue.dataType()).sql
+  }
+
+  private def extractCurrentDefault(default: ColumnDefaultValue): (String, 
Option[Expression]) = {
+    val expr = 
Option(default.getExpression).flatMap(V2ExpressionUtils.toCatalyst)

Review Comment:
   [doubt] presently `toCatalyst` doesn't handle connector scalar udf's is the 
plan to enhance this in future ?



##########
sql/api/src/main/scala/org/apache/spark/sql/types/Metadata.scala:
##########
@@ -120,6 +122,12 @@ sealed class Metadata private[types] (private[types] val 
map: Map[String, Any])
     map(key).asInstanceOf[T]
   }
 
+  private[sql] def getExpression[E](key: String): (String, Option[E]) = {
+    val sql = getString(key)
+    val expr = if (runtimeMap != null) 
runtimeMap.get(key).map(_.asInstanceOf[E]) else None

Review Comment:
   minor 
   
   ```suggestion
       val expr = Option(runtimeMap).flatMap(_.get(key).map(_.asInstanceOf[E]))
   ```



##########
sql/catalyst/src/main/scala/org/apache/spark/sql/connector/catalog/CatalogV2Util.scala:
##########
@@ -597,11 +597,31 @@ private[sql] object CatalogV2Util {
       // Note: the back-fill here is a logical concept. The data source can 
keep the existing
       //       data unchanged and let the data reader to return "exist 
default" for missing
       //       columns.
-      val existingDefault = Literal(default.getValue.value(), 
default.getValue.dataType()).sql
-      
f.withExistenceDefaultValue(existingDefault).withCurrentDefaultValue(default.getSql)
+      val existsDefault = extractExistsDefault(default)
+      val (sql, expr) = extractCurrentDefault(default)
+      val newMetadata = new MetadataBuilder()
+        .withMetadata(f.metadata)
+        .putString(EXISTS_DEFAULT_COLUMN_METADATA_KEY, existsDefault)
+        .putExpression(CURRENT_DEFAULT_COLUMN_METADATA_KEY, sql, expr)
+        .build()
+      f.copy(metadata = newMetadata)
     }.getOrElse(f)
   }
 
+  private def extractExistsDefault(default: ColumnDefaultValue): String = {
+    Literal(default.getValue.value(), default.getValue.dataType()).sql
+  }
+
+  private def extractCurrentDefault(default: ColumnDefaultValue): (String, 
Option[Expression]) = {
+    val expr = 
Option(default.getExpression).flatMap(V2ExpressionUtils.toCatalyst)
+    val sql = Option(default.getSql).orElse(expr.map(_.sql)).getOrElse {

Review Comment:
   [doubt] my understanding was `.sql` is not reliable (based on discussion 
[here](https://github.com/apache/spark/pull/50792#discussion_r2078776399)), 
wondering if this could lead to users using `getMap` or map.get(key), directly 
and extracting the SQL from the map, skip actually checking if there is an 
expression for it and one should use that instead ? essentially if there i an 
entry in the runtimeMap should we let the map.get fail ?



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