Omega359 commented on code in PR #14684:
URL: https://github.com/apache/datafusion/pull/14684#discussion_r1957429055


##########
datafusion/core/src/dataframe/mod.rs:
##########
@@ -1779,37 +1779,82 @@ impl DataFrame {
             .config_options()
             .sql_parser
             .enable_ident_normalization;
+
         let old_column: Column = if ident_opts {
             Column::from_qualified_name(old_name)
         } else {
             Column::from_qualified_name_ignore_case(old_name)
         };
 
-        let (qualifier_rename, field_rename) =
-            match self.plan.schema().qualified_field_from_column(&old_column) {
-                Ok(qualifier_and_field) => qualifier_and_field,
-                // no-op if field not found
-                Err(DataFusionError::SchemaError(
-                    SchemaError::FieldNotFound { .. },
-                    _,
-                )) => return Ok(self),
-                Err(err) => return Err(err),
-            };
-        let projection = self
-            .plan
-            .schema()
-            .iter()
-            .map(|(qualifier, field)| {
-                if qualifier.eq(&qualifier_rename) && field.as_ref() == 
field_rename {
-                    col(Column::from((qualifier, field))).alias(new_name)
-                } else {
-                    col(Column::from((qualifier, field)))
-                }
-            })
-            .collect::<Vec<_>>();
-        let project_plan = LogicalPlanBuilder::from(self.plan)
-            .project(projection)?
-            .build()?;
+        let project_plan = if let LogicalPlan::Projection(Projection {
+            expr,
+            input,
+            schema,
+            ..
+        }) = self.plan
+        {
+            // special case: we already have a projection on top, so we can 
reuse it rather than creating a new one
+            let (qualifier_rename, field_rename) =
+                match schema.qualified_field_from_column(&old_column) {
+                    Ok(qualifier_and_field) => qualifier_and_field,
+                    // no-op if field not found
+                    Err(DataFusionError::SchemaError(
+                        SchemaError::FieldNotFound { .. },
+                        _,
+                    )) => {
+                        self.plan = LogicalPlan::Projection(

Review Comment:
   If the field is not found why are we not just returning Ok(self)? I am 
unsure why we would need to add a projection for the plan in this case



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