Kimahriman commented on code in PR #731:
URL: https://github.com/apache/datafusion-comet/pull/731#discussion_r1700429573


##########
native/spark-expr/src/structs.rs:
##########
@@ -125,3 +125,103 @@ impl PartialEq<dyn Any> for CreateNamedStruct {
             .unwrap_or(false)
     }
 }
+
+#[derive(Debug, Hash)]
+pub struct GetStructField {
+    child: Arc<dyn PhysicalExpr>,
+    ordinal: usize,
+}
+
+impl GetStructField {
+    pub fn new(child: Arc<dyn PhysicalExpr>, ordinal: usize) -> Self {
+        Self { child, ordinal }
+    }
+
+    fn child_field(&self, input_schema: &Schema) -> 
DataFusionResult<Arc<Field>> {
+        match self.child.data_type(input_schema)? {
+            DataType::Struct(fields) => Ok(fields[self.ordinal].clone()),
+            data_type => Err(DataFusionError::Plan(format!(
+                "Expect struct field, got {:?}",
+                data_type
+            ))),
+        }
+    }
+}
+
+impl PhysicalExpr for GetStructField {

Review Comment:
   In this particular case `GetStructField` might not even make sense as a UDF 
because it's created by the Spark analyzer from an `ExtractValue` expression, 
which resolves the name to an ordinal in the struct. It'd be odd to use 
`GetStructField` directly, since you would normally want a nested column by 
name, and not by index. I think the existing `get_field` UDF in DataFusion 
covers that use case, but using this PhysicalExpr on the Spark side seems like 
it makes more sense instead of having to convert the ordinal back into a field 
name to then use `get_field`



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