Re: [PR] Do not rename struct fields when coercing types in `CASE` [datafusion]

["jayzhan211 (via GitHub)" <gi...@apache.org>]

jayzhan211 commented on code in PR #14384:
URL: https://github.com/apache/datafusion/pull/14384#discussion_r1938125583


##########
datafusion/expr-common/src/type_coercion/binary.rs:
##########
@@ -961,23 +961,31 @@ fn struct_coercion(lhs_type: &DataType, rhs_type: &DataType) -> Option<DataType>
                 return None;
             }
 
-            let types = std::iter::zip(lhs_fields.iter(), rhs_fields.iter())
+            let coerced_types = std::iter::zip(lhs_fields.iter(), rhs_fields.iter())
                 .map(|(lhs, rhs)| comparison_coercion(lhs.data_type(), rhs.data_type()))
                 .collect::<Option<Vec<DataType>>>()?;
 
-            let fields = types
+            // preserve the field name and nullability
+            let orig_fields = std::iter::zip(lhs_fields.iter(), rhs_fields.iter());
+
+            let fields: Vec<FieldRef> = coerced_types
                 .into_iter()
-                .enumerate()
-                .map(|(i, datatype)| {
-                    Arc::new(Field::new(format!("c{i}"), datatype, true))
-                })
-                .collect::<Vec<FieldRef>>();
+                .zip(orig_fields)
+                .map(|(datatype, (lhs, rhs))| coerce_fields(datatype, lhs, rhs))
+                .collect();
             Some(Struct(fields.into()))
         }
         _ => None,
     }
 }
 
+/// returns the result of coercing two fields to a common type
+fn coerce_fields(common_type: DataType, lhs: &FieldRef, rhs: &FieldRef) -> FieldRef {
+    let is_nullable = lhs.is_nullable() || rhs.is_nullable();
+    let name = lhs.name(); // pick the name from the left field

Review Comment:
   Yes, I think type union resolution is the correct on for CASE



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