gene-bordegaray commented on code in PR #24670:
URL: https://github.com/apache/datafusion/pull/24670#discussion_r3865597007
##########
datafusion/expr/src/expr_schema.rs:
##########
@@ -71,18 +71,31 @@ pub trait ExprSchemable {
-> Result<(DataType, bool)>;
}
-/// Derives the output field for a cast expression from the source field.
+/// Derives the output field for a cast expression from the source and target
+/// fields. Type-only casts preserve source metadata, while an explicit target
+/// field supplies its own metadata.
+///
/// For `TryCast`, `force_nullable` is `true` since a failed cast returns NULL.
fn cast_output_field(
source_field: &FieldRef,
- target_type: &DataType,
+ target_field: &FieldRef,
force_nullable: bool,
) -> Arc<Field> {
+ // `Cast::new` and `TryCast::new` use this field when only a target
+ // type is known. In that case, retain the source field's metadata.
+ let type_only_target = target_field.name().is_empty()
+ && target_field.is_nullable()
+ && target_field.metadata().is_empty();
+ let metadata = if type_only_target {
Review Comment:
mine can also be wrong. its kinda implicit. I think this difference should
be properly marked but might be a larger change like with an enum:
```rust
enum CastTarget { DataType, Field }
```
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