paleolimbot commented on code in PR #23169:
URL: https://github.com/apache/datafusion/pull/23169#discussion_r3509304476
##########
datafusion/expr/src/expr_schema.rs:
##########
@@ -69,18 +69,43 @@ 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.
+///
+/// Metadata handling:
+/// - Source field metadata is propagated by default
+/// - Extension type metadata keys (`ARROW:extension:name` and
`ARROW:extension:metadata`)
+/// are taken from the target field, overwriting any extension metadata from
the source.
+/// This ensures casting does not incorrectly propagate extension type
identity.
+///
/// 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> {
+ use arrow_schema::extension::{EXTENSION_TYPE_METADATA_KEY,
EXTENSION_TYPE_NAME_KEY};
+
+ // Start with source metadata
+ let mut metadata = source_field.metadata().clone();
Review Comment:
For arguments sake, and because it seemed a lot cleaner to me, I updated the
physical `Cast` to be explicit about its constituents (i.e., `None` for
metadata passes through from source for backward compatibility, `Some(...)`
forces output, like is required for schema alignment). We were never really
casting to a `field`, we were just using a `FieldRef` to store three pieces of
information.
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