friendlymatthew commented on code in PR #9544:
URL: https://github.com/apache/arrow-rs/pull/9544#discussion_r2926310435
##########
arrow-cast/src/cast/mod.rs:
##########
@@ -2276,6 +2305,108 @@ fn cast_struct_fields_in_order(
.collect::<Result<Vec<ArrayRef>, ArrowError>>()
}
+/// Cast a UnionArray to another UnionArray by casting each child array.
+fn cast_union_to_union(
+ array: &UnionArray,
+ from_fields: &UnionFields,
+ to_fields: &UnionFields,
+ _to_mode: UnionMode,
+ cast_options: &CastOptions,
+) -> Result<ArrayRef, ArrowError> {
+ let type_ids = array.type_ids().clone();
+ let offsets = array.offsets().cloned();
+
+ let new_children: Vec<ArrayRef> = from_fields
+ .iter()
+ .map(|(from_id, _from_field)| {
+ let (_, to_field) = to_fields
+ .iter()
+ .find(|(to_id, _)| *to_id == from_id)
+ .ok_or_else(|| {
+ ArrowError::CastError(format!(
+ "Cannot cast union: type_id {from_id} not found in
target union"
+ ))
+ })?;
+ let child = array.child(from_id);
+ cast_with_options(child.as_ref(), to_field.data_type(),
cast_options)
+ })
+ .collect::<Result<_, _>>()?;
Review Comment:
look ups like this can go from n^2 to nlogn if we implement
https://github.com/apache/arrow-rs/pull/8937, since sorting by `type_id` will
give us `logn` searching
##########
arrow-cast/src/cast/mod.rs:
##########
@@ -1180,6 +1194,21 @@ pub fn cast_with_options(
(_, Struct(_)) => Err(ArrowError::CastError(format!(
"Casting from {from_type} to {to_type} not supported"
))),
+ (Union(from_fields, _), Union(to_fields, to_mode)) =>
cast_union_to_union(
+ array.as_any().downcast_ref::<UnionArray>().unwrap(),
+ from_fields,
+ to_fields,
+ *to_mode,
+ cast_options,
+ ),
+ (Union(_, _), _) => cast_union_to_type(
+ array.as_any().downcast_ref::<UnionArray>().unwrap(),
+ to_type,
+ cast_options,
+ ),
+ (_, Union(_, _)) => Err(ArrowError::CastError(format!(
+ "Casting from {from_type} to {to_type} not supported"
+ ))),
Review Comment:
This PR does not support casting scalar types into a Union type
It's not a feature that we currently need and will probably need more
complex work to get it correct. Though, I think this is a cool/valid feature. I
can file an issue if liked
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