Re: [PR] fix: preserve union nullability when introducing nulls [arrow-rs]

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

Jefffrey commented on code in PR #11288:
URL: https://github.com/apache/arrow-rs/pull/11288#discussion_r4190740402


##########
arrow-data/src/transform/union.rs:
##########
@@ -77,23 +77,27 @@ pub(super) fn extend_nulls_dense(
     let DataType::Union(fields, _) = &mutable.data_type else {
         unreachable!()
     };
-    let first_type_id = fields
+    let null_type_id = fields
         .iter()
-        .next()
-        .expect("union must have at least one field")
+        .find(|(_, field)| field.is_nullable())
+        .expect("cannot extend nulls in a union with no nullable field")

Review Comment:
   can these be errors instead of panics, since the function can already return an error



##########
arrow-data/src/transform/union.rs:
##########
@@ -104,16 +108,16 @@ pub(super) fn extend_nulls_sparse(
     let DataType::Union(fields, _) = &mutable.data_type else {
         unreachable!()
     };
-    let first_type_id = fields
+    let null_type_id = fields
         .iter()
-        .next()
-        .expect("union must have at least one field")
+        .find(|(_, field)| field.is_nullable())
+        .expect("cannot extend nulls in a union with no nullable field")

Review Comment:
   same here



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