alamb commented on code in PR #26790:
URL: https://github.com/apache/datafusion/pull/26790#discussion_r4392903340
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datafusion/physical-plan/src/union.rs:
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@@ -1016,31 +1016,29 @@ fn union_schema(inputs: &[Arc<dyn ExecutionPlan>]) ->
Result<SchemaRef> {
// which also uses the left side names.
let base_field = first_schema.field(i).clone();
- // Coerce metadata and nullability across all inputs
-
- inputs
+ // Select the first nullable field, or the first field if none are
+ // nullable, before merging metadata. Otherwise, each non-nullable
+ // candidate would repeat the full metadata merge.
+ let (input_idx, input) = inputs
Review Comment:
so I think my understanding is that this change effectively moves the check
for `Field::isNullable` before the merging.
This is likely to help in situations where a bunch of non nullable inputs
are unioned into a nullable one that is far down the list
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