GaneshPatil7517 opened a new pull request, #19688:
URL: https://github.com/apache/datafusion/pull/19688
## Which issue does this PR close?
Closes #19511
Related to #18882
## Rationale for this change
Currently, `AggregateUDFImpl::is_nullable()` returns `true` by default for
all UDAFs, regardless of input characteristics. This is not ideal because:
1. The same nullability information is already encoded in `return_field()`
2. Most aggregate functions should only be nullable if their inputs are
nullable (e.g., `MIN`, `MAX`, `SUM`)
3. This pattern doesn't align with scalar UDFs, which already use
`return_field_from_args()` for nullability
## What changes are included in this PR?
### Core Changes
- **Deprecated `is_nullable()`** on `AggregateUDFImpl` trait with migration
guidance
- **Updated `udaf_default_return_field()`** to compute nullability from
input fields:
- Output is nullable if ANY input field is nullable
- Output is non-nullable only if ALL inputs are non-nullable
### Tests
Added 4 new tests validating nullability inference:
- `test_return_field_nullability_from_nullable_input`
- `test_return_field_nullability_from_non_nullable_input`
- `test_return_field_nullability_with_mixed_inputs`
- `test_return_field_preserves_return_type`
### Documentation
- New `docs/source/library-user-guide/functions/udf-nullability.md` with
migration guide and examples
- Updated `adding-udfs.md` with reference to nullability documentation
## Are these changes tested?
Yes. All existing tests pass, plus 4 new tests specifically for nullability
behavior.
## Are there any user-facing changes?
**Deprecation warning**: Users implementing `is_nullable()` will see a
deprecation warning directing them to use `return_field()` instead.
**Behavioral change**: Default nullability now depends on input field
nullability rather than always returning `true`. Functions like `COUNT` that
need to always return non-nullable should override `return_field()`.
This is a potentially breaking change for users who rely on the previous
behavior of always-nullable outputs, but the new behavior is more correct and
aligns with scalar UDF patterns.
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