jcsherin opened a new issue, #12373:
URL: https://github.com/apache/datafusion/issues/12373
### Is your feature request related to a problem or challenge?
> Instead of return_type + nullable. I think `field` is a better choice.
Given windowudf is pretty new so not widely used yet (?). It would be nice to
have `field(&self, args: FieldArgs) -> Result<Field>`
```rust
FieldArgs {
arg_type: &[DataType]
schema: Schema // for nullable
}
```
> It is too late for AggregateUDF and ScalarUDF :(
_Originally posted by @jayzhan211 in
https://github.com/apache/datafusion/pull/12030#discussion_r1719736427_
This is a proposal to add a `field` trait method for implementing
user-defined window functions. And also remove (or deprecate) the trait methods
`return_type` and `nullable` from `WindowUDFImpl`.
Both the return data type and nullability of the result from evaluating the
user-defined window function will be specified by the `field` implementation.
### Describe the solution you'd like
The current implementation for a user-defined window function (without
`field` trait method) looks like this:
```rust
impl WindowUDFImpl for RowNumber {
fn return_type(&self, _arg_types: &[DataType]) -> Result<DataType> {
Ok(DataType::UInt64)
}
fn nullable(&self) -> bool {
true
}
}
```
The implementation for a user-defined window function after this change:
```rust
impl WindowUDFImpl for RowNumber {
fn field(&self, field_args: WindowUDFFieldArgs) -> Result<Field> {
Ok(Field::new(
field_args.name(), /* window function display name */
DataType::UInt64, /* result data type */
false /* row number is not nullable */
))
}
}
```
---
1. Add the `field` trait method to `WindowUDFImpl`:
```rust
fn field(&self, field_args: WindowUDFFieldArgs) -> Result<Field>
```
2. Add `WindowUDFFieldArgs`:
```rust
/// Contains metadata necessary for defining the field which represents
/// the final result of evaluating a user-defined window function.
pub struct WindowUDFFieldArgs<'a> {
/// The data types of input expressions to the user-defined window
/// function.
input_types: &'a [DataType],
/// The display name of the user-defined window function.
function_name: &'a str,
}
```
3. Remove/Deprecate `return_type` trait method from `WindowUDFImpl`:
https://github.com/apache/datafusion/blob/502ce4bab6b09989a3412bd99795cba770f143b5/datafusion/expr/src/udwf.rs#L282-L284
4. Remove `nullable` trait method from `WindowUDFImpl`:
https://github.com/apache/datafusion/blob/502ce4bab6b09989a3412bd99795cba770f143b5/datafusion/expr/src/udwf.rs#L347-L354
### Describe alternatives you've considered
Make no changes to `WindowUDFImpl`.
### Additional context
_No response_
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