jayzhan211 commented on PR #14440:
URL: https://github.com/apache/datafusion/pull/14440#issuecomment-2659202892
> > Enum looks good to me but "adding user defined coercion variant" is not
clear to me since I think the Coercible signature is user-defined builtin, they
can change the type they need and set it into their own function 🤔
>
> That is true that `TypeSignature::Coercable` allows users to define
whatever they want.
>
> I was thinking for @shehabgamin 's usecase where he wanted to implement
coercsion that had a slightly different rules about when to cast Integer -->
String. He could implement that with an entirely custom coercion, but it might
be nice to just adjust the rules somehow (and reuse most of the rest of the
logic ) 🤔
Modifying Signature::Coercible alone won't achieve what @shehabgamin wants
because, regardless of the approach, they would still need to:
Fork the entire function
1. Modify its signature
2. Register it in the function map
3. Even if we introduce another variant, we would still maintain a version
for DataFusion, while others would have to go through the same process to
customize it for their needs.
A better solution would be to separate the function signature from the
function itself, as shown below:
```rust
pub struct ScalarFunctionExpr {
fun: Arc<ScalarUDF>,
name: String,
args: Vec<Arc<dyn PhysicalExpr>>,
return_type: DataType,
nullable: bool,
// new
signature: TypeSignature
}
```
This change fundamentally impacts how scalar functions are structured...
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