Jefffrey commented on code in PR #15911:
URL: https://github.com/apache/datafusion/pull/15911#discussion_r2473721449
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datafusion/expr/src/udaf.rs:
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@@ -674,6 +681,31 @@ pub trait AggregateUDFImpl: Debug + Send + Sync {
/// the arguments
fn return_type(&self, arg_types: &[DataType]) -> Result<DataType>;
+ /// What type will be returned by this function, given the arguments?
+ ///
+ /// By default, this function calls [`Self::return_type`] with the
+ /// types of each argument.
+ ///
+ /// # Notes
+ ///
+ /// Most UDFs should implement [`Self::return_type`] and not this
+ /// function as the output type for most functions only depends on the
types
+ /// of their inputs (e.g. `sum(f64)` is always `f64`).
+ ///
+ /// This function can be used for more advanced cases such as:
+ ///
+ /// 1. specifying nullability
+ /// 2. return types based on the **values** of the arguments (rather than
+ /// their **types**.
Review Comment:
Hi @timsaucer, was reading through some documentation and wanted to
understand this point further. Could you elaborate on this? How can this
function return types based on argument values if it only gives access to
`Field`s? (now `FieldRef`s)
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