kosiew commented on code in PR #17085:
URL: https://github.com/apache/datafusion/pull/17085#discussion_r2394701655
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datafusion/expr/src/udaf.rs:
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@@ -762,7 +762,24 @@ pub trait AggregateUDFImpl: Debug + DynEq + DynHash + Send
+ Sync {
/// group during query execution.
///
/// acc_args: [`AccumulatorArgs`] contains information about how the
- /// aggregate function was called.
+ /// aggregate function was called. Use `acc_args.exprs` together with
+ /// `acc_args.schema` to inspect the [`FieldRef`] of each input.
+ ///
+ /// Example: retrieving metadata and return field for input `i`:
+ /// ```ignore
+ /// let metadata = acc_args.schema.field(i).metadata();
+ /// let field = acc_args.exprs[i].return_field(&acc_args.schema)?;
+ /// ```
Review Comment:
Not at all 😄
* `acc_args.schema.field(i)` — returns the raw Arrow `Field` from the
(physical) input schema at position `i` (name, type, nullability, metadata
exactly as in that schema).
* `acc_args.exprs[i].return_field(&acc_args.schema)?` — asks the expression
for the effective `FieldRef` for argument `i` given the full schema. It
incorporates expression semantics (casts, literals, computed types, extension
metadata, nullability changes, etc.) and returns an owned/Arc `FieldRef` (and
can fail), not just a borrowed `&Field`.
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