kosiew commented on code in PR #17085:
URL: https://github.com/apache/datafusion/pull/17085#discussion_r2394519916
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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:
The snippet is meant to illustrate the sentence immediately above it: you
pair `acc_args.exprs` with `acc_args.schema` to recover the full `FieldRef` for
argument *i*.
Pulling the metadata out of `schema.field(i)` is one common use case, and
the follow-up line shows how you would then obtain the complete `FieldRef`
(name, type, metadata) via:
```rust
exprs[i].return_field(&acc_args.schema)
```
...using the same pairing.
I'll tweak the wording.
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