alamb commented on PR #10560:
URL: https://github.com/apache/datafusion/pull/10560#issuecomment-2154554804
> I'm not sure if we really need the error check since we don't have any now
I guess what I was thinking is that the reason there is no check now is
because the rust compiler will ensure the types are correct (specifically that
they are `AggregateUDF`.
With the builder API as it is written now, you can call `order_by` on any
arbitrary Expr (not just the appropriate `Expr::AggregateFunction` variant)
So I was imagining if any of the methods had been called on an invalid Expr,
`build()` would return an error
Maybe something like
```rust
/// Traits to help build aggregate functions
trait AggregateExt {
// return a builder
fn order_by(self, order_by: Vec<Expr>) -> AggregateBuilder;
fn filter(self, filter: Box<Expr>) -> AggregateBuilder;
fn null_treatment(self, null_treatment: NullTreatment) ->
AggregateBuilder;
fn distinct(self) -> AggregateBuilder;
}
pub struct AggregateBuilder {
// if set to none, builder errors
udf: Option<Arc<AggregateUdf>>,
order_by: Vec<Expr>,
....
}
impl AggregateBuilder {
fn order_by(self, order_by: Vec<Expr>) -> AggregateBuilder {
self.order_by = order_by;
self
}
fn filter(self, filter: Box<Expr>) -> AggregateBuilder {..}
fn null_treatment(self, null_treatment: NullTreatment) ->
AggregateBuilder {..}
fn distinct(self) -> AggregateBuilder {..}
// builds up any in progress aggregate
fn build(self) -> Result<Expr> {
let Some(udf) = self.udf else {
return plan_err!("Expr of type XXX is not an aggregate")
}
udf.order_by = self.order_by;
...
Ok(Expr::AggregateFunction(udf))
}
}
impl AggregateExt for Expr {
fn order_by(self, order_by: Vec<Expr>) -> AggregateBuilder {
match self {
Expr::AggregateFunction(mut udaf) => {
AggregateBuilder { udf: Some(udaf) }
}
// wrong type passed -- error when build is called
_ => {
AggregateBuilder { udf: None }
}
}
}
```
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