alamb commented on code in PR #11550:
URL: https://github.com/apache/datafusion/pull/11550#discussion_r1686922713
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datafusion/expr/src/expr_fn.rs:
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@@ -676,6 +679,266 @@ pub fn interval_month_day_nano_lit(value: &str) -> Expr {
Expr::Literal(ScalarValue::IntervalMonthDayNano(interval))
}
+/// Extensions for configuring [`Expr::AggregateFunction`]
+///
+/// Adds methods to [`Expr`] that make it easy to set optional aggregate
options
+/// such as `ORDER BY`, `FILTER` and `DISTINCT`
+///
+/// # Example
+/// ```no_run
+/// # use datafusion_common::Result;
+/// # use datafusion_expr::{AggregateUDF, col, Expr, lit};
+/// # use sqlparser::ast::NullTreatment;
+/// # fn count(arg: Expr) -> Expr { todo!{} }
+/// # fn first_value(arg: Expr) -> Expr { todo!{} }
+/// # fn main() -> Result<()> {
+/// use datafusion_expr::ExprFunctionExt;
+///
+/// // Create COUNT(x FILTER y > 5)
+/// let agg = count(col("x"))
+/// .filter(col("y").gt(lit(5)))
+/// .build()?;
+/// // Create FIRST_VALUE(x ORDER BY y IGNORE NULLS)
+/// let sort_expr = col("y").sort(true, true);
+/// let agg = first_value(col("x"))
+/// .order_by(vec![sort_expr])
+/// .null_treatment(NullTreatment::IgnoreNulls)
+/// .build()?;
+/// # Ok(())
+/// # }
+/// ```
+pub trait ExprFunctionExt {
Review Comment:
> The one problem I see there is that we would end up with Expr implementing
two traits with identical function calls, which would be less user friendly.
I agree that is not a good user experience
> then this PR is ready to go assuming we come to agreement on if this
should be a single trait or broken into two for Aggregate vs Window.
I wonder if we could do something like this:
```rust
trait WindowExt: AggregateExt {
...
}
```
which is kind of like Rust's inheritance -- it basically says that anything
that implements `WindowExt` also implements `AggregateExt`
And then the builder could be extended for window specific functions (and
perhaps `build` made to error if the underlying aggregate function couldn't be
used as a window function)
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