Method on_unique_window_agg

src/expr/src/relation.rs:3171–3191

`on_unique` for window aggregations

(
        window_frame: &WindowFrame,
        arg_expr: MirScalarExpr,
        input_type: &[ReprColumnType],
        return_type: ReprScalarType,
        wrapped_aggr: &AggregateFunc,
    )

    /// `on_unique` for window aggregations
    fn on_unique_window_agg(
        window_frame: &WindowFrame,
        arg_expr: MirScalarExpr,
        input_type: &[ReprColumnType],
        return_type: ReprScalarType,
        wrapped_aggr: &AggregateFunc,
    ) -> (MirScalarExpr, ColumnName) {
        // If the window frame includes the current (single) row, evaluate the wrapped aggregate on
        // that row. Otherwise, return the default value for the aggregate.
        let result_expr = if window_frame.includes_current_row() {
            AggregateExpr {
                func: wrapped_aggr.clone(),
                expr: arg_expr,
                distinct: false, // We have just one input element; DISTINCT doesn't matter.
            }
            .on_unique(input_type)
        } else {
            MirScalarExpr::literal_ok(wrapped_aggr.default(), return_type)
        };
        (result_expr, ColumnName::from("?window_agg?"))
    }

    /// Returns whether the expression is COUNT(*) or not.  Note that
    /// when we define the count builtin in sql::func, we convert