Method using_columns

datafusion/expr/src/logical_plan/plan.rs:492–525

returns all `Using` join columns in a logical plan

(&self)

    /// returns all `Using` join columns in a logical plan
    pub fn using_columns(&self) -> Result<Vec<HashSet<Column>>, DataFusionError> {
        let mut using_columns: Vec<HashSet<Column>> = vec![];

        self.apply_with_subqueries(|plan| {
            if let LogicalPlan::Join(Join {
                join_constraint: JoinConstraint::Using,
                on,
                ..
            }) = plan
            {
                // The join keys in using-join must be columns.
                let columns =
                    on.iter().try_fold(HashSet::new(), |mut accumu, (l, r)| {
                        let Some(l) = l.get_as_join_column() else {
                            return internal_err!(
                                "Invalid join key. Expected column, found {l:?}"
                            );
                        };
                        let Some(r) = r.get_as_join_column() else {
                            return internal_err!(
                                "Invalid join key. Expected column, found {r:?}"
                            );
                        };
                        accumu.insert(l.to_owned());
                        accumu.insert(r.to_owned());
                        Result::<_, DataFusionError>::Ok(accumu)
                    })?;
                using_columns.push(columns);
            }
            Ok(TreeNodeRecursion::Continue)
        })?;

        Ok(using_columns)
    }

    /// returns the first output expression of this `LogicalPlan` node.
    pub fn head_output_expr(&self) -> Result<Option<Expr>> {