Expr::apply_children/map_children & LogicalPlan::map_expressions does not correctly map subquery outer_ref_column expressions

[phillipleblanc]

Describe the bug

The TreeNode implementation for Expr does not handle the outer_ref_column expressions present in the Subquery nodes of Expr::InSubquery, Expr::ScalarSubquery, and Expr::Exists.

The LogicalPlan::map_expressions implementation does not handle the outer_ref_column expression in LogicalPlan::Subquery

To Reproduce

Create a LogicalPlan from a SQL statement with a subquery expression that has an outer column reference, i.e.

SELECT 
    e.employee_id,
    e.first_name,
    e.salary
FROM 
    employees e
WHERE 
    e.salary > (
        SELECT AVG(salary)
        FROM employees e2
        WHERE e2.department_id = e.department_id
    );

I would expect to be able to do something like this:

let plan = sql_to_plan(sql);
plan.apply(|plan| {
  plan.apply_expressions(|expr| {
    match expr {
      Expr::OuterReferenceColumn(_, _) => { do something useful }
      _ => Ok(Transformed::no(expr))
    }
  });
});

Expected behavior

The outer_ref_column expressions are part of the TreeNode visitor implementations in both Expr and LogicalPlan

Additional context

I will not work on this, so someone should feel free to take this. There is a workaround if you know about this behavior, which is to match on the expressions/plans which contain these outer ref column expressions.

[irenjj]

take