helgikrs opened a new issue, #15046:
URL: https://github.com/apache/datafusion/issues/15046
### Describe the bug
Calling `column_refs()` on a subquery expression returns no column
references, even if the subquery contains outer references.
PushDownFilter optimization uses the `column_refs()` to compare against the
columns returned from `prevent_predicate_push_down_columns()` on an extension
node. The effect of this is that a subquery in a filter may get pushed below an
extension node when it shouldn't.
### To Reproduce
```rust
let outer_ref = Expr::OuterReferenceColumn(DataType::Int64,
Column::new(None::<String>, "a"));
let plan = LogicalPlanBuilder::empty(true)
.project(vec![outer_ref.clone()])
.unwrap()
.build()
.unwrap();
let subq = Subquery {
subquery: Arc::new(plan),
outer_ref_columns: vec![outer_ref],
};
let expr = Expr::Exists(Exists::new(subq, false));
println!("{:?}", expr.column_refs());
```
### Expected behavior
`column_refs()` should return the outer columns used by the subquery
expression.
### Additional context
The relevant code is
[here](https://github.com/apache/datafusion/blob/main/datafusion/expr/src/expr.rs#L1533).
It just collects `Expr::Column`s within self.
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