Function expr_to_columns

datafusion/expr/src/utils.rs:276–327

Recursively walk an expression tree, collecting the unique set of columns referenced in the expression

(expr: &Expr, accum: &mut HashSet<Column>)

/// Recursively walk an expression tree, collecting the unique set of columns
/// referenced in the expression
pub fn expr_to_columns(expr: &Expr, accum: &mut HashSet<Column>) -> Result<()> {
    expr.apply(|expr| {
        match expr {
            Expr::Column(qc) => {
                accum.insert(qc.clone());
            }
            // Use explicit pattern match instead of a default
            // implementation, so that in the future if someone adds
            // new Expr types, they will check here as well
            // TODO: remove the next line after `Expr::Wildcard` is removed
            #[expect(deprecated)]
            Expr::Unnest(_)
            | Expr::ScalarVariable(_, _)
            | Expr::Alias(_)
            | Expr::Literal(_, _)
            | Expr::BinaryExpr { .. }
            | Expr::Like { .. }
            | Expr::SimilarTo { .. }
            | Expr::Not(_)
            | Expr::IsNotNull(_)
            | Expr::IsNull(_)
            | Expr::IsTrue(_)
            | Expr::IsFalse(_)
            | Expr::IsUnknown(_)
            | Expr::IsNotTrue(_)
            | Expr::IsNotFalse(_)
            | Expr::IsNotUnknown(_)
            | Expr::Negative(_)
            | Expr::Between { .. }
            | Expr::Case { .. }
            | Expr::Cast { .. }
            | Expr::TryCast { .. }
            | Expr::ScalarFunction(..)
            | Expr::WindowFunction { .. }
            | Expr::AggregateFunction { .. }
            | Expr::GroupingSet(_)
            | Expr::InList { .. }
            | Expr::Exists { .. }
            | Expr::InSubquery(_)
            | Expr::SetComparison(_)
            | Expr::ScalarSubquery(_)
            | Expr::Wildcard { .. }
            | Expr::Placeholder(_)
            | Expr::OuterReferenceColumn { .. }
            | Expr::HigherOrderFunction(_)
            | Expr::Lambda(_)
            | Expr::LambdaVariable(_) => {}
        }
        Ok(TreeNodeRecursion::Continue)
    })
    .map(|_| ())
}

/// Find excluded columns in the schema, if any
/// SELECT * EXCLUDE(col1, col2), would return `vec![col1, col2]`