Inline aliases are not visible within query

[osipovartem]

Some queries rely on alias reuse within the same SELECT

SELECT 'test' AS alias, LENGTH(alias)

DataFusion currently does not resolve alias in such cases during planning, which results in

Schema error: No field named alias

Related to #6543
Suggestion to add visitor to update expressions before planning

#[derive(Debug, Default)]
pub struct InlineAliasesInSelect {}

impl VisitorMut for InlineAliasesInSelect {
    type Break = ();

    fn pre_visit_query(&mut self, query: &mut Query) -> ControlFlow<Self::Break> {
        if let SetExpr::Select(select) = &mut *query.body {
            let mut alias_expr_map = HashMap::new();

            for item in &select.projection {
                if let SelectItem::ExprWithAlias { expr, alias } = item {
                    alias_expr_map.insert(alias.value.clone(), expr.clone());
                }
            }

            for item in &mut select.projection {
                match item {
                    SelectItem::UnnamedExpr(expr) | SelectItem::ExprWithAlias { expr, .. } => {
                        visit_expressions_mut(expr, &mut |e: &mut Expr| {
                            if let Expr::Identifier(ident) = e {
                                if let Some(original) = alias_expr_map.get(&ident.value) {
                                    *e = original.clone();
                                }
                            }
                            ControlFlow::<()>::Continue(())
                        });
                    }
                    _ => {}
                }
            }
        }
        ControlFlow::Continue(())
    }
}

pub fn visit(stmt: &mut Statement) {
    let _ = stmt.visit(&mut InlineAliasesInSelect {});
}

✅ Example

Transforms:

SELECT 'test' AS alias, LENGTH(alias)

Into an equivalent form:

SELECT 'test' AS alias, LENGTH('test')