From 1e33a695356628741667b223e11ed185d79d8690 Mon Sep 17 00:00:00 2001
From: Alex Qyoun-ae <4062971+MazterQyou@users.noreply.github.com>
Date: Fri, 17 Jul 2026 19:24:15 +0400
Subject: [PATCH] fix: Resolve qualified columns during `UNION` type coercion

Signed-off-by: Alex Qyoun-ae <4062971+MazterQyou@users.noreply.github.com>
---
 .../core/src/logical_plan/expr_rewriter.rs    |  7 +++++
 datafusion/core/tests/sql/union.rs            | 31 +++++++++++++++++++
 2 files changed, 38 insertions(+)

diff --git a/datafusion/core/src/logical_plan/expr_rewriter.rs b/datafusion/core/src/logical_plan/expr_rewriter.rs
index a35eb85a0f19e..09ca8d6ad8605 100644
--- a/datafusion/core/src/logical_plan/expr_rewriter.rs
+++ b/datafusion/core/src/logical_plan/expr_rewriter.rs
@@ -619,6 +619,13 @@ pub fn coerce_plan_expr_for_schema(
                         Box::new(e.clone().cast_to(new_type, input.schema())?),
                         alias.clone(),
                     )),
+                    // Projection expressions must be resolved against the input schema:
+                    // the projection's own schema may have had its qualifiers replaced
+                    // (e.g. by `project_with_column_index_alias` during UNION planning),
+                    // so qualified columns in `expr` would no longer resolve against it
+                    (LogicalPlan::Projection(Projection { input, .. }), _) => {
+                        expr.cast_to(new_type, input.schema())
+                    }
                     _ => expr.cast_to(new_type, plan.schema()),
                 }
             } else {
diff --git a/datafusion/core/tests/sql/union.rs b/datafusion/core/tests/sql/union.rs
index 54807b3b2da66..5a42011e3dd2c 100644
--- a/datafusion/core/tests/sql/union.rs
+++ b/datafusion/core/tests/sql/union.rs
@@ -75,3 +75,34 @@ async fn union_all_with_aggregate() -> Result<()> {
     assert_batches_eq!(expected, &actual);
     Ok(())
 }
+
+#[tokio::test]
+async fn union_all_ctes_with_type_coercion() -> Result<()> {
+    let ctx = SessionContext::new();
+    // Type mismatch must enter at the second UNION level: the first UNION
+    // replaces its input projections' schemas with the unqualified union
+    // schema, and the second UNION's coercion used to fail resolving the
+    // still-qualified columns against them
+    let sql = "WITH \
+        a AS (SELECT CAST(1 AS bigint) AS t), \
+        b AS (SELECT CAST(2 AS bigint) AS t), \
+        c AS (SELECT 3.5 AS t) \
+        SELECT 'A' AS l, t FROM a \
+        UNION ALL \
+        SELECT 'B' AS l, t FROM b \
+        UNION ALL \
+        SELECT 'C' AS l, t FROM c";
+    let actual = execute_to_batches(&ctx, sql).await;
+    #[rustfmt::skip]
+    let expected = [
+        "+---+-----+",
+        "| l | t   |",
+        "+---+-----+",
+        "| A | 1   |",
+        "| B | 2   |",
+        "| C | 3.5 |",
+        "+---+-----+",
+    ];
+    assert_batches_eq!(expected, &actual);
+    Ok(())
+}