Raise a plan error on union if column count is not the same between p…

…lans. (#13117)

datafusion/expr/src/logical_plan/builder.rs

@@ -1482,6 +1482,15 @@ pub fn validate_unique_names<'a>(
 /// [`TypeCoercionRewriter::coerce_union`]: https://docs.rs/datafusion-optimizer/latest/datafusion_optimizer/analyzer/type_coercion/struct.TypeCoercionRewriter.html#method.coerce_union
 /// [`coerce_union_schema`]: https://docs.rs/datafusion-optimizer/latest/datafusion_optimizer/analyzer/type_coercion/fn.coerce_union_schema.html
 pub fn union(left_plan: LogicalPlan, right_plan: LogicalPlan) -> Result<LogicalPlan> {
+    if left_plan.schema().fields().len() != right_plan.schema().fields().len() {
+        return plan_err!(
+            "UNION queries have different number of columns: \
+            left has {} columns whereas right has {} columns",
+            left_plan.schema().fields().len(),
+            right_plan.schema().fields().len()
+        );
+    }
+
     // Temporarily use the schema from the left input and later rely on the analyzer to
     // coerce the two schemas into a common one.
 

datafusion/sqllogictest/test_files/type_coercion.slt

@@ -103,11 +103,11 @@ CREATE TABLE orders(
 );
 
 # union_different_num_columns_error() / UNION
-query error Error during planning: Union schemas have different number of fields: query 1 has 1 fields whereas query 2 has 2 fields
+query error DataFusion error: Error during planning: UNION queries have different number of columns: left has 1 columns whereas right has 2 columns
 SELECT order_id FROM orders UNION SELECT customer_id, o_item_id FROM orders
 
 # union_different_num_columns_error() / UNION ALL
-query error Error during planning: Union schemas have different number of fields: query 1 has 1 fields whereas query 2 has 2 fields
+query error DataFusion error: Error during planning: UNION queries have different number of columns: left has 1 columns whereas right has 2 columns
 SELECT order_id FROM orders UNION ALL SELECT customer_id, o_item_id FROM orders
 
 # union_with_different_column_names()