fix: Ensure columns are casted to the correct names with Unions

datafusion/expr/src/expr_rewriter/mod.rs

@@ -261,9 +261,16 @@ fn coerce_exprs_for_schema(
                     #[expect(deprecated)]
                     Expr::Wildcard { .. } => Ok(expr),
                     _ => {
-                        // maintain the original name when casting
-                        let name = dst_schema.field(idx).name();
-                        Ok(expr.cast_to(new_type, src_schema)?.alias(name))
+                        match expr {
+                            // maintain the original name when casting a column, to avoid the
+                            // tablename being added to it when not explicitly set by the query
+                            // (see: https://github.com/apache/datafusion/issues/18818)
+                            Expr::Column(ref column) => {
+                                let name = column.name().to_owned();
+                                Ok(expr.cast_to(new_type, src_schema)?.alias(name))
+                            }
+                            _ => Ok(expr.cast_to(new_type, src_schema)?),
+                        }
                     }
                 }
             } else {

datafusion/optimizer/tests/optimizer_integration.rs

@@ -543,7 +543,7 @@ fn recursive_cte_projection_pushdown() -> Result<()> {
       RecursiveQuery: is_distinct=false
         Projection: test.col_int32 AS id
           TableScan: test projection=[col_int32]
-        Projection: CAST(CAST(nodes.id AS Int64) + Int64(1) AS Int32) AS id
+        Projection: CAST(CAST(nodes.id AS Int64) + Int64(1) AS Int32)
           Filter: nodes.id < Int32(3)
             TableScan: nodes projection=[id]
     "
@@ -567,7 +567,7 @@ fn recursive_cte_with_aliased_self_reference() -> Result<()> {
       RecursiveQuery: is_distinct=false
         Projection: test.col_int32 AS id
           TableScan: test projection=[col_int32]
-        Projection: CAST(CAST(child.id AS Int64) + Int64(1) AS Int32) AS id
+        Projection: CAST(CAST(child.id AS Int64) + Int64(1) AS Int32)
           SubqueryAlias: child
             Filter: nodes.id < Int32(3)
               TableScan: nodes projection=[id]
@@ -630,7 +630,7 @@ fn recursive_cte_projection_pushdown_baseline() -> Result<()> {
         Projection: test.col_int32 AS n
           Filter: test.col_int32 = Int32(5)
             TableScan: test projection=[col_int32]
-        Projection: CAST(CAST(countdown.n AS Int64) - Int64(1) AS Int32) AS n
+        Projection: CAST(CAST(countdown.n AS Int64) - Int64(1) AS Int32)
           Filter: countdown.n > Int32(1)
             TableScan: countdown projection=[n]
     "

datafusion/substrait/tests/cases/logical_plans.rs

@@ -231,8 +231,6 @@ mod tests {
     }
 
     #[tokio::test]
-    // Test still failing, issue tracked in "https://github.com/apache/datafusion/issues/20123".
-    #[ignore]
     async fn duplicate_name_in_union() -> Result<()> {
         let proto_plan =
             read_json("tests/testdata/test_plans/duplicate_name_in_union.substrait.json");