Fix: MemTable LIMIT ignored with reordered projections

Author: RamakrishnaChilaka

datafusion/datasource/src/memory.rs

@@ -249,7 +249,7 @@ impl DataSource for MemorySourceConfig {
                     self.original_schema(),
                     Some(new_projections),
                 )
-                .map(|s| Arc::new(s) as Arc<dyn DataSource>)
+                .map(|s| Arc::new(s.with_limit(self.fetch)) as Arc<dyn DataSource>)
             })
             .transpose()
     }
@@ -897,6 +897,42 @@ mod tests {
         Ok(())
     }
 
+    /// Test that `try_swapping_with_projection` preserves the `fetch` limit.
+    /// Regression test for <https://github.com/apache/datafusion/issues/21176>
+    #[test]
+    fn try_swapping_with_projection_preserves_fetch() {
+        use datafusion_physical_expr::projection::ProjectionExprs;
+
+        let schema = Arc::new(Schema::new(vec![
+            Field::new("a", DataType::Int32, false),
+            Field::new("b", DataType::Utf8, false),
+            Field::new("c", DataType::Int64, false),
+        ]));
+        let partitions: Vec<Vec<RecordBatch>> = vec![vec![batch(10)]];
+        let source = MemorySourceConfig::try_new(&partitions, schema.clone(), None)
+            .unwrap()
+            .with_limit(Some(5));
+
+        assert_eq!(source.fetch, Some(5));
+
+        // Create a projection that reorders columns: [c, a] (indices 2, 0)
+        let projection = ProjectionExprs::from_indices(&[2, 0], &schema);
+        let swapped = source
+            .try_swapping_with_projection(&projection)
+            .unwrap()
+            .expect("should return Some");
+        let new_source = swapped
+            .as_any()
+            .downcast_ref::<MemorySourceConfig>()
+            .unwrap();
+
+        assert_eq!(
+            new_source.fetch,
+            Some(5),
+            "fetch limit must be preserved after projection pushdown"
+        );
+    }
+
     #[tokio::test]
     async fn values_empty_case() -> Result<()> {
         let schema = aggr_test_schema();

datafusion/sqllogictest/test_files/limit.slt

@@ -729,7 +729,7 @@ explain select * from testSubQueryLimit as t1 join (select * from testSubQueryLi
 ----
 logical_plan
 01)Limit: skip=0, fetch=10
-02)--Cross Join: 
+02)--Cross Join:
 03)----SubqueryAlias: t1
 04)------Limit: skip=0, fetch=10
 05)--------TableScan: testsubquerylimit projection=[a, b], fetch=10
@@ -754,7 +754,7 @@ explain select * from testSubQueryLimit as t1 join (select * from testSubQueryLi
 ----
 logical_plan
 01)Limit: skip=0, fetch=2
-02)--Cross Join: 
+02)--Cross Join:
 03)----SubqueryAlias: t1
 04)------Limit: skip=0, fetch=2
 05)--------TableScan: testsubquerylimit projection=[a, b], fetch=2
@@ -867,7 +867,7 @@ limit 1000;
 
 # Config reset
 
-# The SLT runner sets `target_partitions` to 4 instead of using the default, so 
+# The SLT runner sets `target_partitions` to 4 instead of using the default, so
 # reset it explicitly.
 statement ok
 set datafusion.execution.target_partitions = 4;
@@ -927,3 +927,46 @@ DROP TABLE t;
 # Tear down src_table table:
 statement ok
 DROP TABLE src_table;
+
+# LIMIT must work when SELECT projects columns in different order than table schema
+
+statement ok
+CREATE TABLE t21176 (col_a TEXT, col_b DOUBLE, col_c TEXT) AS VALUES
+  ('a-0', 0, 'c-0'), ('a-1', 1, 'c-1'), ('a-2', 2, 'c-2'), ('a-3', 3, 'c-3'),
+  ('a-4', 4, 'c-4'), ('a-5', 5, 'c-5'), ('a-6', 6, 'c-6'), ('a-7', 7, 'c-7'),
+  ('a-8', 8, 'c-8'), ('a-9', 9, 'c-9'), ('a-10', 10, 'c-10'), ('a-11', 11, 'c-11'),
+  ('a-12', 12, 'c-12'), ('a-13', 13, 'c-13'), ('a-14', 14, 'c-14'), ('a-15', 15, 'c-15'),
+  ('a-16', 16, 'c-16'), ('a-17', 17, 'c-17'), ('a-18', 18, 'c-18'), ('a-19', 19, 'c-19');
+
+# Schema-order SELECT with LIMIT should return 5 rows
+query RT rowsort
+SELECT col_b, col_c FROM t21176 LIMIT 5;
+----
+0 c-0
+1 c-1
+2 c-2
+3 c-3
+4 c-4
+
+# Reverse-order SELECT with LIMIT should also return 5 rows (not 20)
+query TR rowsort
+SELECT col_c, col_b FROM t21176 LIMIT 5;
+----
+c-0 0
+c-1 1
+c-2 2
+c-3 3
+c-4 4
+
+# Single column reverse SELECT with LIMIT
+query T rowsort
+SELECT col_c FROM t21176 LIMIT 5;
+----
+c-0
+c-1
+c-2
+c-3
+c-4
+
+statement ok
+DROP TABLE t21176;