test: add test for optimize idempotent behavior in aggregate DataFrame

Author: kosiew

python/tests/test_dataframe.py

@@ -1267,6 +1267,20 @@ def test_optimized_logical_plan(aggregate_df):
     assert expected == plan.display_indent()
 
 
+def test_optimize_idempotent(aggregate_df):
+    """Running optimize twice should yield the same plan.
+
+    Requires PyArrow >=21.0.0 to match CI environment."""
+
+    optimized_once = aggregate_df.optimize()
+    optimized_twice = optimized_once.optimize()
+
+    assert (
+        optimized_once.logical_plan().display_indent()
+        == optimized_twice.logical_plan().display_indent()
+    )
+
+
 def test_execution_plan(aggregate_df):
     plan = aggregate_df.execution_plan()