alamb commented on code in PR #14919:
URL: https://github.com/apache/datafusion/pull/14919#discussion_r1975938366


##########
datafusion/core/tests/physical_optimizer/enforce_sorting.rs:
##########
@@ -3346,3 +3351,62 @@ async fn 
test_window_partial_constant_and_set_monotonicity() -> Result<()> {
 
     Ok(())
 }
+
+#[tokio::test]
+async fn test_preserve_needed_coalesce() -> Result<()> {
+    // Input to EnforceSorting, from our test case.
+    let plan = projection_exec_with_alias(
+        union_exec(vec![parquet_exec_with_stats(); 2]),
+        vec![
+            ("a".to_string(), "a".to_string()),
+            ("b".to_string(), "value".to_string()),
+        ],
+    );
+    let plan = Arc::new(CoalescePartitionsExec::new(plan));
+    let schema = schema();
+    let sort_key = LexOrdering::new(vec![PhysicalSortExpr {
+        expr: col("a", &schema).unwrap(),
+        options: SortOptions::default(),
+    }]);
+    let plan: Arc<dyn ExecutionPlan> =
+        single_partitioned_aggregate(plan, vec![("a".to_string(), 
"a1".to_string())]);
+    let plan = sort_exec(sort_key, plan);
+
+    // Starting plan: as in our test case.
+    assert_eq!(
+        get_plan_string(&plan),
+        vec![
+            "SortExec: expr=[a@0 ASC], preserve_partitioning=[false]",
+            "  AggregateExec: mode=SinglePartitioned, gby=[a@0 as a1], 
aggr=[]",

Review Comment:
   > SinglePartitioned means Single layer of aggregation, but its input is 
Partitioned). However, in your example plan, AggregateExec has a single 
partition (as it is above CoalescePartitions).
   
   I see -- so you are saying that if the input to the AggregateExec has only a 
single partitition, then it should never be `SinglePartitioned`.
   
   So you are saying that the input plan is not valid and thus the error is 
elsewhere (whatever generated this plan).
   
   I will make a PR to clarify the documentation as I agree it is quite 
confusing



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