wiedld commented on code in PR #14919:
URL: https://github.com/apache/datafusion/pull/14919#discussion_r1975887998


##########
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:
   Thank you for taking the time @berkaysynnada .
   
   The test case merged here is a minimal reproducer. The 
`mode=SinglePartitioned` is coming from the actual plans we have running and 
failing in prod: 
https://github.com/influxdata/arrow-datafusion/pull/58#issuecomment-2654249703
   
   A simplified view of it is:
   ```
   "SortExec: expr=[a@0 ASC], preserve_partitioning=[false]",
   "  AggregateExec: mode=SinglePartitioned, gby=[a@0 as a1], aggr=[]",
   "    CoalescePartitionsExec",
   "      ProjectionExec: expr=[a@0 as a, b@1 as value]",
   "        UnionExec",
   "          DataSourceExec: file_groups={1 group: [[x]]}, projection=[a, b, 
c, d, e], file_type=parquet",
   "          DataSourceExec: file_groups={1 group: [[x]]}, projection=[a, b, 
c, d, e], file_type=parquet"
   ```
   
   The coalesce gets removed for the `mode=SinglePartitioned`. Do you think 
that the agg exec is using the wrong mode?



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