[branch-49] fix: string_agg not respecting ORDER BY

datafusion/functions-aggregate/src/string_agg.rs

@@ -178,6 +178,10 @@ impl AggregateUDFImpl for StringAgg {
         )))
     }
 
+    fn reverse_expr(&self) -> datafusion_expr::ReversedUDAF {
+        datafusion_expr::ReversedUDAF::Reversed(string_agg_udaf())
+    }
+
     fn documentation(&self) -> Option<&Documentation> {
         self.doc()
     }

datafusion/sqllogictest/test_files/aggregate.slt

@@ -6028,6 +6028,106 @@ GROUP BY dummy
 ----
 text1
 
+
+# Test string_agg with ORDER BY clasuses (issue #17011)
+statement ok
+create table t (k varchar, v int);
+
+statement ok
+insert into t values ('a', 2), ('b', 3), ('c', 1), ('d', null);
+
+query T
+select string_agg(k, ',' order by k) from t;
+----
+a,b,c,d
+
+query T
+select string_agg(k, ',' order by k desc) from t;
+----
+d,c,b,a
+
+query T
+select string_agg(k, ',' order by v) from t;
+----
+c,a,b,d
+
+query T
+select string_agg(k, ',' order by v nulls first) from t;
+----
+d,c,a,b
+
+query T
+select string_agg(k, ',' order by v desc) from t;
+----
+d,b,a,c
+
+query T
+select string_agg(k, ',' order by v desc nulls last) from t;
+----
+b,a,c,d
+
+query T
+-- odd indexes should appear first, ties solved by v
+select string_agg(k, ',' order by v % 2 == 0, v) from t;
+----
+c,b,a,d
+
+query T
+-- odd indexes should appear first, ties solved by v desc
+select string_agg(k, ',' order by v % 2 == 0, v desc) from t;
+----
+b,c,a,d
+
+query T
+select string_agg(k, ',' order by
+  case
+    when k = 'a' then 3
+    when k = 'b' then 0
+    when k = 'c' then 2
+    when k = 'd' then 1 
+  end)
+from t;
+----
+b,d,c,a
+
+query T
+select string_agg(k, ',' order by
+  case
+    when k = 'a' then 3
+    when k = 'b' then 0
+    when k = 'c' then 2
+    when k = 'd' then 1 
+  end desc)
+from t;
+----
+a,c,d,b
+
+query TT
+explain select string_agg(k, ',' order by v) from t;
+----
+logical_plan
+01)Aggregate: groupBy=[[]], aggr=[[string_agg(t.k, Utf8(",")) ORDER BY [t.v ASC NULLS LAST]]]     
+02)--TableScan: t projection=[k, v]
+physical_plan
+01)AggregateExec: mode=Single, gby=[], aggr=[string_agg(t.k,Utf8(",")) ORDER BY [t.v ASC NULLS LAST]]
+02)--SortExec: expr=[v@1 ASC NULLS LAST], preserve_partitioning=[false]
+03)----DataSourceExec: partitions=1, partition_sizes=[1]
+
+query TT
+explain select string_agg(k, ',' order by v desc) from t;
+----
+logical_plan
+01)Aggregate: groupBy=[[]], aggr=[[string_agg(t.k, Utf8(",")) ORDER BY [t.v DESC NULLS FIRST]]]   
+02)--TableScan: t projection=[k, v]
+physical_plan
+01)AggregateExec: mode=Single, gby=[], aggr=[string_agg(t.k,Utf8(",")) ORDER BY [t.v DESC NULLS FIRST]]
+02)--SortExec: expr=[v@1 DESC], preserve_partitioning=[false]
+03)----DataSourceExec: partitions=1, partition_sizes=[1]
+
+statement ok
+drop table t;
+
+
 # Tests for aggregating with NaN values
 statement ok
 CREATE TABLE float_table (