diff --git a/src/backend/optimizer/plan/analyzejoins.c b/src/backend/optimizer/plan/analyzejoins.c
index 881950e5264..9b694104aa3 100644
--- a/src/backend/optimizer/plan/analyzejoins.c
+++ b/src/backend/optimizer/plan/analyzejoins.c
@@ -1280,8 +1280,9 @@ rel_is_distinct_for(PlannerInfo *root, RelOptInfo *rel, List *clause_list,
 bool
 query_supports_distinctness(Query *query)
 {
-	/* SRFs break distinctness except with DISTINCT, see below */
-	if (query->hasTargetSRFs && query->distinctClause == NIL)
+	/* SRFs break distinctness except with plain DISTINCT, see below */
+	if (query->hasTargetSRFs &&
+		(query->distinctClause == NIL || query->hasDistinctOn))
 		return false;
 
 	/* check for features we can prove distinctness with */
@@ -1333,10 +1334,15 @@ query_is_distinct_for(Query *query, List *distinct_cols)
 	/*
 	 * DISTINCT (including DISTINCT ON) guarantees uniqueness if all the
 	 * columns in the DISTINCT clause appear in colnos and operator semantics
-	 * match.  This is true even if there are SRFs in the DISTINCT columns or
-	 * elsewhere in the tlist.
+	 * match.  With plain DISTINCT this is true even if there are SRFs in the
+	 * tlist, since they are all DISTINCT columns and hence get expanded
+	 * before the Unique step.  But with DISTINCT ON, the planner may postpone
+	 * SRFs that are not DISTINCT ON or ORDER BY columns until after the
+	 * Unique step, which can produce duplicates of the DISTINCT ON columns;
+	 * so we can't rely on DISTINCT ON if there are any tlist SRFs.
 	 */
-	if (query->distinctClause)
+	if (query->distinctClause &&
+		!(query->hasTargetSRFs && query->hasDistinctOn))
 	{
 		foreach(l, query->distinctClause)
 		{
diff --git a/src/test/regress/expected/join.out b/src/test/regress/expected/join.out
index 05f359d3aa7..deca5617ac8 100644
--- a/src/test/regress/expected/join.out
+++ b/src/test/regress/expected/join.out
@@ -6497,6 +6497,39 @@ select d.* from d left join (select distinct * from b) s
          ->  Seq Scan on d
 (9 rows)
 
+-- join removal is not possible when the subquery has DISTINCT ON and a
+-- set-returning function that is not a DISTINCT ON column
+explain (costs off)
+select d.* from d left join
+  (select distinct on (id) id, generate_series(1, 2) as g from b order by id) s
+  on d.a = s.id
+  order by 1, 2;
+                              QUERY PLAN                               
+-----------------------------------------------------------------------
+ Sort
+   Sort Key: d.a, d.b
+   ->  Hash Left Join
+         Hash Cond: (d.a = s.id)
+         ->  Seq Scan on d
+         ->  Hash
+               ->  Subquery Scan on s
+                     ->  ProjectSet
+                           ->  Unique
+                                 ->  Index Only Scan using b_pkey on b
+(10 rows)
+
+select d.* from d left join
+  (select distinct on (id) id, generate_series(1, 2) as g from b order by id) s
+  on d.a = s.id
+  order by 1, 2;
+ a | b 
+---+---
+ 1 | 3
+ 1 | 3
+ 2 | 2
+ 3 | 1
+(4 rows)
+
 -- join removal is not possible here
 explain (costs off)
 select 1 from a t1
diff --git a/src/test/regress/sql/join.sql b/src/test/regress/sql/join.sql
index 450bd5bbf2c..c0ff6c945b0 100644
--- a/src/test/regress/sql/join.sql
+++ b/src/test/regress/sql/join.sql
@@ -2385,6 +2385,18 @@ explain (costs off)
 select d.* from d left join (select distinct * from b) s
   on d.a = s.id;
 
+-- join removal is not possible when the subquery has DISTINCT ON and a
+-- set-returning function that is not a DISTINCT ON column
+explain (costs off)
+select d.* from d left join
+  (select distinct on (id) id, generate_series(1, 2) as g from b order by id) s
+  on d.a = s.id
+  order by 1, 2;
+select d.* from d left join
+  (select distinct on (id) id, generate_series(1, 2) as g from b order by id) s
+  on d.a = s.id
+  order by 1, 2;
+
 -- join removal is not possible here
 explain (costs off)
 select 1 from a t1