diff --git a/src/backend/catalog/index.c b/src/backend/catalog/index.c
index d232a213088..0ee09987812 100644
--- a/src/backend/catalog/index.c
+++ b/src/backend/catalog/index.c
@@ -2050,6 +2050,7 @@ index_constraint_create(Relation heapRelation,
 		HeapTuple	indexTuple;
 		Form_pg_index indexForm;
 		bool		dirty = false;
+		bool		marked_as_primary = false;
 
 		pg_index = table_open(IndexRelationId, RowExclusiveLock);
 
@@ -2063,6 +2064,7 @@ index_constraint_create(Relation heapRelation,
 		{
 			indexForm->indisprimary = true;
 			dirty = true;
+			marked_as_primary = true;
 		}
 
 		if (deferrable && indexForm->indimmediate)
@@ -2075,6 +2077,15 @@ index_constraint_create(Relation heapRelation,
 		{
 			CatalogTupleUpdate(pg_index, &indexTuple->t_self, indexTuple);
 
+			/*
+			 * When we mark an existing index as primary, force a relcache
+			 * flush on its parent table, so that all sessions will become
+			 * aware that the table now has a primary key.  This is important
+			 * because it affects some replication behaviors.
+			 */
+			if (marked_as_primary)
+				CacheInvalidateRelcache(heapRelation);
+
 			InvokeObjectPostAlterHookArg(IndexRelationId, indexRelationId, 0,
 										 InvalidOid, is_internal);
 		}
diff --git a/src/test/regress/expected/publication.out b/src/test/regress/expected/publication.out
index a2aca234ef8..7c7e0226658 100644
--- a/src/test/regress/expected/publication.out
+++ b/src/test/regress/expected/publication.out
@@ -268,6 +268,20 @@ Publications:
     "testpib_ins_trunct"
     "testpub_fortbl"
 
+-- verify relation cache invalidation when a primary key is added using
+-- an existing index
+CREATE TABLE pub_test.testpub_addpk (id int not null, data int);
+ALTER PUBLICATION testpub_default ADD TABLE pub_test.testpub_addpk;
+INSERT INTO pub_test.testpub_addpk VALUES(1, 11);
+CREATE UNIQUE INDEX testpub_addpk_id_idx ON pub_test.testpub_addpk(id);
+-- fail:
+UPDATE pub_test.testpub_addpk SET id = 2;
+ERROR:  cannot update table "testpub_addpk" because it does not have a replica identity and publishes updates
+HINT:  To enable updating the table, set REPLICA IDENTITY using ALTER TABLE.
+ALTER TABLE pub_test.testpub_addpk ADD PRIMARY KEY USING INDEX testpub_addpk_id_idx;
+-- now it should work:
+UPDATE pub_test.testpub_addpk SET id = 2;
+DROP TABLE pub_test.testpub_addpk;
 -- permissions
 SET ROLE regress_publication_user2;
 CREATE PUBLICATION testpub2;  -- fail
diff --git a/src/test/regress/sql/publication.sql b/src/test/regress/sql/publication.sql
index 4f2445ad117..4b7738395ec 100644
--- a/src/test/regress/sql/publication.sql
+++ b/src/test/regress/sql/publication.sql
@@ -149,6 +149,19 @@ ALTER PUBLICATION testpub_default DROP TABLE pub_test.testpub_nopk;
 
 \d+ testpub_tbl1
 
+-- verify relation cache invalidation when a primary key is added using
+-- an existing index
+CREATE TABLE pub_test.testpub_addpk (id int not null, data int);
+ALTER PUBLICATION testpub_default ADD TABLE pub_test.testpub_addpk;
+INSERT INTO pub_test.testpub_addpk VALUES(1, 11);
+CREATE UNIQUE INDEX testpub_addpk_id_idx ON pub_test.testpub_addpk(id);
+-- fail:
+UPDATE pub_test.testpub_addpk SET id = 2;
+ALTER TABLE pub_test.testpub_addpk ADD PRIMARY KEY USING INDEX testpub_addpk_id_idx;
+-- now it should work:
+UPDATE pub_test.testpub_addpk SET id = 2;
+DROP TABLE pub_test.testpub_addpk;
+
 -- permissions
 SET ROLE regress_publication_user2;
 CREATE PUBLICATION testpub2;  -- fail