diff --git a/src/backend/commands/indexcmds.c b/src/backend/commands/indexcmds.c
index 3599c0d8ce..ed19f77ba0 100644
--- a/src/backend/commands/indexcmds.c
+++ b/src/backend/commands/indexcmds.c
@@ -89,6 +89,7 @@ static void RangeVarCallbackForReindexIndex(const RangeVar *relation,
 								Oid relId, Oid oldRelId, void *arg);
 static bool ReindexRelationConcurrently(Oid relationOid, int options);
 static void ReindexPartitionedIndex(Relation parentIdx);
+static void update_relispartition(Oid relationId, bool newval);
 
 /*
  * CheckIndexCompatible
@@ -3388,6 +3389,9 @@ IndexSetParentIndex(Relation partitionIdx, Oid parentOid)
 	if (OidIsValid(parentOid))
 		SetRelationHasSubclass(parentOid, true);
 
+	/* set relispartition correctly on the partition */
+	update_relispartition(partRelid, OidIsValid(parentOid));
+
 	if (fix_dependencies)
 	{
 		/*
@@ -3424,3 +3428,23 @@ IndexSetParentIndex(Relation partitionIdx, Oid parentOid)
 		CommandCounterIncrement();
 	}
 }
+
+/*
+ * Subroutine of IndexSetParentIndex to update the relispartition flag of the
+ * given index to the given value.
+ */
+static void
+update_relispartition(Oid relationId, bool newval)
+{
+	HeapTuple	tup;
+	Relation	classRel;
+
+	classRel = table_open(RelationRelationId, RowExclusiveLock);
+	tup = SearchSysCacheCopy1(RELOID, ObjectIdGetDatum(relationId));
+	Assert(((Form_pg_class) GETSTRUCT(tup))->relispartition != newval);
+	((Form_pg_class) GETSTRUCT(tup))->relispartition = newval;
+	CatalogTupleUpdate(classRel, &tup->t_self, tup);
+	heap_freetuple(tup);
+
+	table_close(classRel, RowExclusiveLock);
+}
diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c
index 66122afb07..b2f8189fa1 100644
--- a/src/backend/commands/tablecmds.c
+++ b/src/backend/commands/tablecmds.c
@@ -525,8 +525,6 @@ static ObjectAddress ATExecAttachPartitionIdx(List **wqueue, Relation rel,
 static void validatePartitionedIndex(Relation partedIdx, Relation partedTbl);
 static void refuseDupeIndexAttach(Relation parentIdx, Relation partIdx,
 					  Relation partitionTbl);
-static void update_relispartition(Relation classRel, Oid relationId,
-					  bool newval);
 static List *GetParentedForeignKeyRefs(Relation partition);
 static void ATDetachCheckNoForeignKeyRefs(Relation partition);
 
@@ -15714,7 +15712,6 @@ AttachPartitionEnsureIndexes(Relation rel, Relation attachrel)
 				if (OidIsValid(constraintOid))
 					ConstraintSetParentConstraint(cldConstrOid, constraintOid,
 												  RelationGetRelid(attachrel));
-				update_relispartition(NULL, cldIdxId, true);
 				found = true;
 
 				CommandCounterIncrement();
@@ -15970,7 +15967,6 @@ ATExecDetachPartition(Relation rel, RangeVar *name)
 
 		idx = index_open(idxid, AccessExclusiveLock);
 		IndexSetParentIndex(idx, InvalidOid);
-		update_relispartition(classRel, idxid, false);
 
 		/* If there's a constraint associated with the index, detach it too */
 		constrOid = get_relation_idx_constraint_oid(RelationGetRelid(partRel),
@@ -16268,7 +16264,6 @@ ATExecAttachPartitionIdx(List **wqueue, Relation parentIdx, RangeVar *name)
 		if (OidIsValid(constraintOid))
 			ConstraintSetParentConstraint(cldConstrId, constraintOid,
 										  RelationGetRelid(partTbl));
-		update_relispartition(NULL, partIdxId, true);
 
 		pfree(attmap);
 
@@ -16401,38 +16396,6 @@ validatePartitionedIndex(Relation partedIdx, Relation partedTbl)
 	}
 }
 
-/*
- * Update the relispartition flag of the given relation to the given value.
- *
- * classRel is the pg_class relation, already open and suitably locked.
- * It can be passed as NULL, in which case it's opened and closed locally.
- */
-static void
-update_relispartition(Relation classRel, Oid relationId, bool newval)
-{
-	HeapTuple	tup;
-	HeapTuple	newtup;
-	Form_pg_class classForm;
-	bool		opened = false;
-
-	if (classRel == NULL)
-	{
-		classRel = table_open(RelationRelationId, RowExclusiveLock);
-		opened = true;
-	}
-
-	tup = SearchSysCache1(RELOID, ObjectIdGetDatum(relationId));
-	newtup = heap_copytuple(tup);
-	classForm = (Form_pg_class) GETSTRUCT(newtup);
-	classForm->relispartition = newval;
-	CatalogTupleUpdate(classRel, &tup->t_self, newtup);
-	heap_freetuple(newtup);
-	ReleaseSysCache(tup);
-
-	if (opened)
-		table_close(classRel, RowExclusiveLock);
-}
-
 /*
  * Return an OID list of constraints that reference the given relation
  * that are marked as having a parent constraints.
diff --git a/src/test/regress/expected/foreign_key.out b/src/test/regress/expected/foreign_key.out
index 3dd33f6aa1..58f6058e70 100644
--- a/src/test/regress/expected/foreign_key.out
+++ b/src/test/regress/expected/foreign_key.out
@@ -2367,3 +2367,15 @@ SELECT tableoid::regclass, * FROM fk;
 (2 rows)
 
 DROP TABLE fk;
+-- test for reported bug: relispartition not set
+-- https://postgr.es/m/CA+HiwqHMsRtRYRWYTWavKJ8x14AFsv7bmAV46mYwnfD3vy8goQ@mail.gmail.com
+CREATE SCHEMA fkpart7
+  CREATE TABLE pkpart (a int) PARTITION BY LIST (a)
+  CREATE TABLE pkpart1 PARTITION OF pkpart FOR VALUES IN (1);
+ALTER TABLE fkpart7.pkpart1 ADD PRIMARY KEY (a);
+ALTER TABLE fkpart7.pkpart ADD PRIMARY KEY (a);
+CREATE TABLE fkpart7.fk (a int REFERENCES fkpart7.pkpart);
+DROP SCHEMA fkpart7 CASCADE;
+NOTICE:  drop cascades to 2 other objects
+DETAIL:  drop cascades to table fkpart7.pkpart
+drop cascades to table fkpart7.fk
diff --git a/src/test/regress/sql/foreign_key.sql b/src/test/regress/sql/foreign_key.sql
index 672fce317f..9c28285717 100644
--- a/src/test/regress/sql/foreign_key.sql
+++ b/src/test/regress/sql/foreign_key.sql
@@ -1657,3 +1657,13 @@ DELETE FROM pk WHERE a = 20;
 UPDATE pk SET a = 90 WHERE a = 30;
 SELECT tableoid::regclass, * FROM fk;
 DROP TABLE fk;
+
+-- test for reported bug: relispartition not set
+-- https://postgr.es/m/CA+HiwqHMsRtRYRWYTWavKJ8x14AFsv7bmAV46mYwnfD3vy8goQ@mail.gmail.com
+CREATE SCHEMA fkpart7
+  CREATE TABLE pkpart (a int) PARTITION BY LIST (a)
+  CREATE TABLE pkpart1 PARTITION OF pkpart FOR VALUES IN (1);
+ALTER TABLE fkpart7.pkpart1 ADD PRIMARY KEY (a);
+ALTER TABLE fkpart7.pkpart ADD PRIMARY KEY (a);
+CREATE TABLE fkpart7.fk (a int REFERENCES fkpart7.pkpart);
+DROP SCHEMA fkpart7 CASCADE;