diff --git a/src/backend/commands/indexcmds.c b/src/backend/commands/indexcmds.c
index f8ee4b0a84..63cc3e3f40 100644
--- a/src/backend/commands/indexcmds.c
+++ b/src/backend/commands/indexcmds.c
@@ -85,6 +85,7 @@ static List *ChooseIndexColumnNames(List *indexElems);
 static void RangeVarCallbackForReindexIndex(const RangeVar *relation,
 								Oid relId, Oid oldRelId, void *arg);
 static void ReindexPartitionedIndex(Relation parentIdx);
+static void update_relispartition(Oid relationId, bool newval);
 
 /*
  * CheckIndexCompatible
@@ -2617,6 +2618,9 @@ IndexSetParentIndex(Relation partitionIdx, Oid parentOid)
 	systable_endscan(scan);
 	relation_close(pg_inherits, RowExclusiveLock);
 
+	/* set relispartition correctly on the partition */
+	update_relispartition(partRelid, OidIsValid(parentOid));
+
 	if (fix_dependencies)
 	{
 		ObjectAddress partIdx;
@@ -2654,3 +2658,23 @@ 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 = heap_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);
+
+	heap_close(classRel, RowExclusiveLock);
+}
diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c
index 65ede339f2..cf61cc1ac4 100644
--- a/src/backend/commands/tablecmds.c
+++ b/src/backend/commands/tablecmds.c
@@ -495,8 +495,6 @@ 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);
 
 
 /* ----------------------------------------------------------------
@@ -15036,7 +15034,6 @@ AttachPartitionEnsureIndexes(Relation rel, Relation attachrel)
 				IndexSetParentIndex(attachrelIdxRels[i], idx);
 				if (OidIsValid(constraintOid))
 					ConstraintSetParentConstraint(cldConstrOid, constraintOid);
-				update_relispartition(NULL, cldIdxId, true);
 				found = true;
 				break;
 			}
@@ -15287,7 +15284,6 @@ 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),
@@ -15562,7 +15558,6 @@ ATExecAttachPartitionIdx(List **wqueue, Relation parentIdx, RangeVar *name)
 		IndexSetParentIndex(partIdx, RelationGetRelid(parentIdx));
 		if (OidIsValid(constraintOid))
 			ConstraintSetParentConstraint(cldConstrId, constraintOid);
-		update_relispartition(NULL, partIdxId, true);
 
 		pfree(attmap);
 
@@ -15712,35 +15707,3 @@ validatePartitionedIndex(Relation partedIdx, Relation partedTbl)
 		relation_close(parentTbl, AccessExclusiveLock);
 	}
 }
-
-/*
- * 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 = heap_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)
-		heap_close(classRel, RowExclusiveLock);
-}