diff --git a/src/backend/catalog/index.c b/src/backend/catalog/index.c
index 55fdde4b24..a819b4197c 100644
--- a/src/backend/catalog/index.c
+++ b/src/backend/catalog/index.c
@@ -1058,6 +1058,7 @@ index_create(Relation heapRelation,
 	if (OidIsValid(parentIndexRelid))
 	{
 		StoreSingleInheritance(indexRelationId, parentIndexRelid, 1);
+		LockRelationOid(parentIndexRelid, ShareUpdateExclusiveLock);
 		SetRelationHasSubclass(parentIndexRelid, true);
 	}
 
diff --git a/src/backend/commands/indexcmds.c b/src/backend/commands/indexcmds.c
index 309389e20d..2caab88aa5 100644
--- a/src/backend/commands/indexcmds.c
+++ b/src/backend/commands/indexcmds.c
@@ -4355,7 +4355,10 @@ IndexSetParentIndex(Relation partitionIdx, Oid parentOid)
 
 	/* set relhassubclass if an index partition has been added to the parent */
 	if (OidIsValid(parentOid))
+	{
+		LockRelationOid(parentOid, ShareUpdateExclusiveLock);
 		SetRelationHasSubclass(parentOid, true);
+	}
 
 	/* set relispartition correctly on the partition */
 	update_relispartition(partRelid, OidIsValid(parentOid));
diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c
index 66cda26a25..8fcb188323 100644
--- a/src/backend/commands/tablecmds.c
+++ b/src/backend/commands/tablecmds.c
@@ -3483,8 +3483,15 @@ findAttrByName(const char *attributeName, const List *columns)
  * SetRelationHasSubclass
  *		Set the value of the relation's relhassubclass field in pg_class.
  *
- * NOTE: caller must be holding an appropriate lock on the relation.
- * ShareUpdateExclusiveLock is sufficient.
+ * It's always safe to set this field to true, because all SQL commands are
+ * ready to see true and then find no children.  On the other hand, commands
+ * generally assume zero children if this is false.
+ *
+ * Caller must hold any self-exclusive lock until end of transaction.  If the
+ * new value is false, caller must have acquired that lock before reading the
+ * evidence that justified the false value.  That way, it properly waits if
+ * another backend is simultaneously concluding no need to change the tuple
+ * (new and old values are true).
  *
  * NOTE: an important side-effect of this operation is that an SI invalidation
  * message is sent out to all backends --- including me --- causing plans
@@ -3499,6 +3506,11 @@ SetRelationHasSubclass(Oid relationId, bool relhassubclass)
 	HeapTuple	tuple;
 	Form_pg_class classtuple;
 
+	Assert(CheckRelationOidLockedByMe(relationId,
+									  ShareUpdateExclusiveLock, false) ||
+		   CheckRelationOidLockedByMe(relationId,
+									  ShareRowExclusiveLock, true));
+
 	/*
 	 * Fetch a modifiable copy of the tuple, modify it, update pg_class.
 	 */
diff --git a/src/backend/storage/lmgr/lmgr.c b/src/backend/storage/lmgr/lmgr.c
index fe3cda2f88..094522acb4 100644
--- a/src/backend/storage/lmgr/lmgr.c
+++ b/src/backend/storage/lmgr/lmgr.c
@@ -335,32 +335,22 @@ CheckRelationLockedByMe(Relation relation, LOCKMODE lockmode, bool orstronger)
 						 relation->rd_lockInfo.lockRelId.dbId,
 						 relation->rd_lockInfo.lockRelId.relId);
 
-	if (LockHeldByMe(&tag, lockmode))
-		return true;
+	return LockHeldByMe(&tag, lockmode, orstronger);
+}
 
-	if (orstronger)
-	{
-		LOCKMODE	slockmode;
+/*
+ *		CheckRelationOidLockedByMe
+ *
+ * Like the above, but takes an OID as argument.
+ */
+bool
+CheckRelationOidLockedByMe(Oid relid, LOCKMODE lockmode, bool orstronger)
+{
+	LOCKTAG		tag;
 
-		for (slockmode = lockmode + 1;
-			 slockmode <= MaxLockMode;
-			 slockmode++)
-		{
-			if (LockHeldByMe(&tag, slockmode))
-			{
-#ifdef NOT_USED
-				/* Sometimes this might be useful for debugging purposes */
-				elog(WARNING, "lock mode %s substituted for %s on relation %s",
-					 GetLockmodeName(tag.locktag_lockmethodid, slockmode),
-					 GetLockmodeName(tag.locktag_lockmethodid, lockmode),
-					 RelationGetRelationName(relation));
-#endif
-				return true;
-			}
-		}
-	}
+	SetLocktagRelationOid(&tag, relid);
 
-	return false;
+	return LockHeldByMe(&tag, lockmode, orstronger);
 }
 
 /*
diff --git a/src/backend/storage/lmgr/lock.c b/src/backend/storage/lmgr/lock.c
index f68c595c8a..0400a50777 100644
--- a/src/backend/storage/lmgr/lock.c
+++ b/src/backend/storage/lmgr/lock.c
@@ -578,11 +578,17 @@ DoLockModesConflict(LOCKMODE mode1, LOCKMODE mode2)
 }
 
 /*
- * LockHeldByMe -- test whether lock 'locktag' is held with mode 'lockmode'
- *		by the current transaction
+ * LockHeldByMe -- test whether lock 'locktag' is held by the current
+ *		transaction
+ *
+ * Returns true if current transaction holds a lock on 'tag' of mode
+ * 'lockmode'.  If 'orstronger' is true, a stronger lockmode is also OK.
+ * ("Stronger" is defined as "numerically higher", which is a bit
+ * semantically dubious but is OK for the purposes we use this for.)
  */
 bool
-LockHeldByMe(const LOCKTAG *locktag, LOCKMODE lockmode)
+LockHeldByMe(const LOCKTAG *locktag,
+			 LOCKMODE lockmode, bool orstronger)
 {
 	LOCALLOCKTAG localtag;
 	LOCALLOCK  *locallock;
@@ -598,7 +604,23 @@ LockHeldByMe(const LOCKTAG *locktag, LOCKMODE lockmode)
 										  &localtag,
 										  HASH_FIND, NULL);
 
-	return (locallock && locallock->nLocks > 0);
+	if (locallock && locallock->nLocks > 0)
+		return true;
+
+	if (orstronger)
+	{
+		LOCKMODE	slockmode;
+
+		for (slockmode = lockmode + 1;
+			 slockmode <= MaxLockMode;
+			 slockmode++)
+		{
+			if (LockHeldByMe(locktag, slockmode, false))
+				return true;
+		}
+	}
+
+	return false;
 }
 
 #ifdef USE_ASSERT_CHECKING
diff --git a/src/include/storage/lmgr.h b/src/include/storage/lmgr.h
index 22b7856ef1..ce15125ac3 100644
--- a/src/include/storage/lmgr.h
+++ b/src/include/storage/lmgr.h
@@ -48,6 +48,8 @@ extern bool ConditionalLockRelation(Relation relation, LOCKMODE lockmode);
 extern void UnlockRelation(Relation relation, LOCKMODE lockmode);
 extern bool CheckRelationLockedByMe(Relation relation, LOCKMODE lockmode,
 									bool orstronger);
+extern bool CheckRelationOidLockedByMe(Oid relid, LOCKMODE lockmode,
+									   bool orstronger);
 extern bool LockHasWaitersRelation(Relation relation, LOCKMODE lockmode);
 
 extern void LockRelationIdForSession(LockRelId *relid, LOCKMODE lockmode);
diff --git a/src/include/storage/lock.h b/src/include/storage/lock.h
index 0017d4b868..cc1f6e78c3 100644
--- a/src/include/storage/lock.h
+++ b/src/include/storage/lock.h
@@ -567,7 +567,8 @@ extern void LockReleaseAll(LOCKMETHODID lockmethodid, bool allLocks);
 extern void LockReleaseSession(LOCKMETHODID lockmethodid);
 extern void LockReleaseCurrentOwner(LOCALLOCK **locallocks, int nlocks);
 extern void LockReassignCurrentOwner(LOCALLOCK **locallocks, int nlocks);
-extern bool LockHeldByMe(const LOCKTAG *locktag, LOCKMODE lockmode);
+extern bool LockHeldByMe(const LOCKTAG *locktag,
+						 LOCKMODE lockmode, bool orstronger);
 #ifdef USE_ASSERT_CHECKING
 extern HTAB *GetLockMethodLocalHash(void);
 #endif