diff --git a/src/backend/commands/cluster.c b/src/backend/commands/cluster.c
index 4c0d6b80d2a..99193f5c886 100644
--- a/src/backend/commands/cluster.c
+++ b/src/backend/commands/cluster.c
@@ -316,7 +316,7 @@ cluster_rel(Relation OldHeap, Oid indexOid, ClusterParams *params)
 	int			save_nestlevel;
 	bool		verbose = ((params->options & CLUOPT_VERBOSE) != 0);
 	bool		recheck = ((params->options & CLUOPT_RECHECK) != 0);
-	Relation	index = NULL;
+	Relation	index;
 
 	Assert(CheckRelationLockedByMe(OldHeap, AccessExclusiveLock, false));
 
@@ -434,10 +434,13 @@ cluster_rel(Relation OldHeap, Oid indexOid, ClusterParams *params)
 	/* Check heap and index are valid to cluster on */
 	if (OidIsValid(indexOid))
 	{
+		/* verify the index is good and lock it */
 		check_index_is_clusterable(OldHeap, indexOid, AccessExclusiveLock);
-		/* Open the index (It should already be locked.) */
+		/* also open it */
 		index = index_open(indexOid, NoLock);
 	}
+	else
+		index = NULL;
 
 	/*
 	 * Quietly ignore the request if this is a materialized view which has not
@@ -615,12 +618,12 @@ mark_index_clustered(Relation rel, Oid indexOid, bool is_internal)
 /*
  * rebuild_relation: rebuild an existing relation in index or physical order
  *
- * OldHeap: table to rebuild --- must be opened and exclusive-locked!
- * index: index to cluster by, or NULL to rewrite in physical order. Must be
- * opened and locked.
+ * OldHeap: table to rebuild.
+ * index: index to cluster by, or NULL to rewrite in physical order.
  *
- * On exit, the heap (and also the index, if one was passed) are closed, but
- * still locked with AccessExclusiveLock.
+ * On entry, heap and index (if one is given) must be open, and
+ * AccessExclusiveLock held on them.
+ * On exit, they are closed, but locks on them are not released.
  */
 static void
 rebuild_relation(Relation OldHeap, Relation index, bool verbose)
@@ -636,6 +639,9 @@ rebuild_relation(Relation OldHeap, Relation index, bool verbose)
 	TransactionId frozenXid;
 	MultiXactId cutoffMulti;
 
+	Assert(CheckRelationLockedByMe(OldHeap, AccessExclusiveLock, false) &&
+		   (index == NULL || CheckRelationLockedByMe(index, AccessExclusiveLock, false)));
+
 	if (index)
 		/* Mark the correct index as clustered */
 		mark_index_clustered(OldHeap, RelationGetRelid(index), true);
@@ -647,18 +653,15 @@ rebuild_relation(Relation OldHeap, Relation index, bool verbose)
 	/*
 	 * Create the transient table that will receive the re-ordered data.
 	 *
-	 * NoLock for the old heap because we already have it locked and want to
-	 * keep unlocking straightforward. The new heap (and its TOAST if one
-	 * exists) will be locked in AccessExclusiveMode on return. Since others
-	 * can't see it yet, we do not care.
+	 * OldHeap is already locked, so no need to lock it again.  make_new_heap
+	 * obtains AccessExclusiveLock on the new heap and its toast table.
 	 */
 	OIDNewHeap = make_new_heap(tableOid, tableSpace,
 							   accessMethod,
 							   relpersistence,
 							   NoLock);
+	Assert(CheckRelationOidLockedByMe(OIDNewHeap, AccessExclusiveLock, false));
 	NewHeap = table_open(OIDNewHeap, NoLock);
-	/* NewHeap already locked by make_new_heap */
-	Assert(CheckRelationLockedByMe(NewHeap, AccessExclusiveLock, false));
 
 	/* Copy the heap data into the new table in the desired order */
 	copy_table_data(NewHeap, OldHeap, index, verbose,
diff --git a/src/backend/commands/matview.c b/src/backend/commands/matview.c
index 4b3d4822872..c12817091ed 100644
--- a/src/backend/commands/matview.c
+++ b/src/backend/commands/matview.c
@@ -319,7 +319,7 @@ RefreshMatViewByOid(Oid matviewOid, bool is_create, bool skipData,
 	OIDNewHeap = make_new_heap(matviewOid, tableSpace,
 							   matviewRel->rd_rel->relam,
 							   relpersistence, ExclusiveLock);
-	LockRelationOid(OIDNewHeap, AccessExclusiveLock);
+	Assert(CheckRelationOidLockedByMe(OIDNewHeap, AccessExclusiveLock, false));
 
 	/* Generate the data, if wanted. */
 	if (!skipData)