diff --git a/src/backend/commands/indexcmds.c b/src/backend/commands/indexcmds.c
index a1c91b5fb8..cb92b1975e 100644
--- a/src/backend/commands/indexcmds.c
+++ b/src/backend/commands/indexcmds.c
@@ -1095,6 +1095,8 @@ DefineIndex(Oid relationId,
 
 						/* Attach index to parent and we're done. */
 						IndexSetParentIndex(cldidx, indexRelationId);
+						/* Also, set child index as the partition. */
+						update_relispartition(NULL, cldidxid, true);
 						if (createdConstraintId != InvalidOid)
 							ConstraintSetParentConstraint(cldConstrOid,
 														  createdConstraintId,
diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c
index aa7328ea40..a50a865d4c 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);
 
@@ -16407,7 +16405,7 @@ validatePartitionedIndex(Relation partedIdx, Relation partedTbl)
  * 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
+void
 update_relispartition(Relation classRel, Oid relationId, bool newval)
 {
 	HeapTuple	tup;
diff --git a/src/include/commands/tablecmds.h b/src/include/commands/tablecmds.h
index 96927b900d..6898fa864c 100644
--- a/src/include/commands/tablecmds.h
+++ b/src/include/commands/tablecmds.h
@@ -57,6 +57,8 @@ extern void ExecuteTruncateGuts(List *explicit_rels, List *relids, List *relids_
 					DropBehavior behavior, bool restart_seqs);
 
 extern void SetRelationHasSubclass(Oid relationId, bool relhassubclass);
+extern void update_relispartition(Relation classRel, Oid relationId,
+					  bool newval);
 
 extern ObjectAddress renameatt(RenameStmt *stmt);