diff --git a/sqlalchemy_mptt/mixins.py b/sqlalchemy_mptt/mixins.py
index 5cdda2d..5cd5b9a 100644
--- a/sqlalchemy_mptt/mixins.py
+++ b/sqlalchemy_mptt/mixins.py
@@ -86,6 +86,7 @@ def parent_id(cls):
             ForeignKey(
                 "{}.{}".format(cls.__tablename__, pk.name), ondelete="CASCADE"
             ),
+            index=True
         )
 
     @declared_attr