diff --git a/lib/Bio/Phylo/Forest/DBTree.pm b/lib/Bio/Phylo/Forest/DBTree.pm
index 5784ff9..f717584 100644
--- a/lib/Bio/Phylo/Forest/DBTree.pm
+++ b/lib/Bio/Phylo/Forest/DBTree.pm
@@ -342,6 +342,6 @@ sub _rs { shift->resultset('Node') }
 __DATA__
 create table node(id int not null,parent int,left int,right int,name varchar(20),length float,height float,primary key(id));
 create index parent_idx on node(parent);
-create index left_idx on node(left);
-create index right_idx on node(right);
+create unique index left_idx on node(left asc);
+create unique index right_idx on node(right asc);
 create index name_idx on node(name);