Someone

arogge=> explain update nst set lvl = (select count(*) from nst b where nst.lft between b.lft and b.rgt) where lvl is null;
                                        QUERY PLAN
------------------------------------------------------------------------------------------
 Seq Scan on nst  (cost=0.00..4055.71 rows=1 width=97)
   Filter: (lvl IS NULL)
   SubPlan
     ->  Aggregate  (cost=1615.19..1615.20 rows=1 width=0)
           ->  Index Scan using lft_idx on nst b  (cost=0.00..1589.04 rows=10461 width=0)
                 Index Cond: ($0 >= lft)
                 Filter: ($0 <= rgt)