test2=# EXPLAIN SELECT * FROM (SELECT * FROM t ORDER BY a) foo ORDER BY a, b;
                               QUERY PLAN                               
------------------------------------------------------------------------
 Incremental Sort  (cost=147027.04..269419.25 rows=1000000 width=20)
   Sort Key: t.a, t.b
   Presorted Key: t.a
   ->  Sort  (cost=136537.84..139037.84 rows=1000000 width=20)
         Sort Key: t.a
         ->  Seq Scan on t  (cost=0.00..16370.00 rows=1000000 width=20)
(6 rows)