BUG #7556 addition info

plan of http://archives.postgresql.org/pgsql-bugs/2012-09/msg00222.php
test=# explain select max(a.info)from sli_test a where a.id not in(select b.id from sli_test2 b where b.id<50000);
                                      QUERY PLAN
---------------------------------------------------------------------------------------
Aggregate  (cost=9243466274.00..9243466274.01 rows=1 width=12)
   ->  Seq Scan on sli_test a  (cost=0.00..9243465024.00 rows=500000 width=12)
         Filter: (NOT (SubPlan 1))
         SubPlan 1
           ->  Materialize  (cost=0.00..18359.60 rows=50919 width=4)
                 ->  Seq Scan on sli_test2 b  (cost=0.00..17906.00 rows=50919 width=4)
                       Filter: (id < 50000)