BUG #11279: unnecessary subplan reduces performance

The following bug has been logged on the website:

Bug reference:      11279
Logged by:          Mason
Email address:      qiao5034@163.com
PostgreSQL version: 9.2.4
Operating system:   linux suse 2.6.32.12-0.7-default
Description:

The following query generates two subplan which is unnecessary.

scripts:

create table a(a1 int, a2 int);
create table b(b1 int, b2 int);
explain
select * from (
select a1, a2,(select SUM(b2) sum_b2 from b where a1 = b1)
from a ) v
where a2 = sum_b2;
                             QUERY PLAN
---------------------------------------------------------------------
 Seq Scan on a  (cost=0.00..327949668.86 rows=44331 width=8)
   Filter: (a2 = (SubPlan 2))
   SubPlan 1
     ->  Aggregate  (cost=36.78..36.79 rows=1 width=4)
           ->  Seq Scan on b  (cost=0.00..36.75 rows=11 width=4)
                 Filter: (a.a1 = b1)
   SubPlan 2
     ->  Aggregate  (cost=36.78..36.79 rows=1 width=4)
           ->  Seq Scan on b b_1  (cost=0.00..36.75 rows=11 width=4)
                 Filter: (a.a1 = b1)
(10 rows)