BUG #3680: memory leak when excuting a SQL "select count(id) from chinatelecom;"

The following bug has been logged online:

Bug reference:      3680
Logged by:          ssurui
Email address:      ssurui@gmail.com
PostgreSQL version: 8.2.3
Operating system:   Redhat LINUX Advance server 3
Description:        memory leak when excuting a SQL "select count(id) from
chinatelecom;"
Details:

I use postgresql 8.2.3 to build a charger system.  chinatelecom is a table
name,use such command to create it :
CREATE TABLE chinatelecom
(
  id serial NOT NULL,
  company_id char(2) DEFAULT 'XX'::bpchar,
  service_id char(2) DEFAULT 'XX'::bpchar,
  inner_flag char(2) DEFAULT 'XX'::bpchar,
  terminal_id char(8) DEFAULT '00000000'::bpchar,
  worker_id char(8) DEFAULT '00000000'::bpchar,
  contract_id varchar(30) DEFAULT '0'::character varying,
  phone_number varchar(30) DEFAULT '0'::character varying,
  money numeric DEFAULT 0,
  trade_code varchar(3) DEFAULT '000'::character varying,
  record_date date DEFAULT ('now'::text)::date,
  la_serial_number varchar(12) DEFAULT '0'::character varying,
  serial_number varchar(12) DEFAULT '0'::character varying,
  record_time char(30),
  CONSTRAINT chinatelecom_pkey PRIMARY KEY (id)
)
WITHOUT OIDS;
ALTER TABLE chinatelecom OWNER TO postgres;
GRANT SELECT, UPDATE, INSERT, DELETE, REFERENCES, TRIGGER ON TABLE
chinatelecom TO postgres;
GRANT SELECT, UPDATE, INSERT, DELETE, REFERENCES, TRIGGER ON TABLE
chinatelecom TO public;

after the system runs about 4 months, the table has 2,000,000 records.

I use pgAdmin III to excute a simple SQL :
select count(*) from chinatelecom;

use top command observe the available memory, I find about 500MB memory is
vanished after the SQL is executed.

please tell me why it happened?