Bad query plan

I have a problem with poor query plan.

My PostgreSQL is "PostgreSQL 8.4.8, compiled by Visual C++ build 1400,
32-bit" installed by EnterpriseDB installer on Windows 7 32 bit.

Steps to reproduce:

Start with fresh installation and execute the following:

drop table if exists small;
drop table if exists large;

CREATE TABLE small
(
  id bigint,
  primary key(id)
);

CREATE TABLE large
(
  id bigint,
  primary key(id)
);

--Insert 100000 rows into large
CREATE or replace FUNCTION populate_large() RETURNS bigint AS $$
DECLARE
    id1 bigint := 0;
BEGIN
    LOOP
        insert into large(id) values(id1);
        id1 := id1 +1;
        if id1>100000 then
            exit;
        end if;
    END LOOP;
    return id1;
END
$$ LANGUAGE plpgsql;

--Insert 1000 rows into small
CREATE or replace FUNCTION populate_small() RETURNS bigint AS $$
DECLARE
    id1 bigint := 0;
BEGIN
    LOOP
        insert into small(id) values(id1);
        id1 := id1 +1;
        if id1>1000 then
            exit;
        end if;
    END LOOP;
    return id1;
END
$$ LANGUAGE plpgsql;

select populate_large(),populate_small();
analyze;

Then execute

explain analyze insert into large(id) select id from small where id
not in(select id from large);

It gives

"Seq Scan on small  (cost=1934.01..823278.28 rows=500 width=8) (actual
time=6263.588..6263.588 rows=0 loops=1)"
"  Filter: (NOT (SubPlan 1))"
"  SubPlan 1"
"    ->  Materialize  (cost=1934.01..3325.02 rows=100001 width=8)
(actual time=0.007..3.012 rows=501 loops=1001)"
"          ->  Seq Scan on large  (cost=0.00..1443.01 rows=100001
width=8) (actual time=0.010..5.810 rows=1001 loops=1)"
"Total runtime: 6263.703 ms"

But

explain analyze insert into large(id) select id from small where not
exists (select id from large l where small.id=l.id);

exeutes much faster:

"Merge Anti Join  (cost=0.00..85.58 rows=1 width=8) (actual
time=15.793..15.793 rows=0 loops=1)"
"  Merge Cond: (small.id = l.id)"
"  ->  Index Scan using small_pkey on small  (cost=0.00..43.27
rows=1001 width=8) (actual time=0.025..3.515 rows=1001 loops=1)"
"  ->  Index Scan using large_pkey on large l  (cost=0.00..3050.28
rows=100001 width=8) (actual time=0.017..2.932 rows=1001 loops=1)"
"Total runtime: 15.863 ms"

Both queries are semantically the same.