Bug: the index on INTEGER field does not work (PG 7.1.2)

Bug: the index on INTEGER field does not work (PG 7.1.2).

Test 1:

1) create table test_int(id int primary key);

2) insert 10000 records in table test_int with perl program (values
1,2,3,...,10000).

3) 500 times execute query

SELECT * FROM test_int WHERE id = random number

(random number puts with perl program)

4) 500 queries executes for 30 seconds

Test 2:

1) create table test_int1(id int); (i.e. without primary key)

2) insert 10000 records in table test_int1 with perl program (values
1,2,3,...,10000).

3) 500 times execute query

SELECT * FROM test_int1 WHERE id = random number

(random number puts with perl program)

4) 500 queries executes for 30 seconds too :~-( ...

P.S. For TEXT field the same operations executes for 30 and 1 seconds
correspondingly.