postgres=# create table t (id int);
CREATE TABLE
postgres=# insert into t select x from generate_series(1,1000000) as x;
INSERT 0 1000000
postgres=# create index on t(id);
CREATE INDEX
postgres=# vacuum analyze t;
VACUUM
postgres=# set seq_page_cost = 1;
SET
postgres=# set random_page_cost = 1;
SET
postgres=# explain select id from t;
                        QUERY PLAN                         
-----------------------------------------------------------
 Seq Scan on t  (cost=0.00..14425.00 rows=1000000 width=4)
(1 row)

postgres=# set seq_page_cost = 10;
SET
postgres=# explain select id from t;
                                   QUERY PLAN                                    
---------------------------------------------------------------------------------
 Index Only Scan using t_id_idx on t  (cost=0.42..17745.42 rows=1000000 width=4)
(1 row)