Hash Join not using hashed index?

I'm using Postgresql 7.02.

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# explain select city.name, country.name from country, city
where city.country_id = country.country_id;
NOTICE: QUERY PLAN:

Hash Join (cost=8.85..16.76 rows=75 width=18)
-> Seq Scan on city (cost=0.00..1.75 rows=75 width=16)
-> Hash (cost=5.53..5.53 rows=253 width=2)
-> Seq Scan on country (cost=0.00..5.53 rows=253 width=2)

EXPLAIN
# create index country_id_idx on country using hash (country_id);
CREATE
# vacuum analyze;
VACUUM
# explain select city.name, country.name from country, city
where city.country_id = country.country_id;
NOTICE: QUERY PLAN:

Hash Join (cost=8.85..16.76 rows=75 width=18)
-> Seq Scan on city (cost=0.00..1.75 rows=75 width=16)
-> Hash (cost=5.53..5.53 rows=253 width=2)
-> Seq Scan on country (cost=0.00..5.53 rows=253 width=2)

EXPLAIN
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Why doesn't PostgreSQL use country_id_idx, but rather rehashing
country_id?