Hash joins don't have anything to do with hash indexes.
A hash join is a join that makes use of a temporary hashtable
built on-the-fly *in memory* for that join.
The planner could choose to use an indexscan on a hash index
as an input for the join, but it'd only be likely to do so
if there is a restriction clause matching the index. In your
example you have only a join WHERE clause.
Plain btree indexes on city.country_id and country.country_id
might work better --- at least they'd offer the option of
a merge join without doing explicit sort.
regards, tom lane
