Two indexes on same column

Hi,

what index is used (and according to what rules) when there are two (or
more) different indexes defined on one column? Assume:

CREATE TABLE example (
id SERIAL PRIMARY KEY,
...);
CREATE INDEX example_id_idx ON example USING hash (id);

By default there are btree index created and the hash index is then
created. So there are two indexes on column "id". Are there described
somewhere what index is used and when? Does it depend on query analyzer
and planner?

Thanks

Vlastik