UNIQUE( col1, col2 ) creates what indexes?

Hi all,

I've created a table:
CREATE TABLE locales (
-- table specific columns
iso639 varchar(2) NOT NULL,
iso3166 varchar(2),
fallback boolean DEFAULT false,

id bigserial,

PRIMARY KEY(id),
UNIQUE(iso639, iso3166)
);

As the manual states this creates an index on the table, but what index is .

EXPLAIN tells me it does an index scan when doing
SELECT * FROM locales WHERE iso639 = 'fr';
or
SELECT * FROM locales WHERE iso639 = 'fr' AND iso3166 = 'CA';
or
SELECT * FROM locales WHERE iso639 = 'fr' AND fallback = TRUE;

but is doing a seq scan when doing
SELECT * FROM locales WHERE iso3166 = 'CA';
or
SELECT * FROM locales WHERE iso3166 = 'CA' AND fallback = TRUE;

At first I thought this might be due to the iso3166 field not being NOT
NULL but that doesn't seem to be the case. Can anyone tell me what I am
missing?

Regards,
Rob