Full outer join

Looking at the docs for 7.1, it appears that full outer joins are supported:
http://www.postgresql.org/devel-corner/docs/user/queries.html

However, I keep getting this error, and I don't know what it means:
ERROR: FULL JOIN is only supported with mergejoinable join conditions

I've tried several variations and keep getting the same error.

Anyone have any ideas?

Details below.

Thanks,

Tim

SELECT *
FROM (stats_http_downloads sh
FULL OUTER JOIN stats_ftp_downloads sf USING (day,filerelease_id)) ;

ERROR: FULL JOIN is only supported with mergejoinable join conditions

SELECT day,filerelease_id
FROM (stats_http_downloads sh
FULL OUTER JOIN stats_ftp_downloads sf USING (day,filerelease_id)) ;

ERROR: FULL JOIN is only supported with mergejoinable join conditions

alexandria=# \d stats_http_downloads
Table "stats_http_downloads"
Attribute | Type | Modifier
----------------+---------+----------------------
day | integer | not null default '0'
filerelease_id | integer | not null default '0'
group_id | integer | not null default '0'
downloads | integer | not null default '0'

alexandria=# \d stats_ftp_downloads
Table "stats_ftp_downloads"
Attribute | Type | Modifier
----------------+---------+----------------------
day | integer | not null default '0'
filerelease_id | integer | not null default '0'
group_id | integer | not null default '0'
downloads | integer | not null default '0'

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