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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