default sql_policy query doesn't use all keys?

Looking at optimizations for sql queries in amavisd-new, 
default 
sql_policy can't find enough key material to use? or is this
impossible 
due to the IN statement?

even the default query uses 'filesort'.

is it the IN statement?

mysql> explain select *,users.id from users left join
policy on 
users.policy_id=policy.id where users.email in ('scheidell',

'scheidelldomain.com', '.') order by users.priority desc;
+----+-------------+--------+--------+---------------+------
---+---------+----------------------+------+----------------
-------------+
| id | select_type | table  | type   | possible_keys | key  
  | key_len 
| ref                  | rows | Extra                      
|
+----+-------------+--------+--------+---------------+------
---+---------+----------------------+------+----------------
-------------+
|  1 | SIMPLE      | users  | range  | email_2,email |
email_2 | 257     
| NULL                 |    3 | Using where; Using filesort
|
|  1 | SIMPLE      | policy | eq_ref | PRIMARY,id    |
PRIMARY | 4       
| mail.users.policy_id |    1 |                            
|
+----+-------------+--------+--------+---------------+------
---+---------+----------------------+------+----------------
-------------+



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