[SQL] Odd error message (" hash indexes do not support whole-index scans")

[Marinos Yannikos]

Shouldn't PostgreSQL behave in a different way in this case? (e.g. just 
not use the index):

                                          Table "public.forum_messages"
 message_id          | integer                  | not null default 
nextval('forum_messages_message_id_seq'::text)
 parent_userid       | integer                  |
 supersededby        | integer                  | default 0

    "forum_messages_pkey" primary key, btree (message_id)
    "idx_supersede" hash (supersededby) WHERE (supersededby > 0)
#  select m1.message_id,m2.message_id,m1.parent_userid,m2.parent_userid 
from forum_messages m1, forum_messages m2 where 
m2.supersededby=m1.message_id and m2.supersededby>0;
ERROR:  hash indexes do not support whole-index scans

Regards,
 Marinos
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