JOIN with ORDER on both tables does a sort when it souldn't

Hi,

I have three tables involved in my problem:

forums_grps [means: Forum-Groups]
- id (PRIMARY KEY)
- title

forums [means: Forums]
- id (PRIMARY KEY)
- forum_group_id (NOT NULL, FOREIGN KEY)
- order (defines listing order of forums in the same forum_group)
INDEX: (forum_group_id, order)

sit_shw_fgr [means: Sites Show Forum-Groups]
- site_id (PRIMARY KEY)
- forum_group_id (PRIMARY KEY, FOREIGN KEY)
- order (defines listing order of shown forum_groups on a site)
INDEX: (site_id, order)

What I want to do is SELECT the forums shown on a given site,
ordered by sit_shw_fgr.order ASC, forums.order ASC. So the query is:

SELECT * FROM sit_shw_fgr JOIN forums
         ON forums.forum_group_id = sit_shw_fgr.forum_group_id
WHERE sit_shw_fgr.site_id = 1
ORDER BY sit_shw_fgr.order ASC, forums.order ASC

If the plan uses a nestloop with both indexes I mentioned, it will get
the results in the correct order. But the planner will only choose this
plan, if I disable all other choices:
SET enable_seqscan TO false;
SET enable_hashjoin TO false;
SET enable_mergejoin TO false;
But even then, it won't realize that the result are in correct order, and
does a sort! Why?

Sort
  Sort Key: sit_shw_fgr.order, forums.order
  -> Nested Loop
     -> Index Scan using sit_shw_fgr_idx_siteid_order on sit_shw_fgr
           Index Cond: (sitid = 1)
     -> Index Scan using forums_idx_forumgroupid_order on forums
           Index Cond: (forums.fgrid = "outer".fgrid)


I'm using PostgreSQL 8.1.8.

Thanks for the answer in advance,
Denes Daniel

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