SELECT DISTINCT triggers sorting operation

Hi everybody,

I'm experiencing an interesting issue with PostgreSQL 8.4.2-r2.
I was running a pretty big database on another server (8.3) pretty fine.
When I moved it to this new server, behaviour changed.

On a complex SELECT DISTINCT query, pgsql now runs a "SORT" operation
although I've never requested it to do so! Considered this a millions
records table, requests usually runned within a second are now executed in
minutes...
Removing DISTINCT clause makes the request get executed instantely.

The interesting part is that "sort operation" is only added when there is
more than one field selected :

database=# EXPLAIN SELECT DISTINCT "articles_article"."id" FROM
"articles_article" LIMIT 8;
QUERY PLAN
----------------------------------------------------------------------------
------------------------------------------
Limit (cost=0.00..28.07 rows=8 width=4)
-> Unique (cost=0.00..7879955.60 rows=2245904 width=4)
-> Index Scan using articles_article_pkey on articles_article
(cost=0.00..7874340.84 rows=2245904 width=4)
(3 rows)

database=# EXPLAIN SELECT DISTINCT "articles_article"."id",
"articles_article"."flux_id" FROM "articles_article" LIMIT 8;
QUERY PLAN
----------------------------------------------------------------------------
-----------------
Limit (cost=614898.16..614898.22 rows=8 width=8)
-> Unique (cost=614898.16..631742.44 rows=2245904 width=8)
-> Sort (cost=614898.16..620512.92 rows=2245904 width=8)
Sort Key: id, flux_id
-> Seq Scan on articles_article (cost=0.00..316550.04
rows=2245904 width=8)
(5 rows)

Tunning the postgresql.conf with these options has not changed anything :
enable_hashagg = on, enable_sort =off.

Any idea how to disable this automatic CPU killing sorting operation?
Thanks,

Gaëtan Allart