trick the query optimiser to skip some optimisations

Hi!

I have a table called 'feed'. It's a big table accessed by many types of
queries, so I have quite a lot of indices on it.

Those that are relevant looks like this:

"feed_user_id_active_id_added_idx" btree (user_id, active_id, added)
"feed_user_id_added_idx" btree (user_id, added DESC)
"feed_user_id_added_idx2" btree (user_id, added DESC) WHERE active_id =
user_id AND type = 1

last one is very small and tailored for the specific query.
"added" field is timestamp, everything else is integers.

That specific query looks like this:

SELECT * FROM feed WHERE user_id = ? AND type = 1 AND active_id = user_id
ORDER BY added DESC LIMIT 31;

But it doesn't use the last index. EXPLAIN shows this:

Limit (cost=0.00..463.18 rows=31 width=50)
-> Index Scan Backward using feed_user_id_active_id_added_idx on
user_feed (cost=0.00..851.66 rows=57 width=50)
Index Cond: ((user_id = 7) AND (active_id = 7))
Filter: (type = 1)

So as we can see optimiser changes "active_id = user_id" to "active_id =
<whatever value user_id takes>". And it brokes my nice fast partial index :(
Can I do something here so optimiser would use the feed_user_id_added_idx2
index? It's around ten times smaller than the 'generic'
feed_user_id_active_id_added_idx index.

I have PostgreSQL 9.2.6 on Debian.

Best regards,
Dmitriy Shalashov