Re: trick the query optimiser to skip some optimisations - Mailing list pgsql-performance

On Wed, Jan 29, 2014 at 3:38 PM, Дмитрий Шалашов <skaurus@gmail.com> wrote:

 

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

 ...

 

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.

How about "where user_id+0=?" 

Cheers,

Jeff