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:
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)
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.
Dmitriy Shalashov
