Hi Elliot, thank you for your answer. I tried this query but it still suffer with index scan on tweet_creation_time_index:
"Sort (cost=4899904.57..4899913.19 rows=3447 width=20) (actual time=37560.938..37562.503 rows=1640 loops=1)" " Sort Key: tt.tweet_id" " Sort Method: quicksort Memory: 97kB" " Buffers: shared hit=1849 read=32788" " -> Nested Loop (cost=105592.06..4899702.04 rows=3447 width=20) (actual time=19151.036..37555.227 rows=1640 loops=1)" " Buffers: shared hit=1849 read=32788" " -> Hash Join (cost=105574.10..116461.68 rows=1679 width=8) (actual time=19099.848..19127.606 rows=597 loops=1)" " Hash Cond: (relationship.followed_id = t.user_id)" " Buffers: shared hit=3 read=31870" " -> Index Only Scan using relationship_id on relationship (cost=0.42..227.12 rows=154 width=8) (actual time=66.102..89.721 rows=106 loops=1)" " Index Cond: (follower_id = 335093362)" " Heap Fetches: 0" " Buffers: shared hit=2 read=3" " -> Hash (cost=83308.25..83308.25 rows=1781234 width=16) (actual time=19031.916..19031.916 rows=1759645 loops=1)" " Buckets: 262144 Batches: 1 Memory Usage: 61863kB" " Buffers: shared hit=1 read=31867" " -> Index Scan using tweet_creation_time_index on tweet t (cost=0.57..83308.25 rows=1781234 width=16) (actual time=48.595..13759.768 rows=1759645 loops=1)" " Index Cond: ((creation_time >= '2013-05-05 00:00:00-03'::timestamp with time zone) AND (creation_time <= '2013-05-06 00:00:00-03'::timestamp with time zone))" " Buffers: shared hit=1 read=31867" " -> Bitmap Heap Scan on tweet_topic tt (cost=17.96..2841.63 rows=723 width=20) (actual time=30.774..30.847 rows=3 loops=597)" " Recheck Cond: (tweet_id = t.id)" " Buffers: shared hit=1846 read=918" " -> Bitmap Index Scan on tweet_topic_pk (cost=0.00..17.78 rows=723 width=0) (actual time=23.084..23.084 rows=3 loops=597)" " Index Cond: (tweet_id = t.id)" " Buffers: shared hit=1763 read=632" You said that I would need B-Tree indexes on the fields that I want the planner to use index only scan, and I think I have them already on the tweet table: "tweet_ios_index" btree (id, user_id, creation_time) Shouldn't the tweet_ios_index be enough to make the scan over tweet_creation_time_index be a index only scan? And, more important, would it be really faster? Thank you very much, Caio On Mon, Nov 4, 2013 at 7:22 PM, Elliot <yields.falseh...@gmail.com> wrote: > On 2013-11-04 16:10, Caio Casimiro wrote: > > Hi Neyman, thank you for your answer. > > Unfortunately this query runs almost at the same time: > > Sort (cost=4877693.98..4877702.60 rows=3449 width=20) (actual > time=25820.291..25821.845 rows=1640 loops=1) > Sort Key: tt.tweet_id > Sort Method: quicksort Memory: 97kB > Buffers: shared hit=1849 read=32788 > -> Nested Loop (cost=247.58..4877491.32 rows=3449 width=20) (actual > time=486.839..25814.120 rows=1640 loops=1) > Buffers: shared hit=1849 read=32788 > -> Hash Semi Join (cost=229.62..88553.23 rows=1681 width=8) > (actual time=431.654..13209.159 rows=597 loops=1) > Hash Cond: (t.user_id = relationship.followed_id) > Buffers: shared hit=3 read=31870 > -> Index Scan using tweet_creation_time_index on tweet t > (cost=0.57..83308.25 rows=1781234 width=16) (actual > time=130.144..10037.764 rows=1759645 loops=1) > Index Cond: ((creation_time >= '2013-05-05 > 00:00:00-03'::timestamp with time zone) AND (creation_time <= '2013-05-06 > 00:00:00-03'::timestamp with time zone)) > Buffers: shared hit=1 read=31867 > -> Hash (cost=227.12..227.12 rows=154 width=8) (actual > time=94.365..94.365 rows=106 loops=1) > Buckets: 1024 Batches: 1 Memory Usage: 3kB > Buffers: shared hit=2 read=3 > -> Index Only Scan using relationship_id on > relationship (cost=0.42..227.12 rows=154 width=8) (actual > time=74.540..94.101 rows=106 loops=1) > Index Cond: (follower_id = 335093362) > Heap Fetches: 0 > Buffers: shared hit=2 read=3 > -> Bitmap Heap Scan on tweet_topic tt (cost=17.96..2841.63 > rows=723 width=20) (actual time=21.014..21.085 rows=3 loops=597) > Recheck Cond: (tweet_id = t.id) > Buffers: shared hit=1846 read=918 > -> Bitmap Index Scan on tweet_topic_pk (cost=0.00..17.78 > rows=723 width=0) (actual time=15.012..15.012 rows=3 loops=597) > Index Cond: (tweet_id = t.id) > Buffers: shared hit=1763 read=632 > Total runtime: 25823.386 ms > > I have noticed that in both queries the index scan on > tweet_creation_time_index is very expensive. Is there anything I can do to > make the planner choose a index only scan? > > > Yes, because that part of the query is kicking back so many rows, many > of which are totally unnecessary anyway - you're first getting all the > tweets in a particular time range, then limiting them down to just users > that are followed. Here's clarification on the approach I mentioned > earlier. All you should really need are basic (btree) indexes on your > different keys (tweet_topic.tweet_id, tweet.id, tweet.user_id, > relationship.follower_id, relationship.followed_id). I also changed the > left join to an inner join as somebody pointed out that your logic amounted > to reducing the match to an inner join anyway. > > SELECT tt.tweet_id, tt.topic, tt.topic_value > FROM tweet_topic AS tt > JOIN tweet AS t > ON tt.tweet_id = t.id > join relationship > on t.user_id = relationship.followed_id > > WHERE creation_time BETWEEN 'D1' AND 'D2' > AND relationship.follower_id = N > ORDER BY tt.tweet_id > ; > >
