Disclaimer: this is a re-post, since I wasn't subscribed the first time I posted. Pardon if this is a duplicate.] The following query is abysmally slow (e.g. 7 hours+). The goal is to find "among the users that follow user #1, who do they also follow?" and to count the latter. SELECT L2.followed_id as followed_id, COUNT(U1.id) AS count FROM users AS U1 INNER JOIN links AS L1 ON L1.follower_id = U1.id INNER JOIN links AS L2 ON L2.follower_id = U1.id WHERE U1.type = 'User::Twitter' AND L1.followed_id = 1 GROUP BY L2.followed_id Here's the rest of the info. === versions psql (9.1.2, server 8.3.14) === schema create_table "users", :force => true do |t| t.string "type" end create_table "links", :force => true do |t| t.integer "followed_id" t.integer "follower_id" end add_index "links", ["follower_id"], :name => "index_links_on_follower_id" add_index "links", ["followed_id", "follower_id"], :name => "index_links_on_followed_id_and_follower_id", :unique => true add_index "links", ["followed_id"], :name => "index_links_on_followed_id" === # of rows users: 2,525,539 links: 4,559,390 === explain "HashAggregate (cost=490089.52..490238.78 rows=11941 width=8)" " -> Hash Join (cost=392604.44..483190.22 rows=1379860 width=8)" " Hash Cond: (f1.follower_id = u1.id)" " -> Bitmap Heap Scan on links f1 (cost=14589.95..55597.70 rows=764540 width=4)" " Recheck Cond: (followed_id = 1)" " -> Bitmap Index Scan on index_links_on_followed_id (cost=0.00..14398.82 rows=764540 width=0)" " Index Cond: (followed_id = 1)" " -> Hash (cost=300976.98..300976.98 rows=4559881 width=12)" " -> Hash Join (cost=94167.40..300976.98 rows=4559881 width=12)" " Hash Cond: (f2.follower_id = u1.id)" " -> Seq Scan on links f2 (cost=0.00..77049.81 rows=4559881 width=8)" " -> Hash (cost=53950.20..53950.20 rows=2526496 width=4)" " -> Seq Scan on users u1 (cost=0.00..53950.20 rows=2526496 width=4)" " Filter: ((type)::text = 'User::Twitter'::text)" === other comments I'm assuming I'm doing something obviously stupid and that the above info will be sufficient for anyone skilled in the art to detect the problem. However, if needed I will gladly invest the time to create a subset of the data in order to run EXPLAIN ANALYZE. (With the whole dataset, it requires > 7 hours to complete the query. I don't want to go down that path again!) - rdp -- Sent via pgsql-performance mailing list (pgsql-performance@xxxxxxxxxxxxxx) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-performance