On Sat, Mar 17, 2012 at 2:56 PM, Robert Poor <rdpoor@xxxxxxxxx> wrote: > 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!) Do you have an index on users.type? -- Sent via pgsql-performance mailing list (pgsql-performance@xxxxxxxxxxxxxx) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-performance
