我想知道为什么要执行
UserView.where(:user_viewable_type => 'Song').order('created_at desc').limit(5)
需要运行1213毫秒。有点慢,不是吗?
在schema.rb中,user_viewable_type
被索引了:
add_index "user_views", ["user_viewable_type"], :name => "index_views_on_viewable_type"
即使在psql中:
"index_views_on_viewable_type" btree (user_viewable_type)
使用.explain
运行返回:
UserView.where(:user_viewable_type => 'Song').order('created_at desc').limit(5).explain
UserView Load (1801.4ms) SELECT "user_views".* FROM "user_views" WHERE "user_views"."user_viewable_type" = 'Song' ORDER BY created_at desc LIMIT 5
EXPLAIN (1.6ms) EXPLAIN SELECT "user_views".* FROM "user_views" WHERE "user_views"."user_viewable_type" = 'Song' ORDER BY created_at desc LIMIT 5
=> "EXPLAIN for: SELECT \"user_views\".* FROM \"user_views\" WHERE \"user_views\".\"user_viewable_type\" = 'Song' ORDER BY created_at desc LIMIT 5\n QUERY PLAN\n-------------------------------------------------------------------------------\n Limit (cost=17113.28..17113.28 rows=5 width=37)\n -> Sort (cost=17113.28..17147.82 rows=69085 width=37)\n Sort Key: created_at\n -> Seq Scan on user_views (cost=0.00..16883.78 rows=69085 width=37)\n Filter: ((user_viewable_type)::text = 'Song'::text)\n(5 rows)\n"
所以我决定拆分查询并删除
order
,我得到了:UserView.where(:user_viewable_type => 'Song').limit(5).explain
UserView Load (1.6ms) SELECT "user_views".* FROM "user_views" WHERE "user_views"."user_viewable_type" = 'Song' LIMIT 5
看起来order('created_at desc')
导致了查询变慢。但是为什么?按顺序排序不应该很快吗?
我尝试在id
列上使用order
:
UserView.where(:user_viewable_type => 'Song').order('id desc').limit(5).explain
UserView Load (44.8ms) SELECT "user_views".* FROM "user_views" WHERE "user_views"."user_viewable_type" = 'Song' ORDER BY id desc LIMIT 5
速度快多了!可能出了什么问题?
我需要提到几件事:
- 如果你还没有注意到,这是一个多态表。
- 过去,我通过Rails迁移将
:live_viewable_type
重命名为:user_viewable_type
,通过t.rename :live_viewable_type, :user_viewable_type
。然而,我认为这不应该是问题,因为索引仍在正确的列上。