我不知道为什么这样做为什么此查询运行缓慢?
UserView.where(:user_viewable_type => 'Song').order('created_at desc').limit(5)
需要1213ms运行。有点慢吗?
在schema.rb user_viewable_type
被索引:
add_index "user_views", ["user_viewable_type"], :name => "index_views_on_viewable_type"
即使在PSQL:
"index_views_on_viewable_type" btree (user_viewable_type)
与.explain
回报运行:
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"
所以我决定打破查询下来并删除order
,我得到:
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')
导致查询变慢。但为什么?难道不能很快下单吗?
我试图在该id
列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
如此之快!什么可能是错的?
几件事我应该提到:
- 如果你还没有注意到,这是一个多态表。
- 过去,我通过经由
t.rename :live_viewable_type, :user_viewable_type
的Rails迁移,将:live_viewable_type
更名为:user_viewable_type
。但是,我认为这不应该成为问题,因为索引仍然在右栏。
将问题中的“解释”替换为“解释分析”,并用换行符将其格式化为代码,以便可读。 – 2012-08-09 17:49:23