답글이 많고 북마크가 있는 주제는 느리게 또는 아예 로드되지 않습니다

martin · 10월 26, 2020, 4:32오전

이 문제는 다음에서 수정되었습니다:

FIX: Prevent slow bookmark first post reminder at query for topic (#11024)

committed 04:30AM - 26 Oct 20 UTC

On forums with a large amount of posts when a user had a bookmark in the topic, …PostgreSQL was using an inefficient query plan to fetch the first post of the topic. When running this ActiveRecord query: ``` topic.posts.with_deleted.where(post_number: 1).first ``` The following query plan was produced: ``` Limit (cost=0.43..583.49 rows=1 width=891) (actual time=3850.515..3850.515 rows=1 loops=1) -> Index Scan using posts_pkey on posts (cost=0.43..391231.51 rows=671 width=891) (actual time=3850.514..3850.514 rows=1 loops=1) Filter: ((topic_id = 160918) AND (post_number = 1)) Rows Removed by Filter: 2274520 Planning time: 0.200 ms Execution time: 3850.559 ms (6 rows) ``` The issue here is the combination of ORDER BY and LIMIT causing the ineficcient Index Scan using posts_pkey on posts to be used. When we correct the AR call to this: ``` topic.posts.with_deleted.find_by(post_number: 1) ``` We end up with a query that still has a LIMIT but no ORDER BY, which in turn creates a much more efficient query plan: ``` Limit (cost=0.43..1.44 rows=1 width=891) (actual time=0.033..0.034 rows=1 loops=1) -> Index Scan using index_posts_on_topic_id_and_post_number on posts (cost=0.43..678.82 rows=671 width=891) (actua l time=0.033..0.033 rows=1 loops=1) Index Cond: ((topic_id = 160918) AND (post_number = 1)) Planning time: 0.167 ms Execution time: 0.072 ms (5 rows) ``` This query plan uses the correct index, `Index Scan using index_posts_on_topic_id_and_post_number on posts`. Note that this is only a problem on forums with a larger amount of posts; tiny forums would not notice the difference. On large forums a query for a topic that takes 1s without a bookmark can take 8-30 seconds, and even end up with 502 errors from nginx.

관련 주제 Bookmarking a Post on a Large Topic creates Absurd Loading Times 를 만든 @Macaw 님에게 cc

글		댓글	조회수
Bookmarking a Post on a Large Topic creates Absurd Loading Times Bug	9	1193	10월 26, 2020
Major slowdown for staff in large topics Product Support	12	2421	6월 3, 2019
Performance improvements on long topics? Product Support	29	5877	7월 12, 2018
Slow Page Loads on User Profiles Feature	14	2730	11월 23, 2020
Topics jumping to wrong post Bug	22	1617	8월 28, 2023

답글이 많고 북마크가 있는 주제는 느리게 또는 아예 로드되지 않습니다

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