Themen laden langsam oder gar nicht, wenn sie viele Antworten haben und der Benutzer Lesezeichen darin gesetzt hat

martin · 26. Oktober 2020 um 04:32

This has been fixed in:

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.

cc @Macaw who made this related topic Bookmarking a Post on a Large Topic creates Absurd Loading Times

Thema		Antworten	Aufrufe
Bookmarking a Post on a Large Topic creates Absurd Loading Times Bug	10	1053	26. Oktober 2020
Major slowdown for staff in large topics Support	13	2231	4. Juni 2019
Performance improvements on long topics? Support	30	5481	20. August 2018
Slow Page Loads on User Profiles Feature	15	2435	23. November 2020
Topics jumping to wrong post Bug	23	1127	29. August 2023

Themen laden langsam oder gar nicht, wenn sie viele Antworten haben und der Benutzer Lesezeichen darin gesetzt hat

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