From 57d06518d419ad067a01ff1dc6836a44d5b4b685 Mon Sep 17 00:00:00 2001
From: Martin Brennan <mjrbrennan@gmail.com>
Date: Mon, 26 Oct 2020 14:30:31 +1000
Subject: [PATCH] FIX: Prevent slow bookmark first post reminder at query for
topic (#11024)
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.
---
lib/topic_view.rb | 8 ++++++--
1 file changed, 6 insertions(+), 2 deletions(-)
diff --git a/lib/topic_view.rb b/lib/topic_view.rb
index c10c99a72fd..49c9e45b49e 100644
--- a/lib/topic_view.rb
+++ b/lib/topic_view.rb
@@ -349,8 +349,12 @@ class TopicView
end
def first_post_bookmark_reminder_at
- @topic.posts.with_deleted.where(post_number: 1).first
- .bookmarks.where(user: @user).pluck_first(:reminder_at)
+ @first_post_bookmark_reminder_at ||= \
+ begin
+ first_post = @topic.posts.with_deleted.find_by(post_number: 1)
+ return if !first_post
+ first_post.bookmarks.where(user: @user).pluck_first(:reminder_at)
+ end
end
MAX_PARTICIPANTS = 24