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PERF: improve performance of unread queries

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Figuring out what unread topics a user has is a very expensive
operation over time.

Users can easily accumulate 10s of thousands of tracking state rows
(1 for every topic they ever visit)

When figuring out what a user has that is unread we need to join
the tracking state records to the topic table. This can very quickly
lead to cases where you need to scan through the entire topic table.

This commit optimises it so we always keep track of the "first" date
a user has unread topics. Then we can easily filter out all earlier
topics from the join.

We use pg functions, instead of nested queries here to assist the
planner.
This commit is contained in:
Sam
2017-05-25 15:07:12 -04:00
parent 6eb6c25816
commit 29fac1ac18
12 changed files with 226 additions and 38 deletions
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25
lib/topic_query.rb
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@ -280,10 +280,21 @@ class TopicQuery
.where("COALESCE(tu.notification_level, :tracking) >= :tracking", tracking: TopicUser.notification_levels[:tracking])
end
def self.unread_filter(list, opts)
def self.unread_filter(list, user_id, opts)
# PERF note
# We use the function first_unread_topic_for here instead of joining
# the table to assist the PostgreSQL query planner
#
# We want the query planner to have the actual value of the first_unread_topic so
# it can pick an appropriate plan. If it does not have this upfront it will just assume
# that the value will be 1/3 of the way through the topic table which makes it use terrible
# indexes for the plan.
#
col_name = opts[:staff] ? "highest_staff_post_number" : "highest_post_number"
list.where("tu.last_read_post_number < topics.#{col_name}")
list
.where("tu.last_read_post_number < topics.#{col_name}")
.where("topics.last_unread_at >= first_unread_topic_for(?)", user_id)
.where("COALESCE(tu.notification_level, :regular) >= :tracking",
regular: TopicUser.notification_levels[:regular], tracking: TopicUser.notification_levels[:tracking])
end
@ -361,7 +372,10 @@ class TopicQuery
end
def unread_results(options={})
result = TopicQuery.unread_filter(default_results(options.reverse_merge(:unordered => true)), staff: @user.try(:staff?))
result = TopicQuery.unread_filter(
default_results(options.reverse_merge(:unordered => true)),
@user&.id,
staff: @user&.staff?)
.order('CASE WHEN topics.user_id = tu.user_id THEN 1 ELSE 2 END')
self.class.results_filter_callbacks.each do |filter_callback|
@ -724,7 +738,10 @@ class TopicQuery
end
def unread_messages(params)
TopicQuery.unread_filter(messages_for_groups_or_user(params[:my_group_ids]), staff: @user.try(:staff?))
TopicQuery.unread_filter(
messages_for_groups_or_user(params[:my_group_ids]),
@user&.id,
staff: @user&.staff?)
.limit(params[:count])
end