PERF: improve performance of unread queries

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.

spec/models/topic_user_spec.rb

@@ -1,6 +1,17 @@
 require 'rails_helper'
 
 describe TopicUser do
+  let :watching do
+    TopicUser.notification_levels[:watching]
+  end
+
+  let :regular do
+    TopicUser.notification_levels[:regular]
+  end
+
+  let :tracking do
+    TopicUser.notification_levels[:tracking]
+  end
 
   describe "#unwatch_categories!" do
     it "correctly unwatches categories" do
@@ -10,9 +21,6 @@ describe TopicUser do
       tracked_topic = Fabricate(:topic)
 
       user = op_topic.user
-      watching = TopicUser.notification_levels[:watching]
-      regular = TopicUser.notification_levels[:regular]
-      tracking = TopicUser.notification_levels[:tracking]
 
       TopicUser.change(user.id, op_topic, notification_level: watching)
       TopicUser.change(user.id, another_topic, notification_level: watching)
@@ -30,6 +38,36 @@ describe TopicUser do
 
   end
 
+  describe "first unread" do
+    it "correctly update first unread as needed" do
+      time1 = 3.days.ago
+      time2 = 2.days.ago
+      time3 = 1.days.ago
+
+      topic1 = Fabricate(:topic, last_unread_at: time1)
+      topic2 = Fabricate(:topic, last_unread_at: time2)
+      topic3 = Fabricate(:topic, last_unread_at: time3)
+
+      user = Fabricate(:user)
+      user.user_stat.update_columns(first_topic_unread_at: Time.zone.now)
+
+      TopicUser.change(user.id, topic3, notification_level: tracking)
+
+      user.user_stat.reload
+      expect(user.user_stat.first_topic_unread_at).to be_within(1.second).of(time3)
+
+      TopicUser.change(user.id, topic2, notification_level: watching)
+
+      user.user_stat.reload
+      expect(user.user_stat.first_topic_unread_at).to be_within(1.second).of(time2)
+
+      TopicUser.change(user.id, topic1, notification_level: regular)
+
+      user.user_stat.reload
+      expect(user.user_stat.first_topic_unread_at).to be_within(1.second).of(time2)
+    end
+  end
+
   describe '#notification_levels' do
     context "verify enum sequence" do
       before do