PERF: Don't display days ago on timeline for megatopics.

Analysis using `pg_stat_statements` showed this query
to be eating up a significant portion of CPU.

app/assets/javascripts/discourse/widgets/topic-timeline.js.es6

@@ -151,8 +151,14 @@ createWidget("timeline-scrollarea", {
     const current = clamp(Math.floor(total * percentage) + 1, 1, total);
 
     const daysAgo = postStream.closestDaysAgoFor(current);
-    const date = new Date();
-    date.setDate(date.getDate() - daysAgo || 0);
+    let date;
+
+    if (daysAgo !== null) {
+      date = new Date();
+      date.setDate(date.getDate() - daysAgo || 0);
+    } else {
+      date = null;
+    }
 
     const result = {
       current,

lib/topic_view.rb

@@ -372,9 +372,18 @@ class TopicView
   # Returns an array of [id, post_number, days_ago] tuples.
   # `days_ago` is there for the timeline calculations.
   def filtered_post_stream
-    @filtered_post_stream ||= @filtered_posts
-      .order(:sort_order)
-      .pluck(:id, :post_number, 'EXTRACT(DAYS FROM CURRENT_TIMESTAMP - created_at)::INT AS days_ago')
+    @filtered_post_stream ||= begin
+      posts = @filtered_posts
+        .order(:sort_order)
+
+      columns = [:id, :post_number]
+
+      if is_mega_topic?
+        columns << 'EXTRACT(DAYS FROM CURRENT_TIMESTAMP - created_at)::INT AS days_ago'
+      end
+
+      posts.pluck(*columns)
+    end
   end
 
   def filtered_post_ids
@@ -554,4 +563,7 @@ class TopicView
     filtered_post_ids.index(closest_post.first) || filtered_post_ids[0]
   end
 
+  def is_mega_topic?
+    !@topic.private_message? && @topic.posts_count >= SiteSetting.auto_close_topics_post_count
+  end
 end

spec/components/topic_view_spec.rb

@@ -549,4 +549,30 @@ describe TopicView do
       end
     end
   end
+
+  describe '#filtered_post_stream' do
+    let!(:post) { Fabricate(:post, topic: topic, user: first_poster) }
+    let!(:post2) { Fabricate(:post, topic: topic, user: evil_trout) }
+    let!(:post3) { Fabricate(:post, topic: topic, user: first_poster) }
+
+    it 'should return the right columns' do
+      expect(topic_view.filtered_post_stream).to eq([
+        [post.id, post.post_number, 0],
+        [post2.id, post2.post_number, 0],
+        [post3.id, post3.post_number, 0]
+      ])
+    end
+
+    describe 'for mega topics' do
+      it 'should return the right columns' do
+        SiteSetting.auto_close_topics_post_count = 2
+
+        expect(topic_view.filtered_post_stream).to eq([
+          [post.id, post.post_number],
+          [post2.id, post2.post_number],
+          [post3.id, post3.post_number]
+        ])
+      end
+    end
+  end
 end