From cbdab71179e60a97137f00c5ed070225dd9c8e4c Mon Sep 17 00:00:00 2001
From: Sam <sam.saffron@gmail.com>
Date: Wed, 20 Jun 2018 18:11:39 +1000
Subject: [PATCH] PERF: stop counting participants on very large topics

This query gets very expensive and can be bypassed on large topics
---
 lib/topic_view.rb | 23 ++++++++++++++++-------
 1 file changed, 16 insertions(+), 7 deletions(-)

diff --git a/lib/topic_view.rb b/lib/topic_view.rb
index 45e7f2bc3df..e064a3ba93f 100644
--- a/lib/topic_view.rb
+++ b/lib/topic_view.rb
@@ -296,17 +296,26 @@ class TopicView
     end
   end
 
+  # if a topic has more that N posts no longer attempt to
+  # get accurate participant count, instead grab cached count
+  # from topic
+  MAX_POSTS_COUNT_PARTICIPANTS = 500
+
   def participant_count
     @participant_count ||=
       begin
         if participants.size == MAX_PARTICIPANTS
-          sql = <<~SQL
-            SELECT COUNT(DISTINCT user_id)
-            FROM posts
-            WHERE id IN (:post_ids)
-            AND user_id IS NOT NULL
-          SQL
-          DB.query_single(sql, post_ids: unfiltered_post_ids).first.to_i
+          if unfiltered_post_ids.length > MAX_POSTS_COUNT_PARTICIPANTS
+            @topic.participant_count
+          else
+            sql = <<~SQL
+              SELECT COUNT(DISTINCT user_id)
+              FROM posts
+              WHERE id IN (:post_ids)
+              AND user_id IS NOT NULL
+            SQL
+            DB.query_single(sql, post_ids: unfiltered_post_ids).first.to_i
+          end
         else
           participants.size
         end