From 1f062ae2fde43a44f407f4af91a190c8c02fb1e0 Mon Sep 17 00:00:00 2001
From: Sam <sam.saffron@gmail.com>
Date: Mon, 15 Feb 2016 18:54:53 +1100
Subject: [PATCH] PERF: improve performance of consistency query

---
 app/models/category_user.rb | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/app/models/category_user.rb b/app/models/category_user.rb
index e7138d91da6..7f0ab17a885 100644
--- a/app/models/category_user.rb
+++ b/app/models/category_user.rb
@@ -95,7 +95,14 @@ class CategoryUser < ActiveRecord::Base
   end
 
   def self.ensure_consistency!
-    exec_sql("DELETE FROM category_users WHERE user_id NOT IN (SELECT id FROM users)")
+    exec_sql <<SQL
+    DELETE FROM category_users
+      WHERE user_id IN (
+        SELECT cu.user_id FROM category_users cu
+        LEFT JOIN users u ON u.id = cu.user_id
+        WHERE u.id IS NULL
+      )
+SQL
   end
 
   private_class_method :apply_default_to_topic, :remove_default_from_topic