PERF: we have no use for topic percent rank

Prepare to remove this column

app/models/topic.rb

@@ -31,6 +31,9 @@ class Topic < ActiveRecord::Base
   def_delegator :notifier, :mute!, :notify_muted!
   def_delegator :notifier, :toggle_mute, :toggle_mute
 
+  # TODO remove 01-01-2019
+  self.ignored_columns = ["percent_rank"]
+
   attr_accessor :allowed_user_ids, :tags_changed, :includes_destination_category
 
   DiscourseEvent.on(:site_setting_saved) do |site_setting|
@@ -1433,7 +1436,6 @@ end
 #  spam_count                :integer          default(0), not null
 #  pinned_at                 :datetime
 #  score                     :float
-#  percent_rank              :float            default(1.0), not null
 #  subtype                   :string
 #  slug                      :string
 #  deleted_by_id             :integer

lib/score_calculator.rb

@@ -20,7 +20,6 @@ class ScoreCalculator
     update_posts_score(opts)
     update_posts_rank(opts)
     update_topics_rank(opts)
-    update_topics_percent_rank(opts)
   end
 
   private
@@ -120,23 +119,6 @@ SQL
     builder.exec
   end
 
-  def update_topics_percent_rank(opts)
-
-    builder = DB.build <<~SQL
-      UPDATE topics SET percent_rank = x.percent_rank
-      FROM (SELECT id, percent_rank()
-            OVER (ORDER BY SCORE DESC) as percent_rank
-            FROM topics) AS x
-            /*where*/
-    SQL
-
-    builder.where("x.id = topics.id AND (topics.percent_rank <> x.percent_rank OR topics.percent_rank IS NULL)")
-
-    filter_topics(builder, opts)
-
-    builder.exec
-  end
-
   def filter_topics(builder, opts)
     return builder unless opts