DEV: remove unused columns from posts and topics

avg_time on posts and topics have not been used in a year.

This uses a re-runnable ddl transaction diasabled migration to
drop the column, cause it touchs very high traffic table and may
deadlock

app/models/post.rb

@@ -10,6 +10,9 @@ class Post < ActiveRecord::Base
   include HasCustomFields
   include LimitedEdit
 
+  # remove line Jan 2021
+  self.ignored_columns = ["avg_time"]
+
   cattr_accessor :plugin_permitted_create_params
   self.plugin_permitted_create_params = {}
 
@@ -1107,7 +1110,6 @@ end
 #  like_count              :integer          default(0), not null
 #  incoming_link_count     :integer          default(0), not null
 #  bookmark_count          :integer          default(0), not null
-#  avg_time                :integer
 #  score                   :float
 #  reads                   :integer          default(0), not null
 #  post_type               :integer          default(1), not null

app/models/topic.rb

@@ -10,6 +10,9 @@ class Topic < ActiveRecord::Base
   include LimitedEdit
   extend Forwardable
 
+  # remove line Jan 2021
+  self.ignored_columns = ["avg_time"]
+
   def_delegator :featured_users, :user_ids, :featured_user_ids
   def_delegator :featured_users, :choose, :feature_topic_users
 
@@ -1567,7 +1570,6 @@ end
 #  featured_user1_id         :integer
 #  featured_user2_id         :integer
 #  featured_user3_id         :integer
-#  avg_time                  :integer
 #  deleted_at                :datetime
 #  highest_post_number       :integer          default(0), not null
 #  image_url                 :string

db/post_migrate/20200430010528_remove_avg_time_from_topics_posts.rb

@@ -0,0 +1,20 @@
+# frozen_string_literal: true
+
+class RemoveAvgTimeFromTopicsPosts < ActiveRecord::Migration[6.0]
+  disable_ddl_transaction!
+
+  def up
+    # this makes it re-runnable and also works if it was not created initially
+    execute <<~SQL
+      ALTER TABLE topics DROP COLUMN IF EXISTS avg_time CASCADE
+    SQL
+
+    execute <<~SQL
+      ALTER TABLE posts DROP COLUMN IF EXISTS avg_time CASCADE
+    SQL
+  end
+
+  def down
+    # do nothing re-runnable
+  end
+end