Make CopyStatusStats migration use batches of 1000 to avoid locks (#8256)

db/migrate/20180812173710_copy_status_stats.rb

@@ -3,15 +3,18 @@ class CopyStatusStats < ActiveRecord::Migration[5.2]
 
   def up
     safety_assured do
+      Status.where.not(id: StatusStat.select('status_id')).select('id').find_in_batches do |statuses|
         execute <<-SQL.squish
           INSERT INTO status_stats (status_id, reblogs_count, favourites_count, created_at, updated_at)
           SELECT id, reblogs_count, favourites_count, created_at, updated_at
           FROM statuses
+          WHERE id IN (#{statuses.map(&:id).join(', ')})
           ON CONFLICT (status_id) DO UPDATE
           SET reblogs_count = EXCLUDED.reblogs_count, favourites_count = EXCLUDED.favourites_count
         SQL
       end
     end
+  end
 
   def down
     # Nothing