[index] Add index on status_stats.created_at

db/migrate/20210103150000_add_index_on_status_stats_created_at.rb

@@ -0,0 +1,7 @@
+class AddIndexOnStatusStatsCreatedAt < ActiveRecord::Migration[5.2]
+  disable_ddl_transaction!
+
+  def change
+    add_index :status_stats, :created_at, algorithm: :concurrently
+  end
+end