[index] Add index on polls id and lock_version

db/migrate/20210117210100_add_index_on_polls.rb

@@ -0,0 +1,7 @@
+class AddIndexOnPolls < ActiveRecord::Migration[5.2]
+  disable_ddl_transaction!
+
+  def change
+    add_index :polls, [:id, :lock_version], algorithm: :concurrently
+  end
+end