delayed jobs: add indices in db.

db/migrate/20190827014726_add_indices_to_delayed_jobs.rb

@@ -0,0 +1,12 @@
+class AddIndicesToDelayedJobs < ActiveRecord::Migration[6.0]
+  def up
+    # the production db already has this index.
+    add_index :delayed_jobs, :locked_at unless index_exists?(:delayed_jobs, :locked_at)
+    add_index :delayed_jobs, :locked_by unless index_exists?(:delayed_jobs, :locked_by)
+  end
+
+  def down
+    remove_index :delayed_jobs, :locked_at if index_exists?(:delayed_jobs, :locked_at)
+    remove_index :delayed_jobs, :locked_by if index_exists?(:delayed_jobs, :locked_by)
+  end
+end