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

db/structure.sql

@@ -5089,6 +5089,20 @@ CREATE INDEX index_comments_on_creator_ip_addr ON public.comments USING btree (c
 CREATE INDEX index_comments_on_post_id ON public.comments USING btree (post_id);
 
 
+--
+-- Name: index_delayed_jobs_on_locked_at; Type: INDEX; Schema: public; Owner: -
+--
+
+CREATE INDEX index_delayed_jobs_on_locked_at ON public.delayed_jobs USING btree (locked_at);
+
+
+--
+-- Name: index_delayed_jobs_on_locked_by; Type: INDEX; Schema: public; Owner: -
+--
+
+CREATE INDEX index_delayed_jobs_on_locked_by ON public.delayed_jobs USING btree (locked_by);
+
+
 --
 -- Name: index_delayed_jobs_on_run_at; Type: INDEX; Schema: public; Owner: -
 --
@@ -7530,6 +7544,7 @@ INSERT INTO "schema_migrations" (version) VALUES
 ('20190109210822'),
 ('20190129012253'),
 ('20190712174818'),
-('20190827013252');
+('20190827013252'),
+('20190827014726');