CREATE INDEX index_logs_on_device_id_and_created_at ON public.logs USING btree (device_id, created_at);

db/migrate/20180726165546_add_more_more_log_indexes.rb

@@ -0,0 +1,5 @@
+class AddMoreMoreLogIndexes < ActiveRecord::Migration[5.2]
+  def change
+    add_index :logs, [:device_id, :created_at]
+  end
+end

db/structure.sql

@@ -1827,6 +1827,13 @@ CREATE INDEX index_logs_on_created_at ON public.logs USING btree (created_at);
 CREATE INDEX index_logs_on_device_id ON public.logs USING btree (device_id);
 
 
+--
+-- Name: index_logs_on_device_id_and_created_at; Type: INDEX; Schema: public; Owner: -
+--
+
+CREATE INDEX index_logs_on_device_id_and_created_at ON public.logs USING btree (device_id, created_at);
+
+
 --
 -- Name: index_logs_on_sent_at; Type: INDEX; Schema: public; Owner: -
 --
@@ -2287,6 +2294,7 @@ INSERT INTO "schema_migrations" (version) VALUES
 ('20180716163108'),
 ('20180719143412'),
 ('20180720021451'),
-('20180726145505');
+('20180726145505'),
+('20180726165546');