Add missing index for device_perf (#11974)

Makes the latency page load faster in case of a large device_perf table

database/migrations/2020_07_29_143221_add_device_perf_index.php

@@ -0,0 +1,32 @@
+<?php
+
+use Illuminate\Database\Migrations\Migration;
+use Illuminate\Database\Schema\Blueprint;
+use Illuminate\Support\Facades\Schema;
+
+class AddDevicePerfIndex extends Migration
+{
+    /**
+     * Run the migrations.
+     *
+     * @return void
+     */
+    public function up()
+    {
+        Schema::table('device_perf', function (Blueprint $table) {
+            $table->index(['device_id', 'timestamp']);
+        });
+    }
+
+    /**
+     * Reverse the migrations.
+     *
+     * @return void
+     */
+    public function down()
+    {
+        Schema::table('device_perf', function (Blueprint $table) {
+            $table->dropIndex(['device_id', 'timestamp']);
+        });
+    }
+}