Improved deferred bindings query performance (#3014)

Credit to @MatheusNaldi. Fixes #2877

modules/system/database/migrations/2017_08_04_121309_Db_Deferred_Bindings_Add_Index_Session.php

@@ -0,0 +1,31 @@
+<?php
+
+use Illuminate\Database\Schema\Blueprint;
+use Illuminate\Database\Migrations\Migration;
+
+class DbDeferredBindingsAddIndexSession extends Migration
+{
+    /**
+     * Run the migrations.
+     *
+     * @return void
+     */
+    public function up()
+    {
+        Schema::table('deferred_bindings', function (Blueprint $table) {
+            $table->index('session_key');
+        });
+    }
+
+    /**
+     * Reverse the migrations.
+     *
+     * @return void
+     */
+    public function down()
+    {
+        Schema::table('deferred_bindings', function (Blueprint $table) {
+            $table->dropIndex(['session_key']);
+        });
+    }
+}