How to have a global variable coming from db in symfony template?

How can I have a global variable in symfony template? I did read this but I prefer to fetch parameter from database, I think this service will be loaded on startup before it can fetch anything from db. Is it possible to do a trick to do so?

EDIT: Update in 2019 with Symfony 3.4+ syntax.

Create a Twig extension where you inject the Entity Manager:

Fuz/AppBundle/Twig/Extension/DatabaseGlobalsExtension.php

<?php

namespace Fuz\AppBundle\Twig\Extension;

use Doctrine\ORM\EntityManager;
use Twig\Extension\AbstractExtension;
use Twig\Extension\GlobalsInterface;

class DatabaseGlobalsExtension extends AbstractExtension implements GlobalsInterface
{

   protected $em;

   public function __construct(EntityManager $em)
   {
      $this->em = $em;
   }

   public function getGlobals()
   {
      return [
          'myVariable' => $this->em->getRepository(FuzAppBundle\Entity::class)->getSomeData(),
      ];
   }
}

Register your extension in your Fuz/AppBundle/Resources/config/services.yml:

services:
    _defaults:
        autowire: true
        autoconfigure: true 

    Fuz\AppBundle\Twig\Extension\DatabaseGlobalsExtension: ~

Now you can do the requests you want using the entity manager.

Don't forget to replace paths and namespaces to match with your application.