Magento 2 log database queries

you can add in one of your modules in the di.xml file this:

<preference for="Magento\Framework\DB\LoggerInterface" type="Magento\Framework\DB\Logger\File"/>

The Magento\Framework\DB\Adapter\Pdo\Mysql class that is used to run the actual queries has a logger member Magento\Framework\DB\LoggerInterface.
By default, the preference for this dependency is set in app/etc/di.xml

<preference for="Magento\Framework\DB\LoggerInterface" type="Magento\Framework\DB\Logger\Quiet"/>

this Magento\Framework\DB\Logger\Quiet does nothing.

<?php
/**
 * Copyright © 2015 Magento. All rights reserved.
 * See COPYING.txt for license details.
 */
namespace Magento\Framework\DB\Logger;

class Quiet implements \Magento\Framework\DB\LoggerInterface
{
    /**
     * {@inheritdoc}
     */
    public function log($str)
    {
    }

    /**
     * {@inheritdoc}
     */
    public function logStats($type, $sql, $bind = [], $result = null)
    {
    }

    /**
     * {@inheritdoc}
     */
    public function critical(\Exception $e)
    {
    }

    /**
     * {@inheritdoc}
     */
    public function startTimer()
    {
    }
}

change the preference to Magento\Framework\DB\Logger\File and you should see the queries logged in var/debug/db.log.
Magento comes with these 2 loggers (Quiet and File) buy default, but you can create your own in case you need a different way of logging queries.