Magento 2 log database queries

At least in newer versions (looking at a 2.2.1 here and now) you can do

bin/magento dev:query-log:enable

and have extensive logs in var/debug/db.log. Dont forget to switch logging off again with

bin/magento dev:query-log:disable

.

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.

To set logAllQueries=true you can add the following code to app/etc/di.xml to change __construct() parameters of Magento\Framework\DB\Logger\File:

<preference for="Magento\Framework\DB\LoggerInterface" type="Magento\Framework\DB\Logger\File"/>
<type name="Magento\Framework\DB\Logger\File">
    <arguments>
        <argument name="logAllQueries" xsi:type="boolean">true</argument>
    </arguments>
</type>

You can also change the other parameters $debugFile, $logQueryTime and $logCallStack in that way.