Log all SQL queries in Magento

In Varien_Db_Adapter_Pdo_Mysql

Magento : lib/varien/Db/Adapter/Pdo/Mysql.php

set

protected $_debug               = true;
protected $_logAllQueries       = true;

and (if nor already there) create the folder defined in

protected $_debugFile           = 'var/debug/sql.txt';

Give read / write permission