In /lib/Varien/Db/Adapter/Pdo/Mysql.php change this property to true:
/**
* Write SQL debug data to file
*
* @var bool
*/
protected $_debug = false;
To enable logging for queries slower than a certain time, alter this property:
/**
* Minimum query duration time to be logged
*
* @var float
*/
protected $_logQueryTime = 0.05;
To log all queries, set this property to true:
/**
* Log all queries (ignored minimum query duration time)
*
* @var bool
*/
protected $_logAllQueries = false;
Optionally, you can log the call stack, to do so change this property to true:
/**
* Add to log call stack data (backtrace)
*
* @var bool
*/
protected $_logCallStack = false;
To change to location of the log file, alter this property:
/**
* Path to SQL debug data log
*
* @var string
*/
protected $_debugFile = 'var/debug/pdo_mysql.log';
Posted by Mike Whitby to Magento (2012-03-01 10:36)