In the past I’ve used the general log in mysql.  That can be a great source of information, but, depending on the version of mysql, it can be a pain to set up.  Other times I’ve inserted logging code into various db classes in order to print out queries that are being run, but I always have to find the right place again.  But today I stumbled on this blog entry - http://blog.nexcess.net/2011/04/15/logging-database-queries-in-magento/ that spells out exactly how you can turn on some (as far as I know) undocumented logging functionality that is built right in to the Magento data access layer.  Kudos to the author and thanks for sharing!

The Change

1. Open the file <magentoroot>/lib/Varien/Db/Adapter/Pdo/Mysql.php.

2. Add the following lines:

$code = 'SQL: ' . $sql . "\r\n";
if ($bind) {
     $code .= 'BIND: ' . print_r($bind, true) . "\r\n";
}
$this->_debugWriteToFile("[".date('Y-m-d H:i:s')."] ".$code);

Add it to the “query” function as shown below:

public function query($sql, $bind = array())
{
     $this->_debugTimer();

     try {
          $sql = (string)$sql;
          if (strpos($sql, ':') !== false || strpos($sql, '?') !== false) {
               $this->_bindParams = $bind;
               $sql = preg_replace_callback('#(([\'"])((\\2)|((.*?[^\\\\])\\2)))#', array($this, 'proccessBindCallback'), $sql);
               $bind = $this->_bindParams;
          }

          $code = 'SQL: ' . $sql . "\r\n";
          if ($bind) {
               $code .= 'BIND: ' . print_r($bind, true) . "\r\n";
          }
          $this->_debugWriteToFile("[".date('Y-m-d H:i:s')."] ".$code);

          $result = parent::query($sql, $bind);
     }
     catch (Exception $e) {
          $this->_debugStat(self::DEBUG_QUERY, $sql, $bind);
          $this->_debugException($e);
     }
     $this->_debugStat(self::DEBUG_QUERY, $sql, $bind, $result);
     return $result;
 }

The Log

By default your sql will be logged to:

<magentoroot>/var/debug/sql.txt

Limitations

1. As mentioned above, the file will likely be overwritten during a Magento upgrade.

2. There’s nothing that will turn logging off (except a Magento upgrade).  So you need to be careful that this is a temporary setting for debugging only.

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