In Varien_Db_Adapter_Pdo_Mysql
Magento 1.4 : 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
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