In order to modify the where condition of a views query we use hook_views_query_alter. This hook should be placed in your custom module.

 //Here, we are going to modify the query, which is an element of the $query variable
function hook_views_query_alter(&$view, &$query) {
  // Example assuming a view with an exposed filter on node title.
  // If the input for the title filter is a positive integer, filter against
  if ($view->name == 'my_view' ) {
    foreach ($query->where as &$condition_group) {
      foreach ($condition_group['conditions'] as &$condition) {
        if ($condition['field'] == 'node.title') {
          $condition = array(
            'field' => 'node.nid',
            'value' => $view->exposed_raw_input['title'],              // can change value 
            'operator' => '=',           // can change operature here if required
          );
        }
      }
    }
  }
}