Drupal 7 Default query search alter

query_search_alter.php

function mymodule_query_alter(QueryAlterableInterface $query){
    $is_search = FALSE;
    foreach ($query->getTables() as $table) {
       if ($table['table'] == 'search_index') {
        $is_search = TRUE;
       }
     }

     if ($is_search) {
        global $language; 
	
        $db_or = db_or();
        $db_or->condition('n.type', 'event', '=');
        $db_or->condition('n.type', 'real_sitio', '=');
        $query->condition($db_or);
	
        $query->condition('n.language' , $language->language, '=');
     } 
  }

This is a bit performance killer so there's a patch for drupal at http://drupal.org/node/1435834 that adds a hook for making the alter directly in the search query:

So finally it would look like:

function mymodule_search_query_search_node_alter(&$query) {
$query->condition('n.type', 'article', '=');
}