Is there any core function to get uid from username in Drupal? Or I should perform a db query? my field is a textfield with '#autocomplete_path' equal to 'user/autocomplete'
You can use the user_load
function. See http://api.drupal.org/api/function/user_load/6
In particular see http://api.drupal.org/api/function/user_load/6#comment-6439
So you would do something like this:
// $name is the user name
$account = user_load(array('name' => check_plain($name)));
// uid is now available as $account->uid