Can failure to increase new user's UID consistently by one be related to this line inside user.module file?

Question

I have noticed at my Drupal 7 that there are gaps between the uid's of new users. The UID of a new user should increase by one from the highest assigned UID in database. However, I have noticed gaps of even a thousand.

Inside user.module file, there is the following code:

// Allow 'uid' to be set by the caller. There is no danger of writing an
      // existing user as drupal_write_record will do an INSERT.
      if (empty($account->uid)) {
        $account->uid = db_next_id(db_query('SELECT MAX(uid) FROM {users}')->fetchField());
      }

I have made the QB query read like this:

$account->uid = db_next_id(db_query('SELECT MAX(uid) FROM users')->fetchField());

It appears that this solves the issue. My question, is there really a mistake inside the core user.module file? Is it safe to modify the file the way I did? Thanks.

UPDATE:

The above does not make a difference after all. I have changed the line to this:

$account->uid = db_query('SELECT MAX(uid) FROM users')->fetchField() + 1;

This works with no reference to db_next_id. For me it is very important to increase the uid's always by one and this is one way to do it.

Answer 1

uid and other identifiers are not sequential since Drupal 7. Identifiers are generated using a sequence API, see this issue: https://www.drupal.org/node/356074

If you check the db_next_id you'll see that the number passed as argument is just de minimum id that is considered valid.

See also this question and answer: What could cause uids to not be numbered consecutively?