What could cause uids to not be numbered consecutively?

Question

I just noticed something weird in a site I'm working on. The first account is user/1 as usual. A second account I created two weeks later is user/20, and a third one a couple days later is user/32. Etc. Two accounts I just created right after each other are numbered consecutively as expected.

There are no custom modules modifying the registration process or anything. I wonder what could cause this, and if there's something I need to fix.

Answer 1

So, it turns out this is indeed normal since Drupal 7: the users table is not auto-increment but uids are created with a sequences API that is shared with actions and batch jobs. Uids can therefore be non-consecutive.

Several reasons for changing back from auto-increment table in D6 are pointed out here by chx.

(From reading chx's response to this blog post)

Answer 2

I noticed the same, I had some gaps between new user ids that even reached at some points 1000 because I was relying on the batch API at my site which shared the same incremental ids for its jobs with the user table. Here is a tested solution that ensures that user id's will always increase by one.

function MYMODULE_user_presave(&$edit, $account, $category) {
  if (isset($account->is_new) && !empty($account->is_new)) {  
   $account->uid = db_query('SELECT MAX(uid) FROM users')->fetchField() + 1;
  }
}