It is now possible to get the last inserted ID from db_merge().

The accepted resolution was to add a getResult() method to the MergeQuery class.

Example implementation:

  $merge = db_merge('block_custom')
    ->key(array(
      'bid' => $delta,
    ))
    ->fields(array(
      'body' => $edit['body']['value'],
      'info' => $edit['info'],
      'title' => $edit['title'],
      'format' => $edit['body']['format'],
    ));
  $result = $merge->execute();
  if ($result === $merge::STATUS_INSERT) {
    $delta = $merge->getResult();
  }

The return value from $merge->execute() would stay the same, but after execution we could get the result from the $merge object.

Multiple previous reports stated that it was not possible, other than selecting the highest ID from the table, which either requires a table lock or causes a race condition.

Similar reports:
https://www.drupal.org/node/1231730
http://drupal.stackexchange.com/questions/58991/how-to-retrieve-last-mys...

Introduced in branch: 
1.0.x
Introduced in version: 
1.0.0
Impacts: 
Module developers
Related Github Issues: 
https://github.com/backdrop/backdrop-issues/issues/328