Apply on existed nodes

If create computed field on existed nodes, you must update all materials for compute values for new fields.
It is possible with this snippet:

<?php
  $query = db_select('node', 'n')
    ->fields('n', array('nid'))
    ->condition('n.type', 'product');
  $result = $query->execute();
  foreach ($result as $row) {
    if ($node = node_load($row->nid)) {
      node_save($node);
    }
  }
?>

If nodes a lot then need wrap this in batch.