Database update with Batch API template

mymodule.php

<?php
/**
 * @file mymodule.install
 * Install hooks for mymodule.
 */

/**
 * Description of what the database update does. Do not use "Implements..." since it adds
 * zero information and makes reading the list of pending database updates in Drush harder
 * to read.
 */
function mymodule_update_7100(&$sandbox) {
  // Number of items to process per loop. This may vary depending on the server's
  // resources. It is better to keep it to a low number so we never hit a PHP
  // memory limit.
  $items_per_loop = 10;

  // Initial setup.
  if (!isset($sandbox['current'])) {
    // This piece of code will run on the first loop. Here we will
    // set the following:
    // 1. $sandbox['current']: defines the current chunk of items to process.
    $sandbox['current'] = 0;
    // 2. $sandbox['chunks']: contains the list of chunks of ids to process.
    $nids = db_query("select nid from {node} where type = 'show'")->fetchCol();
    if (empty($nids)) {
      watchdog('mymodule', 'There are no items to process.', array(), WATCHDOG_NOTICE);
      return;
    }
    // Split the list in chunks to be processed in each loop.
    $sandbox['chunks'] = array_chunk($nids, $items_per_loop);
    // 3. $sandbox['total']: contains the total amount of chunks to process.
    $sandbox['total'] = count($sandbox['chunks']);
  }

  // Extract the current chunk of ids.
  $nids = $sandbox['chunks'][$sandbox['current']];

  // Process the list of ids.
  // [Add your code here]

  // Update batch status.
  $sandbox['current']++;
  $sandbox['#finished'] = ($sandbox['current'] >= $sandbox['total']) ? 1 : ($sandbox['current'] / $sandbox['total']);
}