Update an existing custom schema in Drupal 8

In this tutorial, we will see how to add field to custom schema in Drupal 8 without losing data using a hook_update_N().

Example how to use hook_update_N() :

<?php

use Drupal\Core\Database\Database;

/**
 * @param $sandbox
 */
function mymodule_update_90002(&$sandbox) {
  $spec = [
    'type' => 'varchar',
    'description' => "New Col",
    'length' => 20,
    'not null' => FALSE,
  ];
  $schema = Database::getConnection()->schema();
  $schema->addField('mymodule', 'newcol', $spec);
  $schema->addIndex('mymodule', 'newcol',['newcol'],$spec);

  return t('new col added to mymodule table.');
}

After this, running drush updb or running update.php from the admin interface should detect your hook_update_N() and it should add your new field.