All classes that implement ExtendableInterface

class \Drupal\Core\Database\Driver\mysql\Select
class \Drupal\Core\Database\Driver\pgsql\Select
class \Drupal\Core\Database\Driver\sqlite\Select
class \Drupal\Core\Database\Query\PagerSelectExtender
class \Drupal\Core\Database\Query\Select implements SelectInterface
class \Drupal\Core\Database\Query\SelectExtender implements SelectInterface
class \Drupal\Core\Database\Query\TableSortExtender
class \Drupal\search\SearchQuery
class \Drupal\search\ViewsSearchQuery