def get_pages_by_category(wikidb, category, ns=None):
  query = '''
    SELECT page_namespace, page_title, page_id, cl_sortkey, cl_timestamp
  ''' + '''
    FROM ''' + Page.table_name + '''  /* SLOW_OK */
    JOIN categorylinks ON page_id = cl_from
    WHERE cl_to = %(category)s
  '''

  params = {'category': category}
  if ns is not None:
    query += ' AND page_namespace = %(ns)s'
    params['ns'] = ns

  with wikidb.cursor() as cursor:
    cursor.execute(query, params)
    while True:
      results = cursor.fetchmany(PAGE_BY_CAT_ROWS)
      if not results:
        break
      for result in results:
        yield Page(**result)