使用覆盖索引选择某一天的记录

我想运行这些查询:

select url from weixin_kol_status where created_at>'2015-12-11 00:00:00' and created_at<'2015-12-11 23:59:59';

和

select url from weixin_kol_status where userid in ('...') and created_at>'2015-12-11 00:00:00' and created_at<'2015-12-11 23:59:59';

...使用此表定义:

CREATE TABLE `weixin_kol_status` (
  `id` bigint(20) NOT NULL AUTO_INCREMENT,
  `url` varchar(512) NOT NULL,
  `created_at` datetime NOT NULL,
  `title` varchar(512) NOT NULL DEFAULT '',
  `text` text,
  `attitudes_count` int(11) NOT NULL DEFAULT '0',
  `readcount` int(11) NOT NULL DEFAULT '0',
  `reposts_count` int(11) NOT NULL DEFAULT '0',
  `comments_count` int(11) NOT NULL DEFAULT '0',
  `userid` varchar(32) NOT NULL,
  `screen_name` varchar(32) NOT NULL,
  `type` tinyint(4) NOT NULL DEFAULT '0',
  `ext_data` text,
  `is_topline` tinyint(4) NOT NULL DEFAULT '0',
  `is_business` tinyint(4) NOT NULL DEFAULT '0',
  PRIMARY KEY (`id`),
  UNIQUE KEY `idx_url` (`url`(255)),
  KEY `idx_userid` (`userid`),
  KEY `idx_name` (`screen_name`),
  KEY `idx_created_at` (`created_at`)
) ENGINE=InnoDB AUTO_INCREMENT=328727437 DEFAULT CHARSET=utf8 |


rows = 328727437;

查询需要几分钟.如何优化查询？我怎样才能使用覆盖指数？

执行计划是:

explain select id from weixin_kol_status where created_at>='2015-12-11 00:00:00' and created_at<='2015-12-11 23:59:59'\G;
    *************************** 1. row ***************************
               id: 1
      select_type: SIMPLE
            table: weixin_kol_status
             type: range
    possible_keys: idx_created_at
              key: idx_created_at
          key_len: 5
              ref: NULL
             rows: 1433704
            Extra: Using where; Using index
    1 row in set (0.00 sec)

和

explain select id from weixin_kol_status where created_at='2015-12-11 00:00:00'\G; 
*************************** 1. row ***************************
           id: 1
  select_type: SIMPLE
        table: weixin_kol_status
         type: ref
possible_keys: idx_created_at
          key: idx_created_at
      key_len: 5
          ref: const
         rows: 1
        Extra: Using index
1 row in set (0.00 sec)

但为什么第一个查询Extra: Using where; Using index,第二个查询Extra: Using index.第一个查询没有使用覆盖索引吗？