why the two query has a different execute plan?

[h h]

case one: ucenter.uc_shop can use index uk_userid

root@localhost:dw2 04:50:04>explain select us.id, us.user_id, dcty.id,dcty.is_deleted, dcty.tag_name from ucenter.uc_shop as us left join tqdw.dw_customer_type_yqx as dcty on us.id=dcty.customer_id\G
*************************** 1. row ***************************
id: 1
select_type: SIMPLE
table: us
type: index
possible_keys: NULL
key: uk_userid
key_len: 5
ref: NULL
rows: 272132
Extra: Using index
*************************** 2. row ***************************
id: 1
select_type: SIMPLE
table: dcty
type: ref
possible_keys: customer_id
key: customer_id
key_len: 5
ref: ucenter.us.id
rows: 1
Extra:
2 rows in set (0.00 sec)

case two: ucenter.uc_shop can not use index uk_userid

root@localhost:dw2 04:49:27>explain select * from ucenter.uc_shop as us left join tqdw.dw_customer_type_yqx as dcty on us.id=dcty.customer_id\G *************************** 1. row ***************************
id: 1
select_type: SIMPLE
table: us
type: ALL
possible_keys: NULL
key: NULL
key_len: NULL
ref: NULL
rows: 272132
Extra:
*************************** 2. row ***************************
id: 1
select_type: SIMPLE
table: dcty
type: ref
possible_keys: customer_id
key: customer_id
key_len: 5
ref: ucenter.us.id
rows: 1
Extra:
2 rows in set (0.00 sec)

root@localhost:dw2 04:53:17>show create table ucenter.uc_shop\G
*************************** 1. row ***************************
Table: uc_shop
Create Table: CREATE TABLE `uc_shop` (
`id` int(11) unsigned NOT NULL AUTO_INCREMENT COMMENT '主键ID',
`gmt_create` datetime NOT NULL DEFAULT '1970-01-01 12:00:00' COMMENT '创建时间',
`creator` int(11) NOT NULL DEFAULT '0' COMMENT '创建人ID',
`gmt_modified` datetime DEFAULT '1970-01-01 12:00:00' COMMENT '更新时间',
`modifier` int(11) NOT NULL DEFAULT '0' COMMENT '更新人ID',
`is_deleted` char(1) NOT NULL DEFAULT 'N' COMMENT '是否删除,Y删除,N未删除',
`company_name` varchar(300) NOT NULL DEFAULT '' COMMENT '门店名称',
`company_formal_name` varchar(300) DEFAULT '' COMMENT '门店营业执照名称',
`client_num` varchar(128) DEFAULT '' COMMENT '电商系统客户编码，主要在金蝶系统中使用',
`user_id` int(11) unsigned DEFAULT NULL COMMENT '电商系统原有ID',
`verify_status` int(11) NOT NULL DEFAULT '0' COMMENT '认证状态:-1认证失败，0未认证，1认证中，2认证通过',
`shop_type` int(10) unsigned NOT NULL DEFAULT '2' COMMENT '客户交易类型:1表示B用户，2表示C用户，3表示KA客户',
`shop_status` int(11) NOT NULL DEFAULT '0' COMMENT '门店状态:-1冻结，0正常',
`user_rank` tinyint(3) unsigned NOT NULL DEFAULT '0' COMMENT '会员等级',
`rank_points` int(10) unsigned NOT NULL DEFAULT '0' COMMENT '等级积分值',
`owner_id` int(10) unsigned NOT NULL DEFAULT '0' COMMENT '门店归属小淘汽的组织id，归属淘汽档口的门店组织id为0',
`app_source` varchar(32) DEFAULT '' COMMENT '系统来源',
`memo` varchar(1024) DEFAULT '' COMMENT '备注',
PRIMARY KEY (`id`),
UNIQUE KEY `uk_userid` (`user_id`) USING BTREE COMMENT 'user_id的唯一索引',
UNIQUE KEY `uk_clientnum` (`client_num`)
) ENGINE=InnoDB AUTO_INCREMENT=321275 DEFAULT CHARSET=utf8 COMMENT='UC门店信息表'
1 row in set (0.00 sec)

root@localhost:dw2 04:55:01>show create table tqdw.dw_customer_type_yqx\G
*************************** 1. row ***************************
Table: dw_customer_type_yqx
Create Table: CREATE TABLE `dw_customer_type_yqx` (
`id` int(11) NOT NULL AUTO_INCREMENT,
`creator` varchar(255) DEFAULT '',
`modifier` varchar(255) DEFAULT '',
`gmt_create` datetime DEFAULT NULL,
`gmt_modified` datetime DEFAULT NULL,
`is_deleted` varchar(2) DEFAULT '',
`user_id` int(11) DEFAULT NULL COMMENT '电商系统原有ID',
`customer_id` int(11) DEFAULT NULL,
`company_name` varchar(333) DEFAULT NULL COMMENT '门店名称',
`contacts_name` varchar(333) DEFAULT NULL,
`company_formal_name` varchar(333) DEFAULT NULL,
`contacts_mobile` varchar(25) DEFAULT NULL,
`tag_key` varchar(100) DEFAULT NULL,
`tag_name` varchar(64) DEFAULT NULL,
`province` int(4) DEFAULT NULL,
`city` int(4) DEFAULT NULL,
`district` int(4) DEFAULT NULL,
`avg_month_amount` decimal(12,2) DEFAULT NULL COMMENT '被指定给云修顾问时间后三月平均采购额',
`dispatch_date_service` datetime DEFAULT NULL COMMENT '云修顾问客户入库时间',
`join_in_time` datetime DEFAULT NULL COMMENT '云修客户签约加盟时间',
PRIMARY KEY (`id`),
UNIQUE KEY `customer_id` (`customer_id`),
KEY `mobile_index` (`contacts_mobile`),
KEY `key_index` (`tag_key`),
KEY `user_index` (`user_id`)
) ENGINE=InnoDB AUTO_INCREMENT=8882 DEFAULT CHARSET=utf8 COMMENT='云修客户明细表'
1 row in set (0.00 sec)