why left join can not use unique index?

[h h]

i have two tables saint_customer(table A) ,db_users(table B) ,and have a unique index on each join column,but i am very confused ,why i use table A right join table B ,A's unique index can be used,but when i use table A left join B ,B's unique index can not be used?

root@localhost:qa_ucenter 10:25:59>show create table saint_customer\G
*************************** 1. row ***************************
Table: saint_customer
Create Table: CREATE TABLE `saint_customer` (
`id` int(11) NOT NULL AUTO_INCREMENT COMMENT '主键，客户ID',
`gmt_create` datetime NOT NULL COMMENT '创建时间，入库时间',
`creator` int(11) NOT NULL COMMENT '创建人ID',
`gmt_modified` datetime DEFAULT NULL COMMENT '更新时间',
`modifier` int(11) NOT NULL COMMENT '更新人ID',
`is_deleted` char(1) CHARACTER SET utf8 DEFAULT 'N' COMMENT '是否删除,Y删除，N未删除',
`contacts_id` int(11) DEFAULT NULL COMMENT '主联系人ID',
`province` int(11) DEFAULT '-100' COMMENT '省',
`city` int(11) DEFAULT '-100' COMMENT '市',
`district` int(11) DEFAULT '-100' COMMENT '区',
`street` int(11) DEFAULT '-100' COMMENT '街道',
`address` varchar(256) CHARACTER SET utf8 DEFAULT NULL COMMENT '客户来源',
`zipcode` int(10) DEFAULT NULL COMMENT '邮编',
`industry` varchar(64) CHARACTER SET utf8 DEFAULT NULL COMMENT '产业',
`annual_income` varchar(20) COLLATE utf8_bin DEFAULT NULL COMMENT '年产值',
`ownership` int(2) DEFAULT NULL COMMENT '公司性质:1合资 2国企 3民营 4私营 5个体',
`rating` varchar(64) CHARACTER SET utf8 DEFAULT NULL COMMENT '评分，成熟度',
`origin` int(2) DEFAULT NULL COMMENT '客户来源:1直销膜拜 2客户介绍 3 主动来电',
`chain_store_num` int(11) DEFAULT NULL COMMENT '连锁门店数',
`head_count` varchar(255) CHARACTER SET utf8 DEFAULT NULL COMMENT '员工数',
`company_name` varchar(333) CHARACTER SET utf8 DEFAULT NULL COMMENT '客户公司名',
`contacts_name` varchar(255) CHARACTER SET utf8 DEFAULT NULL COMMENT '联系人姓名',
`contacts_mobile` varchar(25) COLLATE utf8_bin DEFAULT NULL,
`qq` varchar(15) CHARACTER SET utf8 DEFAULT NULL COMMENT 'qq号',
`weixin` varchar(15) CHARACTER SET utf8 DEFAULT NULL COMMENT '微信号',
`imgs` varchar(4096) COLLATE utf8_bin DEFAULT NULL COMMENT '门店图片地址，多个地址用半角逗号给开',
`comment` text COLLATE utf8_bin COMMENT '公司备注',
`introduction` varchar(10000) COLLATE utf8_bin DEFAULT NULL COMMENT '门店简介',
`technical_grade` varchar(10) COLLATE utf8_bin DEFAULT NULL COMMENT '技术等级：初级，中级，高级',
`business_time_begin` time DEFAULT NULL COMMENT '门店开始营业时间',
`business_time_end` time DEFAULT NULL COMMENT '门店结束营业时间',
`user_global_id` varchar(60) COLLATE utf8_bin DEFAULT NULL COMMENT '全局同步ID',
`app_reg` varchar(32) COLLATE utf8_bin DEFAULT NULL COMMENT '注册系统来源',
`store_area` varchar(32) COLLATE utf8_bin DEFAULT NULL COMMENT '门店面积',
`primary_business` varchar(4096) COLLATE utf8_bin DEFAULT NULL COMMENT '主营项目',
`parent_id` int(11) DEFAULT NULL COMMENT '主客户的id',
`store_type` varchar(32) COLLATE utf8_bin DEFAULT NULL COMMENT '门店类型',
`store_district` varchar(255) COLLATE utf8_bin DEFAULT NULL COMMENT '门店位置',
`is_clean` varchar(255) COLLATE utf8_bin DEFAULT NULL COMMENT '门店是否干净',
`company_formal_name` varchar(100) COLLATE utf8_bin NOT NULL DEFAULT '' COMMENT '门店标准名称',
PRIMARY KEY (`id`),
UNIQUE KEY `IDX_CONTACTS_MOBILE` (`contacts_mobile`),
KEY `IDX_GMT_CREATE` (`gmt_create`),
KEY `IDX_DISTRICT` (`district`),
KEY `IDX_CITY` (`city`),
KEY `IDX_PROVINCE` (`province`),
KEY `IDX_COMPANY_NAME` (`company_name`(255)),
KEY `IDX_CONTACTS_NAME` (`contacts_name`)
) ENGINE=InnoDB AUTO_INCREMENT=603973 DEFAULT CHARSET=utf8 COLLATE=utf8_bin COMMENT='客户表'
1 row in set (0.00 sec)


root@localhost:qa_ucenter 10:26:11>show create table db_users\G
*************************** 1. row ***************************
Table: db_users
Create Table: CREATE TABLE `db_users` (
`user_id` mediumint(8) unsigned NOT NULL AUTO_INCREMENT,
`client_id` int(11) NOT NULL COMMENT '所属门店id',
`client_num` varchar(128) NOT NULL COMMENT '客户编号（来自于CRM）',
`client_name` varchar(255) NOT NULL COMMENT '客户名称（来自于CRM）',
`sale_id` int(11) NOT NULL DEFAULT '0' COMMENT '与该客户绑定的销售员',
`bind_time` int(11) NOT NULL DEFAULT '0' COMMENT '销售人员跟客户绑定时间（来自于CRM）',
`is_bind_delete` tinyint(1) NOT NULL DEFAULT '0' COMMENT '绑定关系十分解除（0：未解除 1：已解除）',
`email` varchar(60) DEFAULT NULL,
`user_name` varchar(60) NOT NULL DEFAULT '',
`user_title` varchar(100) DEFAULT NULL COMMENT '名称',
`contact` varchar(20) DEFAULT NULL COMMENT '联系人',
`province` int(11) DEFAULT NULL COMMENT '省份',
`city` int(11) DEFAULT NULL COMMENT '城市',
`district` int(11) unsigned NOT NULL DEFAULT '0',
`street` int(11) NOT NULL DEFAULT '0' COMMENT '街道',
`address` varchar(200) DEFAULT NULL COMMENT '详细地址',
`password` varchar(32) NOT NULL DEFAULT '',
`question` varchar(255) DEFAULT NULL,
`answer` varchar(255) DEFAULT NULL,
`sex` tinyint(1) unsigned NOT NULL DEFAULT '0',
`birthday` date NOT NULL DEFAULT '1970-01-01',
`user_money` decimal(10,2) NOT NULL DEFAULT '0.00',
`frozen_money` decimal(10,2) NOT NULL DEFAULT '0.00',
`pay_points` int(10) unsigned NOT NULL DEFAULT '0',
`rank_points` int(10) unsigned NOT NULL DEFAULT '0',
`address_id` mediumint(8) unsigned NOT NULL DEFAULT '0',
`reg_time` int(10) unsigned NOT NULL DEFAULT '0',
`last_ip` varchar(15) DEFAULT NULL,
`last_login` int(11) unsigned NOT NULL DEFAULT '0',
`last_time` datetime NOT NULL DEFAULT '1970-01-01 12:00:00',
`visit_count` smallint(5) unsigned NOT NULL DEFAULT '0',
`user_rank` tinyint(3) unsigned NOT NULL DEFAULT '0',
`is_special` tinyint(3) unsigned NOT NULL DEFAULT '1' COMMENT '1：代表门店主帐号0：代表副帐号',
`ec_salt` varchar(10) DEFAULT NULL,
`salt` varchar(10) NOT NULL DEFAULT '0',
`parent_id` mediumint(9) NOT NULL DEFAULT '0',
`flag` tinyint(3) unsigned NOT NULL DEFAULT '0',
`alias` varchar(60) NOT NULL DEFAULT '0',
`msn` varchar(60) NOT NULL DEFAULT '0',
`qq` varchar(20) NOT NULL DEFAULT '0',
`office_phone` varchar(20) NOT NULL DEFAULT '0',
`home_phone` varchar(20) NOT NULL DEFAULT '0',
`mobile_phone` varchar(20) NOT NULL,
`verify` char(4) NOT NULL COMMENT '手机验证码',
`is_validated` tinyint(3) unsigned NOT NULL DEFAULT '0',
`credit_line` decimal(10,2) unsigned DEFAULT NULL,
`passwd_question` varchar(50) DEFAULT NULL,
`passwd_answer` varchar(255) DEFAULT NULL,
`zipcode` varchar(11) NOT NULL DEFAULT '000000' COMMENT '邮政编码',
`device_id` varchar(128) NOT NULL COMMENT '客户端设备号',
`short_name` varchar(128) NOT NULL COMMENT '客户简称',
`province_name` varchar(64) NOT NULL DEFAULT '' COMMENT '省份名称',
`city_name` varchar(64) NOT NULL DEFAULT '' COMMENT '城市名称',
`district_name` varchar(64) NOT NULL DEFAULT '' COMMENT '地区名称',
`staff_no` varchar(64) NOT NULL DEFAULT '0' COMMENT '销售员工号',
`comment` varchar(255) DEFAULT NULL,
`user_type` tinyint(4) NOT NULL DEFAULT '1' COMMENT '用户类型：1门店、2普通用户 ',
`verify_status` tinyint(4) NOT NULL DEFAULT '0' COMMENT '认证状态（0:缺省－未认证；1:认证中；2认证成功；－1:认证失败）',
`verify_time` datetime DEFAULT NULL COMMENT '认证时间',
`gmt_modifed` datetime DEFAULT NULL COMMENT '修改时间',
`customer_id` int(11) DEFAULT '0' COMMENT 'CRM的客户ID',
`user_global_id` varchar(32) NOT NULL DEFAULT '' COMMENT 'crm同步主键',
`app_reg` varchar(32) DEFAULT NULL COMMENT '用户创建来源',
`business_licence` varchar(100) DEFAULT '' COMMENT '营业执照',
`verify_feedback` varchar(300) DEFAULT '' COMMENT '小秘书反馈内容',
PRIMARY KEY (`user_id`),
UNIQUE KEY `unique_user_global_id` (`user_global_id`),
UNIQUE KEY `idx_username` (`user_name`),
KEY `email` (`email`),
KEY `parent_id` (`parent_id`),
KEY `flag` (`flag`),
KEY `customer_id_index` (`customer_id`),
KEY `customer_id1_index` (`customer_id`) COMMENT 'dd',
KEY `idx_gmt_modifed` (`gmt_modifed`)
) ENGINE=InnoDB AUTO_INCREMENT=150482 DEFAULT CHARSET=utf8 COMMENT='客户基础信息表'


root@localhost:qa_ucenter 10:27:38>explain select a.*,b.* from saint_customer a right join db_users b on a.contacts_mobile=b.user_name\G
*************************** 1. row ***************************
id: 1
select_type: SIMPLE
table: b
type: ALL
possible_keys: NULL
key: NULL
key_len: NULL
ref: NULL
rows: 119068
Extra:
*************************** 2. row ***************************
id: 1
select_type: SIMPLE
table: a
type: ref
possible_keys: IDX_CONTACTS_MOBILE
key: IDX_CONTACTS_MOBILE
key_len: 78
ref: qa_ucenter.b.user_name
rows: 1
Extra:
2 rows in set (0.00 sec)

root@localhost:qa_ucenter 10:28:02>explain select a.*,b.* from saint_customer a left join db_users b on a.contacts_mobile=b.user_name\G
*************************** 1. row ***************************
id: 1
select_type: SIMPLE
table: a
type: ALL
possible_keys: NULL
key: NULL
key_len: NULL
ref: NULL
rows: 50561
Extra:
*************************** 2. row ***************************
id: 1
select_type: SIMPLE
table: b
type: ALL
possible_keys: idx_username
key: NULL
key_len: NULL
ref: NULL
rows: 119068
Extra:
2 rows in set (0.00 sec)