MySQL字符集不一致导致索引失效的解决办法

create table customer(
  cid int auto_increment primary key,
  cname varchar(50) not null,
  register_time datetime not null,
  recommender varchar(50) character set utf8
) engine=innodb default charset=utf8mb4;

create unique index uk_customer_cname on customer(cname);

insert into customer(cname, register_time, recommender) values('张三', now(), '');
insert into customer(cname, register_time, recommender) values('李四', now(), '张三'),('王五', now(), '李四');

select c.*
from customer c
join customer r on (c.recommender = r.cname )
where r.cid = 1
and c.register_time between now() - interval 1 day and now();

explain
select c.*
from customer c
join customer r on (c.recommender = r.cname )
where r.cname = '张三'
and c.register_time between now() - interval 1 day and now();

id|select_type|table|partitions|type |possible_keys    |key              |key_len|ref  |rows|filtered|Extra      |
--|-----------|-----|----------|-----|-----------------|-----------------|-------|-----|----|--------|-----------|
 1|SIMPLE     |r    |          |const|uk_customer_cname|uk_customer_cname|202    |const|   1|   100.0|Using index|
 1|SIMPLE     |c    |          |ALL  |                 |                 |       |     |   3|   33.33|Using where|

create index idx_customer_cname on customer(recommender);

show warnings\G
*************************** 1. row ***************************
  Level: Note
   Code: 1003
Message: /* select#1 */ select `hrdb`.`c`.`cid` AS `cid`,`hrdb`.`c`.`cname` AS `cname`,`hrdb`.`c`.`register_time` AS `register_time`,`hrdb`.`c`.`recommender` AS `recommender` from `hrdb`.`customer` `c` join `hrdb`.`customer` `r` where ((`hrdb`.`c`.`register_time` between <cache>((now() - interval 1 day)) and <cache>(now())) and (convert(`hrdb`.`c`.`recommender` using utf8mb4) = '张三'))
1 row in set (0.00 sec)

convert(`hrdb`.`c`.`recommender` using utf8mb4) = '张三')

alter table customer modify column recommender varchar(50) character set utf8mb4;

id|select_type|table|partitions|type |possible_keys     |key               |key_len|ref  |rows|filtered|Extra      |
--|-----------|-----|----------|-----|------------------|------------------|-------|-----|----|--------|-----------|
 1|SIMPLE     |r    |          |const|uk_customer_cname |uk_customer_cname |202    |const|   1|   100.0|Using index|
 1|SIMPLE     |c    |          |ref  |idx_customer_cname|idx_customer_cname|203    |const|   1|   33.33|Using where|