MYSQL 条件字段有索引但是不走索引的场景

use muke;                       /* 使用muke这个database */

drop table if exists t1;        /* 如果表t1存在则删除表t1 */

CREATE TABLE `t1` (             /* 创建表t1 */
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `a` varchar(20) DEFAULT NULL,
  `b` int(20) DEFAULT NULL,
  `c` datetime NOT NULL DEFAULT CURRENT_TIMESTAMP,  
  PRIMARY KEY (`id`),
  KEY `idx_a` (`a`) USING BTREE,
  KEY `idx_b` (`b`) USING BTREE,
  KEY `idx_c` (`c`) USING BTREE
) ENGINE=InnoDB  DEFAULT CHARSET=utf8mb4;

drop procedure if exists insert_t1; /* 如果存在存储过程insert_t1，则删除 */
delimiter ;;
create procedure insert_t1()        /* 创建存储过程insert_t1 */
begin
  declare i int;                    /* 声明变量i */
  set i=1;                          /* 设置i的初始值为1 */
  while(i<=10000)do                 /* 对满足i<=10000的值进行while循环 */
    insert into t1(a,b) values(i,i);  /* 写入表t1中a、b两个字段，值都为i当前的值 */
    set i=i+1;                        /* 将i加1 */
  end while;
end;;
delimiter ;
call insert_t1();                    /* 运行存储过程insert_t1 */

update t1 set c = '2019-05-22 00:00:00';  /* 更新表t1的c字段，值都为'2019-05-22 00:00:00' */
update t1 set c = '2019-05-21 00:00:00' where id=10000;     /* 将id为10000的行的c字段改为与其它行都不一样的数据，以便后面实验使用 */

select * from t1 where date(c) ='2019-05-21';

select * from t1 where c>='2019-05-21 00:00:00' and c<='2019-05-21 23:59:59';

select user_name,tele_phone from user_info where tele_phone =11111111111; /* SQL 1 */

select user_name,tele_phone from user_info where tele_phone ='11111111111';

select * from t1 where a like '%1111%';

select * from t1 where a like '1111%';

select * from t1 where b>=1 and b <=2000;

select * from t1 where b>=1 and b <=1000;
select * from t1 where b>=1001 and b <=2000;

select * from t1 where b-1 =1000;

select * from t1 where b =1000 + 1;