[MySQL] 使用force index强制使用索引

1.在测试一个按照时间的范围查询时,尽管增加了索引,发现使用不到索引,可以使用这个来强制使用索引

测试过程为,创建下面的表,以及创建了联合索引

create table delay_delete_users(
id int auto_increment, 
email_id int not null default 0 comment 'email表id',
email varchar(50) not null default '' comment '邮箱前缀',
entid int not null default 0 comment '企业id',
default_domain  varchar(50) not null default '' comment '默认域',
delete_time timestamp comment '删除时间',
clear tinyint not null default 0 comment '0未处理,1已清空',
primary key (id),
key email_entid(email,entid),
key delete_time(delete_time,clear)
)engine innodb;

插入测试数据,进行explain查询

insert into `delay_delete_users` (email,entid,default_domain,delete_time)value('shihan2',23684,'appdev.sinanet.com','2019-12-10 15:49:16');
insert into `delay_delete_users` (email,entid,default_domain,delete_time,clear)value('shihan2',23684,'appdev.sinanet.com','2019-12-10 15:49:16',1);
insert into `delay_delete_users` (email,entid,default_domain,delete_time,clear)value('shihan2',23684,'appdev.sinanet.com','2019-12-12 15:49:16',1);

explain select * from delay_delete_users where delete_time<'2019-12-12' and clear=0; 索引没有使用到,还是进行的全表扫描,看那个扫描行数rows

+----+-------------+--------------------+------------+------+---------------+------+---------+------+------+----------+-------------+
| id | select_type | table              | partitions | type | possible_keys | key  | key_len | ref  | rows | filtered | Extra       |
+----+-------------+--------------------+------------+------+---------------+------+---------+------+------+----------+-------------+
|  1 | SIMPLE      | delay_delete_users | NULL       | ALL  | delete_time   | NULL | NULL    | NULL |    7 |    14.29 | Using where |

explain select * from delay_delete_users force index(delete_time)  where delete_time<'2019-12-12' and clear=0;使用到了索引

+----+-------------+--------------------+------------+-------+---------------+-------------+---------+------+------+----------+-----------------------+
| id | select_type | table              | partitions | type  | possible_keys | key         | key_len | ref  | rows | filtered | Extra                 |
+----+-------------+--------------------+------------+-------+---------------+-------------+---------+------+------+----------+-----------------------+
|  1 | SIMPLE      | delay_delete_users | NULL       | range | delete_time   | delete_time | 4       | NULL |    3 |    14.29 | Using index condition |