[MySQL] 使用force index強制使用索引

1.在測試一個按照時間的範圍查詢時,儘管增加了索引,發現使用不到索引,可以使用這個來強制使用索引

測試過程為,創建下麵的表,以及創建了聯合索引

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 '郵箱首碼',
entid int not null default 0 comment '企業id',
default_domain  varchar(50) not null default '' comment '預設域',
delete_time timestamp comment '刪除時間',
clear tinyint not null default 0 comment '0未處理,1已清空',
primary key (id),
key email_entid(email,entid),
key delete_time(delete_time,clear)
)engine innodb;

插入測試數據,進行explain查詢

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; 索引沒有使用到,還是進行的全表掃描,看那個掃描行數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 |