MySQL分区字段列有必要再单独建索引吗？

前言

大家都知道对于分区字段必须是主键的一部分，那么建了复合主键之后，是否需要对分许字段再单独添加一个索引呢？有没有效果？来验证一下，下面话不多说了，来一起看看详细的介绍吧。

1、新建表effect_new(以创建时间按月分区)

create table `effect_new` (
 `id` bigint(20) not null auto_increment,
 `type` tinyint(4) not null default '0',
 `timezone` varchar(10) default null,
 `date` varchar(10) not null,
 `hour` varchar(2) default null,
 `position` varchar(200) default null,
 `country` varchar(32) not null,
 `create_time` datetime not null default '1970-01-01 00:00:00',
 primary key (`id`,`create_time`),
 key `index_date_hour_coun` (`date`,`hour`,`country`)
) engine=innodb auto_increment=983041 default charset=utf8
partition by range (to_days (`create_time`))
(partition p0 values less than (736754) engine = innodb,
 partition p1 values less than (736785) engine = innodb,
 partition p2 values less than (736815) engine = innodb,
 partition p3 values less than (736846) engine = innodb,
 partition p4 values less than (736876) engine = innodb,
 partition p5 values less than (736907) engine = innodb,
 partition p6 values less than (736938) engine = innodb,
 partition p7 values less than (736968) engine = innodb,
 partition p8 values less than (736999) engine = innodb,
 partition p9 values less than (737029) engine = innodb,
 partition p10 values less than (737060) engine = innodb);

2、插入部分数据数据，

insert into `effect_new` (`id`, `type`, `timezone`, `date`, `hour`, `position`, `country`, `create_time`) values ('1', '0', 'gmt+8', '2017-07-01', '', 'm-noticleanfull-familyrecom-0026', '', '2017-07-02 00:07:02');
insert into `effect_new` (`id`, `type`, `timezone`, `date`, `hour`, `position`, `country`, `create_time`) values ('2', '1', 'gmt+8', '2017-09-30', '23', 'ma5dtjub', 'eg', '2017-10-01 00:00:00');
insert into `effect_new` (`id`, `type`, `timezone`, `date`, `hour`, `position`, `country`, `create_time`) values ('3', '1', 'gmt+8', '2017-09-10', '10', '28', 'dz', '2017-09-11 00:08:20');
insert into `effect_new` (`id`, `type`, `timezone`, `date`, `hour`, `position`, `country`, `create_time`) values ('4', '1', 'gmt+8', '2017-02-03', '20', '32', 'ad', '2017-02-04 00:00:00');
insert into `effect_new` (`id`, `type`, `timezone`, `date`, `hour`, `position`, `country`, `create_time`) values ('5', '0', 'gmt+8', '2017-03-05', '2', null, 'ai', '2017-03-06 02:10:00');
insert into `effect_new` (`id`, `type`, `timezone`, `date`, `hour`, `position`, `country`, `create_time`) values ('6', '0', 'gmt+8', '2017-09-23', '13', 'm-brandsplash-s-0038', 'ag', '2017-09-23 13:00:00');
insert into `effect_new` (`id`, `type`, `timezone`, `date`, `hour`, `position`, `country`, `create_time`) values ('7', '1', null, '2017-10-13', '12', 'bb-main-appad-0018', 'af', '2017-10-14 12:00:00');
insert into `effect_new` (`id`, `type`, `timezone`, `date`, `hour`, `position`, `country`, `create_time`) values ('8', '0', 'gmt+8', '2017-10-28', '2', 'm-chargereminder-s-0040', 'ae', '2017-10-29 00:00:00');
insert into `effect_new` (`id`, `type`, `timezone`, `date`, `hour`, `position`, `country`, `create_time`) values ('9', '1', 'gmt+8', '2017-10-09', null, '30', 'ai', '2017-10-10 00:09:00');
insert into `effect_new` (`id`, `type`, `timezone`, `date`, `hour`, `position`, `country`, `create_time`) values ('10', '0', 'gmt+8', '2017-10-05', '5', ' m-brandsplash', 'la', '2017-10-06 05:10:00');

3、分析语句

explain partitions
select * from effect_new_index
where create_time = '2017-10-14 12:00:00' 

结果为：

id	select_type	table	partitions	tpye	possible_keys	key	key_len	ref	rows	filtered	extra
1	simple	effect_new	p8	all	null	null	null	null	391515	10	using where

4、给表effect_new添加索引idx_ctime

5、分析添加索引后的执行计划

结果为：

id	select_type	table	partitions	tpye	possible_keys	key	key_len	ref	rows	filtered	extra
1	simple	effect_new	p8	ref	idx_ctime	idx_ctime	5	const	60760	100	null

6、结论：

虽然表已经根据此字段分区，但这不能等同于索引。分了区，只能说该字段为某个值的记录会在某个分区里面，但不是索引，还要一顿好找。

有时候，主键不等于分区依据列，这时候主键又想建聚集索引的话，那么必须包含分区依据列，搞成复合主键。那么，这种情况下，分区依据列不就有索引了吗？是的，可是它不够快，如果在这个复合索引里面，分区依据列不排在第一位，就不够快，如果查找语句里常常用分区依据列作为过滤条件，就有必要为分区依据列额外单独建立一个索引。

总结

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