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)
CREATETABLE`effect_new`( `id`bigint(20)NOTNULLAUTO_INCREMENT, `type`tinyint(4)NOTNULLDEFAULT'0', `timezone`varchar(10)DEFAULTNULL, `date`varchar(10)NOTNULL, `hour`varchar(2)DEFAULTNULL, `position`varchar(200)DEFAULTNULL, `country`varchar(32)NOTNULL, `create_time`datetimeNOTNULLDEFAULT'1970-01-0100:00:00', PRIMARYKEY(`id`,`create_time`), KEY`index_date_hour_coun`(`date`,`hour`,`country`) )ENGINE=InnoDBAUTO_INCREMENT=983041DEFAULTCHARSET=utf8 PARTITIONBYRANGE(TO_DAYS(`create_time`)) (PARTITIONp0VALUESLESSTHAN(736754)ENGINE=InnoDB, PARTITIONp1VALUESLESSTHAN(736785)ENGINE=InnoDB, PARTITIONp2VALUESLESSTHAN(736815)ENGINE=InnoDB, PARTITIONp3VALUESLESSTHAN(736846)ENGINE=InnoDB, PARTITIONp4VALUESLESSTHAN(736876)ENGINE=InnoDB, PARTITIONp5VALUESLESSTHAN(736907)ENGINE=InnoDB, PARTITIONp6VALUESLESSTHAN(736938)ENGINE=InnoDB, PARTITIONp7VALUESLESSTHAN(736968)ENGINE=InnoDB, PARTITIONp8VALUESLESSTHAN(736999)ENGINE=InnoDB, PARTITIONp9VALUESLESSTHAN(737029)ENGINE=InnoDB, PARTITIONp10VALUESLESSTHAN(737060)ENGINE=InnoDB);
2、插入部分数据数据,
INSERTINTO`effect_new`(`id`,`type`,`timezone`,`date`,`hour`,`position`,`country`,`create_time`)VALUES('1','0','GMT+8','2017-07-01','','M-NotiCleanFull-FamilyRecom-0026','','2017-07-0200:07:02'); INSERTINTO`effect_new`(`id`,`type`,`timezone`,`date`,`hour`,`position`,`country`,`create_time`)VALUES('2','1','GMT+8','2017-09-30','23','Ma5dtJub','EG','2017-10-0100:00:00'); INSERTINTO`effect_new`(`id`,`type`,`timezone`,`date`,`hour`,`position`,`country`,`create_time`)VALUES('3','1','GMT+8','2017-09-10','10','28','DZ','2017-09-1100:08:20'); INSERTINTO`effect_new`(`id`,`type`,`timezone`,`date`,`hour`,`position`,`country`,`create_time`)VALUES('4','1','GMT+8','2017-02-03','20','32','AD','2017-02-0400:00:00'); INSERTINTO`effect_new`(`id`,`type`,`timezone`,`date`,`hour`,`position`,`country`,`create_time`)VALUES('5','0','GMT+8','2017-03-05','2',NULL,'AI','2017-03-0602:10:00'); INSERTINTO`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','2017-09-2313:00:00'); INSERTINTO`effect_new`(`id`,`type`,`timezone`,`date`,`hour`,`position`,`country`,`create_time`)VALUES('7','1',NULL,'2017-10-13','12','BB-Main-AppAd-0018','AF','2017-10-1412:00:00'); INSERTINTO`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','2017-10-2900:00:00'); INSERTINTO`effect_new`(`id`,`type`,`timezone`,`date`,`hour`,`position`,`country`,`create_time`)VALUES('9','1','GMT+8','2017-10-09',NULL,'30','AI','2017-10-1000:09:00'); INSERTINTO`effect_new`(`id`,`type`,`timezone`,`date`,`hour`,`position`,`country`,`create_time`)VALUES('10','0','GMT+8','2017-10-05','5','M-BrandSplash','LA','2017-10-0605:10:00');
3、分析语句
EXPLAINPARTITIONS select*fromeffect_new_index wherecreate_time='2017-10-1412:00:00'
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 | Usingwhere |
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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