【MySQL】分區字段列是否有必要再單獨建索引

對於分區字段必須是主鍵的一部分，那麼建了複合主鍵以後，是否須要對分許字段再單獨添加一個索引呢？有沒有效果？下面來驗證一下

一、新建表effect_new(以建立時間按月分區)

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);

二、插入部分數據數據，

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');

三、分析語句

EXPLAIN PARTITIONS
select * from effect_new_index
where create_time = '2017-10-14 12:00:00'

結果爲：

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

四、給表effect_new添加索引idx_ctime

五、分析添加索引後的執行計劃

結果爲：

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

六、結論：

雖然表已經根據此字段分區，但這不能等同於索引。分了區，只能說該字段爲某個值的記錄會在某個分區裏面，但不是索引，還要一頓好找。

有時候，主鍵不等於分區依據列，這時候主鍵又想建彙集索引的話，那麼必須包含分區依據列，搞成複合主鍵。那麼，這種狀況下，分區依據列不就有索引了嗎？是的，但是它不夠快，若是在這個複合索引裏面，分區依據列不排在第一位，就不夠快，若是查找語句裏經常用分區依據列做爲過濾條件，就有必要爲分區依據列額外單獨創建一個索引。