MySQL Select具有多个联接，首先对联接的表执行完整表扫描
我有以下疑问：MySQL Select具有多个联接，首先对联接的表执行完整表扫描,mysql,join,indexing,inner-join,Mysql,Join,Indexing,Inner Join,我有以下疑问： SELECT Impressions.id AS `Impressions__id`, Impressions.timestamp AS `Impressions__timestamp`, Impressions.name AS `Impressions__name`, Impressions.lat AS `Impressions__lat`, Impressions.lng AS `Impressions__lng`, Impression
SELECT
Impressions.id AS `Impressions__id`,
Impressions.timestamp AS `Impressions__timestamp`,
Impressions.name AS `Impressions__name`,
Impressions.lat AS `Impressions__lat`,
Impressions.lng AS `Impressions__lng`,
Impressions.personas_count AS `Impressions__personas_count`,
Impressions.modified AS `Impressions__modified`,
Beacons.id AS `Beacons__id`,
Beacons.uuid AS `Beacons__uuid`,
Beacons.major AS `Beacons__major`,
Beacons.minor_dec AS `Beacons__minor_dec`,
Beacons.minor_hex AS `Beacons__minor_hex`,
Beacons.impressions_count AS `Beacons__impressions_count`,
Beacons.created AS `Beacons__created`,
Beacons.modified AS `Beacons__modified`,
Zones.id AS `Zones__id`,
Zones.location_id AS `Zones__location_id`,
Zones.beacon_id AS `Zones__beacon_id`,
Zones.fixture_no AS `Zones__fixture_no`,
Zones.placement AS `Zones__placement`,
Zones.floor AS `Zones__floor`,
Zones.impressions_count AS `Zones__impressions_count`,
Zones.ignore_further_incidents AS `Zones__ignore_further_incidents`,
Zones.is_reviewed AS `Zones__is_reviewed`,
Zones.review_date AS `Zones__review_date`,
Zones.created AS `Zones__created`,
Zones.modified AS `Zones__modified`,
Locations.id AS `Locations__id`,
Locations.retailer_id AS `Locations__retailer_id`,
Locations.google_place_id AS `Locations__google_place_id`,
Locations.regional_name AS `Locations__regional_name`,
Locations.location AS `Locations__location`,
Locations.store_no AS `Locations__store_no`,
Locations.lat AS `Locations__lat`,
Locations.lng AS `Locations__lng`,
Locations.address1 AS `Locations__address1`,
Locations.address2 AS `Locations__address2`,
Locations.address3 AS `Locations__address3`,
Locations.city AS `Locations__city`,
Locations.state AS `Locations__state`,
Locations.postal_code AS `Locations__postal_code`,
Locations.region_id AS `Locations__region_id`,
Locations.country_id AS `Locations__country_id`,
Locations.zones_count AS `Locations__zones_count`,
Locations.contacts_count AS `Locations__contacts_count`,
Locations.created AS `Locations__created`,
Locations.modified AS `Locations__modified`,
Devices.id AS `Devices__id`,
Devices.os AS `Devices__os`,
Devices.bluetooth_enabled AS `Devices__bluetooth_enabled`,
Devices.impressions_count AS `Devices__impressions_count`,
Devices.modified AS `Devices__modified`,
Regions.id AS `Regions__id`,
Regions.country_name AS `Regions__country_name`,
Regions.subdiv AS `Regions__subdiv`,
Regions.subdiv_name AS `Regions__subdiv_name`,
Regions.level_name AS `Regions__level_name`,
Regions.alt_names AS `Regions__alt_names`,
Regions.subdiv_star AS `Regions__subdiv_star`,
Regions.subdiv_id AS `Regions__subdiv_id`,
Regions.country_id AS `Regions__country_id`,
Regions.country_code_2 AS `Regions__country_code_2`,
Regions.country_code_3 AS `Regions__country_code_3`,
Countries.id AS `Countries__id`,
Countries.country_name AS `Countries__country_name`,
Countries.alt_names AS `Countries__alt_names`,
Countries.code2 AS `Countries__code2`,
Countries.code3 AS `Countries__code3`,
Countries.iso_cc AS `Countries__iso_cc`,
Countries.fips_code AS `Countries__fips_code`,
Countries.fips_country_name AS `Countries__fips_country_name`,
Countries.un_region AS `Countries__un_region`,
Countries.un_subregion AS `Countries__un_subregion`,
Countries.comments AS `Countries__comments`,
Countries.created AS `Countries__created`,
Countries.modified AS `Countries__modified`
FROM
impressions Impressions
inner join beacons Beacons ON Beacons.id = (Impressions.beacon_id)
inner JOIN zones Zones ON Zones.id = (Impressions.zone_id)
inner JOIN devices Devices ON Devices.id = (Impressions.device_id)
INNER JOIN locations Locations ON Locations.id = (Zones.location_id)
LEFT JOIN regions Regions ON Regions.id = (Locations.region_id)
LEFT JOIN countries Countries ON Countries.id = (Locations.country_id)
ORDER BY
Impressions.timestamp desc
LIMIT
15 OFFSET 15
EXPLAIN+----+-------------+-------------+--------+---------------------------------------+----------------+---------+---------------------------------+-------+---------------------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------------+--------+---------------------------------------+----------------+---------+---------------------------------+-------+---------------------------------+
| 1 | SIMPLE | Devices | ALL | PRIMARY | NULL | NULL | NULL | 43274 | Using temporary; Using filesort |
| 1 | SIMPLE | Impressions | ref | zone_idx,device_id_idx2,beacon_id_idx | device_id_idx2 | 8 | gen1_d2go.Devices.id | 3 | NULL |
| 1 | SIMPLE | Zones | eq_ref | PRIMARY,fk_location_idx,comp | PRIMARY | 8 | gen1_d2go.Impressions.zone_id | 1 | NULL |
| 1 | SIMPLE | Beacons | eq_ref | PRIMARY | PRIMARY | 8 | gen1_d2go.Impressions.beacon_id | 1 | NULL |
| 1 | SIMPLE | Locations | eq_ref | PRIMARY | PRIMARY | 8 | gen1_d2go.Zones.location_id | 1 | NULL |
| 1 | SIMPLE | Regions | eq_ref | PRIMARY | PRIMARY | 4 | gen1_d2go.Locations.region_id | 1 | NULL |
| 1 | SIMPLE | Countries | eq_ref | PRIMARY | PRIMARY | 4 | gen1_d2go.Locations.country_id | 1 | NULL |
+----+-------------+-------------+--------+---------------------------------------+----------------+---------+---------------------------------+-------+---------------------------------+
7 rows in set (0.00 sec)
设备印象设备创建+----+-------------+-------------+--------+---------------------------------------+----------------+---------+---------------------------------+-------+---------------------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------------+--------+---------------------------------------+----------------+---------+---------------------------------+-------+---------------------------------+
| 1 | SIMPLE | Devices | ALL | PRIMARY | NULL | NULL | NULL | 43274 | Using temporary; Using filesort |
| 1 | SIMPLE | Impressions | ref | zone_idx,device_id_idx2,beacon_id_idx | device_id_idx2 | 8 | gen1_d2go.Devices.id | 3 | NULL |
| 1 | SIMPLE | Zones | eq_ref | PRIMARY,fk_location_idx,comp | PRIMARY | 8 | gen1_d2go.Impressions.zone_id | 1 | NULL |
| 1 | SIMPLE | Beacons | eq_ref | PRIMARY | PRIMARY | 8 | gen1_d2go.Impressions.beacon_id | 1 | NULL |
| 1 | SIMPLE | Locations | eq_ref | PRIMARY | PRIMARY | 8 | gen1_d2go.Zones.location_id | 1 | NULL |
| 1 | SIMPLE | Regions | eq_ref | PRIMARY | PRIMARY | 4 | gen1_d2go.Locations.region_id | 1 | NULL |
| 1 | SIMPLE | Countries | eq_ref | PRIMARY | PRIMARY | 4 | gen1_d2go.Locations.country_id | 1 | NULL |
+----+-------------+-------------+--------+---------------------------------------+----------------+---------+---------------------------------+-------+---------------------------------+
7 rows in set (0.00 sec)
CREATE TABLE `impressions` (
`id` bigint(20) unsigned NOT NULL AUTO_INCREMENT,
`device_id` bigint(20) unsigned NOT NULL,
`beacon_id` bigint(20) unsigned NOT NULL,
`zone_id` bigint(20) unsigned NOT NULL,
`timestamp` datetime NOT NULL,
`google_place_id` bigint(20) unsigned DEFAULT NULL,
`name` varchar(60) DEFAULT NULL,
`lat` decimal(12,7) DEFAULT NULL,
`lng` decimal(12,7) DEFAULT NULL,
`personas_count` int(10) unsigned DEFAULT '0',
`created` datetime DEFAULT NULL,
`modified` datetime DEFAULT NULL,
PRIMARY KEY (`id`,`timestamp`),
KEY `zone_idx` (`zone_id`),
KEY `device_id_idx2` (`device_id`),
KEY `beacon_id_idx` (`beacon_id`),
KEY `timestamp_idx` (`id`,`timestamp`),
KEY `ALL` (`id`,`timestamp`,`name`,`lat`,`lng`,`personas_count`,`modified`),
CONSTRAINT `beacon_id` FOREIGN KEY (`beacon_id`) REFERENCES `beacons` (`id`) ON DELETE NO ACTION ON UPDATE NO ACTION,
CONSTRAINT `device2` FOREIGN KEY (`device_id`) REFERENCES `devices` (`id`) ON DELETE NO ACTION ON UPDATE NO ACTION,
CONSTRAINT `zone_FK` FOREIGN KEY (`zone_id`) REFERENCES `zones` (`id`) ON DELETE NO ACTION ON UPDATE NO ACTION
) ENGINE=InnoDB AUTO_INCREMENT=303907 DEFAULT CHARSET=utf8;
CREATE TABLE `devices` (
`id` bigint(20) unsigned NOT NULL AUTO_INCREMENT,
`device_id` bigint(20) unsigned NOT NULL,
`advertiser_id` char(36) NOT NULL,
`os` varchar(80) DEFAULT NULL,
`bluetooth_enabled` tinyint(1) DEFAULT NULL,
`impressions_count` int(10) unsigned DEFAULT '0',
`created` datetime DEFAULT NULL,
`modified` datetime DEFAULT NULL,
PRIMARY KEY (`id`),
KEY `advertiser_idx` (`advertiser_id`),
KEY `ad_dev` (`device_id`,`advertiser_id`),
KEY `device_id` (`device_id`)
) ENGINE=InnoDB AUTO_INCREMENT=53628 DEFAULT CHARSET=utf8;
FORCE INDEX（timestamp_idx）Current database: gen1_d2go
+----+-------------+-------------+--------+------------------------------+---------------+---------+---------------------------------+------+-------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------------+--------+------------------------------+---------------+---------+---------------------------------+------+-------+
| 1 | SIMPLE | Impressions | index | NULL | timestamp_idx | 5 | NULL | 15 | NULL |
| 1 | SIMPLE | Zones | eq_ref | PRIMARY,fk_location_idx,comp | PRIMARY | 8 | gen1_d2go.Impressions.zone_id | 1 | NULL |
| 1 | SIMPLE | Beacons | eq_ref | PRIMARY | PRIMARY | 8 | gen1_d2go.Impressions.beacon_id | 1 | NULL |
| 1 | SIMPLE | Locations | eq_ref | PRIMARY | PRIMARY | 8 | gen1_d2go.Zones.location_id | 1 | NULL |
| 1 | SIMPLE | Regions | eq_ref | PRIMARY | PRIMARY | 4 | gen1_d2go.Locations.region_id | 1 | NULL |
| 1 | SIMPLE | Countries | eq_ref | PRIMARY | PRIMARY | 4 | gen1_d2go.Locations.country_id | 1 | NULL |
| 1 | SIMPLE | Devices | eq_ref | PRIMARY | PRIMARY | 8 | gen1_d2go.Impressions.device_id | 1 | NULL |
+----+-------------+-------------+--------+------------------------------+---------------+---------+---------------------------------+------+-------+
7 rows in set (0.01 sec)
Impressions时间戳开始的索引。这样，优化器将有希望决定以时间戳
顺序扫描印象
，从而避免排序等
附带的教训。。。您有3个索引，以id，timestamp
开头。一个是主键
。这意味着另外两个是不必要的
因此，可以进行额外的加速：
ALTER TABLE Impressions
DROP INDEX timestamp_idx, -- as already mentioned
DROP INDEX ALL, -- ditto
DROP PRIMARY KEY, -- to rearrange it
ADD PRIMARY KEY(timestamp, id), -- thus
ADD INDEX(id); -- and keep AUTO_INCREMENT happy
为什么?？通过让PK以时间戳开始，查询可以扫描数据，而不是在某些索引和数据之间跳跃。这将加快相关查询的速度。警告：这可能会影响其他查询
其他注释
CHAR（36）
闻起来像UUID，对吗？但是使用utf8，它需要108个字节！。更改为CHAR（36）字符集ascii NOT NULL
，这样它只需要36个字节。（或者您可以转换为“二进制（16）”以节省更多；但这是另一回事，需要更多的代码。）
除非您有数十亿行，BIGINT
（8字节）对于ID来说是多余的INT UNSIGNED
仅为4个字节
更小可以通过各种方式转换为更快。关于全表扫描，您无法对其进行更多优化。
请参阅此处以了解更多信息
我很确定我会看到什么-但是请你用强制索引发布执行计划好吗？@PaulSpiegel用新的解释更新了文档抱歉，我没有答案。我只是想知道更多的信息，所以可能有人能帮上忙。我还面临着愚蠢的执行计划的问题，不得不使用STRIGHT_JOIN或FORCE INDEX来修复它们。所以我对答案很感兴趣。直接加入也有效。我只是想知道为什么mysql不会合作这是有意义的-我没有看到timestamp\u idx
以id
开始，因此与PK和ALL
索引是冗余的。但我仍然不明白，为什么使用该指数会带来如此大的性能提升。我认为MySQL仍然需要将完整的索引读入临时表，然后对其进行排序，然后才能决定选择哪15行。但是在执行计划中没有这样的人。想想一长串的人，按姓氏排序，然后按姓氏排序。但是你只有这个人的名字。