我有以下查询：
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
此查询大约需要6秒钟才能运行。 EXPLAIN输出如下：
+----+-------------+-------------+--------+---------------------------------------+----------------+---------+---------------------------------+-------+---------------------------------+
| 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)
我不明白为什么它支持Devices表的完整扫描。这些表都已编入索引，CREATE和Impressions的{{1}}语句如下：
展示次数
Devices设备
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;
踢球者是：
当我在FROM展示后使用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;
时，效果很好。它使用该索引，并在大约0.078秒内运行。我不知道为什么它试图避免使用该索引，或者首先从该表中进行选择。
已更新
包含EXPLAIN和FORCE INDEX
FORCE INDEX (timestamp_idx)答案 0 :(得分：1)
Impressions需要使用timestamp启动的索引。这样，优化器有望决定以Impressions顺序扫描timestamp，从而避免排序等。
侧面课程......您有 3 索引，以id, timestamp开头。一个是PRIMARY KEY。这意味着其他两个是不必要的。
因此可以获得额外的加速：
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
为什么呢？通过使用时间戳使PK start ，查询可以扫描数据，而不是在某些索引和数据之间跳转。这将加速有问题的查询。警告：可能会伤害其他查询。
其他说明......
CHAR(36)闻起来像UUID，对吗？但是使用utf8，它只需要108个字节！更改为CHAR(36) CHARACTER SET ascii NOT NULL，这样它只需要36个字节。 （或者你可以转换为`BINARY（16）以获得更多节省;但这是另一个故事，还有更多代码。）
除非您有数十亿行，否则BIGINT（8字节）对于ID来说是过度的。 INT UNSIGNED只有4个字节。
较小的转化为各种方式更快。</ p>
答案 1 :(得分：0)
关于全表扫描，您无法对其进行更优化。 请参阅此处了解更多信息 https://cryptkcoding.com/blog/2012/04/06/how-to-optimize-mysql-join-queries-through-indexing/