【发布时间】：2010-12-21 21:00:01
【问题描述】：
- 什么时候应该在数据库中使用复合索引？
- 使用 综合指数）？
- 为什么要使用复合索引？
例如，我有一个homes 表：
CREATE TABLE IF NOT EXISTS `homes` (
`home_id` int(10) unsigned NOT NULL auto_increment,
`sqft` smallint(5) unsigned NOT NULL,
`year_built` smallint(5) unsigned NOT NULL,
`geolat` decimal(10,6) default NULL,
`geolng` decimal(10,6) default NULL,
PRIMARY KEY (`home_id`),
KEY `geolat` (`geolat`),
KEY `geolng` (`geolng`),
) ENGINE=InnoDB ;
对geolat 和geolng 使用复合索引是否有意义，这样：
我替换：
KEY `geolat` (`geolat`),
KEY `geolng` (`geolng`),
与：
KEY `geolat_geolng` (`geolat`, `geolng`)
如果是这样：
- 为什么？
- 使用复合索引会对性能产生什么影响？
更新：
由于许多人表示它完全取决于我执行的查询，以下是执行的最常见查询：
SELECT * FROM homes
WHERE geolat BETWEEN ??? AND ???
AND geolng BETWEEN ??? AND ???
更新 2：
使用以下数据库架构：
CREATE TABLE IF NOT EXISTS `homes` (
`home_id` int(10) unsigned NOT NULL auto_increment,
`primary_photo_group_id` int(10) unsigned NOT NULL default '0',
`customer_id` bigint(20) unsigned NOT NULL,
`account_type_id` int(11) NOT NULL,
`address` varchar(128) collate utf8_unicode_ci NOT NULL,
`city` varchar(64) collate utf8_unicode_ci NOT NULL,
`state` varchar(2) collate utf8_unicode_ci NOT NULL,
`zip` mediumint(8) unsigned NOT NULL,
`price` mediumint(8) unsigned NOT NULL,
`sqft` smallint(5) unsigned NOT NULL,
`year_built` smallint(5) unsigned NOT NULL,
`num_of_beds` tinyint(3) unsigned NOT NULL,
`num_of_baths` decimal(3,1) unsigned NOT NULL,
`num_of_floors` tinyint(3) unsigned NOT NULL,
`description` text collate utf8_unicode_ci,
`geolat` decimal(10,6) default NULL,
`geolng` decimal(10,6) default NULL,
`display_status` tinyint(1) NOT NULL,
`date_listed` timestamp NOT NULL default CURRENT_TIMESTAMP,
`contact_email` varchar(100) collate utf8_unicode_ci NOT NULL,
`contact_phone_number` varchar(15) collate utf8_unicode_ci NOT NULL,
PRIMARY KEY (`home_id`),
KEY `customer_id` (`customer_id`),
KEY `city` (`city`),
KEY `num_of_beds` (`num_of_beds`),
KEY `num_of_baths` (`num_of_baths`),
KEY `geolat` (`geolat`),
KEY `geolng` (`geolng`),
KEY `account_type_id` (`account_type_id`),
KEY `display_status` (`display_status`),
KEY `sqft` (`sqft`),
KEY `price` (`price`),
KEY `primary_photo_group_id` (`primary_photo_group_id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8 COLLATE=utf8_unicode_ci AUTO_INCREMENT=8 ;
使用以下 SQL：
EXPLAIN SELECT homes.home_id,
address,
city,
state,
zip,
price,
sqft,
year_built,
account_type_id,
num_of_beds,
num_of_baths,
geolat,
geolng,
photo_id,
photo_url_dir
FROM homes
LEFT OUTER JOIN home_photos ON homes.home_id = home_photos.home_id
AND homes.primary_photo_group_id = home_photos.home_photo_group_id
AND home_photos.home_photo_type_id = 2
WHERE homes.display_status = true
AND homes.geolat BETWEEN -100 AND 100
AND homes.geolng BETWEEN -100 AND 100
解释返回：
id select_type table type possible_keys key key_len ref rows Extra
----------------------------------------------------------------------------------------------------------
1 SIMPLE homes ref geolat,geolng,display_status display_status 1 const 2 Using where
1 SIMPLE home_photos ref home_id,home_photo_type_id,home_photo_group_id home_photo_group_id 4 homes.primary_photo_group_id 4
我不太明白如何阅读 EXPLAIN 命令。这看起来是好是坏。现在，我没有使用 geolat 和 geolng 的复合索引。我应该是吗？
【问题讨论】：
标签： mysql indexing composite-index