问题描述

1. 什么时候应该在数据库中使用复合索引?
2. 使用 a 的性能后果是什么?复合索引)?
3. 为什么要使用复合索引?

例如，我有一个 homes 表:

For example, I have a homes table:

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，例如:

Does it make sense for me to use a composite index for both geolat and geolng, such that:

我替换:

  KEY `geolat` (`geolat`),
  KEY `geolng` (`geolng`),

与:

KEY `geolat_geolng` (`geolat`, `geolng`)

如果是这样:

• 为什么?
• 使用复合索引对性能的影响是什么?

更新:

由于许多人表示它完全取决于我执行的查询，以下是最常见的查询:

Since many people have stated it entirely dependent upon the queries I perform, below is the most common query performed:

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

EXPLAIN 返回:

EXPLAIN returns:

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，我没有对 geolat 和 geolng 使用复合索引.我应该吗?

I don't quite understand how to read the EXPLAIN command. Does this look good or bad. Right now, I am NOT using a composite index for geolat and geolng. Should I be?

推荐答案

当您使用可从中受益的查询时，您应该使用复合索引.如下所示的复合索引:

You should use a composite index when you are using queries that benefit from it. A composite index that looks like this:

index( column_A, column_B, column_C )

将有利于使用这些字段进行连接、过滤和有时选择的查询.它还将有益于使用该组合中最左侧列子集的查询.所以上面的索引也会满足需要的查询

will benefit a query that uses those fields for joining, filtering, and sometimes selecting. It will also benefit queries that use left-most subsets of columns in that composite. So the above index will also satisfy queries that need

index( column_A, column_B, column_C )
index( column_A, column_B )
index( column_A )

但它不会(至少不会直接，如果没有更好的索引，它可能会部分帮助)对需要的查询有帮助

But it will not (at least not directly, maybe it can help partially if there are no better indices) help for queries that need

index( column_A, column_C )

注意 column_B 是如何丢失的.

Notice how column_B is missing.

在您的原始示例中，两个维度的复合索引最有利于查询两个维度或最左边的维度本身，而不是最右边的维度本身.如果您总是查询两个维度，那么复合索引是可行的方法，这并不重要(最有可能).

In your original example, a composite index for two dimensions will mostly benefit queries that query on both dimensions or the leftmost dimension by itself, but not the rightmost dimension by itself. If you're always querying two dimensions, a composite index is the way to go, doesn't really matter which is first (most probably).

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