本期来谈谈覆盖索引与延迟关联。在此之前，我们先简单建立一个订单表 Orders 用于举例说明。表中共包含 3 个字段：
id
int
product_id
name
CREATE
TABLE
`orders`
`id`
int
10
NOT
NULL
COMMENT
'订单 ID'
`product_id`
int
10
DEFAULT
NULL
COMMENT
'商品 ID'
`name`
varchar
255
CHARACTER
SET
COLLATE
DEFAULT
NULL
COMMENT
'订单名称'
KEY
`id`
KEY
`product_idx`
`product_id`
USING
ENGINE
InnoDB
DEFAULT
CHARSET
COLLATE
覆盖索引
什么是覆盖索引？
根据索引
不包含
覆盖索引
MyISAM
product_id
SELECT product_id FROM orders
EXPLAIN
SELECT
FROM
----+-------------+--------+------------+-------+---------------+-------------+---------+------+------+----------+-------------+
----+-------------+--------+------------+-------+---------------+-------------+---------+------+------+----------+-------------+
----+-------------+--------+------------+-------+---------------+-------------+---------+------+------+----------+-------------+
set
0.00
id
SELECT id, product_id FROM orders WHERE product_id = 1
product_id
product_id = 1
通过该子结点指针读取磁盘上的数据行
id
由于 MyISAM 的叶子结点存储着指向数据行的指针，该查询多了一步回表操作，无法使用覆盖索引。
EXPLAIN
SELECT
id
FROM
WHERE
1
----+-------------+--------+------------+------+---------------+-------------+---------+-------+------+----------+-------+
----+-------------+--------+------------+------+---------------+-------------+---------+-------+------+----------+-------+
----+-------------+--------+------------+------+---------------+-------------+---------+-------+------+----------+-------+
set
0.00
MyISAM 索引结构
InnoDB
二级索引的叶子结点保存着行的主键值
InnoDB 二级索引的叶子结点包含行主键值
SELECT id, product_id FROM orders WHERE product_id = 1
EXPLAIN
SELECT
id
FROM
WHERE
1
----+-------------+--------+------------+------+---------------+-------------+---------+-------+------+----------+-------------+
----+-------------+--------+------------+------+---------------+-------------+---------+-------+------+----------+-------------+
----+-------------+--------+------------+------+---------------+-------------+---------+-------+------+----------+-------------+
set
0.01
Extra
Using index
product_id
product_id = 1
id
查询轨迹并未进行回表取值。
延迟关联
deferred join
在查询的第一阶段 MySQL 使用覆盖索引，再通过该覆盖索引查询到的结果到外层查询匹配需要的所有列值。
这样说有些抽象，我们来看看下面的例子。
用延迟关联优化分页(LIMIT)
LIMIT
LIMIT 10000, 20
EXPLAIN
SELECT
FROM
LIMIT
10000
20
----+-------------+--------+------------+------+---------------+------+---------+------+------+----------+-------+
----+-------------+--------+------------+------+---------------+------+---------+------+------+----------+-------+
----+-------------+--------+------------+------+---------------+------+---------+------+------+----------+-------+
尽可能使用索引覆盖扫描，而不是查询所有列
EXPLAIN
SELECT
FROM
AS
JOIN
SELECT
id
FROM
LIMIT
10000
20
AS
ON
----+-------------+------------+------------+-------+---------------+-------------+---------+------------+------+----------+-------------+
----+-------------+------------+------------+-------+---------------+-------------+---------+------------+------+----------+-------------+
----+-------------+------------+------------+-------+---------------+-------------+---------+------------+------+----------+-------------+
这样一来，MySQL 在 SQL 语句的「内层」进行扫描时使用了覆盖索引，「外层」再通过索引树找到相关的数据行，直接减少了扫描的数据量。
总结
只需扫描索引，无须回表
deferred join
参考资料
《高性能 MySQL》
[1]
参考资料
[1]
https://book.douban.com/subject/23008813/
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