MySQL

Question

"High Performance MySQL"书中说，

The ORDER BY clause also has the same limitation as lookup queries: it needs to form a leftmost prefix of the index. In all other cases, MySQL uses a filesort.

这里有一个 table 有一个索引 (rental_date, inventory_id, customer_id):

CREATE TABLE rental (
...
PRIMARY KEY (rental_id),
UNIQUE KEY rental_date (rental_date,inventory_id,customer_id),
KEY idx_fk_inventory_id (inventory_id),
KEY idx_fk_customer_id (customer_id),
KEY idx_fk_staff_id (staff_id),
...
);

MySQL uses the rental_date index to order the following query, as you can see from the lack of a filesort in EXPLAIN:

> mysql> EXPLAIN SELECT
> rental_id, staff_id FROM sakila.rental
> -> WHERE rental_date = '2005-05-25'
> -> ORDER BY inventory_id, customer_id\G
> *************************** 1. row *************************** 
> type: ref 
> possible_keys: rental_date 
> key: rental_date 
> rows: 1 
> Extra: Using where

This works, even though the ORDER BY clause isn’t itself a leftmost prefix of the index, because we specified an equality condition for the first column in the index.

和

This query has range condition on the first column, so MySQL doesn't use the rest of index:

EXPLAIN 
SELECT `rental_id`, `staff_id` FROM `sakila`.`rental`
WHERE `rental_date` > '2005-05-25'
ORDER BY `inventory_id`, `customer_id`;
*************************** 1. row *************************** 
...
key: NULL Extra: Using where; using filesort
...

问题：为什么查询第一列有范围条件，MySQL没有使用索引？

匹配 'rental_date' > '2005-05-25' 的第一个位置将使用 B 树索引找到。然后 MySQL 可以按顺序扫描子节点（inventory_id，customer_id），我想。有什么问题吗？

Answer 1

假设索引是这样的：

rental_date, inventory_id, customer_id
======================================
...
2005-05-25, 10, 10
2005-05-25, 20, 20
2005-05-25, 30, 30
2005-05-26, 20, 20
2005-05-26, 40, 40
2005-05-27, 10, 10
2005-05-27, 30, 30
...

在第一个查询中 mysql 使用索引查找第一个条目“2005-05-25”。由于 rental_date 是索引的第一个字段，因此相同日期值的其他字段（inventory_id、customer_id）按排序顺序排列。

但是在第二个查询中想象一下 2005-05-26、2005-05-27 会发生什么。突然，inventory_id、customer_id 没有排序。它们仅针对特定的 rental_date 值进行排序。所以 mysql 最后必须对它们进行排序。

MySQL - 范围条件和排序依据

MySQL - RANGE CONDITION and ORDER BY

indexing

sql-order-by