以下是查询的结果，该查询花了将近8秒，并且有可用的索引但未使用。只有40,000条记录。
mysql> SELECT * FROM (`phppos_customers`)
JOIN `phppos_people` ON `phppos_customers`.`person_id`=`phppos_people`.`person_id`
WHERE `deleted` = 0 ORDER BY `last_name` asc LIMIT 20
-> ;
** PERSONAL DATA REMOVED **
mysql> EXPLAIN SELECT * FROM (`phppos_customers`) JOIN `phppos_people` ON `phppos_customers`.`person_id`=`phppos_people`.`person_id` WHERE `deleted` = 0 ORDER BY `last_name` asc LIMIT 20
-> ;
+----+-------------+------------------+--------+-------------------+---------+---------+--------------------------------+-------+---------------------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+------------------+--------+-------------------+---------+---------+--------------------------------+-------+---------------------------------+
| 1 | SIMPLE | phppos_customers | ref | person_id,deleted | deleted | 4 | const | 22545 | Using temporary; Using filesort |
| 1 | SIMPLE | phppos_people | eq_ref | PRIMARY | PRIMARY | 4 | pos.phppos_customers.person_id | 1 | |
+----+-------------+------------------+--------+-------------------+---------+---------+--------------------------------+-------+---------------------------------+
2 rows in set (0.00 sec)
mysql> describe phppos_customers;
+----------------+--------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+----------------+--------------+------+-----+---------+-------+
| person_id | int(10) | NO | MUL | NULL | |
| account_number | varchar(255) | YES | UNI | NULL | |
| company_name | varchar(255) | NO | | NULL | |
| taxable | int(1) | NO | | 1 | |
| deleted | int(1) | NO | MUL | 0 | |
+----------------+--------------+------+-----+---------+-------+
5 rows in set (0.01 sec)
mysql> describe phppos_people;
+--------------+--------------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra |
+--------------+--------------+------+-----+---------+----------------+
| first_name | varchar(255) | NO | MUL | NULL | |
| last_name | varchar(255) | NO | MUL | NULL | |
| phone_number | varchar(255) | NO | | NULL | |
| email | varchar(255) | NO | MUL | NULL | |
| address_1 | varchar(255) | NO | | NULL | |
| address_2 | varchar(255) | NO | | NULL | |
| city | varchar(255) | NO | | NULL | |
| state | varchar(255) | NO | | NULL | |
| zip | varchar(255) | NO | | NULL | |
| country | varchar(255) | NO | | NULL | |
| comments | text | NO | | NULL | |
| person_id | int(10) | NO | PRI | NULL | auto_increment |
+--------------+--------------+------+-----+---------+----------------+
12 rows in set (0.00 sec)
编辑：show create table phppos_customers;
| phppos_customers | CREATE TABLE `phppos_customers` (
`person_id` int(10) NOT NULL,
`account_number` varchar(255) COLLATE utf8_unicode_ci DEFAULT NULL,
`company_name` varchar(255) COLLATE utf8_unicode_ci NOT NULL,
`taxable` int(1) NOT NULL DEFAULT '1',
`deleted` int(1) NOT NULL DEFAULT '0',
UNIQUE KEY `account_number` (`account_number`),
KEY `person_id` (`person_id`),
KEY `deleted` (`deleted`),
CONSTRAINT `phppos_customers_ibfk_1` FOREIGN KEY (`person_id`) REFERENCES `phppos_people` (`person_id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8 COLLATE=utf8_unicode_ci |
编辑：show create table phppos_people;
| phppos_people | CREATE TABLE `phppos_people` (
`first_name` varchar(255) COLLATE utf8_unicode_ci NOT NULL,
`last_name` varchar(255) COLLATE utf8_unicode_ci NOT NULL,
`phone_number` varchar(255) COLLATE utf8_unicode_ci NOT NULL,
`email` varchar(255) COLLATE utf8_unicode_ci NOT NULL,
`address_1` varchar(255) COLLATE utf8_unicode_ci NOT NULL,
`address_2` varchar(255) COLLATE utf8_unicode_ci NOT NULL,
`city` varchar(255) COLLATE utf8_unicode_ci NOT NULL,
`state` varchar(255) COLLATE utf8_unicode_ci NOT NULL,
`zip` varchar(255) COLLATE utf8_unicode_ci NOT NULL,
`country` varchar(255) COLLATE utf8_unicode_ci NOT NULL,
`comments` text COLLATE utf8_unicode_ci NOT NULL,
`person_id` int(10) NOT NULL AUTO_INCREMENT,
PRIMARY KEY (`person_id`),
KEY `first_name` (`first_name`),
KEY `last_name` (`last_name`),
KEY `email` (`email`)
) ENGINE=InnoDB AUTO_INCREMENT=45870 DEFAULT CHARSET=utf8 COLLATE=utf8_unicode_ci |
编辑显示指数：
mysql> show index from phppos_customers;
+------------------+------------+----------------+--------------+----------------+-----------+-------------+----------+--------+------+------------+---------+
| Table | Non_unique | Key_name | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment |
+------------------+------------+----------------+--------------+----------------+-----------+-------------+----------+--------+------+------------+---------+
| phppos_customers | 0 | account_number | 1 | account_number | A | 2 | NULL | NULL | YES | BTREE | |
| phppos_customers | 1 | person_id | 1 | person_id | A | 46217 | NULL | NULL | | BTREE | |
| phppos_customers | 1 | deleted | 1 | deleted | A | 2 | NULL | NULL | | BTREE | |
+------------------+------------+----------------+--------------+----------------+-----------+-------------+----------+--------+------+------------+---------+
3 rows in set (0.00 sec)
mysql> show index from phppos_people;
+---------------+------------+------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+
| Table | Non_unique | Key_name | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment |
+---------------+------------+------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+
| phppos_people | 0 | PRIMARY | 1 | person_id | A | 45699 | NULL | NULL | | BTREE | |
| phppos_people | 1 | first_name | 1 | first_name | A | 45699 | NULL | NULL | | BTREE | |
| phppos_people | 1 | last_name | 1 | last_name | A | 45699 | NULL | NULL | | BTREE | |
| phppos_people | 1 | email | 1 | email | A | 45699 | NULL | NULL | | BTREE | |
+---------------+------------+------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+
4 rows in set (0.04 sec)
答案 0 :(得分：1)
查看解释输出，这就是mysql现在所做的事情：
phppos_customers表开始，使用deleted上的索引查找deleted = 0 phppos_people查找相同person_id（加入）的信息。last_name排序，不带任何索引，以帮助排序，因为它是已经连接的结果集，并且必须对所有行进行排序才能找到前20个。这看起来不像是运行此查询的最佳方式，由于某些原因，优化器选择了错误的连接顺序。
尝试使用STRAIGHT_JOIN强制执行不同的连接顺序：
SELECT * FROM `phppos_people` STRAIGHT_JOIN `phppos_customers`
ON `phppos_customers`.`person_id`=`phppos_people`.`person_id`
WHERE `deleted` = 0
ORDER BY `last_name` ASC LIMIT 20;
这次mysql应该：
phppos_people开头，使用该列的索引按last_name排序。phppos_customers并检查是否deleted = 0。