索引列仍然需要8秒，不使用索引（mysql 5）

Question

以下是查询结果，需要将近8秒，并且有索引可用但尚未使用。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：秀创建表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：显示创建表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SHOW指标：

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) 

Answer 1

望着这个解释输出是什么MySQL的现在：

1. 开始phppos_customers表并使用上deleted索引查找与行deleted = 0
2. 对于每一行（剩下22k）找到phppos_people的信息相同person_id（加入）。
3. 在临时表和排序last_name没有任何指标将所有的东西，以帮助选，因为它是一个已经加入结果集，所有行必须进行排序，以找到第20位

这不看就像运行此查询的最佳方式一样，出于某种原因，优化器选择了错误的连接顺序。

尝试强制使用不同STRAIGHT_JOIN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; 

这一次的MySQL应该：

1. 开始phppos_people使用该列指数last_name排序。
2. 对于每一行加入phppos_customers并检查是否为deleted = 0。
3. 使用限制优化，首先找到20后停止。