使用索引导致的MySQL延迟； 使用临时； 使用文件排序

Question

我有一个查询，该查询将两个表连接起来并按主键排序数据。 这导致MySQL非常流行的问题“使用索引；使用临时文件；使用文件排序”。

该问题在我的具有约40万条记录的生产表中导致严重的延迟问题。

这是更多信息：

我有两个表：Doctor和Area。 Doctor表具有指向Area的外键。

医生：

+-----------------------------+---------------+------+-----+---------+----------------+
| Field                       | Type          | Null | Key | Default | Extra          |
+-----------------------------+---------------+------+-----+---------+----------------+
| id                          | int(11)       | NO   | PRI | NULL    | auto_increment |
| area_id                     | int(11)       | NO   | MUL | NULL    |                |
+-----------------------------+---------------+------+-----+---------+----------------+

医生索引：

+---------------+------------+------------------------+--------------+------------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| Table         | Non_unique | Key_name               | Seq_in_index | Column_name      | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | Index_comment |
+---------------+------------+------------------------+--------------+------------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| doctor        |          0 | PRIMARY                |            1 | id               | A         |        5546 |     NULL | NULL   |      | BTREE      |         |               |
| doctor        |          1 | doctor_dfd0e917        |            1 | area_id          | A         |          29 |     NULL | NULL   |      | BTREE      |         |               |
+---------------+------------+------------------------+--------------+------------------+-----------+-------------+----------+--------+------+------------+---------+---------------+

区域：

+------------------------+-------------+------+-----+---------+----------------+
| Field                  | Type        | Null | Key | Default | Extra          |
+------------------------+-------------+------+-----+---------+----------------+
| id                     | int(11)     | NO   | PRI | NULL    | auto_increment |
+------------------------+-------------+------+-----+---------+----------------+

面积索引：

+---------------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| Table         | Non_unique | Key_name | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | Index_comment |
+---------------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| area          |          0 | PRIMARY  |            1 | id          | A         |          24 |     NULL | NULL   |      | BTREE      |         |               |
+---------------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+

我正在尝试运行以下查询：

SELECT `doctor`.`id`, 
       `area`.`id` 
FROM 
       `doctor` 
INNER JOIN 
       `area` ON (`doctor`.`area_id` = `area`.`id`) 
ORDER BY 
      `doctor`.`id` DESC LIMIT 100;

EXPLAIN返回以下内容（带有问题的“使用”索引；“使用临时”；“使用文件排序”）：

+----+-------------+---------------+-------+------------------------+------------------------+---------+--------------+------+----------------------------------------------+
| id | select_type | table         | type  | possible_keys          | key                    | key_len | ref          | rows | Extra                                        |
+----+-------------+---------------+-------+------------------------+------------------------+---------+--------------+------+----------------------------------------------+
|  1 | SIMPLE      | area          | index | PRIMARY                | PRIMARY                | 4       | NULL         |   24 | Using index; Using temporary; Using filesort |
|  1 | SIMPLE      | doctor        | ref   | doctor_dfd0e917        | doctor_dfd0e917        | 4       | area.id      |  191 | Using index                                  |
+----+-------------+---------------+-------+------------------------+------------------------+---------+--------------+------+----------------------------------------------+

如果删除ORDER BY子句，则会达到预期的效果：

+----+-------------+---------------+-------+------------------------+------------------------+---------+--------------+------+----------------------------------------------+
| id | select_type | table         | type  | possible_keys          | key                    | key_len | ref          | rows | Extra                                        |
+----+-------------+---------------+-------+------------------------+------------------------+---------+--------------+------+----------------------------------------------+
|  1 | SIMPLE      | area          | index | PRIMARY                | PRIMARY                | 4       | NULL         |   24 | Using index                                  |
|  1 | SIMPLE      | doctor        | ref   | doctor_dfd0e917        | doctor_dfd0e917        | 4       | area.id      |  191 | Using index                                  |
+----+-------------+---------------+-------+------------------------+------------------------+---------+--------------+------+----------------------------------------------+

即使使用主键，为什么ORDER BY子句也会在这里引起问题？

先感谢您。

Answer 1

看来每个医生只有一个区域。 查看此查询的工作方式：

SELECT d.id,
       (SELECT a.id FROM area a ON a.id = d.area_id) as area_id
FROM doctor d 
ORDER BY d.id DESC
LIMIT 100;

如果要使用inner join测试表中是否存在医生，请添加：

SELECT d.id,
       (SELECT a.id FROM area a ON a.id = d.area_id) as area_id
FROM doctor d 
WHERE EXISTS (SELECT 1 FROM area a ON a.id = d.area_id)
ORDER BY d.id DESC
LIMIT 100;

这两个都很有可能会按顺序扫描doctors表，并根据需要从area获取信息。