巧妙转换字段利用索引

某后台用户管理sql(后台人员反映用户操作界面越来越慢,抓到该sql)

SELECT c.dropshipper_level,c.customers_id,c.customers_firstname,c.is_wholesale, c.customers_email_address,
    a.entry_country_id, ci.customers_info_date_account_created, ci.customers_info_number_of_logons ,admin_id
FROM customers c LEFT JOIN address_book a ON c.customers_id = a.customers_id
LEFT JOIN customers_info AS ci ON ci.customers_info_id=c.customers_id
WHERE 1 ORDER BY ci.customers_info_date_account_created DESC LIMIT 0, 20

explain后发现

| id | select_type | table | type   | possible_keys                 | key                           | key_len | ref                          | rows   | Extra                           |
+----+-------------+-------+--------+-------------------------------+-------------------------------+---------+------------------------------+--------+---------------------------------+
|  1 | SIMPLE      | c     | ALL    | NULL                          | NULL                          | NULL    | NULL                         | 4064464 | Using temporary; Using filesort |
|  1 | SIMPLE      | a     | ref    | idx_address_book_customers_id | idx_address_book_customers_id | 4       | banggood_work.c.customers_id |      2 |                                 |
|  1 | SIMPLE      | ci    | eq_ref | PRIMARY                       | PRIMARY                       | 4       | banggood_work.c.customers_id |      1 |                                 |

customers表，即c表为全表扫描！而customers_info(ci)表的customers_info_date_account_created字段是有索引存在的，但是因为这个sql为left join，是用c表驱动ci，所以导致ci的customers_info_date_account_created字段上的索引无法使用！

但是我们可以稍微转换一下思路，customers_info_date_account_created字段的意思为账号创建时间，整个sql实现的功能是按照账号创建时间拿出最新的20个用户的信息。而我们知道,大部分系统的账号的客户id(customers_id)为自增所得，这就存在一个关系，账号创建时间的先后顺序，基本上对应了customers_id的大小关系！所以，后面的ci.customers_info_date_account_created DESC LIMIT 0, 20可以使用ORDER BY c.customers_id DESC LIMIT 0, 20进行替换

替换后sql如下

mysql> EXPLAIN
     SELECT c.dropshipper_level,c.customers_id,c.customers_firstname,c.is_wholesale, c.customers_email_address,
    a.entry_country_id, ci.customers_info_date_account_created, ci.customers_info_number_of_logons ,admin_id
     FROM customers c LEFT JOIN address_book a ON c.customers_id = a.customers_id
    LEFT JOIN customers_info AS ci ON ci.customers_info_id=c.customers_id
     WHERE 1 ORDER BY c.customers_id DESC LIMIT 0, 20;
+----+-------------+-------+--------+-------------------------------+-------------------------------+---------+------------------------------+------+-------+
| id | select_type | table | type   | possible_keys                 | key                           | key_len | ref                          | rows | Extra |
+----+-------------+-------+--------+-------------------------------+-------------------------------+---------+------------------------------+------+-------+
|  1 | SIMPLE      | c     | index  | NULL                          | PRIMARY                       | 4       | NULL                         |   10 |       |
|  1 | SIMPLE      | a     | ref    | idx_address_book_customers_id | idx_address_book_customers_id | 4       | banggood_work.c.customers_id |    2 |       |
|  1 | SIMPLE      | ci    | eq_ref | PRIMARY                       | PRIMARY                       | 4       | banggood_work.c.customers_id |    1 |       |

customers表已经使用索引。

且两次profiling对比发现，前者在执行过程中存在Copying to tmp table on disk状态，执行时间大约为20s；

而后者的Copying to tmp table on disk状态不复存在，执行时间大致为0.01s。

注意：

对于order by类型的sql，如果直接order by该字段，无法使用索引，可以试着寻找该字段的对应关系，相互转换后，类比的实现功能！