正如在other questions中所建立的那样，使用＆＃34;子查询＆＃34;在MySQL中删除会导致它变慢，而相同的＆＃34;选择＆＃34;查询执行速度快：
MariaDB [as_01_import]> explain select * from invoice_payment where invoice_id in (select id from dochead where system_id = 5786);
+------+-------------+-----------------+------+---------------------------------------+----------------------------+---------+-------------------------+------+-------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+------+-------------+-----------------+------+---------------------------------------+----------------------------+---------+-------------------------+------+-------------+
| 1 | PRIMARY | dochead | ref | PRIMARY,dochead_system_id | dochead_system_id | 4 | const | 891 | Using index |
| 1 | PRIMARY | invoice_payment | ref | invoice_payment_invoice_fk,invoice_id | invoice_payment_invoice_fk | 4 | as_01_import.dochead.id | 1 | |
+------+-------------+-----------------+------+---------------------------------------+----------------------------+---------+-------------------------+------+-------------+
MariaDB [as_01_import]> explain delete from invoice_payment where invoice_id in (select id from dochead where system_id = 5786);
+------+--------------------+-----------------+-----------------+---------------------------+---------+---------+------+---------+-------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+------+--------------------+-----------------+-----------------+---------------------------+---------+---------+------+---------+-------------+
| 1 | PRIMARY | invoice_payment | ALL | NULL | NULL | NULL | NULL | 1235451 | Using where |
| 2 | DEPENDENT SUBQUERY | dochead | unique_subquery | PRIMARY,dochead_system_id | PRIMARY | 4 | func | 1 | Using where |
+------+--------------------+-----------------+-----------------+---------------------------+---------+---------+------+---------+-------------+
2 rows in set (0.44 sec)
知道JOIN可以使用索引，我想问专家：
什么阻止MySQL / MariaDB在DELETE中使用SUBQUERY中的索引？这是一个实现问题还是存在概念问题？有没有计划解决这个问题？影响其他SQL供应商的问题是否相同？
答案 0 :(得分：2)
子查询是派生表，未实现。它们表现在临时表中。
正如我在this answer写的那样：
文档Derived Tables in MySQL 5.7很好地描述了它 版本5.6和5.7，后者将不提供任何惩罚 物化衍生表输出的变化被合并 进入外部查询。在以前的版本中，大量的开销是 使用派生的临时表忍受。
答案 1 :(得分：1)
请勿使用IN ( SELECT ... )。相反，请使用此处所述的多表DELETE：http://dev.mysql.com/doc/refman/5.5/en/delete.html
这样做会在适用的情况下使用索引。