MySQL:5.7中delete长期处于preparing状态简析

一、问题展示

实际这里是是一个delete in语句。

mysql> show processlist;
+----+------+-----------+------+---------+------+-----------+-----------------------------------------------------------------------------+-----------+---------------+
| Id | User | Host      | db   | Command | Time | State     | Info                                                                        | Rows_sent | Rows_examined |
+----+------+-----------+------+---------+------+-----------+-----------------------------------------------------------------------------+-----------+---------------+
|  2 | root | localhost | ttt  | Query   |   14 | preparing | delete from testout2 where id not in(select id from testin2 where name='t') |         0 |         37151 |
|  7 | root | localhost | NULL | Query   |    0 | starting  | show processlist                                                            |         0 |             0 |
+----+------+-----------+------+---------+------+-----------+-----------------------------------------------------------------------------+-----------+---------------+
2 rows in set (0.00 sec)

mysql> show processlist;
+----+------+-----------+------+---------+------+-----------+-----------------------------------------------------------------------------+-----------+---------------+
| Id | User | Host      | db   | Command | Time | State     | Info                                                                        | Rows_sent | Rows_examined |
+----+------+-----------+------+---------+------+-----------+-----------------------------------------------------------------------------+-----------+---------------+
|  2 | root | localhost | ttt  | Query   |   15 | preparing | delete from testout2 where id not in(select id from testin2 where name='t') |         0 |         37744 |
|  7 | root | localhost | NULL | Query   |    0 | starting  | show processlist                                                            |         0 |             0 |
+----+------+-----------+------+---------+------+-----------+-----------------------------------------------------------------------------+-----------+---------------+
2 rows in set (0.01 sec)

大概就是这样一个语句，处于preparing 状态，问delete为什么会处于这个状态？

二、简析

对于preparing状态一般归结为优化阶段，并还没有进入实际的语句执行阶段。但是这里有所不同。并且我们知道一般delete执行慢一般状态会处于，

• updating
• 子查询的sending data中

但是这里是preparing状态。那么为什么会处于这种状态呢？简单分析一下。

我们知道5.7的delete in实际上还是用的DEPENDENT SUBQUERY。这个类似exists的操作，都会用外层查询驱动内层。稍微跟了一下5.7的delete in（不做详细研究），发现所有的query block并没有完全完成优化，实际上这种子查询会在调用Item_subselect::exec的时候通过

• if (!unit->is_optimized() && unit->optimize(thd))

进行判断，如果unit->is_optimized()判断为false，则会调入unit->optimize(thd)，对subquery的query block进行优化，而优化结束后查询的状态会被设置为preparing。也就出现了这种现象，如下：

T@2: | | | | | THD::enter_stage: 'init' /opt/percona-server-locks-detail-5.7.22/sql/sql_delete.cc:79
T@2: | | | | | | | THD::enter_stage: 'System lock' /opt/percona-server-locks-detail-5.7.22/sql/lock.cc:323
T@2: | | | | | THD::enter_stage: 'updating' /opt/percona-server-locks-detail-5.7.22/sql/sql_delete.cc:425
T@2: | | | | | | | | | | THD::enter_stage: 'optimizing' /opt/percona-server-locks-detail-5.7.22/sql/sql_optimizer.cc:151
T@2: | | | | | | | | | | THD::enter_stage: 'statistics' /opt/percona-server-locks-detail-5.7.22/sql/sql_optimizer.cc:386
T@2: | | | | | | | | | | THD::enter_stage: 'preparing' /opt/percona-server-locks-detail-5.7.22/sql/sql_optimizer.cc:494
T@2: | | | | | THD::enter_stage: 'end' /opt/percona-server-locks-detail-5.7.22/sql/sql_delete.cc:538

Item_in_subselect::val_bool
  ->st_select_lex_unit::optimize
    ->st_select_lex::optimize
      ->JOIN::optimize
        ->THD::enter_stage(stage_preparing)

这种现象当前看起来是5.7（5.7以下没有测试）中DML语句带in/not in常见的，也可以归结为状态转换稍微考虑欠妥，可能开发大佬也没意识到这个状态已经变为DBA诊断问题的一个主要手段。
而到了8.0状态已经更加精细，有了一些细微的变化，而且8.0.21后支持DML 中的in使用semi join(not in还不行，但是状态是executing）优化。所以我们简单的将其归结为正在执行类似sending data状态即可。需要优化的是将这种delete改写为联合delete形式。