我的不太大的表挂在ALTER命令上。它可能是什么？
仅150k行，42个字段总计142 MByte。 InnoDB存储引擎和服务器版本：5.5.44-MariaDB MariaDB Server。 1个字段，＆＃39; slotindex＆＃39;，是主键：bigint（20）和BTREE类型。
命令：
MariaDB [mydb]> ALTER TABLE `runs` CHANGE `p_w_trans_x` `p_w_tran_x` FLOAT NOT NULL;
Stage: 1 of 2 'copy to tmp table' 65.7% of stage done
Stage: 2 of 2 'Enabling keys' 0% of stage done
在这个阶段2将永远完全挂起。
进程列表如下：
MariaDB [(none)]> show full processlist;
+--------+------+-----------------+-----------+---------+-------+---------------------------------+---------------------------------------------------------------------+----------+
| Id | User | Host | db | Command | Time | State | Info | Progress |
+--------+------+-----------------+-----------+---------+-------+---------------------------------+---------------------------------------------------------------------+----------+
| 274226 | root | localhost:45423 | edc_proxy | Sleep | 16043 | | NULL | 0.000 |
| 274319 | root | localhost | myDB | Query | 99 | Waiting for table metadata lock | ALTER TABLE `runs` CHANGE `p_w_trans_x` `p_w_tran_x` FLOAT NOT NULL | 0.000 |
| 274416 | root | localhost | NULL | Query | 0 | NULL | show full processlist | 0.000 |
+--------+------+-----------------+-----------+---------+-------+---------------------------------+---------------------------------------------------------------------+----------+
这个answer建议检查information_schema表，而不是那里：
MariaDB [INFORMATION_SCHEMA]> SELECT * FROM INNODB_LOCK_WAITS;
Empty set (0.00 sec)
MariaDB [INFORMATION_SCHEMA]> SELECT * FROM INNODB_LOCKS ;
Empty set (0.00 sec)
MariaDB [INFORMATION_SCHEMA]> SELECT * FROM INNODB_TRX;
+----------+-----------+---------------------+-----------------------+------------------+------------+---------------------+-----------+---------------------+-------------------+-------------------+------------------+-----------------------+-----------------+-------------------+-------------------------+---------------------+-------------------+------------------------+----------------------------+---------------------------+---------------------------+
| trx_id | trx_state | trx_started | trx_requested_lock_id | trx_wait_started | trx_weight | trx_mysql_thread_id | trx_query | trx_operation_state | trx_tables_in_use | trx_tables_locked | trx_lock_structs | trx_lock_memory_bytes | trx_rows_locked | trx_rows_modified | trx_concurrency_tickets | trx_isolation_level | trx_unique_checks | trx_foreign_key_checks | trx_last_foreign_key_error | trx_adaptive_hash_latched | trx_adaptive_hash_timeout |
+----------+-----------+---------------------+-----------------------+------------------+------------+---------------------+-----------+---------------------+-------------------+-------------------+------------------+-----------------------+-----------------+-------------------+-------------------------+---------------------+-------------------+------------------------+----------------------------+---------------------------+---------------------------+
| 83A8B36E | RUNNING | 2016-12-08 11:13:02 | NULL | NULL | 0 | 274226 | NULL | NULL | 0 | 0 | 0 | 376 | 0 | 0 | 0 | REPEATABLE READ | 1 | 1 | NULL | 0 | 10000 |
+----------+-----------+---------------------+-----------------------+------------------+------------+---------------------+-----------+---------------------+-------------------+-------------------+------------------+-----------------------+-----------------+-------------------+-------------------------+---------------------+-------------------+------------------------+----------------------------+---------------------------+---------------------------+
1 row in set (0.00 sec)
来自show engine innodb status;的交易部分：
------------
TRANSACTIONS
------------
Trx id counter 83A8F071
Purge done for trx's n:o < 83A8CA86 undo n:o < 0
History list length 1490
LIST OF TRANSACTIONS FOR EACH SESSION:
---TRANSACTION 0, not started
MySQL thread id 274543, OS thread handle 0x7fbb863e6700, query id 85356480 localhost root
show engine innodb status
---TRANSACTION 83A8EB07, not started
mysql tables in use 1, locked 2
MySQL thread id 274542, OS thread handle 0x7fbb843f6700, query id 85354935 localhost root Waiting for table metadata lock
ALTER TABLE `runs` CHANGE `p_w_trans_x` `p_w_tran_x` FLOAT NOT NULL
---TRANSACTION 83A8B36E, ACTIVE 24627 sec
MySQL thread id 274226, OS thread handle 0x7fbb845f5700, query id 85337236 localhost 127.0.0.1 root
Trx read view will not see trx with id >= 83A8B36F, sees < 83A8B36D
----------------------------
END OF INNODB MONITOR OUTPUT
============================
任何有关进一步调查，绕过问题和解决问题的建议都表示赞赏！
答案 0 :(得分：2)
元数据锁是一种隐式（从用户角度来看）锁定，可防止DDL对表进行操作，因为其他东西需要表保留其当前形式。在这种情况下，它是一个已经运行的事务。
任务1：如果您在线程274226上终止连接，则您的alter将成功。
mysql> KILL 274226;
这里的问题，如information_schema.innodb_trx所示，是这个线程已经运行了几个小时的事务，我们可以推断该表已被该事务引用。在没有transactiom仍然具有MVCC视图或涉及该表的任何锁定之前，不能更改表。此事务包含一个视图，我们可以再次推断可能影响此表，如最后一行所示：
--TRANSACTION 83A8B36E, ACTIVE 24627 sec
MySQL thread id 274226, OS thread handle 0x7fbb845f5700, query id 85337236 localhost 127.0.0.1 root
Trx read view will not see trx with id >= 83A8B36F, sees < 83A8B36D
请注意，Sleep不是真正的命令，在此上下文中，它只是任何空闲连接的占位符状态。所有连接都在做某事，在这种情况下，“某事”正在休眠 - 换句话说，空闲并等待另一个查询。但是空闲连接仍然是一个连接，如果你的代码（或查询浏览器工具）让事务运行，它就会继续运行。
任务2：找到导致该事务运行的错误或错误。在实时应用程序中，让正在运行的事务可能会造成更大的混乱。
答案 1 :(得分：0)
我遇到了一个与表锁非常相似的问题，但最终它变成了 MySQL Workbench。 MySQL Workbench 中的任何 RENAME TABLE 或 ALTER TABLE 命令都只会要求元锁定。我登录到服务器并能够执行这些类型的查询没问题。当前版本的 Workbench 是 8.0.22。
我已经筛选了所有这些查询，当它们没有显示任何问题时，我完全被难住了：
SHOW OPEN TABLES;
SHOW ENGINE inndodb STATUS;
SELECT * FROM INNODB_LOCK_WAITS;
SELECT * FROM INNODB_LOCKS;
SELECT * FROM INNODB_TRX;
SHOW FULL PROCESSLIST;