MariaDB InnoDB表：如何找到导致"waiting for table metadata lock"的语句

Question

我如何确定什么SQL语句是线程ID显示在元数据锁信息行(SELECT * FROM information_schema.metadata_lock_info)上的MariaDB?

Server version: 10.0.15-MariaDB MariaDB Server

所有相关的问题都深入到"Waiting for table metadata lock";从MySQL的角度来看，但这对MariaDB没有帮助，因为他们的自省实现与我所知道的不同。在谷歌上搜索不会出现很多。

A "显示完整的processlist"给出如下行:

| 57295 | main  | localhost | joints | Execute |   50 | Waiting for table metadata lock | select ...

显示语句，但也不显示它有锁。因此，我打开了元数据锁信息，如下所示[0]。这只提供锁持有者的线程ID，而不提供语句:

MariaDB [joints]> SELECT * FROM information_schema.metadata_lock_info;
+-----------+--------------------------+-----------------+----------------------+--------------+----------------+
| THREAD_ID | LOCK_MODE                | LOCK_DURATION   | LOCK_TYPE            | TABLE_SCHEMA | TABLE_NAME     |
+-----------+--------------------------+-----------------+----------------------+--------------+----------------+
|     57322 | MDL_INTENTION_EXCLUSIVE  | MDL_EXPLICIT    | Global read lock     |              |                |
|     57322 | MDL_SHARED_NO_READ_WRITE | MDL_EXPLICIT    | Table metadata lock  | joints       | 16_study       |
|     57322 | MDL_INTENTION_EXCLUSIVE  | MDL_EXPLICIT    | Schema metadata lock | joints       |                |
|     57269 | MDL_SHARED_READ          | MDL_TRANSACTION | Table metadata lock  | joints       | authentication |
|     57301 | MDL_SHARED_READ          | MDL_TRANSACTION | Table metadata lock  | joints       | authentication |
|     57280 | MDL_SHARED_READ          | MDL_TRANSACTION | Table metadata lock  | joints       | authentication |
|     57317 | MDL_SHARED_READ          | MDL_TRANSACTION | Table metadata lock  | joints       | ship           |
|     57271 | MDL_SHARED_READ          | MDL_TRANSACTION | Table metadata lock  | joints       | administration |
|     57264 | MDL_SHARED_READ          | MDL_TRANSACTION | Table metadata lock  | joints       | server         |
+-----------+--------------------------+-----------------+----------------------+--------------+----------------+

我真正想看到的是"join"。这两个输出在发生锁定时的状态。我没有看到连接这两个"表"中的数据的方法。因为前者看起来并不是一张桌子。我想避免得到:

ERROR 1933 (HY000): Target is not running an EXPLAINable command

当尝试实时执行时，由于线程在检查时结束。

[0] https://mariadb.com/kb/en/mariadb/metadata_lock_info/

Answer 1

THREAD_ID映射到information_schema.PROCESSLIST.ID (show [full] processlist;的第一列)。即:

SELECT * FROM information_schema.METADATA_LOCK_INFO AS mli
JOIN information_schema.PROCESSLIST AS pl ON mli.THREAD_ID = pl.ID

我倾向于像下面这样的东西，以便更容易看到正在发生的事情(换行符不能很好地与cli一起工作):

SELECT
  mli.THREAD_ID, mli.LOCK_MODE, mli.LOCK_TYPE, 
  CAST(GROUP_CONCAT(DISTINCT CONCAT(mli.TABLE_SCHEMA, '.', mli.TABLE_NAME) ORDER BY mli.TABLE_SCHEMA, mli.TABLE_NAME SEPARATOR 'n') AS CHAR) AS locked_tables, 
  pl.USER, pl.HOST, pl.DB, pl.COMMAND, pl.TIME, pl.STATE, pl.INFO, pl.QUERY_ID, pl.TID
FROM information_schema.METADATA_LOCK_INFO AS mli
JOIN information_schema.PROCESSLIST AS pl ON mli.THREAD_ID = pl.ID
GROUP BY mli.THREAD_ID, mli.LOCK_MODE, mli.LOCK_TYPE
ORDER BY time DESC, pl.ID;

特别有趣的是，当pl.COMMAND = 'Sleep'表示一些连接池或其他(主要是只读的)程序持有对它们有锁的打开连接时。