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

Question

如何确定 MariaDB 上元数据锁定信息行 (SELECT * FROM information_schema.metadata_lock_info) 中显示的线程 ID 的 SQL 语句是什么？

Server version: 10.0.15-MariaDB MariaDB Server

所有相关问题都从 MySQL 的角度深入到“等待 table 元数据锁定”，但这对 MariaDB 没有帮助，因为它们的内省实现方式与我所能实现的不同告诉。谷歌搜索并没有出现很多。

“显示完整进程列表”给出的行如下：

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

它确实显示了语句，但也没有显示它有锁。因此，我按照此处 [0] 的说明打开了元数据锁定信息。这里只提供锁持有者的线程ID，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         |
+-----------+--------------------------+-----------------+----------------------+--------------+----------------+

我真正想要的是在发生锁定时看到这两个输出的“连接”。我看不到从这两个“table”中加入数据的方法，因为前者似乎不是 table。我想避免得到：

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

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; 中的第一列。即：

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

我更喜欢下面这样的东西，这样可以更容易地看到正在发生的事情（虽然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 pl.COMMAND = 'Sleep'，因为这表明某些连接池或其他（主要是只读的）程序正在持有打开的连接，这些连接上有锁。

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

MariaDB InnoDB table: how to find statement causing "waiting for table metadata lock"

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