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mysql-plugin-mdl-info

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mysql information plugin to show all mdl locks

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mysql-plugin-mdl-info
=======================

This plugin is used to show all metadata locks (MDL) info of mysql 5.5+.
The behavior of metadata locks has changed in mysql 5.5 . DDL statements may be blocked by MDL in some unobvious way like `begin; select * from some table;` and has no way to find out by whom the table is locked. This plugin used a trick way to fetch the MDL information and now, you can know who lock the tables through a information schema table.

usage
-----

first, compile the plugin and install in to plugin dir

cp -r src /path/to/mysql-src/plugin/mdl_info
    cd /path/to/mysql-src
    cmake . -DBUILD_CONFIG=mysql_release
    cd plugin/mdl_info
    make
    make install
    
CAUTION: mysql plugins MUST be built using the same version of the source code and the same build arguments. If mysqld is built as a debug version without cmake parameter -DBUILD_CONFIG, the parameter must not be added when compiling plugins.    
    
then, load the plugin into mysql

mysql> INSTALL PLUGIN MDL_INFO SONAME 'mdl_info.so';
    
thus, you can found `MDL_INFO` table in `information_schema` database
    
    mysql> lock tables plugin read;
    Query OK, 0 rows affected (0.00 sec)
    
    mysql> select * from information_schema.MDL_INFO;
    +-----------+----------+-------------+-----------+----------+--------+
    | THREAD_ID | DURATION | TYPE        | NAMESPACE | DATABASE | NAME   |
    +-----------+----------+-------------+-----------+----------+--------+
    |         4 | EXPLICIT | SHARED_READ | TABLE     | mysql    | plugin |
    +-----------+----------+-------------+-----------+----------+--------+
    1 row in set (0.00 sec)
    
    mysql> unlock tables;
    Query OK, 0 rows affected (0.00 sec)
    
    mysql> lock tables plugin write;
    Query OK, 0 rows affected (0.00 sec)
    
    mysql> select * from information_schema.MDL_INFO;
    +-----------+----------+----------------------+-----------+----------+--------+
    | THREAD_ID | DURATION | TYPE                 | NAMESPACE | DATABASE | NAME   |
    +-----------+----------+----------------------+-----------+----------+--------+
    |         4 | EXPLICIT | INTENTION_EXCLUSIVE  | GLOBAL    |          |        |
    |         4 | EXPLICIT | SHARED_NO_READ_WRITE | TABLE     | mysql    | plugin |
    |         4 | EXPLICIT | INTENTION_EXCLUSIVE  | SCHEMA    | mysql    |        |
    +-----------+----------+----------------------+-----------+----------+--------+
    3 rows in set (0.00 sec)
    
    mysql> unlock tables;
    Query OK, 0 rows affected (0.00 sec)
    
    mysql> begin;
    Query OK, 0 rows affected (0.00 sec)
    
    mysql> select * from plugin;
    +-----------+--------------+
    | name      | dl           |
    +-----------+--------------+
    | MDL_INFO | mdl_info.so |
    +-----------+--------------+
    1 row in set (0.01 sec)
    
    mysql> select * from information_schema.MDL_INFO;
    +-----------+-------------+-------------+-----------+----------+--------+
    | THREAD_ID | DURATION    | TYPE        | NAMESPACE | DATABASE | NAME   |
    +-----------+-------------+-------------+-----------+----------+--------+
    |         4 | TRANSACTION | SHARED_READ | TABLE     | mysql    | plugin |
    +-----------+-------------+-------------+-----------+----------+--------+
    1 row in set (0.01 sec)
    
    mysql> rollback;
    Query OK, 0 rows affected (0.00 sec)
    
    mysql> flush tables with read lock;
    Query OK, 0 rows affected (0.00 sec)
    
    mysql> select * from information_schema.MDL_INFO;
    +-----------+----------+--------+-----------+----------+------+
    | THREAD_ID | DURATION | TYPE   | NAMESPACE | DATABASE | NAME |
    +-----------+----------+--------+-----------+----------+------+
    |         4 | EXPLICIT | SHARED | COMMIT    |          |      |
    |         4 | EXPLICIT | SHARED | GLOBAL    |          |      |
    +-----------+----------+--------+-----------+----------+------+
    2 rows in set (0.00 sec)
    
    mysql> unlock tables;
    Query OK, 0 rows affected (0.00 sec)