查看 Waiting for table metadata lock 相關信息

mysql 有兩個視圖記錄 metadata_lock  相關 的信息：

performance_schema.metadata_locks

sys.schema_table_lock_waits

但是默認是情況下，即使出現了 Waiting for table metadata lock ，兩個視圖裏面也沒有任何的記錄 。

如果想在兩個視圖中看到相關信息，需要啓用相關的設置：
 

primary:performance_schema 11:38:34>update setup_consumers set enabled='YES' where name ='global_instrumentation';
Query OK, 0 rows affected (0.01 sec)
Rows matched: 1  Changed: 0  Warnings: 0

primary:performance_schema 11:40:28>update setup_instruments set enabled='YES' where name='wait/lock/metadata/sql/mdl';
Query OK, 1 row affected (0.01 sec)
Rows matched: 1  Changed: 1  Warnings: 0

 

下面我們用一個案例來進行分析：

session 1	session 2
begin; select * from hlb limit 1;
	alter table hlb add cxx int;

 

該視圖記錄的是被阻塞會話的信息，因此我們需要 kill 9 來rollback 會話 9 。

primary:performance_schema 11:57:32>SELECT * from sys.schema_table_lock_waits\G
*************************** 1. row ***************************
               object_schema: bak
                 object_name: hlb
           waiting_thread_id: 31         --等待的線程ID
                 waiting_pid: 6          --等待的連接PID
             waiting_account: root@localhost    
           waiting_lock_type: EXCLUSIVE    --等待的鎖類型
       waiting_lock_duration: TRANSACTION
               waiting_query: alter table bak.hlb add cxxx int
          waiting_query_secs: 4           --鎖等待時間
 waiting_query_rows_affected: 0
 waiting_query_rows_examined: 0
          blocking_thread_id: 31
                blocking_pid: 6
            blocking_account: root@localhost  
          blocking_lock_type: SHARED_UPGRADABLE  --等待的鎖類型
      blocking_lock_duration: TRANSACTION
     sql_kill_blocking_query: KILL QUERY 6
sql_kill_blocking_connection: KILL 6
*************************** 2. row ***************************
               object_schema: bak
                 object_name: hlb
           waiting_thread_id: 31
                 waiting_pid: 6
             waiting_account: root@localhost
           waiting_lock_type: EXCLUSIVE
       waiting_lock_duration: TRANSACTION
               waiting_query: alter table bak.hlb add cxxx int
          waiting_query_secs: 4
 waiting_query_rows_affected: 0
 waiting_query_rows_examined: 0
          blocking_thread_id: 34
                blocking_pid: 9
            blocking_account: root@localhost
          blocking_lock_type: SHARED_READ
      blocking_lock_duration: TRANSACTION
     sql_kill_blocking_query: KILL QUERY 9
sql_kill_blocking_connection: KILL 9
2 rows in set (0.01 sec)

另外MDL 是在mysql server 層的鎖，而innodb 層也有表級別上的鎖（IS/IX）,但兩者並不是一回事

MDL 鎖等待超時閾值 由選項 lock_wait_timeout 控制，默認值是一年，我們可以將它調低