Waiting for table metadata lock

【問題描述】：Mysql5.6的主從環境，在主庫上添加字段始終添加不了，一直沒有返回值。
【排查分析】：
1、在測試環境上執行沒有問題，很快就返回。因為數據量不大，也就10w左右（線上也是10w左右）
2、在執行alter語句時，在另外的終端通過show processlist發現 有等待TML鎖

3、通過如下sql查線程ID

SELECT distinct b.trx_id blocking_trx_id,
b.trx_mysql_thread_id 源頭鎖thread_id,
SUBSTRING(p. HOST, 1, INSTR(p. HOST, ':') - 1) blocking_host,
SUBSTRING(p. HOST, INSTR(p. HOST, ':') + 1) blocking_port,
IF(p.COMMAND = 'Sleep', p.TIME, 0) idel_in_trx,
b.trx_query blocking_query,
r.trx_id waiting_trx_id,
r.trx_mysql_thread_id waiting_thread,
TIMESTAMPDIFF(SECOND, r.trx_wait_started, CURRENT_TIMESTAMP) wait_time,
r.trx_query waiting_query,
l.lock_table waiting_table_lock
FROM information_schema.INNODB_LOCKS l
LEFT JOIN information_schema.INNODB_LOCK_WAITS w
ON w.requested_lock_id = l.lock_id
LEFT JOIN information_schema.INNODB_TRX b
ON b.trx_id = w.blocking_trx_id
LEFT JOIN information_schema.INNODB_TRX r
ON r.trx_id = w.requesting_trx_id
LEFT JOIN information_schema. PROCESSLIST p
ON p.ID = b.trx_mysql_thread_id
ORDER BY wait_time DESC;

如果沒有返回值 ，使用下面的sql來查：

select i.trx_mysql_thread_id
from  information_schema.innodb_trx i,
  (select 
         id, time
     from
         information_schema.processlist
     where
         time = (select 
                 max(time)
             from
                 information_schema.processlist
             where
                 state = 'Waiting for table metadata lock'
                     and substring(info, 1, 5) in ('alter' , 'optim', 'repai', 'lock ', 'drop ', 'creat'))) p
  where timestampdiff(second, i.trx_started, now()) > p.time
  and i.trx_mysql_thread_id  not in (connection_id(),p.id);

根據返回的結果繼續查相關的session：
select * from information_schema.processlist where id=104018;

可以根據情況對該會話進行處理。