Waiting for table metadata lock

【问题描述】：Mysql5.6的主从环境，在主库上添加字段始终添加不了，一直没有返回值。
【排查分析】：
1、在测试环境上执行没有问题，很快就返回。因为数据量不大，也就10w左右（线上也是10w左右）
2、在执行alter语句时，在另外的终端通过show processlist发现 有等待TML锁

3、通过如下sql查线程ID

SELECT distinct b.trx_id blocking_trx_id,b.trx_mysql_thread_id 源头锁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_lockFROM information_schema.INNODB_LOCKS lLEFT JOIN information_schema.INNODB_LOCK_WAITS wON w.requested_lock_id = l.lock_idLEFT JOIN information_schema.INNODB_TRX bON b.trx_id = w.blocking_trx_idLEFT JOIN information_schema.INNODB_TRX rON r.trx_id = w.requesting_trx_idLEFT JOIN information_schema. PROCESSLIST pON p.ID = b.trx_mysql_thread_idORDER BY wait_time DESC;

如果没有返回值 ，使用下面的sql来查：

select i.trx_mysql_thread_idfrom  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);

根据返回的结果继续查相关的session：
select * from information_schema.processlist where id=104018;

可以根据情况对该会话进行处理。