【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_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;

如果没有返回值 ，使用下面的sql来查：

select i.trx_mysql_thread_id
from  information_schema.innodb_trx i,(select id, timefrominformation_schema.processlistwheretime = (select max(time)frominformation_schema.processlistwherestate = 'Waiting for table metadata lock'and substring(info, 1, 5) in ('alter' , 'optim', 'repai', 'lock ', 'drop ', 'creat'))) pwhere timestampdiff(second, i.trx_started, now()) > p.timeand 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。