问题
mysql> status
--------------
mysql Ver 14.14 Distrib 5.7.25, for Linux (x86_64) using EditLine wrapper
Connection id: 17190053
Current database: gkbb
Current user: root@10.9.165.246
SSL: Not in use
Current pager: less
Using outfile: ''
Using delimiter: ;
Server version: 5.5.5-10.1.26-MariaDB MariaDB Server
Protocol version: 10
Connection: 10.9.108.125 via TCP/IP
Server characterset: utf8
Db characterset: utf8
Client characterset: utf8
Conn. characterset: utf8
TCP port: 3306
Uptime: 13 days 21 hours 14 min 38 sec
Threads: 276 Questions: 31378648 Slow queries: 212 Opens: 2977 Flush tables: 1 Open tables: 2761 Queries per second avg: 26.155
--------------
mysql> show global variables like "innodb_version";
+----------------+-------------+
| Variable_name | Value |
+----------------+-------------+
| innodb_version | 5.6.36-82.1 |
+----------------+-------------+
1 row in set (0.06 sec)
PS: 查看 status 还有一个快捷方式 \s。
编辑测试库表结构（添加字段），卡住，任何操作都不行了，等一个多小时，还是不行。。
还一度怀疑是不是表结构设计问题，字段、数据是不是太多了。
过程
偶尔想起看看会话情况：
SELECT * FROM information_schema.processlist WHERE db = 'mydb';
或命令：
mysqladmin -uroot -p123456 processlistmysql -uroot -p123456 -e 'SHOW PROCESSLIST'
看到里面好几个会话的状态都是 wait for table metadata lock，这就有点奇怪了，之前没有见过。
网上的资料显示：
为了在并发环境下维护表元数据的数据一致性，在表上有活动事务（显式或隐式）的时候，不可以对元数据进行写入操作。因此 MySQL 引入了 metadata lock ，来保护表的元数据信息。
因此在对表进行上述操作时，如果表上有活动事务（未提交或回滚），请求写入的会话会等待在 Metadata lock wait 。
如果资料没错，那么就是说，如果有事务没有结束，DDL 操作请求 MDL（metadata lock）时会卡住这张表。
我想起我们的服务中确实存在会话没有关闭的情况。
- 用了 SQLAlchemy 做 ORM
- 每次查询都使用一个会话，包括 SELECT
- 增删改操作都立即 commit 了，SELECT 却没有（记得是有个什么原因特意如此）
合理怀疑：这个查询 SESSION 没有关闭，导致 ALTER 语句进入 MDL 等待状态，然后导致了表无法进行任何操作（包括查询，至于为什么这样，我不知道）。
本地复现
- 开两个终端，分别建立 MySQL 连接。
- 其中一个终端（A）：
SET SESSION auto_commit = 0;SELECT * FROM test.test LIMIT 1;- 另一个终端（B）只需要：
TRUNCATE test.test;，然后发现：卡住了。
PS:
- DDL 需要 metadata 锁。
TRUNCATE属于 DDL，可能因为其非事务性（不支持提交和回滚）。参考：https://dba.stackexchange.com/questions/36607/why-is-truncate-ddl
现在，回到终端 A：
mysql> select * from information_schema.processlist where db = 'test';
+----+------+-----------+------+---------+------+---------------------------------+----------------------------------------------------------------+
| ID | USER | HOST | DB | COMMAND | TIME | STATE | INFO |
+----+------+-----------+------+---------+------+---------------------------------+----------------------------------------------------------------+
| 3 | root | localhost | test | Query | 0 | executing | select * from information_schema.processlist where db = 'test' |
| 5 | root | localhost | test | Query | 6111 | Waiting for table metadata lock | truncate test |
+----+------+-----------+------+---------+------+---------------------------------+----------------------------------------------------------------+
2 rows in set (0.00 sec)
mysql> select * from information_schema.innodb_trx\G
*************************** 1. row ***************************
trx_id: 421232684444408
trx_state: RUNNING
trx_started: 2019-03-29 16:06:14
trx_requested_lock_id: NULL
trx_wait_started: NULL
trx_weight: 0
trx_mysql_thread_id: 3
trx_query: select * from information_schema.innodb_trx
trx_operation_state: NULL
trx_tables_in_use: 0
trx_tables_locked: 0
trx_lock_structs: 0
trx_lock_memory_bytes: 1136
trx_rows_locked: 0
trx_rows_modified: 0
trx_concurrency_tickets: 0
trx_isolation_level: REPEATABLE READ
trx_unique_checks: 1
trx_foreign_key_checks: 1
trx_last_foreign_key_error: NULL
trx_adaptive_hash_latched: 0
trx_adaptive_hash_timeout: 0
trx_is_read_only: 0
trx_autocommit_non_locking: 0
1 row in set (0.00 sec)
mysql> show engine innodb status\G
*************************** 1. row ***************************
Type: InnoDB
Name:
Status:
=====================================
2019-03-29 19:04:40 0x7f1bcc1d6700 INNODB MONITOR OUTPUT
=====================================
Per second averages calculated from the last 3 seconds
-----------------
BACKGROUND THREAD
-----------------
srv_master_thread loops: 7 srv_active, 0 srv_shutdown, 11228 srv_idle
srv_master_thread log flush and writes: 11234
----------
SEMAPHORES
----------
OS WAIT ARRAY INFO: reservation count 16
OS WAIT ARRAY INFO: signal count 10
RW-shared spins 0, rounds 27, OS waits 12
RW-excl spins 0, rounds 32, OS waits 0
RW-sx spins 0, rounds 0, OS waits 0
Spin rounds per wait: 27.00 RW-shared, 32.00 RW-excl, 0.00 RW-sx
------------
TRANSACTIONS
------------
Trx id counter 54542
Purge done for trx's n:o < 54542 undo n:o < 0 state: running but idle
History list length 53
LIST OF TRANSACTIONS FOR EACH SESSION:
---TRANSACTION 421232684445328, not started
0 lock struct(s), heap size 1136, 0 row lock(s)
---TRANSACTION 421232684443488, not started
0 lock struct(s), heap size 1136, 0 row lock(s)
--------
FILE I/O
--------
I/O thread 0 state: waiting for completed aio requests (insert buffer thread)
I/O thread 1 state: waiting for completed aio requests (log thread)
I/O thread 2 state: waiting for completed aio requests (read thread)
I/O thread 3 state: waiting for completed aio requests (read thread)
I/O thread 4 state: waiting for completed aio requests (read thread)
I/O thread 5 state: waiting for completed aio requests (read thread)
I/O thread 6 state: waiting for completed aio requests (write thread)
I/O thread 7 state: waiting for completed aio requests (write thread)
I/O thread 8 state: waiting for completed aio requests (write thread)
I/O thread 9 state: waiting for completed aio requests (write thread)
Pending normal aio reads: [0, 0, 0, 0] , aio writes: [0, 0, 0, 0] ,
ibuf aio reads:, log i/o's:, sync i/o's:
Pending flushes (fsync) log: 0; buffer pool: 0
639 OS file reads, 99 OS file writes, 21 OS fsyncs
0.00 reads/s, 0 avg bytes/read, 0.00 writes/s, 0.00 fsyncs/s
-------------------------------------
INSERT BUFFER AND ADAPTIVE HASH INDEX
-------------------------------------
Ibuf: size 1, free list len 0, seg size 2, 4 merges
merged operations:
insert 0, delete mark 0, delete 0
discarded operations:
insert 0, delete mark 0, delete 0
Hash table size 34679, node heap has 0 buffer(s)
Hash table size 34679, node heap has 0 buffer(s)
Hash table size 34679, node heap has 0 buffer(s)
Hash table size 34679, node heap has 0 buffer(s)
Hash table size 34679, node heap has 0 buffer(s)
Hash table size 34679, node heap has 0 buffer(s)
Hash table size 34679, node heap has 0 buffer(s)
Hash table size 34679, node heap has 0 buffer(s)
0.00 hash searches/s, 0.00 non-hash searches/s
---
LOG
---
Log sequence number 26417867
Log flushed up to 26417867
Pages flushed up to 26417867
Last checkpoint at 26417858
0 pending log flushes, 0 pending chkp writes
17 log i/o's done, 0.00 log i/o's/second
----------------------
BUFFER POOL AND MEMORY
----------------------
Total large memory allocated 137428992
Dictionary memory allocated 133441
Buffer pool size 8192
Free buffers 7710
Database pages 482
Old database pages 0
Modified db pages 0
Pending reads 0
Pending writes: LRU 0, flush list 0, single page 0
Pages made young 0, not young 0
0.00 youngs/s, 0.00 non-youngs/s
Pages read 446, created 41, written 72
0.00 reads/s, 0.00 creates/s, 0.00 writes/s
No buffer pool page gets since the last printout
Pages read ahead 0.00/s, evicted without access 0.00/s, Random read ahead 0.00/s
LRU len: 482, unzip_LRU len: 0
I/O sum[0]:cur[0], unzip sum[0]:cur[0]
--------------
ROW OPERATIONS
--------------
0 queries inside InnoDB, 0 queries in queue
1 read views open inside InnoDB
Process ID=4492, Main thread ID=139757271107328, state: sleeping
Number of rows inserted 6, updated 0, deleted 0, read 20
0.00 inserts/s, 0.00 updates/s, 0.00 deletes/s, 0.00 reads/s
----------------------------
END OF INNODB MONITOR OUTPUT
============================
1 row in set (0.00 sec)
表明：事务存在，TRUNCATE 锁等待。
如果，kill 3 干掉这个没有 commit 的查询 SESSION，TRUNCATE 就会正常执行下去。
结论
- 不管怎样，应该保持事务的短小精干，快速执行和退出。
PS：业务代码中SELECT之前为什么不提交的问题需要进一步检查。 - DDL 需要 MDL，而没有结束的事务会阻止 MDL。
更多细节，需要更多深入了解。
