这是一个随机行为（重新启动mysql会话后未发生），当我将DDL查询包装在事务T1中并同时在同一表上使用select查询启动另一个事务T2时，DDL查询会陷入等待表元数据锁定的状态，而其他任何选择查询都将等待DDL查询完成。但是在提交T2之后，DDL应该获得元数据锁定并完成操作，但它仍在等待表元数据锁定状态。
连接1查询：
mysql> begin;
Query OK, 0 rows affected (0.00 sec)
mysql> ALTER TABLE merchants ADD COLUMN temp6 varchar(255);
连接2查询：
mysql> begin;
Query OK, 0 rows affected (0.00 sec)
mysql> select * from merchants where account_id=null;
Empty set (0.00 sec)
mysql> commit;
Query OK, 0 rows affected (0.00 sec)
查询顺序为：
Server version: 8.0.13 MySQL Community Server - GPL
mysql> select @@autocommit;
+--------------+
| @@autocommit |
+--------------+
| 1 |
+--------------+
1 row in set (0.00 sec)
mysql> SELECT
OBJECT_TYPE,
OBJECT_SCHEMA,
OBJECT_NAME,
LOCK_TYPE,
LOCK_STATUS,
THREAD_ID,
PROCESSLIST_ID,
PROCESSLIST_INFO
FROM performance_schema.metadata_locks
INNER JOIN performance_schema.threads ON THREAD_ID = OWNER_THREAD_ID
WHERE PROCESSLIST_ID <> CONNECTION_ID();
+-------------+-------------------------+-----------------------------------+---------------------+-------------+-----------+----------------+-----------------------------------------------------+
| OBJECT_TYPE | OBJECT_SCHEMA | OBJECT_NAME | LOCK_TYPE | LOCK_STATUS | THREAD_ID | PROCESSLIST_ID | PROCESSLIST_INFO |
+-------------+-------------------------+-----------------------------------+---------------------+-------------+-----------+----------------+-----------------------------------------------------+
| GLOBAL | NULL | NULL | INTENTION_EXCLUSIVE | GRANTED | 126 | 87 | ALTER TABLE merchants ADD COLUMN temp6 varchar(255) |
| SCHEMA | merchant_onboarding_dev | NULL | INTENTION_EXCLUSIVE | GRANTED | 126 | 87 | ALTER TABLE merchants ADD COLUMN temp6 varchar(255) |
| TABLE | merchant_onboarding_dev | merchants | SHARED_UPGRADABLE | GRANTED | 126 | 87 | ALTER TABLE merchants ADD COLUMN temp6 varchar(255) |
| BACKUP LOCK | NULL | NULL | INTENTION_EXCLUSIVE | GRANTED | 126 | 87 | ALTER TABLE merchants ADD COLUMN temp6 varchar(255) |
| TABLESPACE | NULL | merchant_onboarding_dev/merchants | INTENTION_EXCLUSIVE | GRANTED | 126 | 87 | ALTER TABLE merchants ADD COLUMN temp6 varchar(255) |
| TABLE | merchant_onboarding_dev | #sql-63_57 | EXCLUSIVE | GRANTED | 126 | 87 | ALTER TABLE merchants ADD COLUMN temp6 varchar(255) |
| TABLE | merchant_onboarding_dev | merchants | EXCLUSIVE | PENDING | 126 | 87 | ALTER TABLE merchants ADD COLUMN temp6 varchar(255) |
+-------------+-------------------------+-----------------------------------+---------------------+-------------+-----------+----------------+-----------------------------------------------------+
7 rows in set (0.00 sec)
mysql> show full processlist;
+----+-----------------+-----------------+-------------------------+---------+--------+---------------------------------+-----------------------------------------------------+
| Id | User | Host | db | Command | Time | State | Info |
+----+-----------------+-----------------+-------------------------+---------+--------+---------------------------------+-----------------------------------------------------+
| 4 | event_scheduler | localhost | NULL | Daemon | 115824 | Waiting on empty queue | NULL |
| 62 | root | localhost:59116 | merchant_onboarding_dev | Sleep | 107 | | NULL |
| 63 | root | localhost:59117 | merchant_onboarding_dev | Sleep | 107 | | NULL |
| 65 | root | localhost:59119 | NULL | Sleep | 1 | | NULL |
| 79 | root | localhost | merchant_onboarding_dev | Query | 0 | starting | show full processlist |
| 81 | root | localhost | merchant_onboarding_dev | Sleep | 838 | | NULL |
| 83 | root | localhost | merchant_onboarding_dev | Sleep | 821 | | NULL |
| 87 | root | localhost | merchant_onboarding_dev | Query | 842 | Waiting for table metadata lock | ALTER TABLE merchants ADD COLUMN temp6 varchar(255) |
+----+-----------------+-----------------+-------------------------+---------+--------+---------------------------------+-----------------------------------------------------+
8 rows in set (0.00 sec)
mysql> SHOW ENGINE INNODB STATUS;
=====================================
2019-04-08 02:30:45 0x700006ef3000 INNODB MONITOR OUTPUT
=====================================
Per second averages calculated from the last 20 seconds
-----------------
BACKGROUND THREAD
-----------------
srv_master_thread loops: 242 srv_active, 0 srv_shutdown, 73418 srv_idle
srv_master_thread log flush and writes: 0
----------
SEMAPHORES
----------
OS WAIT ARRAY INFO: reservation count 87
OS WAIT ARRAY INFO: signal count 516
RW-shared spins 722, rounds 723, OS waits 1
RW-excl spins 1110, rounds 3537, OS waits 28
RW-sx spins 0, rounds 0, OS waits 0
Spin rounds per wait: 1.00 RW-shared, 3.19 RW-excl, 0.00 RW-sx
------------
TRANSACTIONS
------------
Trx id counter 15753
Purge done for trx's n:o < 15749 undo n:o < 0 state: running but idle
History list length 11
LIST OF TRANSACTIONS FOR EACH SESSION:
---TRANSACTION 281479647891952, not started
mysql tables in use 1, locked 1
0 lock struct(s), heap size 1136, 0 row lock(s)
---TRANSACTION 281479647891040, not started
0 lock struct(s), heap size 1136, 0 row lock(s)
---TRANSACTION 281479647893776, not started
0 lock struct(s), heap size 1136, 0 row lock(s)
---TRANSACTION 281479647892864, not started
0 lock struct(s), heap size 1136, 0 row lock(s)
---TRANSACTION 281479647890128, not started
0 lock struct(s), heap size 1136, 0 row lock(s)
---TRANSACTION 281479647889216, not started
0 lock struct(s), heap size 1136, 0 row lock(s)
--------
FILE I/O
--------
I/O thread 0 state: waiting for i/o request (insert buffer thread)
I/O thread 1 state: waiting for i/o request (log thread)
I/O thread 2 state: waiting for i/o request (read thread)
I/O thread 3 state: waiting for i/o request (read thread)
I/O thread 4 state: waiting for i/o request (read thread)
I/O thread 5 state: waiting for i/o request (read thread)
I/O thread 6 state: waiting for i/o request (write thread)
I/O thread 7 state: waiting for i/o request (write thread)
I/O thread 8 state: waiting for i/o request (write thread)
I/O thread 9 state: waiting for i/o request (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
950 OS file reads, 52924 OS file writes, 40911 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, 0 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 3 buffer(s)
Hash table size 34679, node heap has 3 buffer(s)
Hash table size 34679, node heap has 7 buffer(s)
Hash table size 34679, node heap has 2 buffer(s)
Hash table size 34679, node heap has 2 buffer(s)
Hash table size 34679, node heap has 1 buffer(s)
Hash table size 34679, node heap has 2 buffer(s)
Hash table size 34679, node heap has 4 buffer(s)
0.00 hash searches/s, 0.00 non-hash searches/s
---
LOG
---
Log sequence number 89894761
Log buffer assigned up to 89894761
Log buffer completed up to 89894761
Log written up to 89894761
Log flushed up to 89894761
Added dirty pages up to 89894761
Pages flushed up to 89894761
Last checkpoint at 89894761
42886 log i/o's done, 0.00 log i/o's/second
----------------------
BUFFER POOL AND MEMORY
----------------------
Total large memory allocated 137428992
Dictionary memory allocated 1037251
Buffer pool size 8191
Free buffers 3332
Database pages 4835
Old database pages 1764
Modified db pages 0
Pending reads 0
Pending writes: LRU 0, flush list 0, single page 0
Pages made young 634, not young 1
0.00 youngs/s, 0.00 non-youngs/s
Pages read 911, created 3927, written 8771
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: 4835, 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
0 read views open inside InnoDB
Process ID=99, Main thread ID=0x700006689000 , state=sleeping
Number of rows inserted 176434, updated 4249, deleted 10814, read 2383183
0.00 inserts/s, 0.00 updates/s, 0.00 deletes/s, 0.00 reads/s
----------------------------
END OF INNODB MONITOR OUTPUT
============================
我无法理解的是，为什么即使持有锁定的事务已提交，DDL查询仍要一直等待表元数据锁定。
答案 0 :(得分：2)
在MySQL中，大多数DDL语句实际上会生成一个隐式提交。 ALTER TABLE语句确实属于此类。来自the documentation：
本节中列出的语句（及其任何同义词）隐式结束当前会话中任何活动的事务，就像您在执行该语句之前进行了COMMIT一样。
大多数这些语句在执行后也会导致隐式提交。
此internal documentation涉及了更多细节...事情会变得一团糟：
此外，某些DDL语句发出临时事务提交：例如，
ALTER TABLE在将数据从原始表复制到内部临时表之后发出提交。
因此，当您执行ALTER TABLE时，MySQL会隐式尝试在该语句中进行提交，但由于另一个事务正在进行中而无法执行。该文档没有精确地涵盖这种用例，但是我怀疑服务器不能正确处理这种情况，并且会产生您所看到的怪异行为。该文档本身指出，这种行为是错误定义的。
底线：运行ALTER TABLE语句时，不能依靠事务来处理并发。因此，当表忙于其他事务时，您不想ALTER一个表：这是不安全的。在运行此类语句之前，您要确保未在表上设置任何锁定。是的，在处理类似于生产型的数据库时肯定会带来痛苦的约束，但是看起来这正是MySQL的工作方式……