当包裹在事务中时，Mysql DDL 查询卡在等待表元数据锁中

Question

This is a random behavior(didn't reoccur after restarting the mysql session) that when I wrapped the DDL query in a transaction T1 and at same time I initiate another transaction T2 with a select query on same table, DDL query gets stuck waiting for table metadata lock which is expected, and any other select queries waits for DDL query to finish. But after committing T2, DDL is supposed to get metadata lock and finish, but it remains waiting for table metadata lock state.

Connection 1 Query:

mysql> begin;                            
Query OK, 0 rows affected (0.00 sec)

mysql> ALTER TABLE merchants ADD COLUMN temp6 varchar(255);

Connection 2 Query:

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)

Sequence of queries is :

1. Connection 1 : begin;
2. Connection 2 : begin;
3. Connection 2 : select * from merchants where account_id=null;
4. Connection 1 : ALTER TABLE merchants ADD COLUMN temp6 varchar(255);
5. Connection 2 : commit;

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
============================

What I am not able to understand is why DDL query kept on waiting for table metadata lock even though the transaction which held the lock commits.

Answer 1

In MySQL, most DDL statements actually generate an implicit commit . The ALTER TABLE statement does fall in this category. From the documentation :

The statements listed in this section (and any synonyms for them) implicitly end any transaction active in the current session, as if you had done a COMMIT before executing the statement.

Most of these statements also cause an implicit commit after executing.

This internal documentation goes into more details... and things tend to get messy:

In addition, some DDL statements issue interim transaction commits: for example, ALTER TABLE issues a commit after data is copied from the original table to the internal temporary table.

So while you execute the ALTER TABLE , MySQL implicitly tries to commit within the statement, but can't since another transaction is in progress. The documentation does not precisely covers this use case, but I would suspect that the server does not properly handle this situation, and generates the weird behavior that you are seeing. The documentation itself states that this type of behavior is badly defined .

Bottom line: you cannot rely on transactions to handle concurrency when running an ALTER TABLE statement. As a consequence, you don't want to ALTER a table while it is busy on some other transaction: this is not safe. Before running such statement, you want to make sure that no lock is set on the table. Yes, that surely incurs painful constraints when dealing with production-like databases, but it looks like it's just how MySQL does it...