如何使用MySQL观测MDL锁，相信很多没有经验的人对此束手无策，为此本文总结了问题出现的原因和解决方法，通过这篇文章希望你能解决这个问题。
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在给一个小表加字段的时候，一直拿不到锁，报错：
ERROR 1205 (HY000): Lock wait timeout exceeded; try restarting transaction
期间show processlist查看，ddl在等待MDL锁。MySQL 的 Metadata Lock 机制是为了保护并发访问数据对象的一致性，并且showps也看不到活动的事务。
如何查到MDL锁的源头呢？innodb_locks是看不到MDL锁的信息的。performance_schema下面有一张表叫metadata_locks
mysql> desc metadata_locks;
+-----------------------+---------------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+-----------------------+---------------------+------+-----+---------+-------+
| OBJECT_TYPE | varchar(64) | NO | | NULL | |
| OBJECT_SCHEMA | varchar(64) | YES | | NULL | |
| OBJECT_NAME | varchar(64) | YES | | NULL | |
| OBJECT_INSTANCE_BEGIN | bigint(20) unsigned | NO | | NULL | |
| LOCK_TYPE | varchar(32) | NO | | NULL | |
| LOCK_DURATION | varchar(32) | NO | | NULL | |
| LOCK_STATUS | varchar(32) | NO | | NULL | |
| SOURCE | varchar(64) | YES | | NULL | |
| OWNER_THREAD_ID | bigint(20) unsigned | YES | | NULL | |
| OWNER_EVENT_ID | bigint(20) unsigned | YES | | NULL | |
+-----------------------+---------------------+------+-----+---------+-------+
10 rows in set (0.00 sec)
默认需要开启metadata 的 instrument，才能拿到MDL锁的信息。
call sys.ps_setup_enable_instrument('wait/lock/metadata/sql/mdl%')
或者：
UPDATE performance_schema.setup_consumers SET ENABLED = 'YES' WHERE NAME ='global_instrumentation';
UPDATE performance_schema.setup_instruments SET ENABLED = 'YES' WHERE NAME ='wait/lock/metadata/sql/mdl';开启后重新执行DDL语句，然后观测
mysql> select * from performance_schema.metadata_locks where object_name='t'\G
*************************** 1. row ***************************
OBJECT_TYPE: TABLE
OBJECT_SCHEMA: ming
OBJECT_NAME: t
OBJECT_INSTANCE_BEGIN: 87772208
LOCK_TYPE: SHARED_READ
LOCK_DURATION: TRANSACTION
LOCK_STATUS: GRANTED
SOURCE:
OWNER_THREAD_ID: 29
OWNER_EVENT_ID: 990
*************************** 2. row ***************************
OBJECT_TYPE: TABLE
OBJECT_SCHEMA: ming
OBJECT_NAME: t
OBJECT_INSTANCE_BEGIN: 140151695148016
LOCK_TYPE: SHARED_UPGRADABLE
LOCK_DURATION: TRANSACTION
LOCK_STATUS: GRANTED
SOURCE:
OWNER_THREAD_ID: 31
OWNER_EVENT_ID: 178
*************************** 3. row ***************************
OBJECT_TYPE: TABLE
OBJECT_SCHEMA: ming
OBJECT_NAME: t
OBJECT_INSTANCE_BEGIN: 140151695148992
LOCK_TYPE: EXCLUSIVE
LOCK_DURATION: TRANSACTION
LOCK_STATUS: PENDING
SOURCE:
OWNER_THREAD_ID: 31
OWNER_EVENT_ID: 237
3 rows in set (0.00 sec)
可以看到已经会话被授予（GRANTED）了SHARED_READ类型的MDL锁。DDL会话先被授予了SHARED_UPGRADABLE，然后需要被授予EXCLUSIVE的锁，但是拿不到，所以处于PENDING状态。所以只要找到GRANTED的会话是哪个，然后杀掉就可以了。
MDL_SHARED_READ,
/*
A shared metadata lock for cases when there is an intention to modify
(and not just read) data in the table.
A connection holding SW lock can read table metadata and modify or read
table data (after acquiring appropriate table and row-level locks).
To be used for tables to be modified by INSERT, UPDATE, DELETE
statements, but not LOCK TABLE ... WRITE or DDL). Also taken by
SELECT ... FOR UPDATE.
*/
MDL_SHARED_UPGRADABLE,
/*
A shared metadata lock for cases when we need to read data from table
and block all concurrent modifications to it (for both data and metadata).
Used by LOCK TABLES READ statement.
通过perforamnce_schema.threads查找：
mysql> select * from performance_schema.threads where thread_id=29\G
*************************** 1. row ***************************
THREAD_ID: 29
NAME: thread/sql/one_connection
TYPE: FOREGROUND
PROCESSLIST_ID: 2
PROCESSLIST_USER: root
PROCESSLIST_HOST: localhost
PROCESSLIST_DB: ming
PROCESSLIST_COMMAND: Sleep
PROCESSLIST_TIME: 107
PROCESSLIST_STATE: NULL
PROCESSLIST_INFO: NULL
PARENT_THREAD_ID: 1
ROLE: NULL
INSTRUMENTED: YES
HISTORY: YES
CONNECTION_TYPE: Socket
THREAD_OS_ID: 2822
1 row in set (0.00 sec)
取processlist_id，kill掉该会话即可。
mysql> kill 2;
Query OK, 0 rows affected (0.00 sec)
mysql> alter table t add c4 int;
Query OK, 0 rows affected (3 min 58.00 sec)
Records: 0 Duplicates: 0 Warnings: 0
查看pstack： 阻塞源头
Thread 9 (Thread 0x7f77c3043700 (LWP 2822)):
#0 0x00007f77f1debf0d in poll () from /lib64/libc.so.6
#1 0x00000000012c48df in vio_io_wait (vio=, event=, timeout=28800000) at /export/home/pb2/build/sb_0-32013917-1545390211.74/mysql-5.7.25/vio/viosocket.c:786
#2 0x00000000012c49d3 in vio_socket_io_wait (vio=, event=) at /export/home/pb2/build/sb_0-32013917-1545390211.74/mysql-5.7.25/vio/viosocket.c:77
#3 0x00000000012c5008 in vio_read (vio=0x505ab10, buf=0x5372800 "\001", size=4) at /export/home/pb2/build/sb_0-32013917-1545390211.74/mysql-5.7.25/vio/viosocket.c:132
#4 0x0000000000c6e4a3 in net_read_raw_loop (net=0x50683f8, count=4) at /export/home/pb2/build/sb_0-32013917-1545390211.74/mysql-5.7.25/sql/net_serv.cc:672
#5 0x0000000000c6ed3b in net_read_packet_header (net=0x50683f8) at /export/home/pb2/build/sb_0-32013917-1545390211.74/mysql-5.7.25/sql/net_serv.cc:756
#6 net_read_packet (net=, complen=0x7f77c3042cf8) at /export/home/pb2/build/sb_0-32013917-1545390211.74/mysql-5.7.25/sql/net_serv.cc:822
#7 0x0000000000c6efec in my_net_read (net=0x50683f8) at /export/home/pb2/build/sb_0-32013917-1545390211.74/mysql-5.7.25/sql/net_serv.cc:899
#8 0x0000000000c7c88c in Protocol_classic::read_packet (this=0x5067c98) at /export/home/pb2/build/sb_0-32013917-1545390211.74/mysql-5.7.25/sql/protocol_classic.cc:808
#9 0x0000000000c7b622 in Protocol_classic::get_command (this=0x5067c98, com_data=0x7f77c3042da0, cmd=0x7f77c3042dcc) at /export/home/pb2/build/sb_0-32013917-1545390211.74/mysql-5.7.25/sql/protocol_classic.cc:965
#10 0x0000000000d1e3d7 in do_command (thd=0x5066c40) at /export/home/pb2/build/sb_0-32013917-1545390211.74/mysql-5.7.25/sql/sql_parse.cc:964
#11 0x0000000000defb14 in handle_connection (arg=) at /export/home/pb2/build/sb_0-32013917-1545390211.74/mysql-5.7.25/sql/conn_handler/connection_handler_per_thread.cc:306
#12 0x000000000125d3f4 in pfs_spawn_thread (arg=0x50008f0) at /export/home/pb2/build/sb_0-32013917-1545390211.74/mysql-5.7.25/storage/perfschema/pfs.cc:2190
#13 0x00007f77f333ce25 in start_thread () from /lib64/libpthread.so.0
#14 0x00007f77f1df6bad in clone () from /lib64/libc.so.6
被阻塞者：
Thread 7 (Thread 0x7f77c2fdf700 (LWP 2869)):
#0 0x00007f77f3340d42 in pthread_cond_timedwait@@GLIBC_2.3.2 () from /lib64/libpthread.so.0
#1 0x0000000000c67d40 in native_cond_timedwait (abstime=0x7f77c2fda790, mutex=0x7f779c000b78, cond=0x7f779c000ba8) at /export/home/pb2/build/sb_0-32013917-1545390211.74/mysql-5.7.25/include/thr_cond.h:129
#2 my_cond_timedwait (abstime=0x7f77c2fda790, mp=0x7f779c000b78, cond=0x7f779c000ba8) at /export/home/pb2/build/sb_0-32013917-1545390211.74/mysql-5.7.25/include/thr_cond.h:182
#3 inline_mysql_cond_timedwait (src_line=1861, src_file=0x140f5d0 "/export/home/pb2/build/sb_0-32013917-1545390211.74/mysql-5.7.25/sql/mdl.cc", abstime=0x7f77c2fda790, mutex=0x7f779c000b78, that=0x7f779c000ba8) at /export/home/pb2/build/sb_0-32013917-1545390211.74/mysql-5.7.25/include/mysql/psi/mysql_thread.h:1229
#4 MDL_wait::timed_wait (this=0x7f779c000b78, owner=0x7f779c000ae0, abs_timeout=0x7f77c2fda790, set_status_on_timeout=false, wait_state_name=) at /export/home/pb2/build/sb_0-32013917-1545390211.74/mysql-5.7.25/sql/mdl.cc:1861
#5 0x0000000000c6a917 in MDL_context::acquire_lock (this=0x7f779c000b78, mdl_request=0x7f77c2fda810, lock_wait_timeout=) at /export/home/pb2/build/sb_0-32013917-1545390211.74/mysql-5.7.25/sql/mdl.cc:3655
#6 0x0000000000c6b68f in MDL_context::upgrade_shared_lock (this=0x7f779c000b78, mdl_ticket=0x7f779c0103f0, new_type=MDL_EXCLUSIVE, lock_wait_timeout=31536000) at /export/home/pb2/build/sb_0-32013917-1545390211.74/mysql-5.7.25/sql/mdl.cc:3919
#7 0x0000000000d82ad7 in mysql_inplace_alter_table (thd=0x7f779c000ae0, table_list=0x7f779c005fe0, table=0x7f779c01af60, altered_table=0x7f779c068980, ha_alter_info=0x7f77c2fdc1e0, inplace_supported=HA_ALTER_INPLACE_NO_LOCK_AFTER_PREPARE, target_mdl_request=0x7f77c2fdc9a0, alter_ctx=0x7f77c2fdac40) at /export/home/pb2/build/sb_0-32013917-1545390211.74/mysql-5.7.25/sql/sql_table.cc:7449
#8 0x0000000000d8550e in mysql_alter_table (thd=0x7f779c000ae0, new_db=0x7f779c068980 "\360\222\006\234w\177", new_name=0x7f77c2fdc1e0 "\300\323\375\302w\177", create_info=0x7f77c2fdd3c0, table_list=0x7f779c005fe0, alter_info=0x7f77c2fdd4c0) at /export/home/pb2/build/sb_0-32013917-1545390211.74/mysql-5.7.25/sql/sql_table.cc:9798
#9 0x0000000000e8de2c in Sql_cmd_alter_table::execute (this=, thd=0x7f779c000ae0) at /export/home/pb2/build/sb_0-32013917-1545390211.74/mysql-5.7.25/sql/sql_alter.cc:327
#10 0x0000000000d17f2a in mysql_execute_command (thd=0x7f779c000ae0, first_level=true) at /export/home/pb2/build/sb_0-32013917-1545390211.74/mysql-5.7.25/sql/sql_parse.cc:4835
#11 0x0000000000d1c3fd in mysql_parse (thd=0x7f779c000ae0, parser_state=) at /export/home/pb2/build/sb_0-32013917-1545390211.74/mysql-5.7.25/sql/sql_parse.cc:5570
#12 0x0000000000d1d625 in dispatch_command (thd=0x7f779c000ae0, com_data=0x7f77c2fdeda0, command=COM_QUERY) at /export/home/pb2/build/sb_0-32013917-1545390211.74/mysql-5.7.25/sql/sql_parse.cc:1484
#13 0x0000000000d1e4d4 in do_command (thd=0x7f779c000ae0) at /export/home/pb2/build/sb_0-32013917-1545390211.74/mysql-5.7.25/sql/sql_parse.cc:1025
#14 0x0000000000defb14 in handle_connection (arg=) at /export/home/pb2/build/sb_0-32013917-1545390211.74/mysql-5.7.25/sql/conn_handler/connection_handler_per_thread.cc:306
#15 0x000000000125d3f4 in pfs_spawn_thread (arg=0x5d629c0) at /export/home/pb2/build/sb_0-32013917-1545390211.74/mysql-5.7.25/storage/perfschema/pfs.cc:2190#16 0x00007f77f333ce25 in start_thread () from /lib64/libpthread.so.0
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