MySQL 理解MDL Lock

2022-07-30 23:12:13

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MySQL 理解MDL Lock：

一、MDL Lock综述
MySQL中MDL锁一直是一个比较让人比较头疼的问题，我们谈起堵塞一般更加倾向于InnoDB层的row lock（gap lock/next key lock/key lock），因为它很好理解也很好观察。而对于MDL Lock考虑就少一些，因为它实在不好观察，只有出现问题查看show processlist的时候，可以看到简单的所谓的‘Waiting for table metadata lock’之类的状态，其实MDL Lock是MySQL上层一个非常复杂的子系统，有自己的死锁检测机制。
大家一般说是不是锁表了很大一部分就和MDL Lock有关，可见的它的关键性和严重性，笔者也是根据自己的需求学习了一些，且没有能力阅读全部的代码，但是笔者通过增加一个TICKET的打印函数让语句的MDL Lock加锁流程全部打印出来方便学习，下面从一些基础概念说起然后告诉大家笔者是如何做的打印功能，最后对每种MDL TYPE可能出现的语句进行测试和分析。如果大家对基本概念和增加打印函数不感兴趣可直接参考第五部分语句加MDL Lock测试和分析，希望这些测试能够帮助到大家诊断问题。

刚好最近笔者遇到一次MDL Lock出现死锁的情况会在下篇文章中给出案例，本文只看理论。

处于层次：MySQL层，实际上早在open_table函数中MDL LOCK就开始获取了。
最早获取阶段: THD::enter_stage: ‘Opening tables’
调用栈帧
#0 open_table_get_mdl_lock(thd=0x7fffd0000df0,ot_ctx=0x7fffec06fb00,
table_list=0x7fffd00067d8,flags=0,mdl_ticket=0x7fffec06f950)
at/root/MySQL5.7.14/percona-server-5.7.14-7/sql/sql_base.cc:2789
#1 0x0000000001516e17inopen_table(thd=0x7fffd0000df0,
table_list=0x7fffd00067d8,ot_ctx=0x7fffec06fb00)
at/root/MySQL5.7.14/percona-server-5.7.14-7/sql/sql_base.cc:3237
死锁检测出错码
{ "ER_LOCK_DEADLOCK", 1213, "Deadlock found when trying to get lock; try restarting transaction" },
ERROR 1213 (40001): Deadlock found when trying to get lock; try restarting transaction
可以发现MDL Lock的死锁抛错和Innodb死锁一模一样，不同的只是‘show engine innodb status’没有死锁信息。
二、重要数据结构和概念
1、MDL Lock类型
我们主要研究的类型如下：

MDL_INTENTION_EXCLUSIVE(IX)
MDL_SHARED(S)
MDL_SHARED_HIGH_PRIO(SH)
MDL_SHARED_READ(SR)
MDL_SHARED_WRITE(SW)
MDL_SHARED_WRITE_LOW_PRIO(SWL)
MDL_SHARED_UPGRADABLE(SU)
MDL_SHARED_READ_ONLY(SRO)
MDL_SHARED_NO_WRITE(SNW)
MDL_SHARED_NO_READ_WRITE(SNRW)
MDL_EXCLUSIVE(X)

第五部分会对每种类型进行详细的测试和解释。

2、MDL Lock namespace
在MDL中MDL_KEY按照namespace+DB+OBJECT_NAME的方式进行表示，所谓的namespace也比较重要下面是namespace的分类：

GLOBAL is used for the global read lock.
TABLESPACE is for tablespaces.
SCHEMA is for schemas (aka databases).
TABLE is for tables and views.
FUNCTION is for stored functions.
PROCEDURE is for stored procedures.
TRIGGER is for triggers.
EVENT is for event scheduler events.
COMMIT is for enabling the global read lock to block commits.
USER_LEVEL_LOCK is for user-level locks.
LOCKING_SERVICE is for the name plugin RW-lock service
本文我们主要对GLOBAL/SCHEMA/TABLE namespace进行描述，而对于COMMIT namespace是提交的时候会用到的如果遇到等待，状态为‘Waiting for commit lock’，一般为FTWRL堵塞COMMIT。可参考我的《深入理解MySQL主从原理》15节。其他namespace不做描述。

3、MDL Lock实现分类
scope lock:一般对应全局MDL Lock，如flush table with read lock 会获取namespace space：GLOBAL type：S和namespace space：COMMIT type：S的MDL Lock。它包含GLOBAL, COMMIT, TABLESPACE和SCHEMA
object lock:如其名字所示，对象级别的MDL Lock，比如TABLE级别的MDL Lock，这也是本文的讨论核心。它包含其他的namespace。
下面是源码注释：

  /**
    Helper struct which defines how different types of locks are handled
    for a specific MDL_lock. In practice we use only two strategies: "scoped"
    lock strategy for locks in GLOBAL, COMMIT, TABLESPACE and SCHEMA namespaces
    and "object" lock strategy for all other namespaces.
  */
4、MDL Lock兼容矩阵
MySQL：理解MDL Lock

MySQL：理解MDL Lock

这里兼容矩阵是学习锁堵塞的重点，类型很多比Innodb row lock类型要多很多，不用记住，只需要遇到能知道。

5、MDL Lock duration（MDL Lock持续周期）
这个对应源码的enum_mdl_duration，通常我们需要关注MDL Lock是事务提交后释放还是语句结束后释放，实际上就是这个，这对MDL lock堵塞的范围很重要。我直接复制源码的解释。

MDL_STATEMENT：Locks with statement duration are automatically released at the end of statement or transaction.
MDL_TRANSACTION：Locks with transaction duration are automatically released at the end of transaction.
MDL_EXPLICIT:Locks with explicit duration survive the end of statement and transaction.They have to be released explicitly by calling MDL_context::release_lock().
6、MDL Lock的FAST PATH（unobtrusive）和SLOW PATH（obtrusive）
使用两种不同的方式目的在于优化MDL Lock的实现，下面是源码的注释，可做适当了解：

A) “unobtrusive” lock types
1) Each type from this set should be compatible with all other types from the set (including itself).
2) These types should be common for DML operations Our goal is to optimize acquisition and release of locks of this type by avoiding complex checks and manipulations on m_waiting/m_granted bitmaps/lists. We replace them with a check of and increment/decrement of integer counters.We call the latter type of acquisition/release “fast path”.Use of “fast path” reduces the size of critical section associated with MDL_lock::m_rwlock lock in the common case and thus increases scalability.The amount by which acquisition/release of specific type “unobtrusive” lock increases/decreases packed counter in MDL_lock::m_fast_path_state is returned by this function.
B) “obtrusive” lock types
1) Granted or pending lock of those type is incompatible withsome other types of locks or with itself.
2) Not common for DML operations These locks have to be always acquired involving manipulations on m_waiting/m_granted bitmaps/lists, i.e. we have to use “slow path” for them. Moreover in the presence of active/pending locks from “obtrusive” set we have to acquire using “slow path” even locks of”unobtrusive” type.
7、MDL_request结构部分属性
也就是通过语句解析后需要获得的MDL Lock的需求，然后通过这个类对象在MDL子系统中进行MDL Lock申请，大概包含如下一些属性：

/** Type of metadata lock. */
  enum enum_mdl_type type; //需求的类型
  /** Duration for requested lock. */
  enum enum_mdl_duration duration; //持续周期
  /**
    Pointers for participating in the list of lock requests for this context.
  */
  MDL_request *next_in_list; //双向链表实现
  MDL_request **prev_in_list;
  /**
    Pointer to the lock ticket object for this lock request.
    Valid only if this lock request is satisfied.
  */
  MDL_ticket *ticket; //注意这里如果申请成功(没有等待)，会指向一个实际的TICKET，否则为NULL
  /** A lock is requested based on a fully qualified name and type. */
8、MDL_key结构部分属性
就是实际的namespace+DB+OBJECT_NAME，整个放到一个char数组里面，他会在MDL_LOCK和MDL_REQUEST中出现。

private:
uint16m_length;
uint16m_db_name_length;
charm_ptr[MAX_MDLKEY_LENGTH];//放到了这里
9、MDL_ticket结构部分属性
如同门票一样，如果获取了MDL Lock必然给MDL_request返回一张门票，如果等待则不会分配。源码MDL_context::acquire_lock可以观察到。部分属性如下：

/**
    Pointers for participating in the list of lock requests for this context.
    Context private.正如解释这里是context中链表链表的形成，是线程私有的
  */
  MDL_ticket *next_in_context;
  MDL_ticket **prev_in_context;
  /**
    Pointers for participating in the list of satisfied/pending requests
    for the lock. Externally accessible.正如解释这里是MDL_LOCK中链表链表的形成，是全局的
  */
  MDL_ticket *next_in_lock;
  MDL_ticket **prev_in_lock;
/**
    Context of the owner of the metadata lock ticket. Externally accessible.
    很明显这里指向了这个ticket的拥有者也就是MDL_context，它是线程的属性
  */
  MDL_context *m_ctx;
  /**
    Pointer to the lock object for this lock ticket. Externally accessible.
    很明显这里是一个指向MDL_LOCK的一个指针
  */
  MDL_lock *m_lock;
  /**
    Indicates that ticket corresponds to lock acquired using "fast path"
    algorithm. Particularly this means that it was not included into
    MDL_lock::m_granted bitmap/list and instead is accounted for by
    MDL_lock::m_fast_path_locks_granted_counter
    这里就代表了是否是FAST PATH从注释来看fast path方式不会在MDL LOCK中
    占用granted位图和链表取而代之代之的是一个统计器m_fast_path_locks_granted_counter
    这样一来开销肯定更小
  */
  bool m_is_fast_path;
  /**
    Indicates that ticket corresponds to lock request which required
    storage engine notification during its acquisition and requires
    storage engine notification after its release.
  */
10、MDL_lock结构部分属性
每一个MDL_key都会对应一个MDL_lock，其中包含了所谓的GRANTED链表和WAIT链表，考虑它的复杂性，可以直接参考源码注释也非常详细，这里给出我所描述的几个属性如下：

/** The key of the object (data) being protected. */
  MDL_key key;
/** List of granted tickets for this lock. */
  Ticket_list m_granted;
/** Tickets for contexts waiting to acquire a lock. */
  Ticket_list m_waiting;
11、MDL_context结构部分属性
这是整个MySQL线程和MDL Lock子系统进行交互的一个所谓的上下文结构,其中包含了很多方法和属性，我比较关注的属性如下：

/**
    If our request for a lock is scheduled, or aborted by the deadlock
    detector, the result is recorded in this class.
  */
  MDL_wait m_wait;
/**
    Lists of all MDL tickets acquired by this connection.
    这是一个不同MDL lock持续时间的一个链表数组。实际就是
    MDL_STATEMENT一个链表
    MDL_TRANSACTION一个链表
    MDL_EXPLICIT一个链表
  */
Ticket_list m_tickets[MDL_DURATION_END];
//这是一个父类指针指向子类对象，虚函数重写的典型,实际他就指向了一个线程
/*
class THD :public MDL_context_owner,
           public Query_arena,
           public Open_tables_state
*/
MDL_context_owner *m_owner;
12、所有等待状态
源码给出了所有的等待标记如下：

PSI_stage_info MDL_key::m_namespace_to_wait_state_name[NAMESPACE_END]=
{
  {0, "Waiting for global read lock", 0},
  {0, "Waiting for tablespace metadata lock", 0},
  {0, "Waiting for schema metadata lock", 0},
  {0, "Waiting for table metadata lock", 0},
  {0, "Waiting for stored function metadata lock", 0},
  {0, "Waiting for stored procedure metadata lock", 0},
  {0, "Waiting for trigger metadata lock", 0},
  {0, "Waiting for event metadata lock", 0},
  {0, "Waiting for commit lock", 0},
  {0, "User lock", 0}, /* Be compatible with old status. */
  {0, "Waiting for locking service lock", 0},
  {0, "Waiting for backup lock", 0},
  {0, "Waiting for binlog lock", 0}
};
我们常见的是：

“Waiting for table metadata lock”：通常就是namespace TABLE级别的MDL Lock，具体根据兼容矩阵判断参考第五节。
“Waiting for global read lock”：通常就是namespace GLOBAL级别的MDL Lock，通常和flush table with read lock有关，参考第五节。
“Waiting for commit lock”：通常就是namespace COMMIT级别的MDL Lock，通常和flush table with read lock有关，参考第五节。
三、增加打印函数my_print_ticket
学习MDL Lock最好的方式当然是获取一条语句锁加的所有MDL Lock，包含加锁、升级、降级和释放的流程。虽然5.7加入诊断MDL Lock的方法：

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';
select * from performance_schema.metadata_locks
但是对于每个语句获取的所有MDL Lock的流程仍然不好观察，因此我加入了打印函数：

/*p_ticket in parameter*/
int my_print_ticket(const MDL_ticket* p_ticket)
并且在mdl_ticket类中增加了这个函数原型为友元函数：

friend int my_print_ticket(const MDL_ticket* p_ticket);
主要捕获MDL Lock的加锁信息打印到err日志中，包含的信息如下：

线程id：通过p_ticket->m_ctx->get_thd(); 获取
mdl lock database name：通过p_ticket->m_lock->key.db_name()获取
mdl lock object name：通过p_ticket->m_lock->key.name()获取
mdl lock namespace：通过p_ticket->m_lock->key.mdl_namespace()获取
mdl lock fast path：通过p_ticket->m_is_fast_path获取判断是则输出否则不输出
mdl lock type：通过p_ticket->m_type获取
mdl lock duration：通过p_ticket->m_duration获取
上面这些信息都在前面进行过描述了。具体的输出信息如下：

2017-08-03T07:34:21.720583Z 3 [Note] (>MDL PRINT) Thread id is 3:
2017-08-03T07:34:21.720601Z 3 [Note] (->MDL PRINT) DB_name is:test
2017-08-03T07:34:21.720619Z 3 [Note] (-->MDL PRINT) OBJ_name is:test
2017-08-03T07:34:21.720637Z 3 [Note] (--->MDL PRINT) Namespace is:TABLE
2017-08-03T07:34:21.720655Z 3 [Note] (---->MDL PRINT) Fast path is:(Y)
2017-08-03T07:34:21.720673Z 3 [Note] (----->MDL PRINT) Mdl type is:MDL_SHARED_WRITE(SW)
2017-08-03T07:34:21.720692Z 3 [Note] (------>MDL PRINT) Mdl duration is:MDL_TRANSACTION
这实际上和metadata_locks中的信息差不多，如下：

MySQL>   select * from performance_schema.metadata_locks/G
*************************** 1. row ***************************
          OBJECT_TYPE: TABLE
        OBJECT_SCHEMA: test
          OBJECT_NAME: test
OBJECT_INSTANCE_BEGIN: 140734412907760
            LOCK_TYPE: SHARED_WRITE
        LOCK_DURATION: TRANSACTION
          LOCK_STATUS: GRANTED
               SOURCE: sql_parse.cc:6314
      OWNER_THREAD_ID: 39
       OWNER_EVENT_ID: 241
一旦有了这个函数我们只需要在加锁、升级、降级和释放的位置进行适当添加就可以了。

四、在合适的位置增加my_print_ticket打印函数
既然我们要研究MDL Lock的加锁?升级?降级，那么我们就必要找到他们的函数入口，然后在合适的位置增加打印函数my_print_ticket进行观察，下面标示出打印位置。

1、加锁：MDL_context::acquire_lock
bool
MDL_context::acquire_lock(MDL_request *mdl_request, ulong lock_wait_timeout)
{
  if (mdl_request->ticket) //获取成功获得ticket
  {
    /*
      We have managed to acquire lock without waiting.
      MDL_lock, MDL_context and MDL_request were updated
      accordingly, so we can simply return success.
    */
    //REQUESET获取TICKET成功此处打印
    return FALSE;
  }
  /*
    Our attempt to acquire lock without waiting has failed.
    As a result of this attempt we got MDL_ticket with m_lock
    member pointing to the corresponding MDL_lock object which
    has MDL_lock::m_rwlock write-locked.
  */
  //获取不成功加入MDL_lock 等待队列
  lock= ticket->m_lock;
  lock->m_waiting.add_ticket(ticket);
  will_wait_for(ticket); //死锁检测
  /* There is a shared or exclusive lock on the object. */
  DEBUG_SYNC(get_thd(), "mdl_acquire_lock_wait");
  find_deadlock();
  //此处打印TICKET进入了等待流程
  if (lock->needs_notification(ticket) || lock->needs_connection_check())
  {
   }
  done_waiting_for();//等待完成对死锁检测等待图进行调整去掉本等待边edge(无向图)
  //当然到这里也是通过等待后获得成功了状态为GRANTED
  DBUG_ASSERT(wait_status == MDL_wait::GRANTED);
  m_tickets[mdl_request->duration].push_front(ticket);
  mdl_request->ticket= ticket;
  MySQL_mdl_set_status(ticket->m_psi, MDL_ticket::GRANTED);
  //此处打印通过等待REQUEST获得了TICKET
  return FALSE;
}
2、降级：void MDL_ticket::downgrade_lock(enum_mdl_type new_type)
void MDL_ticket::downgrade_lock(enum_mdl_type new_type)
{
  /* Only allow downgrade from EXCLUSIVE and SHARED_NO_WRITE. */
  DBUG_ASSERT(m_type == MDL_EXCLUSIVE ||
              m_type == MDL_SHARED_NO_WRITE);
//此处打印出降级前的TICKET
  if (m_hton_notified)
  {
    MySQL_mdl_set_status(m_psi, MDL_ticket::POST_RELEASE_NOTIFY);
    m_ctx->get_owner()->notify_hton_post_release_exclusive(&m_lock->key);
    m_hton_notified= false;
    MySQL_mdl_set_status(m_psi, MDL_ticket::GRANTED);
  }
//函数结尾答应出降级后的TICKET
}
3、升级：MDL_context::upgrade_shared_lock(MDL_ticket *mdl_ticket,enum_mdl_type new_type, ulong lock_wait_timeout)
bool
MDL_context::upgrade_shared_lock(MDL_ticket *mdl_ticket,
                                 enum_mdl_type new_type,
                                 ulong lock_wait_timeout)
{
  MDL_REQUEST_INIT_BY_KEY(&mdl_new_lock_request,
                          &mdl_ticket->m_lock->key, new_type,
                          MDL_TRANSACTION);//构造一个request
//此处打印出来的TICKET类型
  if (acquire_lock(&mdl_new_lock_request, lock_wait_timeout)) //尝试使用新的LOCK_TYPE进行加锁
    DBUG_RETURN(TRUE);
  is_new_ticket= ! has_lock(mdl_svp, mdl_new_lock_request.ticket);
  lock= mdl_ticket->m_lock;
  //下面进行一系列对MDL_LOCK的维护并且对所谓的合并操作
  /* Code below assumes that we were upgrading to "obtrusive" type of lock. */
  DBUG_ASSERT(lock->is_obtrusive_lock(new_type));
  /* Merge the acquired and the original lock. @todo: move to a method. */
  MySQL_prlock_wrlock(&lock->m_rwlock);
  if (is_new_ticket)
  {
    m_tickets[MDL_TRANSACTION].remove(mdl_new_lock_request.ticket);
    MDL_ticket::destroy(mdl_new_lock_request.ticket);
  }
//此处打印出来的升级后TICKET类型
  DBUG_RETURN(FALSE);
}
4、释放：略
五、常见MDL Lock类型加锁测试
1、MDL_INTENTION_EXCLUSIVE(IX)
这个锁会在很多操作的时候都会出现，比如做任何一个DML/DDL操作都会触发，实际上DELTE/UPDATE/INSERT/FOR UPDATE等DML操作会在GLOBAL 上加IX锁，然后才会在本对象上加锁。而DDL 语句至少会在GLOBAL 上加IX锁，对象所属 SCHEMA上加IX锁，本对象加锁。

下面是 DELETE 触发的 GLOABL IX MDL LOCK：

2017-08-03T18:22:38.092205Z 3 [Note] (acquire_lock)THIS MDL LOCK acquire ok!
2017-08-03T18:22:38.092242Z 3 [Note] (>MDL PRINT) Thread id is 3:
2017-08-03T18:22:38.092276Z 3 [Note] (--->MDL PRINT) Namespace is:GLOBAL
2017-08-03T18:22:38.092310Z 3 [Note] (---->MDL PRINT) Fast path is:(Y)
2017-08-03T18:22:38.092344Z 3 [Note] (----->MDL PRINT) Mdl type is:MDL_INTENTION_EXCLUSIVE(IX)
2017-08-03T18:22:38.092380Z 3 [Note] (------>MDL PRINT) Mdl duration is:MDL_STATEMENT
2017-08-03T18:22:38.092551Z 3 [Note] (------->MDL PRINT) Mdl status is:EMPTY
我们注意一样它的持续周期为语句级别。

下面是 ALETER 语句触发的GLOABL IX MDL Lock：

2017-08-03T18:46:05.894871Z 3 [Note] (acquire_lock)THIS MDL LOCK acquire ok!
2017-08-03T18:46:05.894915Z 3 [Note] (>MDL PRINT) Thread id is 3:
2017-08-03T18:46:05.894948Z 3 [Note] (--->MDL PRINT) Namespace is:GLOBAL
2017-08-03T18:46:05.894980Z 3 [Note] (---->MDL PRINT) Fast path is:(Y)
2017-08-03T18:46:05.895012Z 3 [Note] (----->MDL PRINT) Mdl type is:MDL_INTENTION_EXCLUSIVE(IX)
2017-08-03T18:46:05.895044Z 3 [Note] (------>MDL PRINT) Mdl duration is:MDL_STATEMENT
2017-08-03T18:46:05.895076Z 3 [Note] (------->MDL PRINT) Mdl status is:EMPTY
所以这个MDL Lock无所不在，而只有是否兼容问题，如果不兼容则堵塞。scope lock的IX类型一般都是兼容的除非遇到S类型，下面讨论。

2、MDL_SHARED(S)
这把锁一般用在flush tables with read lock中，如下：

MySQL> flush tables with read lock;
Query OK, 0 rows affected (0.01 sec)
2017-08-03T18:19:11.603911Z 3 [Note] (acquire_lock)THIS MDL LOCK acquire ok!
2017-08-03T18:19:11.603947Z 3 [Note] (>MDL PRINT) Thread id is 3:
2017-08-03T18:19:11.603971Z 3 [Note] (--->MDL PRINT) Namespace is:GLOBAL
2017-08-03T18:19:11.603994Z 3 [Note] (----->MDL PRINT) Mdl type is:MDL_SHARED(S)
2017-08-03T18:19:11.604045Z 3 [Note] (------>MDL PRINT) Mdl duration is:MDL_EXPLICIT
2017-08-03T18:19:11.604073Z 3 [Note] (------->MDL PRINT) Mdl status is:EMPTY
2017-08-03T18:19:11.604133Z 3 [Note] (acquire_lock)THIS MDL LOCK acquire ok!
2017-08-03T18:19:11.604156Z 3 [Note] (>MDL PRINT) Thread id is 3:
2017-08-03T18:19:11.604194Z 3 [Note] (--->MDL PRINT) Namespace is:COMMIT
2017-08-03T18:19:11.604217Z 3 [Note] (----->MDL PRINT) Mdl type is:MDL_SHARED(S)
2017-08-03T18:19:11.604240Z 3 [Note] (------>MDL PRINT) Mdl duration is:MDL_EXPLICIT
2017-08-03T18:19:11.604310Z 3 [Note] (------->MDL PRINT) Mdl status is:EMPTY
我们注意到其namspace为GLOBAL和COMMIT显然他们是scope lock ，他们的TYPE为S，那么很显然根据兼容性原则scope lock的MDL IX和MDL S 不兼容， flush tables with read lock 就会堵塞所有DELTE/UPDATE/INSERT/FOR UPDATE等DML和DDL操作，并且也会堵塞commit操作。

3、MDL_SHARED_HIGH_PRIO(SH)
这个锁基本上大家也是经常用到只是没感觉到而已，比如我们一般desc操作，兼容矩阵如下：
MySQL：理解MDL Lock

操作记录如下：

MySQL> desc test.testsort10;
2017-08-03T19:06:05.843277Z 4 [Note] (acquire_lock)THIS MDL LOCK acquire ok!
2017-08-03T19:06:05.843324Z 4 [Note] (>MDL PRINT) Thread id is 4:
2017-08-03T19:06:05.843359Z 4 [Note] (->MDL PRINT) DB_name is:test
2017-08-03T19:06:05.843392Z 4 [Note] (-->MDL PRINT) OBJ_name is:testsort10
2017-08-03T19:06:05.843425Z 4 [Note] (--->MDL PRINT) Namespace is:TABLE
2017-08-03T19:06:05.843456Z 4 [Note] (---->MDL PRINT) Fast path is:(Y)
2017-08-03T19:06:05.843506Z 4 [Note] (----->MDL PRINT) Mdl type is:MDL_SHARED_HIGH_PRIO(SH)
2017-08-03T19:06:05.843538Z 4 [Note] (------>MDL PRINT) Mdl duration is:MDL_TRANSACTION
2017-08-03T19:06:05.843570Z 4 [Note] (------->MDL PRINT) Mdl status is:EMPTY
这中类型的优先级比较高，但是其和X不兼容。注意持续时间为MDL_TRANSACTION 。

4、MDL_SHARED_READ(SR)
这把锁一般用在非当前读取的select中，兼容性如下：
MySQL：理解MDL Lock

操作记录如下：

MySQL> select * from test.testsort10 limit 1;
2017-08-03T19:13:52.338764Z 4 [Note] (acquire_lock)THIS MDL LOCK acquire ok!
2017-08-03T19:13:52.338813Z 4 [Note] (>MDL PRINT) Thread id is 4:
2017-08-03T19:13:52.338847Z 4 [Note] (->MDL PRINT) DB_name is:test
2017-08-03T19:13:52.338883Z 4 [Note] (-->MDL PRINT) OBJ_name is:testsort10
2017-08-03T19:13:52.338917Z 4 [Note] (--->MDL PRINT) Namespace is:TABLE
2017-08-03T19:13:52.338950Z 4 [Note] (---->MDL PRINT) Fast path is:(Y)
2017-08-03T19:13:52.339025Z 4 [Note] (----->MDL PRINT) Mdl type is:MDL_SHARED_READ(SR)
2017-08-03T19:13:52.339062Z 4 [Note] (------>MDL PRINT) Mdl duration is:MDL_TRANSACTION
2017-08-03T19:13:52.339097Z 4 [Note] (------->MDL PRINT) Mdl status is:EMPTY
这里还是要提及一下平时我们偶尔会出现select也堵住的情况(比如DDL的某个阶段需要对象MDL X锁)。我们不得不抱怨MySQL居然会堵塞select其实这里也就是object mdl lock X 和SR 不兼容的问题(参考前面的兼容矩阵)。注意持续时间为MDL_TRANSACTION 。

5、MDL_SHARED_WRITE(SW)
这把锁一般用于DELTE/UPDATE/INSERT/FOR UPDATE等操作对table的加锁(当前读)，不包含DDL操作，但是要注意DML操作实际上还会有一个GLOBAL的IX的锁，前面已经提及过了，这把锁只是对象上的，兼容性如下：

MySQL：理解MDL Lock

操作记录如下：

MySQL> select * from test.testsort10 limit 1 for update;
2017-08-03T19:25:41.218428Z 4 [Note] (acquire_lock)THIS MDL LOCK acquire ok!
2017-08-03T19:25:41.218461Z 4 [Note] (>MDL PRINT) Thread id is 4:
2017-08-03T19:25:41.218493Z 4 [Note] (->MDL PRINT) DB_name is:test
2017-08-03T19:25:41.218525Z 4 [Note] (-->MDL PRINT) OBJ_name is:testsort10
2017-08-03T19:25:41.218557Z 4 [Note] (--->MDL PRINT) Namespace is:TABLE
2017-08-03T19:25:41.218588Z 4 [Note] (---->MDL PRINT) Fast path is:(Y)
2017-08-03T19:25:41.218620Z 4 [Note] (----->MDL PRINT) Mdl type is:MDL_SHARED_WRITE(SW)
2017-08-03T19:25:41.218677Z 4 [Note] (------>MDL PRINT) Mdl d