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MYSQL METADATA LOCK(MDL LOCK)MDL鎖問題分析

一、前言

MYSQL中MDL鎖一直是一個(gè)比較讓人比較頭疼的問題，我們談起鎖一般更加傾向于INNODB下層的gap lock、next key lock、row lock等，因?yàn)樗芎美斫猓埠芎糜^察，而對(duì)于MDL LOCK卻了解得很少，因?yàn)樗鼘?shí)在不好觀察，只有出現(xiàn)問題查看show processlist勉強(qiáng)可以看到

簡(jiǎn)單的所謂的Waiting for table metadata lock之類的狀態(tài)，其實(shí)MDL LOCK是MYSQL上層一個(gè)非常復(fù)雜的子系統(tǒng)，有自己的死鎖檢測(cè)機(jī)制

(無向圖?)而大家一般口中的是不是鎖表了其實(shí)就是指的它，可見的它的關(guān)鍵性和嚴(yán)重性，筆者也是根據(jù)自己的需求學(xué)習(xí)了一些(冰山一角)，而沒有能力閱讀全部的代碼，但是筆者通過增加一個(gè)TICKET的打印函數(shù)讓語句的MDL LOCK加鎖流程全部打印出來方便學(xué)習(xí)研究，下面從一些基礎(chǔ)說起然后告訴大家修改了哪些東西，最后對(duì)每種MDL TYPE進(jìn)行測(cè)試和分析,如果大家對(duì)基本概念和增加打印函數(shù)不感興趣可直接參考第五部分加鎖測(cè)試,但是如果不了解基礎(chǔ)知識(shí)可能看起來有點(diǎn)困難。

剛好最近遇到一次MDL LOCK出現(xiàn)死鎖的情況會(huì)在下篇文章中給出案例，這里只看理論

----處于層次:MYSQL SERVER層次，實(shí)際上早在open_table函數(shù)中MDL LOCK就開始獲取了，可以說他是最早獲取的LOCK結(jié)構(gòu)

----最早獲取階段: THD::enter_stage: 'Opening tables'

調(diào)用棧幀

#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 0x0000000001516e17 in open_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

----死鎖檢測(cè)出錯(cuò)碼:

{ "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

MDL LOCK的死鎖拋錯(cuò)和INNODB死鎖一模一樣不同的只是SHOW ENGINE INNODB 沒有死鎖信息。

----涉及代碼:mdl.h mdl.cc

二、基礎(chǔ)重要的數(shù)據(jù)結(jié)構(gòu)(類)和概念

1、MDL TYPE

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)

后面會(huì)對(duì)每種TYPE進(jìn)行詳細(xì)的測(cè)試，最后也會(huì)給出源碼中解釋

2、MDL NAMESPACE

在MDL中MDL_KEY按照NAMESPACE+DB+OBJECT_NAME的方式進(jìn)行表示，所謂的namespace也不叫重要

下面是NAMESPACE的分類

- 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

3、實(shí)現(xiàn)分類

scope lock:一般對(duì)應(yīng)全局MDL LOCK 如flush table with read lock 為namespace space:GLOBAL type:S

object lock:如其名字，對(duì)象級(jí)別的MDL LOCK，比如TABLE

下面是源碼中的注釋：

 /**
  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兼容矩陣

scope lock:
     | Type of active  |
 Request |  scoped lock  |
  type  | IS(*) IX  S X |
 ---------+------------------+
 IS    | +   +  + + |
 IX    | +   +  - - |
 S    | +   -  + - |
 X    | +   -  - - |

object lock:
    Request | Granted requests for lock      |
    type  | S SH SR SW SWLP SU SRO SNW SNRW X |
   ----------+---------------------------------------------+
   S     | +  +  +  +  +  +  +  +  +  - |
   SH    | +  +  +  +  +  +  +  +  +  - |
   SR    | +  +  +  +  +  +  +  +  -  - |
   SW    | +  +  +  +  +  +  -  -  -  - |
   SWLP   | +  +  +  +  +  +  -  -  -  - |
   SU    | +  +  +  +  +  -  +  -  -  - |
   SRO    | +  +  +  -  -  +  +  +  -  - |
   SNW    | +  +  +  -  -  -  +  -  -  - |
   SNRW   | +  +  -  -  -  -  -  -  -  - |
   X     | -  -  -  -  -  -  -  -  -  - |

5、MDL duration及MDL持續(xù)到什么時(shí)候

這個(gè)也不多用過多解釋，看源碼注釋即可

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 FAST PATH（unobtrusive） OR SLOW PATH（obtrusive）

使用兩種不同的方式目的在于優(yōu)化MDL lock的實(shí)現(xiàn)，下面是源碼的注釋

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 with
        some 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類

也就是通過語句解析后需要獲得的MDL LOCK的需求，然后通過這個(gè)類對(duì)象在MDL子系統(tǒng)
中進(jìn)行MDL LOCK申請(qǐng)，大概包含如下一些屬性

/** Type of metadata lock. */
 enum enum_mdl_type type; //需求的類型
 /** Duration for requested lock. */
 enum enum_mdl_duration duration; //持續(xù)時(shí)間
 /**
  Pointers for participating in the list of lock requests for this context.
 */
 MDL_request *next_in_list; //雙向鏈表實(shí)現(xiàn)
 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; //注意這里如果申請(qǐng)成功(沒有等待)，會(huì)指向一個(gè)實(shí)際的TICKET，否則為NULL
 /** A lock is requested based on a fully qualified name and type. */
 MDL_key key;//注意這里是一個(gè)MDL_KEY類型，主要的就是前面說的NAMESPACE+DB+OBJECT_NAME

MDL_key類，就是實(shí)際的NAMESPACE+DB+OBJECT_NAME，整個(gè)放到一個(gè)char數(shù)組里面，他會(huì)是MDL_LOCK和MDL_REQUEST中出現(xiàn)

private:

uint16 m_length;

uint16 m_db_name_length;

char m_ptr[MAX_MDLKEY_LENGTH];//放到了這里

8、MDL_ticket

如同門票一樣，如果獲取了MDL LOCK必然給MDL_request返回一張門票，如果等待則不會(huì)源碼MDL_context::acquire_lock

可以觀察到。當(dāng)然這也是我主要觀察的一個(gè)類

/**
Pointers for participating in the list of lock requests for this context.
Context private.正如解釋這里是context中鏈表鏈表的形成，是線程私有的
*/
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.正如解釋這里是MDL_LOCK中鏈表鏈表的形成，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.
很明顯這里指向了這個(gè)ticket的擁有者也就是MDL_context，它是線程的屬性
*/
MDL_context *m_ctx;
/**
Pointer to the lock object for this lock ticket. Externally accessible.
很明顯這里是一個(gè)指向MDL_LOCK的一個(gè)指針
*/
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
這里就代表了是否是FAST PATH從注釋來看fast path方式不會(huì)在MDL LOCK中
占用granted位圖和鏈表取而代之代之的是一個(gè)統(tǒng)計(jì)器m_fast_path_locks_granted_counter
這樣一來開銷肯定更小
*/
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.
*/
bool m_hton_notified;

9、MDL_lock

每一個(gè)MDL_key都會(huì)對(duì)應(yīng)一個(gè)MDL_lock，其中包含了所謂的GRANTED鏈表和WAIT鏈表，考慮它的復(fù)雜性，可以直接參考源碼注釋也非常詳細(xì)，這里給出我所描述的幾個(gè)屬性。

/** 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;

10、MDL_context

這是整個(gè)MYSQL 線程和MDL lock子系統(tǒng)進(jìn)行交互的一個(gè)所謂的上下文結(jié)構(gòu)其中包含了很多方法和屬性，我比較關(guān)注的屬性如下：

/**
  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.
  這是一個(gè)不同MDL lock持續(xù)時(shí)間的一個(gè)鏈表數(shù)組。實(shí)際就是
  MDL_STATEMENT一個(gè)鏈表
  MDL_TRANSACTION一個(gè)鏈表
  MDL_EXPLICIT一個(gè)鏈表
 */
Ticket_list m_tickets[MDL_DURATION_END];
//這是一個(gè)父類指針指向子類對(duì)象，虛函數(shù)重寫的典型,實(shí)際他就指向了一個(gè)線程
/*
class THD :public MDL_context_owner,
      public Query_arena,
      public Open_tables_state
*/
MDL_context_owner *m_owner;

11、MDL_wait

這個(gè)類主要是當(dāng)前ticket獲取狀態(tài)

enum_wait_status m_wait_status;

包含

EMPTY 初始化

GRANTED 獲取成功

VICTIM 死鎖

TIMEOUT 超時(shí)

KILLED KILLED

12、等待標(biāo)記

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}
};

三、增加MDL LOCK打印函數(shù)

研究MDL LOCK鎖最好的方式當(dāng)然是能夠獲取MDL 加鎖、升級(jí)、降級(jí)的流程，因?yàn)樵创a太龐大了，不可能面面俱到
雖然5.7加入了

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

的方式進(jìn)行MDL LOCK的查看，但是如果要觀察一個(gè)語句到底獲取了哪些MDL LOCK還是顯得無力所以筆者在mdl.cc中加入了一個(gè)函數(shù)原型如下

/*p_ticket in parameter*/

int my_print_ticket(const MDL_ticket* p_ticket)

并且在mdl_ticket類中增加了這個(gè)函數(shù)原型為友元函數(shù)，否則私有成員獲取不到，而給出的公有方法比較繁雜

friend int my_print_ticket(const MDL_ticket* p_ticket);

主要獲取MDL LOCK的如下信息打印到mysql err日志中:

線程id 通過p_ticket->m_ctx->get_thd(); 獲取

mdl lock database name 通過p_ticket->m_lock->key.db_name()獲取

mdl lock object name 通過p_ticket->m_lock->key.name()獲取

mdl lock namespace 通過p_ticket->m_lock->key.mdl_namespace()獲取

mdl lock fast path 通過p_ticket->m_is_fast_path獲取判斷是則輸出否則不輸出

mdl lock type 通過p_ticket->m_type獲取

mdl lock duration 通過p_ticket->m_duration獲取

輸出信息如下：

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

實(shí)際上和metadata_locks中的信息差不多，這是我這里的Thread id 是show processlist出來的id，但是我可以獲得鎖獲取的歷史信息,我這里同時(shí)沒有 LOCK_STATUS: GRANTED，但是可以在MDL_context::acquire_lock 邏輯上可以判斷出來

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

四、在合適的位置增加打印函數(shù)進(jìn)行觀察

既然我們要研究MDL LOCK的加鎖\升級(jí)\降級(jí)、那么我們就必要找到他們的函數(shù)入口，然后在合適的位置增加打印函數(shù)進(jìn)行觀察，下面標(biāo)示出打印位置，刪除了大部分的源代碼，需要參考請(qǐng)自行查看源碼

1、加鎖：MDL_context::acquire_lock

bool
MDL_context::acquire_lock(MDL_request *mdl_request, ulong lock_wait_timeout)
{
 if (mdl_request->ticket) //獲取成功獲得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獲取TICKET成功 此處打印
  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.
 */
 //獲取不成功加入MDL_lock 等待隊(duì)列
 lock= ticket->m_lock;
 lock->m_waiting.add_ticket(ticket);
 will_wait_for(ticket); //死鎖檢測(cè)
 /* There is a shared or exclusive lock on the object. */
 DEBUG_SYNC(get_thd(), "mdl_acquire_lock_wait");
 find_deadlock(); 
 //此處打印TICKET進(jìn)入了等待流程
 if (lock->needs_notification(ticket) || lock->needs_connection_check())
 {
  }
 done_waiting_for();//等待完成對(duì)死鎖檢測(cè)等待圖進(jìn)行調(diào)整去掉本等待邊edge(無向圖)
 //當(dāng)然到這里也是通過等待后獲得成功了狀態(tài)為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);
 //此處打印通過等待REQUEST獲得了TICKET
 return FALSE;
}

2、降級(jí)：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);
//此處打印出降級(jí)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);
 }
//函數(shù)結(jié)尾答應(yīng)出降級(jí)后的TICKET
}

3、升級(jí)：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);//構(gòu)造一個(gè)request
 //此處打印出來的TICKET類型 
 if (acquire_lock(mdl_new_lock_request, lock_wait_timeout)) //嘗試使用新的LOCK_TYPE進(jìn)行加鎖
  DBUG_RETURN(TRUE);
 is_new_ticket= ! has_lock(mdl_svp, mdl_new_lock_request.ticket);
 lock= mdl_ticket->m_lock;
 //下面進(jìn)行一系列對(duì)MDL_LOCK的維護(hù)并且對(duì)所謂的合并操作
 /* 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);
 }
 //此處打印出來的升級(jí)后TICKET類型 
 DBUG_RETURN(FALSE);
}

當(dāng)然我現(xiàn)在只是在這些地方進(jìn)行了打印，以后如果需要在其他地方答應(yīng)加上函數(shù)就可以了。

五、各種MDL LOCK TYPE加鎖測(cè)試

1、MDL_INTENTION_EXCLUSIVE(IX)

這個(gè)鎖會(huì)在很多操作的時(shí)候都會(huì)出現(xiàn)比如做任何一個(gè)DML/DDL 操作都會(huì)觸發(fā)，實(shí)際上DELTE/UPDATE/INSERT/FOR UPDATE等DML操作會(huì)在GLOBAL 上加IX鎖然后才會(huì)在本對(duì)象上加鎖而DDL 語句至少會(huì)在GLOBAL 上加IX鎖，對(duì)象所屬 SCHEMA上加IX鎖，本對(duì)象加鎖下面是 DELETE 觸發(fā)的 GLOABL IX MDL LOCK

2017-08-03T18:22:38.092100Z 3 [Note] Test2:open_tables_for_query()
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

下面是 ALETER 語句觸發(fā)的GLOABL IX MDL LOCK以及SCHEMA級(jí)別的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 
2017-08-03T18:46:05.895116Z 3 [Note] (acquire_lock)THIS MDL LOCK acquire ok!
2017-08-03T18:46:05.895147Z 3 [Note] (>MDL PRINT) Thread id is 3: 
2017-08-03T18:46:05.895206Z 3 [Note] (->MDL PRINT) DB_name is:test 
2017-08-03T18:46:05.895243Z 3 [Note] (-->MDL PRINT) OBJ_name is: 
2017-08-03T18:46:05.895276Z 3 [Note] (--->MDL PRINT) Namespace is:SCHEMA 
2017-08-03T18:46:05.895325Z 3 [Note] (---->MDL PRINT) Fast path is:(Y)
2017-08-03T18:46:05.895357Z 3 [Note] (----->MDL PRINT) Mdl type is:MDL_INTENTION_EXCLUSIVE(IX) 
2017-08-03T18:46:05.895390Z 3 [Note] (------>MDL PRINT) Mdl duration is:MDL_TRANSACTION 
2017-08-03T18:46:05.895421Z 3 [Note] (------->MDL PRINT) Mdl status is:EMPTY

所以這個(gè)MDL LOCK 無所不在，而只有是否兼容問題，如果不兼容則堵塞。SCOPED 的IX類型一般都是兼容的除非遇到
S類型

2、MDL_SHARED(S)

這把鎖一般用在flush tables with read lock中

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

我們注意到其namspace為GLOBAL和COMMIT顯然他們是SCOPED LOCK，他們的TYPE為S，那么很顯然根據(jù)兼容性原則
SCOPED 的MDL IX和MDL S 不兼容， flush tables with read lock; 就會(huì)堵塞所有DELTE/UPDATE/INSERT/FOR UPDATE
等DML和DDL操作(因?yàn)檫@些操作都需要GLOBAL MDL IX鎖)

3、MDL_SHARED_HIGH_PRIO(SH)

這個(gè)鎖基本上大家也是經(jīng)常用到只是沒感覺到而已，比如我們一般desc操作

兼容性：

    Request | Granted requests for lock         |
    type  | S SH SR SW SWLP SU SRO SNW SNRW X |
   ----------+---------------------------------------------+
   SH    | +  +  +  +  +  +  +  +  +  - |
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

這中類型的優(yōu)先級(jí)比較高，但是其和X不兼容。也很好理解比如在rename 階段肯定不能進(jìn)行desc操作。

4、MDL_SHARED_READ(SR)

這把鎖一般用在非當(dāng)前讀取的select中兼容性：

    Request | Granted requests for lock         |
    type  | S SH SR SW SWLP SU SRO SNW SNRW X |
   ----------+---------------------------------------------+
   SR    | +  +  +  +  +     +   +    +  -  - |
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

這里還是要提及一下平時(shí)我們偶爾會(huì)出現(xiàn)select也堵住的情況(比如DDL的某個(gè)階段需要對(duì)象MDL X鎖)。我們不得不抱怨
MYSQL居然會(huì)堵塞select其實(shí)這里也就是object mdl lock X 和SR 不兼容的問題(參考前面的兼容矩陣)。

5、MDL_SHARED_WRITE(SW)

這把鎖一般用于DELTE/UPDATE/INSERT/FOR UPDATE等操作對(duì)table的加鎖(當(dāng)前讀)，不包含DDL操作
但是要注意DML操作實(shí)際上會(huì)有一個(gè)GLOBAL的IX的鎖，前面已經(jīng)提及過了，這把鎖只是對(duì)象上的

兼容性：

    Request | Granted requests for lock         |
    type  | S SH SR SW SWLP SU SRO SNW SNRW X |
   ----------+---------------------------------------------+
   SW    | +  +  +  +  +  +  -  -  -  - |
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 duration is:MDL_TRANSACTION 
2017-08-03T19:25:41.218874Z 4 [Note] (------->MDL PRINT) Mdl status is:EMPTY

6、MDL_SHARED_WRITE_LOW_PRIO(SWL)

這把鎖很少用到源碼注釋只有

Used by DML statements modifying

tables and using the LOW_PRIORITY clause

會(huì)用到

兼容性：

    Request | Granted requests for lock         |
    type  | S SH SR SW SWLP SU SRO SNW SNRW X |
   ----------+---------------------------------------------+
   SWLP   | +  +  +  +  +  +  -  -  -  - |
mysql> update LOW_PRIORITY test.testsort10 set id1=1000 where id1= 96282;
2017-08-03T19:32:47.433507Z 4 [Note] (acquire_lock)THIS MDL LOCK acquire ok!
2017-08-03T19:32:47.433521Z 4 [Note] (>MDL PRINT) Thread id is 4: 
2017-08-03T19:32:47.433533Z 4 [Note] (->MDL PRINT) DB_name is:test 
2017-08-03T19:32:47.433547Z 4 [Note] (-->MDL PRINT) OBJ_name is:testsort10 
2017-08-03T19:32:47.433560Z 4 [Note] (--->MDL PRINT) Namespace is:TABLE 
2017-08-03T19:32:47.433572Z 4 [Note] (---->MDL PRINT) Fast path is:(Y)
2017-08-03T19:32:47.433594Z 4 [Note] (----->MDL PRINT) Mdl type is:MDL_SHARED_WRITE_LOW_PRIO(SWL) 
2017-08-03T19:32:47.433607Z 4 [Note] (------>MDL PRINT) Mdl duration is:MDL_TRANSACTION 
2017-08-03T19:32:47.433620Z 4 [Note] (------->MDL PRINT) Mdl status is:EMPTY

7、MDL_SHARED_UPGRADABLE(SU)

這把鎖一般在ALTER TABLE語句中用到，他可以升級(jí)為SNW, SNRW，X，同時(shí)至少X鎖也可以降級(jí)為SU

實(shí)際上在INNODB ONLINE DDL中非常依賴于他,DML(SW)和SELECT(SR)都不會(huì)堵塞

兼容性：

    Request | Granted requests for lock         |
    type  | S SH SR SW SWLP SU SRO SNW SNRW X |
   ----------+---------------------------------------------+
   SU    | +  +  +  +  +  -  +  -  -  - |

我們有必要研究一下他的兼容性，可以看到 OBJECT LOCK中(SELECT)SR (DML)SW都是允許的，而在SCOPED LOCK中
雖然DML DDL都會(huì)在GLOBAL 上鎖但是其類型都是IX所以這個(gè)SU鎖不堵塞DML/SELECT 讀寫操作進(jìn)入INNODB引擎層，它是ONLINE DDL的根基，如果不兼容你都進(jìn)入不了INNODB引擎層，更談不上什么ONLINE DDL，注意我這里說的ALGORITHM=INPLACE 并且不設(shè)置LOCK

(For DDL operations with LOCK=DEFAULT, or with the LOCK clause omitted, MySQL uses the lowest level
of locking that is available for that kind of operation, allowing concurrent queries, DML, or both wherever
possible. This is the setting to use when making pre-planned, pre-tested changes that you know will not
cause any availability problems based on the workload for that table
When an operation on the primary key uses ALGORITHM=INPLACE, even though the data is still copied, it
is more efficient than using ALGORITHM=COPY because:
? No undo logging or associated redo logging is required for ALGORITHM=INPLACE. These operations add
overhead to DDL statements that use ALGORITHM=COPY.
? The secondary index entries are pre-sorted, and so can be loaded in order.
? The change buffer is not used, because there are no random-access inserts into the secondary indexes.
)

如下面的語句

mysql> alter table testsort12 add column it int not null;
Query OK, 0 rows affected (6.27 sec)
Records: 0 Duplicates: 0 Warnings: 0

我簡(jiǎn)單的分析一下：

2017-08-03T19:46:54.781453Z 3 [Note] (acquire_lock)THIS MDL LOCK acquire ok!
2017-08-03T19:46:54.781487Z 3 [Note] (>MDL PRINT) Thread id is 3: 
2017-08-03T19:46:54.781948Z 3 [Note] (->MDL PRINT) DB_name is:test 
2017-08-03T19:46:54.781990Z 3 [Note] (-->MDL PRINT) OBJ_name is:testsort12 
2017-08-03T19:46:54.782026Z 3 [Note] (--->MDL PRINT) Namespace is:TABLE 
2017-08-03T19:46:54.782060Z 3 [Note] (----->MDL PRINT) Mdl type is:MDL_SHARED_UPGRADABLE(SU) 
2017-08-03T19:46:54.782096Z 3 [Note] (------>MDL PRINT) Mdl duration is:MDL_TRANSACTION 
2017-08-03T19:46:54.782175Z 3 [Note] (------->MDL PRINT) Mdl status is:EMPTY 
2017-08-03T19:46:54.803898Z 3 [Note] (upgrade_shared_lock)THIS MDL LOCK will upgrade
2017-08-03T19:46:54.804201Z 3 [Note] (upgrade_shared_lock)THIS MDL LOCK upgrade TO
2017-08-03T19:46:54.804240Z 3 [Note] (>MDL PRINT) Thread id is 3: 
2017-08-03T19:46:54.804254Z 3 [Note] (->MDL PRINT) DB_name is:test 
2017-08-03T19:46:54.804267Z 3 [Note] (-->MDL PRINT) OBJ_name is:testsort12 
2017-08-03T19:46:54.804280Z 3 [Note] (--->MDL PRINT) Namespace is:TABLE 
2017-08-03T19:46:54.804293Z 3 [Note] (----->MDL PRINT) Mdl type :MDL_EXCLUSIVE(X) 
2017-08-03T19:46:54.804306Z 3 [Note] (------>MDL PRINT) Mdl duration is:MDL_TRANSACTION 
2017-08-03T19:46:54.804319Z 3 [Note] (------->MDL PRINT) Mdl status is:EMPTY 
2017-08-03T19:46:54.855563Z 3 [Note] (downgrade_lock)THIS MDL LOCK will downgrade
2017-08-03T19:46:54.855693Z 3 [Note] (downgrade_lock) to this MDL lock
2017-08-03T19:46:54.855706Z 3 [Note] (>MDL PRINT) Thread id is 3: 
2017-08-03T19:46:54.855717Z 3 [Note] (->MDL PRINT) DB_name is:test 
2017-08-03T19:46:54.856053Z 3 [Note] (-->MDL PRINT) OBJ_name is:testsort12 
2017-08-03T19:46:54.856069Z 3 [Note] (--->MDL PRINT) Namespace is:TABLE 
2017-08-03T19:46:54.856082Z 3 [Note] (----->MDL PRINT) Mdl type is:MDL_SHARED_UPGRADABLE(SU) 
2017-08-03T19:46:54.856094Z 3 [Note] (------>MDL PRINT) Mdl duration is:MDL_TRANSACTION 
2017-08-03T19:46:54.856214Z 3 [Note] (------->MDL PRINT) Mdl status is:EMPTY 
2017-08-03T19:47:00.260166Z 3 [Note] (upgrade_shared_lock)THIS MDL LOCK will upgrade
2017-08-03T19:47:00.304057Z 3 [Note] (upgrade_shared_lock)THIS MDL LOCK upgrade TO
2017-08-03T19:47:00.304090Z 3 [Note] (>MDL PRINT) Thread id is 3: 
2017-08-03T19:47:00.304105Z 3 [Note] (->MDL PRINT) DB_name is:test 
2017-08-03T19:47:00.304119Z 3 [Note] (-->MDL PRINT) OBJ_name is:testsort12 
2017-08-03T19:47:00.304132Z 3 [Note] (--->MDL PRINT) Namespace is:TABLE 
2017-08-03T19:47:00.304181Z 3 [Note] (----->MDL PRINT) Mdl type is:MDL_EXCLUSIVE(X) 
2017-08-03T19:47:00.304196Z 3 [Note] (------>MDL PRINT) Mdl duration is:MDL_TRANSACTION 
2017-08-03T19:47:00.304211Z 3 [Note] (------->MDL PRINT) Mdl status is:EMPTY 
2017-08-03T19:47:01.032329Z 3 [Note] (acquire_lock)THIS MDL LOCK acquire ok!

首先獲得testsort12表上的

2017-08-03T19:46:54.781487 獲得 MDL_SHARED_UPGRADABLE(SU)
2017-08-03T19:46:54.804293 升級(jí) MDL_EXCLUSIVE(X) 
2017-08-03T19:46:54.855563 降級(jí) MDL_SHARED_UPGRADABLE(SU)
2017-08-03T19:47:00.304057 升級(jí) MDL_EXCLUSIVE(X)

因?yàn)椴还苋绾芜@個(gè)alter操作還是比較費(fèi)時(shí)的，從時(shí)間我們看到2017-08-03T19:46:54降級(jí)完成到2017-08-03T19:47:00這段時(shí)間
實(shí)際上是最耗時(shí)的實(shí)際上這里就是實(shí)際的COPY操作，但是這個(gè)過程實(shí)際在MDL SU模式下所以不會(huì)堵塞DML/SELECT操作。
這里再給大家提個(gè)醒所謂的ONLINE DDL只是在COPY階段不堵塞DML/SELECT操作，還是盡量在數(shù)據(jù)庫壓力小的時(shí)候，
比如如果有DML沒有提交或者SELECT沒有做完這個(gè)時(shí)候SW SR必然堵塞X，而X能夠堵塞一切且為高優(yōu)先級(jí)。這樣導(dǎo)致
的現(xiàn)象就是由于DML未提交堵塞DDL操作而DDL操作堵塞一切操作，基本對(duì)于這個(gè)TABLE的表全部堵塞。而對(duì)于ALGORITHM=COPY 其他部分差不多，但是在COPY階段用的是SNW鎖，接下來我就先來看看SNW鎖

8、MDL_SHARED_NO_WRITE(SNW)

SU可以升級(jí)為SNW而SNW可以升級(jí)為X，如前面所提及用于ALGORITHM=COPY 中，保護(hù)數(shù)據(jù)的一致性。
先看看它的兼容性

   Request | Granted requests for lock          
    type  | S SH SR SW SWLP SU SRO SNW SNRW X |
   ----------+---------------------------------------------+
   SNW    | +  +  +  -  -  -  +  -  -  - |

可以看到SR可以但是SW不行，當(dāng)然也就堵塞了DML(SW)而SELECT(SR)不會(huì)堵塞，下面我只是給出了關(guān)鍵部分

mysql> alter table testsort12 add column ik int not null, ALGORITHM=COPY ;
2017-08-03T20:07:58.413215Z 3 [Note] (upgrade_shared_lock)THIS MDL LOCK upgrade TO
2017-08-03T20:07:58.413241Z 3 [Note] (>MDL PRINT) Thread id is 3: 
2017-08-03T20:07:58.413257Z 3 [Note] (->MDL PRINT) DB_name is:test 
2017-08-03T20:07:58.413273Z 3 [Note] (-->MDL PRINT) OBJ_name is:testsort12 
2017-08-03T20:07:58.413292Z 3 [Note] (--->MDL PRINT) Namespace is:TABLE 
2017-08-03T20:07:58.413308Z 3 [Note] (----->MDL PRINT) Mdl type is:MDL_SHARED_NO_WRITE(SNW) 
2017-08-03T20:07:58.413325Z 3 [Note] (------>MDL PRINT) Mdl duration is:MDL_TRANSACTION 
2017-08-03T20:07:58.413341Z 3 [Note] (------->MDL PRINT) Mdl status is:EMPTY 
2017-08-03T20:08:25.392006Z 3 [Note] (upgrade_shared_lock)THIS MDL LOCK upgrade TO
2017-08-03T20:08:25.392024Z 3 [Note] (>MDL PRINT) Thread id is 3: 
2017-08-03T20:08:25.392086Z 3 [Note] (->MDL PRINT) DB_name is:test 
2017-08-03T20:08:25.392159Z 3 [Note] (-->MDL PRINT) OBJ_name is:testsort12 
2017-08-03T20:08:25.392199Z 3 [Note] (--->MDL PRINT) Namespace is:TABLE 
2017-08-03T20:08:25.392214Z 3 [Note] (----->MDL PRINT) Mdl type is:MDL_EXCLUSIVE(X) 
2017-08-03T20:08:25.392228Z 3 [Note] (------>MDL PRINT) Mdl duration is:MDL_TRANSACTION 
2017-08-03T20:08:25.392242Z 3 [Note] (------->MDL PRINT) Mdl status is:EMPTY 
2017-08-03T20:07:58.413308 獲得了MDL_SHARED_NO_WRITE(SNW) 
2017-08-03T20:08:25.392006 升級(jí)為MDL_EXCLUSIVE(X)

這2017-08-03T20:07:58.413308到2017-08-03T20:08:25.392006就是實(shí)際COPY的時(shí)間，可見整個(gè)COPY期間只能DML
而不能SELECT，也是ALGORITHM=COPY和ALGORITHM=INPLACE一個(gè)關(guān)鍵區(qū)別。

9、MDL_SHARED_READ_ONLY(SRO)

用于LOCK TABLES READ 語句

兼容性如下

    Request | Granted requests for lock         |
    type  | S SH SR SW SWLP SU SRO SNW SNRW X |
   ----------+---------------------------------------------+
   SRO    | +  +  +  -  -  +  +  +  -  - |
堵塞DML(SW)但是SELECT(SR)還是可以的。
mysql> lock table testsort12 read;
Query OK, 0 rows affected (0.01 sec)
2017-08-03T21:08:27.267947Z 3 [Note] (acquire_lock)THIS MDL LOCK acquire ok!
2017-08-03T21:08:27.267979Z 3 [Note] (>MDL PRINT) Thread id is 3: 
2017-08-03T21:08:27.268009Z 3 [Note] (->MDL PRINT) DB_name is:test 
2017-08-03T21:08:27.268040Z 3 [Note] (-->MDL PRINT) OBJ_name is:testsort12 
2017-08-03T21:08:27.268070Z 3 [Note] (--->MDL PRINT) Namespace is:TABLE 
2017-08-03T21:08:27.268113Z 3 [Note] (----->MDL PRINT) Mdl type is:MDL_SHARED_READ_ONLY(SRO) 
2017-08-03T21:08:27.268145Z 3 [Note] (------>MDL PRINT) Mdl duration is:MDL_TRANSACTION 
2017-08-03T21:08:27.268175Z 3 [Note] (------->MDL PRINT) Mdl status is:EMPTY

10、MDL_SHARED_NO_READ_WRITE(SNRW)

用于LOCK TABLES WRITE語句

兼容性：

    Request | Granted requests for lock         |
    type  | S SH SR SW SWLP SU SRO SNW SNRW X |
   ----------+---------------------------------------------+
   SNRW   | +  +  -  -  -  -  -  -  -  - |

可以看到DML(SW)和SELECT(SR)都被堵塞只有SH還可以，還可以DESC(SH) 。

mysql> lock table testsort12 write;
Query OK, 0 rows affected (0.00 sec)
2017-08-03T21:13:07.113347Z 3 [Note] (acquire_lock)THIS MDL LOCK acquire ok!
2017-08-03T21:13:07.113407Z 3 [Note] (>MDL PRINT) Thread id is 3: 
2017-08-03T21:13:07.113435Z 3 [Note] (--->MDL PRINT) Namespace is:GLOBAL 
2017-08-03T21:13:07.113458Z 3 [Note] (---->MDL PRINT) Fast path is:(Y)
2017-08-03T21:13:07.113482Z 3 [Note] (----->MDL PRINT) Mdl type is:MDL_INTENTION_EXCLUSIVE(IX) 
2017-08-03T21:13:07.113505Z 3 [Note] (------>MDL PRINT) Mdl duration is:MDL_STATEMENT 
2017-08-03T21:13:07.113604Z 3 [Note] (------->MDL PRINT) Mdl status is:EMPTY 
2017-08-03T21:13:07.113637Z 3 [Note] (acquire_lock)THIS MDL LOCK acquire ok!
2017-08-03T21:13:07.113660Z 3 [Note] (>MDL PRINT) Thread id is 3: 
2017-08-03T21:13:07.113681Z 3 [Note] (->MDL PRINT) DB_name is:test 
2017-08-03T21:13:07.113703Z 3 [Note] (-->MDL PRINT) OBJ_name is: 
2017-08-03T21:13:07.113725Z 3 [Note] (--->MDL PRINT) Namespace is:SCHEMA 
2017-08-03T21:13:07.113746Z 3 [Note] (---->MDL PRINT) Fast path is:(Y)
2017-08-03T21:13:07.113768Z 3 [Note] (----->MDL PRINT) Mdl type is:MDL_INTENTION_EXCLUSIVE(IX) 
2017-08-03T21:13:07.113791Z 3 [Note] (------>MDL PRINT) Mdl duration is:MDL_TRANSACTION 
2017-08-03T21:13:07.113813Z 3 [Note] (------->MDL PRINT) Mdl status is:EMPTY 
2017-08-03T21:13:07.113842Z 3 [Note] (acquire_lock)THIS MDL LOCK acquire ok!
2017-08-03T21:13:07.113865Z 3 [Note] (>MDL PRINT) Thread id is 3: 
2017-08-03T21:13:07.113887Z 3 [Note] (->MDL PRINT) DB_name is:test 
2017-08-03T21:13:07.113922Z 3 [Note] (-->MDL PRINT) OBJ_name is:testsort12 
2017-08-03T21:13:07.113945Z 3 [Note] (--->MDL PRINT) Namespace is:TABLE 
2017-08-03T21:13:07.113975Z 3 [Note] (----->MDL PRINT) Mdl type is:MDL_SHARED_NO_READ_WRITE(SNRW) 
2017-08-03T21:13:07.113998Z 3 [Note] (------>MDL PRINT) Mdl duration is:MDL_TRANSACTION 
2017-08-03T21:13:07.114021Z 3 [Note] (------->MDL PRINT) Mdl status is:EMPTY

除此之外lock table 還需要GLOBAL和SCHEMA上的IX鎖，換句話說flush tables with read lock; 會(huì)堵塞
lock table testsort12 write;但是lock table testsort12 read 卻不會(huì)堵塞。

11、MDL_EXCLUSIVE(X)

用于各種DDL操作，注釋為CREATE/DROP/RENAME TABLE操作，實(shí)際上基本全部的DDL都會(huì)涉及到這個(gè)鎖，如上面分析的
add column操作，但是持續(xù)時(shí)間一般比較短暫。

兼容性：

   Request | Granted requests for lock         |
    type  | S SH SR SW SWLP SU SRO SNW SNRW X |
   ----------+---------------------------------------------+
   X     | -  -  -  -  -  -  -  -  -  - |

沒有上面意外堵塞一切，也被一切所堵塞

比如剛才的add column操作

2017-08-03T19:46:54.804240Z 3 [Note] (>MDL PRINT) Thread id is 3: 
2017-08-03T19:46:54.804254Z 3 [Note] (->MDL PRINT) DB_name is:test 
2017-08-03T19:46:54.804267Z 3 [Note] (-->MDL PRINT) OBJ_name is:testsort12 
2017-08-03T19:46:54.804280Z 3 [Note] (--->MDL PRINT) Namespace is:TABLE 
2017-08-03T19:46:54.804293Z 3 [Note] (----->MDL PRINT) Mdl type is:MDL_EXCLUSIVE(X) 
2017-08-03T19:46:54.804306Z 3 [Note] (------>MDL PRINT) Mdl duration is:MDL_TRANSACTION 
2017-08-03T19:46:54.804319Z 3 [Note] (------->MDL PRINT) Mdl status is:EMPTY

六、源碼注釋部分

enum enum_mdl_type {
 /*
  An intention exclusive metadata lock. Used only for scoped locks.
  Owner of this type of lock can acquire upgradable exclusive locks on
  individual objects.
  Compatible with other IX locks, but is incompatible with scoped S and
  X locks.
 */
 MDL_INTENTION_EXCLUSIVE= 0,
 /*
  A shared metadata lock.
  To be used in cases when we are interested in object metadata only
  and there is no intention to access object data (e.g. for stored
  routines or during preparing prepared statements).
  We also mis-use this type of lock for open HANDLERs, since lock
  acquired by this statement has to be compatible with lock acquired
  by LOCK TABLES ... WRITE statement, i.e. SNRW (We can't get by by
  acquiring S lock at HANDLER ... OPEN time and upgrading it to SR
  lock for HANDLER ... READ as it doesn't solve problem with need
  to abort DML statements which wait on table level lock while having
  open HANDLER in the same connection).
  To avoid deadlock which may occur when SNRW lock is being upgraded to
  X lock for table on which there is an active S lock which is owned by
  thread which waits in its turn for table-level lock owned by thread
  performing upgrade we have to use thr_abort_locks_for_thread()
  facility in such situation.
  This problem does not arise for locks on stored routines as we don't
  use SNRW locks for them. It also does not arise when S locks are used
  during PREPARE calls as table-level locks are not acquired in this
  case.
 */
 MDL_SHARED,
 /*
  A high priority shared metadata lock.
  Used for cases when there is no intention to access object data (i.e.
  data in the table).
  "High priority" means that, unlike other shared locks, it is granted
  ignoring pending requests for exclusive locks. Intended for use in
  cases when we only need to access metadata and not data, e.g. when
  filling an INFORMATION_SCHEMA table.
  Since SH lock is compatible with SNRW lock, the connection that
  holds SH lock lock should not try to acquire any kind of table-level
  or row-level lock, as this can lead to a deadlock. Moreover, after
  acquiring SH lock, the connection should not wait for any other
  resource, as it might cause starvation for X locks and a potential
  deadlock during upgrade of SNW or SNRW to X lock (e.g. if the
  upgrading connection holds the resource that is being waited for).
 */
 MDL_SHARED_HIGH_PRIO,
 /*
  A shared metadata lock for cases when there is an intention to read data
  from table.
  A connection holding this kind of lock can read table metadata and read
  table data (after acquiring appropriate table and row-level locks).
  This means that one can only acquire TL_READ, TL_READ_NO_INSERT, and
  similar table-level locks on table if one holds SR MDL lock on it.
  To be used for tables in SELECTs, subqueries, and LOCK TABLE ... READ
  statements.
 */
 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_WRITE,
 /*
  A version of MDL_SHARED_WRITE lock which has lower priority than
  MDL_SHARED_READ_ONLY locks. Used by DML statements modifying
  tables and using the LOW_PRIORITY clause.
 */
 MDL_SHARED_WRITE_LOW_PRIO,
 /*
  An upgradable shared metadata lock which allows concurrent updates and
  reads of table data.
  A connection holding this kind of lock can read table metadata and read
  table data. It should not modify data as this lock is compatible with
  SRO locks.
  Can be upgraded to SNW, SNRW and X locks. Once SU lock is upgraded to X
  or SNRW lock data modification can happen freely.
  To be used for the first phase of ALTER TABLE.
 */
 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.
 */
 MDL_SHARED_READ_ONLY,
 /*
  An upgradable shared metadata lock which blocks all attempts to update
  table data, allowing reads.
  A connection holding this kind of lock can read table metadata and read
  table data.
  Can be upgraded to X metadata lock.
  Note, that since this type of lock is not compatible with SNRW or SW
  lock types, acquiring appropriate engine-level locks for reading
  (TL_READ* for MyISAM, shared row locks in InnoDB) should be
  contention-free.
  To be used for the first phase of ALTER TABLE, when copying data between
  tables, to allow concurrent SELECTs from the table, but not UPDATEs.
 */
 MDL_SHARED_NO_WRITE,
 /*
  An upgradable shared metadata lock which allows other connections
  to access table metadata, but not data.
  It blocks all attempts to read or update table data, while allowing
  INFORMATION_SCHEMA and SHOW queries.
  A connection holding this kind of lock can read table metadata modify and
  read table data.
  Can be upgraded to X metadata lock.
  To be used for LOCK TABLES WRITE statement.
  Not compatible with any other lock type except S and SH.
 */
 MDL_SHARED_NO_READ_WRITE,
 /*
  An exclusive metadata lock.
  A connection holding this lock can modify both table's metadata and data.
  No other type of metadata lock can be granted while this lock is held.
  To be used for CREATE/DROP/RENAME TABLE statements and for execution of
  certain phases of other DDL statements.
 */
 MDL_EXCLUSIVE,
 /* This should be the last !!! */
 MDL_TYPE_END};
/** Duration of metadata lock. */
enum enum_mdl_duration {
 /**
  Locks with statement duration are automatically released at the end
  of statement or transaction.
 */
 MDL_STATEMENT= 0,
 /**
  Locks with transaction duration are automatically released at the end
  of transaction.
 */
 MDL_TRANSACTION,
 /**
  Locks with explicit duration survive the end of statement and transaction.
  They have to be released explicitly by calling MDL_context::release_lock().
 */
 MDL_EXPLICIT,
 /* This should be the last ! */
 MDL_DURATION_END };
/**
  Object namespaces.
  Sic: when adding a new member to this enum make sure to
  update m_namespace_to_wait_state_name array in mdl.
  Different types of objects exist in different namespaces
   - 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
  Note that although there isn't metadata locking on triggers,
  it's necessary to have a separate namespace for them since
  MDL_key is also used outside of the MDL subsystem.
  Also note that requests waiting for user-level locks get special
  treatment - waiting is aborted if connection to client is lost.
 */
 enum enum_mdl_namespace { GLOBAL=0,
              TABLESPACE,
              SCHEMA,
              TABLE,
              FUNCTION,
              PROCEDURE,
              TRIGGER,
              EVENT,
              COMMIT,
              USER_LEVEL_LOCK,
              LOCKING_SERVICE,
              BACKUP,
              BINLOG,
              /* This should be the last ! */
              NAMESPACE_END };

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