解決MySQL 5.7中定位DDL被阻塞的問題
阿新 • • 發佈：2020-01-09
在上篇文章《MySQL表結構變更，不可不知的Metadata Lock》中，我們介紹了MDL引入的背景，及基本概念，從“道”的層面知道了什麼是MDL。下面就從“術”的層面看看如何定位MDL的相關問題。
在MySQL 5.7中，針對MDL，引入了一張新表performance_schema.metadata_locks，該表可對外展示MDL的相關資訊，包括其作用物件，型別及持有等待情況。
開啟MDL的instrument
但是相關instrument並沒有開啟（MySQL 8.0是預設開啟的），其可通過如下兩種方式開啟，
臨時生效
修改performance_schema.setup_instrume nts表
UPDATE performance_schema.setup_instruments SET ENABLED = 'YES',TIMED = 'YES' WHERE NAME = 'wait/lock/metadata/sql/mdl';
永久生效
在配置檔案中設定
[mysqld] performance-schema-instrument='wait/lock/metadata/sql/mdl=ON'
測試場景
下面結合一個簡單的Demo，來看看在MySQL 5.7中如何定位DDL操作的阻塞問題。
session1> begin; Query OK,0 rows affected (0.00 sec) session1> delete from slowtech.t1 where id=2; Query OK,1 row affected (0.00 sec) session1> select * from slowtech.t1; +------+------+ | id | name | +------+------+ | 1 | a | +------+------+ 1 row in set (0.00 sec) session1> update slowtech.t1 set name='c' where id=1; Query OK,1 row affected (0.00 sec) Rows matched: 1 Changed: 1 Warnings: 0 session2> alter table slowtech.t1 add c1 int; ##被阻塞 session3> show processlist; +----+------+-----------+------+---------+------+---------------------------------+------------------------------------+ | Id | User | Host | db | Command | Time | State | Info | +----+------+-----------+------+---------+------+---------------------------------+------------------------------------+ | 2 | root | localhost | NULL | Sleep | 51 | | NULL | | 3 | root | localhost | NULL | Query | 0 | starting | show processlist | | 4 | root | localhost | NULL | Query | 9 | Waiting for table metadata lock | alter table slowtech.t1 add c1 int | +----+------+-----------+------+---------+------+---------------------------------+------------------------------------+ 3 rows in set (0.00 sec) session3> select object_type,object_schema,object_name,lock_type,lock_duration,lock_status,owner_thread_id from performance_schema.metadata_locks; +-------------+--------------------+----------------+---------------------+---------------+-------------+-----------------+ | object_type | object_schema | object_name | lock_type | lock_duration | lock_status | owner_thread_id | +-------------+--------------------+----------------+---------------------+---------------+-------------+-----------------+ | TABLE | slowtech | t1 | SHARED_WRITE | TRANSACTION | GRANTED | 27 | | GLOBAL | NULL | NULL | INTENTION_EXCLUSIVE | STATEMENT | GRANTED | 29 | | SCHEMA | slowtech | NULL | INTENTION_EXCLUSIVE | TRANSACTION | GRANTED | 29 | | TABLE | slowtech | t1 | SHARED_UPGRADABLE | TRANSACTION | GRANTED | 29 | | TABLE | slowtech | t1 | EXCLUSIVE | TRANSACTION | PENDING | 29 | | TABLE | performance_schema | metadata_locks | SHARED_READ | TRANSACTION | GRANTED | 28 | +-------------+--------------------+----------------+---------------------+---------------+-------------+-----------------+ 6 rows in set (0.00 sec)
這裡，重點關注lock_status，"PENDING"代表執行緒在等待MDL，而"GRANTED"則代表執行緒持有MDL。
如何找出引起阻塞的會話
結合owner_thread_id，可以可到，是29號執行緒在等待27號執行緒的MDL，此時，可kill掉52號執行緒。
但需要注意的是，owner_thread_id給出的只是執行緒ID，並不是show processlist中的ID。如果要查詢執行緒對應的processlist id，需查詢performance_schema.threads表。
session3> select * from performance_schema.threads where thread_id in (27,29)\G
*************************** 1. row ***************************
THREAD_ID: 27
NAME: thread/sql/one_connection
TYPE: FOREGROUND
PROCESSLIST_ID: 2
PROCESSLIST_USER: root
PROCESSLIST_HOST: localhost
PROCESSLIST_DB: NULL
PROCESSLIST_COMMAND: Sleep
PROCESSLIST_TIME: 214
PROCESSLIST_STATE: NULL
PROCESSLIST_INFO: NULL
PARENT_THREAD_ID: 1
ROLE: NULL
INSTRUMENTED: YES
HISTORY: YES
CONNECTION_TYPE: Socket
THREAD_OS_ID: 9800
*************************** 2. row ***************************
THREAD_ID: 29
NAME: thread/sql/one_connection
TYPE: FOREGROUND
PROCESSLIST_ID: 4
PROCESSLIST_USER: root
PROCESSLIST_HOST: localhost
PROCESSLIST_DB: NULL
PROCESSLIST_COMMAND: Query
PROCESSLIST_TIME: 172
PROCESSLIST_STATE: Waiting for table metadata lock
PROCESSLIST_INFO: alter table slowtech.t1 add c1 int
PARENT_THREAD_ID: 1
ROLE: NULL
INSTRUMENTED: YES
HISTORY: YES
CONNECTION_TYPE: Socket
THREAD_OS_ID: 9907
2 rows in set (0.00 sec)將這兩張表結合，借鑑sys.innodb_lock _waits的輸出，實際上我們也可以直觀地呈現MDL的等待關係。
SELECT
a.OBJECT_SCHEMA AS locked_schema,a.OBJECT_NAME AS locked_table,"Metadata Lock" AS locked_type,c.PROCESSLIST_ID AS waiting_processlist_id,c.PROCESSLIST_TIME AS waiting_age,c.PROCESSLIST_INFO AS waiting_query,c.PROCESSLIST_STATE AS waiting_state,d.PROCESSLIST_ID AS blocking_processlist_id,d.PROCESSLIST_TIME AS blocking_age,d.PROCESSLIST_INFO AS blocking_query,concat('KILL ',d.PROCESSLIST_ID) AS sql_kill_blocking_connection
FROM
performance_schema.metadata_locks a
JOIN performance_schema.metadata_locks b ON a.OBJECT_SCHEMA = b.OBJECT_SCHEMA
AND a.OBJECT_NAME = b.OBJECT_NAME
AND a.lock_status = 'PENDING'
AND b.lock_status = 'GRANTED'
AND a.OWNER_THREAD_ID <> b.OWNER_THREAD_ID
AND a.lock_type = 'EXCLUSIVE'
JOIN performance_schema.threads c ON a.OWNER_THREAD_ID = c.THREAD_ID
JOIN performance_schema.threads d ON b.OWNER_THREAD_ID = d.THREAD_ID\G
*************************** 1. row ***************************
locked_schema: slowtech
locked_table: t1
locked_type: Metadata Lock
waiting_processlist_id: 4
waiting_age: 259
waiting_query: alter table slowtech.t1 add c1 int
waiting_state: Waiting for table metadata lock
blocking_processlist_id: 2
blocking_age: 301
blocking_query: NULL
sql_kill_blocking_connection: KILL 2
1 row in set (0.00 sec)
輸出一目瞭然，DDL操作如果要獲得MDL，執行kill 2即可。
官方的sys.schematablelock_waits
實際上，MySQL 5.7在sys庫中也集成了類似功能，同樣的場景，其輸出如下，
mysql> select * from sys.schema_table_lock_waits\G
*************************** 1. row ***************************
object_schema: slowtech
object_name: t1
waiting_thread_id: 29
waiting_pid: 4
waiting_account: root@localhost
waiting_lock_type: EXCLUSIVE
waiting_lock_duration: TRANSACTION
waiting_query: alter table slowtech.t1 add c1 int
waiting_query_secs: 446
waiting_query_rows_affected: 0
waiting_query_rows_examined: 0
blocking_thread_id: 27
blocking_pid: 2
blocking_account: root@localhost
blocking_lock_type: SHARED_READ
blocking_lock_duration: TRANSACTION
sql_kill_blocking_query: KILL QUERY 2
sql_kill_blocking_connection: KILL 2
*************************** 2. row ***************************
object_schema: slowtech
object_name: t1
waiting_thread_id: 29
waiting_pid: 4
waiting_account: root@localhost
waiting_lock_type: EXCLUSIVE
waiting_lock_duration: TRANSACTION
waiting_query: alter table slowtech.t1 add c1 int
waiting_query_secs: 446
waiting_query_rows_affected: 0
waiting_query_rows_examined: 0
blocking_thread_id: 29
blocking_pid: 4
blocking_account: root@localhost
blocking_lock_type: SHARED_UPGRADABLE
blocking_lock_duration: TRANSACTION
sql_kill_blocking_query: KILL QUERY 4
sql_kill_blocking_connection: KILL 4
2 rows in set (0.00 sec)
具體分析下官方的輸出，
只有一個alter table操作，卻產生了兩條記錄，而且兩條記錄的kill物件竟然還不一樣，對錶結構不熟悉及不仔細看記錄內容的話，難免會kill錯物件。
不僅如此，如果有N個查詢被DDL操作堵塞，則會產生N*2條記錄。在阻塞操作較多的情況下，這N*2條記錄完全是個噪音。
而之前的SQL，無論有多少操作被阻塞，一個alter table操作，就只會輸出一條記錄。
如何檢視阻塞會話已經執行過的操作
但上面這個SQL也有遺憾，其blocking_query為NULL，而在會話1中，其明明已經執行了三個SQL。
這個與performance_schema.threads（類似於show processlist）有關，其只會輸出當前正在執行的SQL，對於已經執行過的，實際上是沒辦法看到。
但在線上，kill是一個需要謹慎的操作，畢竟你很難知道kill的是不是業務關鍵操作？又或者，是個批量update操作？那麼，有沒有辦法抓到該事務之前的操作呢？
答案，有。
即Performance Schema中記錄Statement Event（操作事件）的表，具體包括
events_statements_current,events_statements_history,events_statements_history_long,prepared_statements_instances。
常用的是前面三個。
三者的表結構完全一致，其中，events_statements_history又包含了events_statements_current的操作，所以我們這裡會使用events_statements_history。
終極SQL如下，
SELECT
locked_schema,locked_table,locked_type,waiting_processlist_id,waiting_age,waiting_query,waiting_state,blocking_processlist_id,blocking_age,substring_index(sql_text,"transaction_begin;",-1) AS blocking_query,sql_kill_blocking_connection
FROM
(
SELECT
b.OWNER_THREAD_ID AS granted_thread_id,a.OBJECT_SCHEMA AS locked_schema,d.PROCESSLIST_ID) AS sql_kill_blocking_connection
FROM
performance_schema.metadata_locks a
JOIN performance_schema.metadata_locks b ON a.OBJECT_SCHEMA = b.OBJECT_SCHEMA
AND a.OBJECT_NAME = b.OBJECT_NAME
AND a.lock_status = 'PENDING'
AND b.lock_status = 'GRANTED'
AND a.OWNER_THREAD_ID <> b.OWNER_THREAD_ID
AND a.lock_type = 'EXCLUSIVE'
JOIN performance_schema.threads c ON a.OWNER_THREAD_ID = c.THREAD_ID
JOIN performance_schema.threads d ON b.OWNER_THREAD_ID = d.THREAD_ID
) t1,(
SELECT
thread_id,group_concat( CASE WHEN EVENT_NAME = 'statement/sql/begin' THEN "transaction_begin" ELSE sql_text END ORDER BY event_id SEPARATOR ";" ) AS sql_text
FROM
performance_schema.events_statements_history
GROUP BY thread_id
) t2
WHERE
t1.granted_thread_id = t2.thread_id \G
*************************** 1. row ***************************
locked_schema: slowtech
locked_table: t1
locked_type: Metadata Lock
waiting_processlist_id: 4
waiting_age: 294
waiting_query: alter table slowtech.t1 add c1 int
waiting_state: Waiting for table metadata lock
blocking_processlist_id: 2
blocking_age: 336
blocking_query: delete from slowtech.t1 where id=2;select * from slowtech.t1;update slowtech.t1 set name='c' where id=1
sql_kill_blocking_connection: KILL 2
1 row in set,1 warning (0.00 sec)
從上面的輸出可以看到，blocking_query中包含了會話1中當前事務的所有操作，按執行的先後順序輸出。
需要注意的是，預設情況下，events_statements_history只會保留每個執行緒最近的10個操作，如果事務中進行的操作較多，實際上也是沒辦法抓全的。
總結
以上所述是小編給大家介紹的解決MySQL 5.7中如何定位DDL被阻塞的問題，希望對大家有所幫助，如果大家有任何疑問請給我留言，小編會及時回覆大家的。在此也非常感謝大家對我們網站的支援！