MySQL 5.7中如何定位DDL被阻塞的問題

時間 2019-11-05

標籤 mysql 5.7 如何定位 ddl 阻塞問題欄目 MySQL 简体版

原文原文鏈接

在上篇文章《MySQL表結構變動，不可不知的Metadata Lock》中，咱們介紹了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。mysql

在MySQL 5.7中，針對MDL，引入了一張新表performance_schema.metadata_locks，該表可對外展現MDL的相關信息，包括其做用對象，類型及持有等待狀況。sql

開啓MDL的instrumentsession

可是相關instrument並無開啓（MySQL 8.0是默認開啓的），其可經過以下兩種方式開啓，測試

臨時生效spa

修改performance_schema.setup_instrume nts表，但實例重啓後，又會恢復爲默認值。線程

UPDATE performance_schema.setup_instruments SET ENABLED = 'YES', TIMED = 'YES'
WHERE NAME = 'wait/lock/metadata/sql/mdl';

永久生效

在配置文件中設置

[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操做的阻塞問題。code

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)

這裏，重點關注lock_status，"PENDING"表明線程在等待MDL，而"GRANTED"則表明線程持有MDL。orm

如何找出引發阻塞的會話對象

結合owner_thread_id，能夠可到，是29號線程在等待27號線程的MDL，此時，可kill掉52號線程。blog

但須要注意的是，owner_thread_id給出的只是線程ID，並非show processlist中的ID。若是要查找線程對應的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，實際上咱們也能夠直觀地呈現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操做若是要得到MDL，執行kill 2便可。

官方的sys.schematablelock_waits

實際上，MySQL 5.7在sys庫中也集成了相似功能，一樣的場景，其輸出以下，

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操做，卻產生了兩條記錄，並且兩條記錄的kill對象居然還不同，對錶結構不熟悉及不仔細看記錄內容的話，不免會kill錯對象。

不只如此，若是有N個查詢被DDL操做堵塞，則會產生N*2條記錄。在阻塞操做較多的狀況下，這N*2條記錄徹底是個噪音。

而以前的SQL，不管有多少操做被阻塞，一個alter table操做，就只會輸出一條記錄。

如何查看阻塞會話已經執行過的操做

但上面這個SQL也有遺憾，其blocking_query爲NULL，而在會話1中，其明明已經執行了三個SQL。

這個與performance_schema.threads（相似於show processlist）有關，其只會輸出當前正在運行的SQL，對於已經執行過的，其實是沒辦法看到。

但在線上，kill是一個須要謹慎的操做，畢竟你很難知道kill的是否是業務關鍵操做？又或者，是個批量update操做？那麼，有沒有辦法抓到該事務以前的操做呢？

答案，有。

即Performance Schema中記錄Statement Event（操做事件）的表，具體包括events_statements_current,events_statements_history,events_statements_history_long,prepared_statements_instances。

經常使用的是前面三個。

三者的表結構徹底一致，其中，events_statements_history又包含了events_statements_current的操做，因此咱們這裏會使用events_statements_history。

終極SQL以下，

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,
            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
    ) 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)