MySQL如何查看元数据锁阻塞在哪里

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MySQL如何查看元数据锁阻塞在哪里

操作步骤：

1、session 1 执行：

   start transaction;
   select *from t1;

2、session 2 在第1步执行完后执行：

  drop table t1;

此时session 2的drop语句被阻塞。那么怎么分析查看元数据锁呢？

方法：

1）执行show processlist;，可以看到drop语句在等待元数据锁

mysql> show processlist; 
+----+-------------+-----------+------+---------+---------+-----------------------------------------------------------------------------+------------------+ 
| Id | User    | Host   | db  | Command | Time  | State                                    | Info       | 
+----+-------------+-----------+------+---------+---------+-----------------------------------------------------------------------------+------------------+ 
| 5 | system user |      | NULL | Connect | 1050234 | Waiting for master to send event                      | NULL       | 
| 6 | system user |      | NULL | Connect | 983193 | Slave has read all relay log; waiting for the slave I/O thread to update it | NULL       | 
| 8 | root    | localhost | yzs | Sleep  |   93 |                                       | NULL       | 
| 9 | root    | localhost | yzs | Query  |    3 | Waiting for table metadata lock                       | drop table t1  | 
| 10 | root    | localhost | NULL | Query  |    0 | init                                    | show processlist | 
+----+-------------+-----------+------+---------+---------+-----------------------------------------------------------------------------+------------------+ 
5 rows in set (0.00 sec)

2）可以看到当前正在运行的事务的线程是trx_mysql_thread_id:8，那么这个线程在干什么呢？

mysql> select *from information_schema.innodb_trx\G 
*************************** 1. row *************************** 
          trx_id: 17683 
         trx_state: RUNNING 
        trx_started: 2017-10-18 05:32:46 
   trx_requested_lock_id: NULL 
     trx_wait_started: NULL 
        trx_weight: 0 
    trx_mysql_thread_id: 8 
         trx_query: NULL 
    trx_operation_state: NULL 
     trx_tables_in_use: 0 
     trx_tables_locked: 0 
     trx_lock_structs: 0 
   trx_lock_memory_bytes: 320 
      trx_rows_locked: 0 
     trx_rows_modified: 0 
  trx_concurrency_tickets: 0 
    trx_isolation_level: REPEATABLE READ 
     trx_unique_checks: 1 
  trx_foreign_key_checks: 1 
trx_last_foreign_key_error: NULL 
 trx_adaptive_hash_latched: 0 
 trx_adaptive_hash_timeout: 10000 
     trx_is_read_only: 0 
trx_autocommit_non_locking: 0 
1 row in set (0.03 sec)

3）可以看到这个线程执行的是select语句，如果执行show engine innodb status;可以看到该事务处于sleep状态，也就是说这个事务语句执行完了，但是没有提交。

执行kill 8，将该事务的线程杀掉就可以了。或者检查业务的SQL语句，检查下是否有未提交的SQL语句。

mysql> select *from performance_schema.events_statements_current\G 
*************************** 1. row *************************** 
       THREAD_ID: 27 
        EVENT_ID: 15 
      END_EVENT_ID: 15 
       EVENT_NAME: statement/sql/select 
         SOURCE: mysqld.cc:962 
      TIMER_START: 1050544992900922000 
       TIMER_END: 1050544993740836000 
       TIMER_WAIT: 839914000 
       LOCK_TIME: 196000000 
        SQL_TEXT: select *from t1 
         DIGEST: 1aa32397c8ec37230aed78ef16126571 
      DIGEST_TEXT: SELECT * FROM `t1`  
     CURRENT_SCHEMA: yzs 
      OBJECT_TYPE: NULL 
     OBJECT_SCHEMA: NULL 
      OBJECT_NAME: NULL 
 OBJECT_INSTANCE_BEGIN: NULL 
      MYSQL_ERRNO: 0 
   RETURNED_SQLSTATE: NULL 
      MESSAGE_TEXT: NULL 
         ERRORS: 0 
        WARNINGS: 0 
     ROWS_AFFECTED: 0 
       ROWS_SENT: 10 
     ROWS_EXAMINED: 10 
CREATED_TMP_DISK_TABLES: 0 
   CREATED_TMP_TABLES: 0 
    SELECT_FULL_JOIN: 0 
 SELECT_FULL_RANGE_JOIN: 0 
      SELECT_RANGE: 0 
   SELECT_RANGE_CHECK: 0 
      SELECT_SCAN: 1 
   SORT_MERGE_PASSES: 0 
       SORT_RANGE: 0 
       SORT_ROWS: 0 
       SORT_SCAN: 0 
     NO_INDEX_USED: 1 
   NO_GOOD_INDEX_USED: 0 
    NESTING_EVENT_ID: NULL 
   NESTING_EVENT_TYPE: NULL

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