Waiting for table metadata lock

問題：

開發同學反饋一張小表加字段很長時間不能完成，processlist中STATE爲Waiting for table metadata lock 請求支援。

問題分析：

想了一下有兩種情況：

1.加字段的這個表上有查詢長時間未能結束。

2.對這個表有事務操作未完成。

驗證：

mysql> select * from information_schema.processlist;
+----+------+-----------+------+---------+------+---------------------------------+----------------------------------------------+
| ID | USER | HOST      | DB   | COMMAND | TIME | STATE                           | INFO                                         |
+----+------+-----------+------+---------+------+---------------------------------+----------------------------------------------+
|  9 | root | localhost | test | Query   |  690 | Waiting for table metadata lock | alter table t add column c1 int              |
| 10 | root | localhost | NULL | Query   |    0 | executing                       | select * from information_schema.processlist |
|  8 | root | localhost | test | Sleep   |  716 |                                 | NULL                                         |
+----+------+-----------+------+---------+------+---------------------------------+----------------------------------------------+
3 rows in set (0.00 sec)

mysql> select * from information_schema.innodb_trx\G
*************************** 1. row ***************************
                    trx_id: 421534325778256
                 trx_state: RUNNING
               trx_started: 2018-11-02 19:19:51
     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: 1136
           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: 0
          trx_is_read_only: 0
trx_autocommit_non_locking: 0
1 row in set (0.00 sec)

  從information_schema.processlist 上看，並沒有對更新表的查詢操作。

  從nformation_schema.innodb_trx 上發現有一個記錄，但並沒有具體的SQL.

 

解決：

通過PERFORMANCE_SCHEMA.threads， PERFORMANCE_SCHEMA.events_statements_current 關聯查詢找到未提交事務的thread_id, 並kill 掉。加字段的操作很快完成。

mysql> SELECT concat('kill  ',t.PROCESSLIST_ID ,';') processlist_id, t.thread_id, PROCESSLIST_INFO, e.SQL_TEXT FROM PERFORMANCE_SCHEMA.threads t, PERFORMANCE_SCHEMA.events_statements_current e WHERE e.THREAD_ID = t.THREAD_ID AND t.PROCESSLIST_ID != 'null' AND t.PROCESSLIST_ID IS NOT NULL  AND PROCESSLIST_INFO IS NULL;
+----------------+-----------+------------------+--------------------+
| processlist_id | thread_id | PROCESSLIST_INFO | SQL_TEXT           |
+----------------+-----------+------------------+--------------------+
| kill  8;       |        33 | NULL             | select name from t |
+----------------+-----------+------------------+--------------------+
1 row in set, 1 warning (0.00 sec)

mysql> kill 8;
Query OK, 0 rows affected (0.00 sec)

mysql> select * from information_schema.processlist;
+----+------+-----------+------+---------+------+-----------+----------------------------------------------+
| ID | USER | HOST      | DB   | COMMAND | TIME | STATE     | INFO                                         |
+----+------+-----------+------+---------+------+-----------+----------------------------------------------+
|  9 | root | localhost | test | Sleep   |  714 |           | NULL                                         |
| 10 | root | localhost | NULL | Query   |    0 | executing | select * from information_schema.processlist |
+----+------+-----------+------+---------+------+-----------+----------------------------------------------+
2 rows in set (0.00 sec)