经常会遇到这样一个场景：
业务那边觉得数据库“很慢”，上去通过show processlist查看发现大量State为在等待lock，如：
Waiting for table metadata/level lock等


比如在执行一个DDL时，发现被hang住，查看到目前进程状态，有MDL

mysql> SHOW PROCESSLIST;
+-----+------+-----------+------+---------+------+---------------------------------+--------------------------------------------+
| Id | User | Host | db | Command | Time | State| Info|
+-----+------+-----------+------+---------+------+---------------------------------+--------------------------------------------+
| 585 | root | localhost | test | Sleep | 1658 || NULL |
| 586 | root | localhost | test | Query | 1654 | Waiting for table metadata lock | alter table t change name name varchar(32) |
| 590 | root | localhost | test | Query | 0| starting| show processlist|
+-----+------+-----------+------+---------+------+---------------------------------+--------------------------------------------+
3 rows in set (0.00 sec)

在实验环境里，我们很快就能定位到，应该是id为585这个线程，但是无法知道正在执行什么sql：
通过information_schema.innodb_trx\G，也不能查询到具体执行了什么sql。
通过简单的kill的确可以解决眼前的问题，但如果继续遇到该问题，也难以定位具体内容。


但其实，performance_schema.events_statements_current提供了相关信息，此处复现一下：

session1> BEGIN;
Query OK, 0 rows affected (0.00 sec)

session1> UPDATE t SET name='fasdfsad';
Query OK, 3 rows affected (0.00 sec)
Rows matched: 3Changed: 3Warnings: 0

session2> ALTER TABLE t CHANGE name name varchar(32)

发现被hang住


查看一下是否有事务未提交，可以发现的确有：
该事务内的语句执行完毕（处于Sleep），但未提交，就会看不到对应的trx_query：

session3> SELECT * FROM information_schema.innodb_trx\G
*************************** 1. row ***************************
trx_id: 9614
trx_state: RUNNING
trx_started: 2017-09-19 15:58:05
trx_requested_lock_id: NULL
trx_wait_started: NULL
trx_weight: 2
trx_mysql_thread_id: 585
trx_query: NULL
trx_operation_state: NULL
trx_tables_in_use: 0
trx_tables_locked: 1
trx_lock_structs: 2
trx_lock_memory_bytes: 1136
trx_rows_locked: 4
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)

只能根据trx_mysql_thread_id看到未提交的事务的process id，看一下processlist，INFO内也没有具体内容：

session3> SHOW PROCESSLIST;
+-----+------+-----------+------+---------+------+---------------------------------+--------------------------------------------+
| Id | User | Host| db | Command | Time | State| Info|
+-----+------+-----------+------+---------+------+---------------------------------+--------------------------------------------+
| 585 | root | localhost | test | Sleep | 42 || NULL |
| 586 | root | localhost | test | Query | 37 | Waiting for table metadata lock | ALTER TABLE t CHANGE name name varchar(32) |
| 590 | root | localhost | test | Query | 0 | starting| SHOW PROCESSLIST|
+-----+------+-----------+------+---------+------+---------------------------------+--------------------------------------------+
3 rows in set (0.00 sec)

但只要打开了P_S，就可以通过performance_schema.events_statements_current来查看到对应的sql，包括已经执行完，但没有提交的。

session3> SELECT sql_text FROM performance_schema.events_statements_current;
+-------------------------------------------------------------------+
| sql_text |
+-------------------------------------------------------------------+
| UPDATE t SET name='fasdfsad' |
| ALTER TABLE t CHANGE name name varchar(32) |
| select sql_text from performance_schema.events_statements_current |
+-------------------------------------------------------------------+
3 rows in set (0.00 sec)

当然，在复杂的生产环境中，光凭上面的语句查出来的信息，是远远不够的。

通过如下语句，可以扩展show processlist的显示结果，并提供对应的SQL。

SELECT b.processlist_id, c.db, a.sql_text, c.command, c.time, c.state
FROM performance_schema.events_statements_current a JOIN performance_schema.threads b USING(thread_id)
JOIN information_schema.processlist c ON b.processlist_id = c.id
WHERE a.sql_text NOT LIKE '%performance%';

结果：

+----------------+------+--------------------------------------------+---------+------+---------------------------------+
| processlist_id | db | sql_text | command | time | state |
+----------------+------+--------------------------------------------+---------+------+---------------------------------+
| 585| test | UPDATE t SET name='fasdfsad'| Sleep | 243 | |
| 586| test | ALTER TABLE t CHANGE name name varchar(32) | Query | 238 | Waiting for table metadata lock |
+----------------+------+--------------------------------------------+---------+------+---------------------------------+
2 rows in set (0.01 sec)

也可以很容易定位到执行的内容是做了一个update操作未提交。


作者微信公众号（持续更新）