什么是长事务
运行时间比较长，长时间未提交的事务，也可以称之为大事务。这类事务往往会造成大量的阻塞和锁超时，容易造成主从延迟，要尽量避免使用长事务。
下面我将演示下如何开启事务及模拟长事务：
#假设我们有一张stu_tb表，结构及数据如下 stu_id 唯一索引 mysql> select * from stu_tb; +--------------+--------+----------+---------------------+---------------------+ | id | stu_id | stu_name | create_time | update_time | +--------------+--------+----------+---------------------+---------------------+ | 1 | 1001 | from1 | 2019-09-15 14:27:34 | 2019-09-15 14:27:34 | | 2 | 1002 | dfsfd | 2019-09-15 14:27:34 | 2019-09-15 14:27:34 | | 3 | 1003 | fdgfg | 2019-09-15 14:27:34 | 2019-09-15 14:27:34 | | 4 | 1004 | sdfsdf | 2019-09-15 14:27:34 | 2019-09-15 14:27:34 | +--------------+--------+----------+---------------------+---------------------+ 4 rows in set (0.00 sec) #显式开启事务，可用begin或start transactionmysql > start transaction; Query OK, 0 rows affected (0.00 sec) mysql> select * from stu_tb where stu_id = 1002 for update; #如果我们不及时提交上个事务，那么这个事务就变成了长事务，当其他会话要操作这条数据时，就会一直等待。
如何找到长事务
遇到事务等待问题时，我们首先要做的是找到正在执行的事务。 information_schema.INNODB_TRX 表中包含了当前innodb内部正在运行的事务信息，这个表中给出了事务的开始时间，我们可以稍加运算即可得到事务的运行时间。
SELECT t.*, to_seconds(now()) - to_seconds(t.trx_started) idle_time FROM information_schema.innodb_trx t; *************************** 1. row *************************** trx_id: 6168 trx_state: RUNNING trx_started: 2019-09-16 11:08:27 trx_requested_lock_id: NULL trx_wait_started: NULL trx_weight: 3 trx_mysql_thread_id: 11 trx_query: NULL trx_operation_state: NULL trx_tables_in_use: 0 trx_tables_locked: 1 trx_lock_structs: 3 trx_lock_memory_bytes: 1136 trx_rows_locked: 2 trx_rows_modified: 0 trx_concurrency_tickets: 0 trx_isolation_level: REPEATABLE READ trx_unique_checks: 1 trx_foreign_key_checks: 1trx_last_foreign_key_error: NULL trx_adaptive_hash_latched: 0 trx_adaptive_hash_timeout: 0 trx_is_read_only: 0trx_autocommit_non_locking: 0 idle_time: 170
在结果中idle_time是计算产生的，也是事务的持续时间。但事务的trx_query是NUL，这并不是说事务什么也没执行，一个事务可能包含多个SQL，如果SQL执行完毕就不再显示了。当前事务正在执行，innodb也不知道这个事务后续还有没有sql,啥时候会commit。 因此trx_query不能提供有意义的信息。
如果我们想看到这个事务执行过的SQL，看是否可以杀掉长事务，怎么办呢？我们可以联合其他系统表查询得到，具体查询SQL如下：
SELECT now(), ( UNIX_TIMESTAMP(now()) - UNIX_TIMESTAMP(a.trx_started) ) diff_sec, b.id, b. USER, b. HOST, b.db, d.SQL_TEXT FROM .innodb_trx a INNER JOIN PERFORMANCE_SCHEMA .`PROCESSLIST` b ON a.TRX_MYSQL_THREAD_ID = b.id AND b.command = 'Sleep' INNER JOIN PERFORMANCE_SCHEMA .threads c ON b.id = c.PROCESSLIST_ID INNER JOIN PERFORMANCE_SCHEMA .events_statements_current d ON d.THREAD_ID = c.THREAD_ID; +---------------------+----------+----+------+-----------+--------+-----------------------------------------------------+ | now() | diff_sec | id | user | host | db | SQL_TEXT | +---------------------+----------+----+------+-----------+--------+-----------------------------------------------------+ | 2019-09-16 14:06:26 | 54 | 17 | root | localhost | testdb | select * from stu_tb where stu_id = 1002 for update | +---------------------+----------+----+------+-----------+--------+-----------------------------------------------------+
上述结果中diff_sec和上面idle_time表示意思相同，都是代表此事务持续的秒数。SQL_TEXT表示该事务刚执行的SQL。但是呢，上述语句只能查到事务最后执行的SQL，我们知道，一个事务里可能包含多个SQL，那我们想查询这个未提交的事务执行过哪些SQL，是否可以满足呢，答案是结合events_statements_history系统表也可以满足需求。下面语句将会查询出该事务执行过的所有SQL：
SELECT ps.id 'PROCESS ID', ps. USER, ps. HOST, esh.EVENT_ID, trx.trx_started, esh.event_name 'EVENT NAME', esh.sql_text 'SQL', ps.time FROM PERFORMANCE_SCHEMA .events_statements_history esh JOIN PERFORMANCE_SCHEMA .threads th ON esh.thread_id = th.thread_id JOIN information_schema. PROCESSLIST ps ON ps.id = th.processlist_id LEFT JOIN information_schema.innodb_trx trx ON trx.trx_mysql_thread_id = ps.id WHERE trx.trx_id IS NOT NULL AND ps. USER != 'SYSTEM_USER' ORDER BY esh.EVENT_ID; +------------+------+-----------+----------+---------------------+------------------------------+-----------------------------------------------------+------+ | PROCESS ID | USER | HOST | EVENT_ID | trx_started | EVENT NAME | SQL | time | +------------+------+-----------+----------+---------------------+------------------------------+-----------------------------------------------------+------+ | 20 | root | localhost | 1 | 2019-09-16 14:18:44 | statement/sql/select | select @@version_comment limit 1 | 60 | | 20 | root | localhost | 2 | 2019-09-16 14:18:44 | statement/sql/begin | start transaction | 60 | | 20 | root | localhost | 3 | 2019-09-16 14:18:44 | statement/sql/select | SELECT DATABASE() | 60 | | 20 | root | localhost | 4 | 2019-09-16 14:18:44 | statement/com/Init DB | NULL | 60 | | 20 | root | localhost | 5 | 2019-09-16 14:18:44 | statement/sql/show_databases | show databases | 60 | | 20 | root | localhost | 6 | 2019-09-16 14:18:44 | statement/sql/show_tables | show tables | 60 | | 20 | root | localhost | 7 | 2019-09-16 14:18:44 | statement/com/Field List | NULL | 60 | | 20 | root | localhost | 8 | 2019-09-16 14:18:44 | statement/com/Field List | NULL | 60 | | 20 | root | localhost | 9 | 2019-09-16 14:18:44 | statement/sql/select | select * from stu_tb | 60 | | 20 | root | localhost | 10 | 2019-09-16 14:18:44 | statement/sql/select | select * from stu_tb where stu_id = 1002 for update | 60 | +------------+------+-----------+----------+---------------------+------------------------------+-----------------------------------------------------+------+
从上述结果中我们可以看到该事务从一开始到现在执行过的所有SQL，当我们把该事务相关信息都查询清楚后，我们就可以判定该事务是否可以杀掉，以免影响其他事务造成等待现象。
在这里稍微拓展下，长事务极易造成阻塞或者死锁现象，通常情况下我们可以首先查询 sys.innodb_lock_waits 视图确定有没有事务阻塞现象：
#假设一个事务执行 select * from stu_tb where stu_id = 1002 for update #另外一个事务执行 update stu_tb set stu_name = 'wang' where stu_id = 1002 mysql> select * from sys.innodb_lock_waits\G *************************** 1. row *************************** wait_started: 2019-09-16 14:34:32 wait_age: 00:00:03 wait_age_secs: 3 locked_table: `testdb`.`stu_tb` locked_index: uk_stu_id locked_type: RECORD waiting_trx_id: 6178 waiting_trx_started: 2019-09-16 14:34:32 waiting_trx_age: 00:00:03 waiting_trx_rows_locked: 1 waiting_trx_rows_modified: 0 waiting_pid: 19 waiting_query: update stu_tb set stu_name = 'wang' where stu_id = 1002 waiting_lock_id: 6178:47:4:7 waiting_lock_mode: X blocking_trx_id: 6177 blocking_pid: 20 blocking_query: NULL blocking_lock_id: 6177:47:4:7 blocking_lock_mode: X blocking_trx_started: 2019-09-16 14:18:44 blocking_trx_age: 00:15:51 blocking_trx_rows_locked: 2 blocking_trx_rows_modified: 0 sql_kill_blocking_query: KILL QUERY 20sql_kill_blocking_connection: KILL 20
上述结果显示出被阻塞的SQL以及锁的类型，更强大的是杀掉会话的语句也给出来了。但是并没有找到阻塞会话执行的SQL，如果我们想找出更详细的信息，可以使用下面语句：
SELECT tmp.*, c.SQL_Text blocking_sql_text, p. HOST blocking_host FROM ( SELECT r.trx_state wating_trx_state, r.trx_id waiting_trx_id, r.trx_mysql_thread_Id waiting_thread, r.trx_query waiting_query, b.trx_state blocking_trx_state, b.trx_id blocking_trx_id, b.trx_mysql_thread_id blocking_thread, b.trx_query blocking_query FROM information_schema.innodb_lock_waits w INNER JOIN information_schema.innodb_trx b ON b.trx_id = w.blocking_trx_id INNER JOIN information_schema.innodb_trx r ON r.trx_id = w.requesting_trx_id ) tmp, information_schema. PROCESSLIST p, PERFORMANCE_SCHEMA .events_statements_current c, PERFORMANCE_SCHEMA .threads t WHERE tmp.blocking_thread = p.id AND t.thread_id = c.THREAD_ID AND t.PROCESSLIST_ID = p.id *************************** 1. row *************************** wating_trx_state: LOCK WAIT waiting_trx_id: 6180 waiting_thread: 19 waiting_query: update stu_tb set stu_name = 'wang' where stu_id = 1006blocking_trx_state: RUNNING blocking_trx_id: 6177 blocking_thread: 20 blocking_query: NULL blocking_sql_text: select * from stu_tb where stu_id = 1006 for update blocking_host: localhost
上面结果显得更加清晰，我们可以清楚的看到阻塞端及被阻塞端事务执行的语句，有助于我们排查并确认是否可以杀掉阻塞的会话。
监控长事务
现实工作中我们需要监控下长事务，定义一个阈值，比如说30s 执行时间超过30s的事务即为长事务，要求记录并告警出来，提醒管理人员去处理。下面给出监控脚本，各位可以参考下，根据需求改动使用：
#!/bin/bash # Describe: monitor long transaction /usr/local/mysql/bin/mysql -N -uroot -pxxxxxx -e "select now(),(UNIX_TIMESTAMP(now()) - UNIX_TIMESTAMP(a.trx_started)) diff_sec,b.id,b.user,b.host,b.db,d.SQL_TEXT from information_schema.innodb_trx a inner join information_schema.PROCESSLIST b on a.TRX_MYSQL_THREAD_ID=b.id and b.command = 'Sleep' inner join performance_schema.threads c ON b.id = c.PROCESSLIST_ID inner join performance_schema.events_statements_current d ON d.THREAD_ID = c.THREAD_ID;" | while read A B C D E F G H do if [ "$C" -gt 30 ] then echo $(date +"%Y-%m-%d %H:%M:%S") echo "processid[$D] $E@$F in db[$G] hold transaction time $C SQL:$H" fi done >> /tmp/longtransaction.txt
简单说明一下，这里的-gt 30是30秒钟的意思，只要超过了30秒钟就认定是长事务，可以根据实际需要自定义。将该脚本加入定时任务中即可执行。
总结
1、查询所有正在运行的事务及运行时间
SELECT t.*, to_seconds(now()) - to_seconds(t.trx_started) idle_time FROM information_schema.innodb_trx t;
2、查询事务详细信息及执行的SQL
SELECT now(), ( UNIX_TIMESTAMP(now()) - UNIX_TIMESTAMP(a.trx_started) ) diff_sec, b.id, b. USER, b. HOST, b.db, d.SQL_TEXT FROM .innodb_trx a INNER JOIN PERFORMANCE_SCHEMA .`PROCESSLIST` b ON a.TRX_MYSQL_THREAD_ID = b.id AND b.command = 'Sleep' INNER JOIN PERFORMANCE_SCHEMA .threads c ON b.id = c.PROCESSLIST_ID INNER JOIN PERFORMANCE_SCHEMA .events_statements_current d ON d.THREAD_ID = c.THREAD_ID;
3、查询事务执行过的所有历史SQL记录
SELECT ps.id 'PROCESS ID', ps. USER, ps. HOST, esh.EVENT_ID, trx.trx_started, esh.event_name 'EVENT NAME', esh.sql_text 'SQL', ps.time FROM PERFORMANCE_SCHEMA .events_statements_history esh JOIN PERFORMANCE_SCHEMA .threads th ON esh.thread_id = th.thread_id JOIN information_schema. PROCESSLIST ps ON ps.id = th.processlist_id LEFT JOIN information_schema.innodb_trx trx ON trx.trx_mysql_thread_id = ps.id WHERE trx.trx_id IS NOT NULL AND ps. USER != 'SYSTEM_USER' ORDER BY esh.EVENT_ID;
4、简单查询事务锁
select * from sys.innodb_lock_waits
5、查询事务锁详细信息
SELECT tmp.*, c.SQL_Text blocking_sql_text, p. HOST blocking_host FROM ( SELECT r.trx_state wating_trx_state, r.trx_id waiting_trx_id, r.trx_mysql_thread_Id waiting_thread, r.trx_query waiting_query, b.trx_state blocking_trx_state, b.trx_id blocking_trx_id, b.trx_mysql_thread_id blocking_thread, b.trx_query blocking_query FROM information_schema.innodb_lock_waits w INNER JOIN information_schema.innodb_trx b ON b.trx_id = w.blocking_trx_id INNER JOIN information_schema.innodb_trx r ON r.trx_id = w.requesting_trx_id ) tmp, information_schema. PROCESSLIST p, PERFORMANCE_SCHEMA .events_statements_current c, PERFORMANCE_SCHEMA .threads t WHERE tmp.blocking_thread = p.id AND t.thread_id = c.THREAD_ID AND t.PROCESSLIST_ID = p.id
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