写在前面：在个别时候可能需要查看当前最新的事务ID，以便做一些业务逻辑上的判断（例如利用事务ID变化以及前后时差，统计每次事务的响应时长等用途）。
通常地，我们有两种方法可以查看当前的事务ID：
1、执行SHOW ENGINE INNODB STATUS，查看事务相关信息
===================================== 150303 17:16:11 INNODB MONITOR OUTPUT ===================================== Per second averages calculated from the last 15 seconds ... ------------ TRANSACTIONS Trx id counter 3359877657 -- 当前最大事务ID Purge done for trx's n:o < 3359877468 undo n:o < 0 state: running History list length 324 LIST OF TRANSACTIONS FOR EACH SESSION: ---TRANSACTION 0, not started -- 该会话中执行SHOW ENGINE INNODB STATUS，不会产生事务，所以事务ID为0 MySQL thread id 4692367, OS thread handle 0x51103940, query id 677284426 xx.173ops.com 10.x.x.x yejr init SHOW /*!50000 ENGINE*/ INNODB STATUS ---TRANSACTION 3359877640, not started --非活跃事务，还未开始 mysql tables in use 1, locked 0 MySQL thread id 4678384, OS thread handle 0x41a57940, query id 677284427 xx.173ops.com 10.x.x.x yejr System lock select polinfo0_.Fid as Fid39_0_, ... ---TRANSACTION 3359877652, not started MySQL thread id 4678383, OS thread handle 0x50866940, query id 677284420 xx.173ops.com 10.x.x.x yejr cleaning up ---TRANSACTION 3359877635, ACTIVE 1358 sec, thread declared inside InnoDB 5000 --活跃长事务，运行了1358秒还未结束，要引起注意，可能会导致大量锁等待发生 mysql tables in use 1, locked 1 1 lock struct(s), heap size 376, 0 row lock(s), undo log entries 1 MySQL thread id 3120717, OS thread handle 0x529b4940, query id 677284351 xx.173ops.com 10.x.x.x yejr query end insert into t_live_room ...
2、查看INFORMATION_SCHEMA.INNODB_TRX、INNODB_LOCKS、INNODB_LOCK_WAITS 三个表，通过这些信息能快速发现哪些事务在阻塞其他事务
#先查询 INNODB_TRX 表，看看都有哪些事务
mysql> SELECT * FROM INFORMATION_SCHEMA.INNODB_TRX\G
*************************** 1. row ***************************
trx_id: 17778 -- 当前事务ID
trx_state: LOCK WAIT -- 处于锁等待状态，也就是等待其他会话释放锁资源
trx_started: 2015-03-04 10:40:26
trx_requested_lock_id: 17778:82:3:6 -- 欲请求的锁
trx_wait_started: 2015-03-04 10:40:26
trx_weight: 2 -- 大意是该锁影响了2行记录
trx_mysql_thread_id: 657 -- processlist中的线程ID
trx_query: update trx_fee set fee=rand()*1000 where id= 4
trx_operation_state: starting index read
trx_tables_in_use: 1
trx_tables_locked: 1
trx_lock_structs: 2
trx_lock_memory_bytes: 360
trx_rows_locked: 1
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
*************************** 2. row ***************************
trx_id: 17773
trx_state: RUNNING
trx_started: 2015-03-04 10:40:23
trx_requested_lock_id: NULL
trx_wait_started: NULL
trx_weight: 10
trx_mysql_thread_id: 656
trx_query: NULL
trx_operation_state: NULL
trx_tables_in_use: 0
trx_tables_locked: 0
trx_lock_structs: 2
trx_lock_memory_bytes: 360
trx_rows_locked: 9
trx_rows_modified: 8
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
#再看 INNODB_LOCKS 表，看看都有什么锁
mysql> select * from information_schema.INNODB_LOCKS\G *************************** 1. row *************************** lock_id: 17778:82:3:6 --当前锁ID lock_trx_id: 17778 --该锁对应的事务ID lock_mode: X -- 锁类型，排它锁X lock_type: RECORD --锁范围，记录锁：record lock，其他锁范围：间隙锁：gap lock，或者next-key lock（记录锁+间隙锁） lock_table: `test`.`trx_fee` lock_index: PRIMARY --加载在哪个索引上的锁 lock_space: 82 lock_page: 3 lock_rec: 6 lock_data: 4 *************************** 2. row *************************** lock_id: 17773:82:3:6 lock_trx_id: 17773 lock_mode: X lock_type: RECORD lock_table: `test`.`trx_fee` lock_index: PRIMARY lock_space: 82 lock_page: 3 lock_rec: 6 lock_data: 4
#最后看 INNODB_LOCK_WAITS 表，看看当前都有哪些锁等待
mysql> select * from information_schema.INNODB_LOCK_WAITS\G
*************************** 1. row ***************************
requesting_trx_id: 17778 --请求锁的事务ID（等待方）
requested_lock_id: 17778:82:3:6 -- 请求锁ID
blocking_trx_id: 17773 -- 阻塞该锁的事务ID（当前持有方，待释放）
blocking_lock_id: 17773:82:3:6 -- 持有的锁ID
关于INFORMATION_SCHEMA中和InnoDB有关的表用途描述，可以查看手册：21.29 INFORMATION_SCHEMA Tables for InnoDB
3、利用percona分支的特性，查看当前最新事务ID，该特性从5.6.11-60.3版本开始引入，执行下面的2个命令即可查看
mysqladmin ext | grep Innodb_max_trx_id 或者 mysql> show global status like 'Innodb_max_trx_id';
最后，交代下问题的来源其实是这样的，有位朋友和我讨论问题，说在java连接池中，发现2个事务的事务ID是一样的，测试的SQL代码：
begin;update trx set un=rand() where id=round(rand()*10)+1;select * from information_schema.INNODB_TRX; commit;select sleep(0.01);begin;update trx set un=rand() where id=round(rand()*10)+1;select * from information_schema.INNODB_TRX;commit;
这串代码不能折行，中间的 sleep 停留 不能太大，也就是模拟足够快的情况下，检查2次事务的ID是否有变化。可以发现，时间足够短的话，2次查询到的事务ID是一样的，并没有发生变化。大家也可以在自己的环境下试试。
