为什么MySQL没有负载,但交易却跑不动?

2023年 10月 30日 66.8k 0

5.7版本中查询锁竞争

  在MySQL 8.0 之前,必须SET GLOBAL innodb_status_output_locks=ON后才能在SHOW ENGINE INNODB STATUS中查到数据锁,例如下面这个事务:

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


    mysql> select * from t1 where intcol1=0 for update;
    ...
    900 rows in set (0.00 sec)

    在SHOW ENGINE INNODB STATUS中查到

      ---TRANSACTION 7827, ACTIVE 11 sec
      222 lock struct(s), heap size 24784, 5720 row lock(s)
      MySQL thread id 9912, OS thread handle 139967683151616, query id 11123 localhost root
      TABLE LOCK table `mysqlslap`.`t1` trx id 7827 lock mode IX
      RECORD LOCKS space id 25 page no 37 n bits 1056 index in_1 of table `mysqlslap`.`t1` trx id 7827 lock_mode X


      ...

      修改了900行,却锁住了5720行。查询space id为25对应的对象:

        mysql> select * from information_schema.INNODB_SYS_DATAFILES where space=25;
        +-------+--------------------+
        | SPACE | PATH |
        +-------+--------------------+
        | 25 | ./mysqlslap/t1.ibd |
        +-------+--------------------+
        1 row in set (0.00 sec)

        在另外一个session里执行

          mysql> update t1 set intcol1=1 where intcol1=0;
          ERROR 1205 (HY000): Lock wait timeout exceeded; try restarting transaction

          查询锁的情况



            mysql> select * from information_schema.innodb_lock_waits;
            +-------------------+-------------------+-----------------+------------------+
            | requesting_trx_id | requested_lock_id | blocking_trx_id | blocking_lock_id |
            +-------------------+-------------------+-----------------+------------------+
            | 7829 | 7829:25:4:2 | 7827 | 7827:25:4:2 |
            +-------------------+-------------------+-----------------+------------------+
            1 row in set, 1 warning (0.00 sec)


            mysql> select * from information_schema.innodb_locks;
            +-------------+-------------+-----------+-----------+------------------+-----------------+------------+-----------+----------+----------------+
            | lock_id | lock_trx_id | lock_mode | lock_type | lock_table | lock_index | lock_space | lock_page | lock_rec | lock_data |
            +-------------+-------------+-----------+-----------+------------------+-----------------+------------+-----------+----------+----------------+
            | 7829:25:4:2 | 7829 | X | RECORD | `mysqlslap`.`t1` | GEN_CLUST_INDEX | 25 | 4 | 2 | 0x000000000200 |
            | 7827:25:4:2 | 7827 | X | RECORD | `mysqlslap`.`t1` | GEN_CLUST_INDEX | 25 | 4 | 2 | 0x000000000200 |
            +-------------+-------------+-----------+-----------+------------------+-----------------+------------+-----------+----------+----------------+
            2 rows in set, 1 warning (0.00 sec)

            查询阻塞的线程:

              SELECT b.trx_mysql_thread_id AS 'blocked_thread_id'
              ,b.trx_query AS 'blocked_sql_text'
              ,c.trx_mysql_thread_id AS 'blocker_thread_id'
              ,c.trx_query AS 'blocker_sql_text'
              ,( Unix_timestamp() - Unix_timestamp(c.trx_started) )
              AS 'blocked_time'
              FROM information_schema.innodb_lock_waits a
              INNER JOIN information_schema.innodb_trx b
              ON a.requesting_trx_id = b.trx_id
              INNER JOIN information_schema.innodb_trx c
              ON a.blocking_trx_id = c.trx_id
              WHERE ( Unix_timestamp() - Unix_timestamp(c.trx_started) ) > 4;
              +-------------------+-----------------------------------------+-------------------+------------------+--------------+
              | blocked_thread_id | blocked_sql_text | blocker_thread_id | blocker_sql_text | blocked_time |
              +-------------------+-----------------------------------------+-------------------+------------------+--------------+
              | 9921 | update t1 set intcol1=1 where intcol1=0 | 9917 | NULL | 782 |
              +-------------------+-----------------------------------------+-------------------+------------------+--------------+
              1 row in set, 1 warning (0.00 sec)



              根据线程号查询执行的SQL

                SELECT a.sql_text,
                c.id,
                d.trx_started
                FROM performance_schema.events_statements_current a
                join performance_schema.threads b
                ON a.thread_id = b.thread_id
                join information_schema.processlist c
                ON b.processlist_id = c.id
                join information_schema.innodb_trx d
                ON c.id = d.trx_mysql_thread_id
                where c.id=9917
                ORDER BY d.trx_started\G
                *************************** 1. row ***************************
                sql_text: select * from t1 where intcol1=0 for update
                id: 9917
                trx_started: 2023-05-26 13:24:59
                1 row in set (0.00 sec)

                注意这里查询出的SQL是阻塞事务的最后一条SQL,并不一定是阻塞的源头。

                解决锁竞争

                解决方法是针对where中的条件增加索引,使MySQL服务层的过滤能在存储层完成,例如

                  mysql> create index in_1 on t1(intcol1);
                  Query OK, 0 rows affected (0.04 sec)
                  Records: 0 Duplicates: 0 Warnings: 0


                  mysql> show create table t1\G
                  *************************** 1. row ***************************
                  Table: t1
                  Create Table: CREATE TABLE `t1` (
                  `intcol1` int(32) DEFAULT NULL,
                  `intcol2` int(32) DEFAULT NULL,
                  `charcol1` varchar(128) DEFAULT NULL,
                  `charcol2` varchar(128) DEFAULT NULL,
                  `charcol3` varchar(128) DEFAULT NULL,
                  KEY `in_1` (`intcol1`)
                  ) ENGINE=InnoDB DEFAULT CHARSET=latin1
                  1 row in set (0.00 sec)



                  再锁住同样的行

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


                    mysql> select * from t1 where intcol1=0 for update;
                    ...
                    900 rows in set (0.00 sec)

                    在SHOW ENGINE INNODB STATUS中查到

                      ---TRANSACTION 7841, ACTIVE 15 sec
                      155 lock struct(s), heap size 24784, 1801 row lock(s)
                      MySQL thread id 9917, OS thread handle 139966608627456, query id 11227 localhost root
                      TABLE LOCK table `mysqlslap`.`t1` trx id 7841 lock mode IX
                      RECORD LOCKS space id 25 page no 37 n bits 1056 index in_1 of table `mysqlslap`.`t1` trx id 7841 lock_mode X




                      ...

                      被锁住的记录从之前的5720条减少到1801条。

                      有索引后执行计划也不同,加索引之前的执行计划

                        mysql> explain select * from t1 where intcol1=0 for update;
                        +----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------------+
                        | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
                        +----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------------+
                        | 1 | SIMPLE | t1 | NULL | ALL | NULL | NULL | NULL | NULL | 6143 | 10.00 | Using where |
                        +----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------------+
                        1 row in set, 1 warning (0.00 sec)

                        加了索引后的执行计划是:

                          mysql> explain select * from t1 where intcol1=0 for update;
                          +----+-------------+-------+------------+------+---------------+------+---------+-------+------+----------+-------+
                          | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
                          +----+-------------+-------+------------+------+---------------+------+---------+-------+------+----------+-------+
                          | 1 | SIMPLE | t1 | NULL | ref | in_1 | in_1 | 5 | const | 900 | 100.00 | NULL |
                          +----+-------------+-------+------------+------+---------------+------+---------+-------+------+----------+-------+
                          1 row in set, 1 warning (0.00 sec)



                          对比一下可以发现后者的Extra字段中没有“Using where”,因为过滤再存储层已经完成了。生产中可以先 select 出 主键id,再用 主键id 去 update

                          8.0版本从 MySQL 8.0 开始,performance_schema.data_locks显示 InnoDB 数据锁。具体参见拙作《MySQL 8.0运维于优化》第18章第3节“优化索引”。

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