执行UPDATE时,WEHRE条件列虽已有索引,但还会锁全表,肿么回事?
叶师傅有次上课过程中执行UPDATE测试案例时,发现虽然WHERE条件列已有索引,有时候能利用二级索引进行更新(且只锁定相应必要的几行记录),但有时候却变成了根据主键进行更新,且会锁全表。我们先来看看下面的例子。
测试表 t1
CREATE TABLE `t1` ( `c1` int(10) unsigned NOT NULL DEFAULT '0', `c2` int(10) unsigned NOT NULL DEFAULT '0', `c3` int(10) unsigned NOT NULL DEFAULT '0', `c4` int(10) unsigned NOT NULL DEFAULT '0', PRIMARY KEY (`c1`), KEY `c2` (`c2`) ) ENGINE=InnoDB DEFAULT CHARSET=utf8表中数据
+----+----+----+----+ | c1 | c2 | c3 | c4 | +----+----+----+----+ | 0 | 0 | 0 | 0 | | 1 | 1 | 1 | 0 | | 3 | 3 | 3 | 0 | | 4 | 2 | 2 | 0 | | 6 | 8 | 5 | 0 | | 7 | 6 | 6 | 10 | | 10 | 10 | 4 | 0 | +----+----+----+----+case1:根据二级索引UPDATE,不锁全表
先看执行计划
yejr@imysql.com [yejr]>desc update t1 set c4=123 where c2>=8\G *************************** 1. row *************************** id: 1 select_type: UPDATE table: t1 partitions: NULL type: range possible_keys: c2 key: c2 key_len: 4 ref: const rows: 2 filtered: 100.00 Extra: Using where启动两个session执行UPDATE测试
session1session 2mysql> begin; mysql> update t1 set c4=123 where c2>=8;Query OK, 2 rows affected (0.00 sec)Rows matched: 2 Changed: 2 Warnings: 0mysql> begin;mysql> select * from t1 where c2 = 7 for update;...1 row in set (0.00 sec)#直接可得到结果,不会被阻塞case2:根据PRIMARY KEY更新,锁全表
yejr@imysql.com [yejr]>desc update t1 set c4=123 where c2>=6\G *************************** 1. row *************************** id: 1 select_type: UPDATE table: t1 partitions: NULL type: index possible_keys: c2 key: PRIMARY key_len: 4 ref: NULL rows: 7 filtered: 100.00 Extra: Using where我们能看到本次执行计划是 根据主键索引进行更新,且会锁全表。
同样地,启动两个session执行UPDATE测试
session1session 2mysql> begin; mysql> update t1 set c4=123 where c2>=6;Query OK, 3 rows affected (0.00 sec)Rows matched: 3 Changed: 3 Warnings: 0mysql> begin;mysql> select * from t1 where c2 = 3 for update;#无法得到结果,被阻塞了mysql> rollback; #执行rollback,释放锁... 1 row in set (4.23 sec) #session1释放锁后才能得到结果查看行锁等待情况
yejr@imysql.com [yejr]>select * from sys.innodb_lock_waits\G *************************** 1. row *************************** wait_started: 2017-08-15 15:20:20 wait_age: 00:00:17 wait_age_secs: 17 locked_table: `yejr`.`t1` locked_index: PRIMARY <--主键上加锁 locked_type: RECORD waiting_trx_id: 268350 waiting_trx_started: 2017-08-15 15:20:20 waiting_trx_age: 00:00:17 waiting_trx_rows_locked: 2 waiting_trx_rows_modified: 0 waiting_pid: 13 waiting_query: select * from t1 where c2 = 3 for update waiting_lock_id: 268350:387:3:4 waiting_lock_mode: X blocking_trx_id: 268349 blocking_pid: 12 blocking_query: NULL blocking_lock_id: 268349:387:3:4 blocking_lock_mode: X blocking_trx_started: 2017-08-15 15:20:18 blocking_trx_age: 00:00:19 blocking_trx_rows_locked: 8 <-- 所有记录都被加锁了 blocking_trx_rows_modified: 3 <---持有锁的事务更新了3行记录 sql_kill_blocking_query: KILL QUERY 12 sql_kill_blocking_connection: KILL 12好了,案例说完了,也该说原因了。
肾好的同学可能记得我说过一个结论:当MySQL预估扫描行数超过全表总数约 20% ~ 30% 时,即便有二级索引,也会直接升级为全表扫描。
这个结论的原因并不难理解,二级索引的顺序和主键顺序一般来说是不一样的,根据二级索引的顺序回表读数据时,实际上对于主键则很可能是随机扫描,因此当需要随机扫描的数量超过一定比例时(一般是20% ~ 30%),则优化器会决定直接改成全表扫描。
上述说法出处:WHERE Clause Optimization(https://dev.mysql.com/doc/refman/5.7/en/where-optimization.html)
Each table index is queried, and the best index is used unless the optimizer believes that it is more efficient to use a table scan. At one time, a scan was used based on whether the best index spanned more than 30% of the table, but a fixed percentage no longer determines the choice between using an index or a scan. The optimizer now is more complex and bases its estimate on additional factors such as table size, number of rows, and I/O block size.不过,上面这个结论是针对读数据的情况,UPDATE/DELETE修改数据时是否也这样呢?
答案是肯定的,要不然上面的测试结果怎么解释......
按照官方开发者的说法,当优化器评估根据二级索引更新行数超过约50%(从上面测试结果来看,其实20% ~ 30%就已经是这样了,不过这个比例并不是固定值,会根据各种代价计算自动调整)就会改成走主键索引,并且锁全表,这么做是既定的策略,原因和上面一样,也是为了提高检索效率。
老调重弹,几点建议:
不管检索还是更新,都尽可能利用索引;
不要一次性检索或更新大批量数据,建议分城多批次;
事务尽快提交,降低行锁持有时间及其影响。
本文提到的问题,我也提了个BUG#87021,有兴趣的可以去看看。
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