UPDATE能走索引还会锁全表吗

导读

执行UPDATE时,WEHRE条件列虽已有索引,但还会锁全表,肿么回事?

问题描述

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

session1 session 2
mysql> begin;
mysql> update t1 set c4=123 where c2>=8;

Query OK, 2 rows affected (0.00 sec)
Rows matched: 2 Changed: 2 Warnings: 0

mysql> 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

session1 session 2
mysql> begin;
mysql> update t1 set c4=123 where c2>=6;

Query OK, 3 rows affected (0.00 sec)
Rows matched: 3 Changed: 3 Warnings: 0

mysql> 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

问题分析

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

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。

总结

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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叶金荣

Oracle MySQL ACE Director,腾讯云TVP成员

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