`UPDATE ... WHERE ... ` InnoDB 中的多行锁定

Question

我正在使用 InnoDB 引擎为 MySQL 数据库 v5.7.16 实现基于 table 的自定义序列生成器。
sequence_table 如下所示：

+-------------+-----------+
|sequence_name|next_value |
+-------------+-----------+
|  first_seq  |     1     |
+-------------+-----------+
|  second_seq |     1     |
+-------------+-----------+

sequence_name 列是主键。
此序列 table 包含针对不同消费者的多个序列。

我使用以下策略进行序列更新：

Select 当前序列值：select next_val from sequence_table where sequence_name=?。
将分配大小添加到当前序列值。
如果当前值与第一步中选择的值匹配，则更新序列值：update sequence_table set next_val=? where sequence_name=? and next_val=?。
如果更新成功return增加的序列值，否则从步骤1开始重复该过程。

文档包含以下信息：

UPDATE ... WHERE ... sets an exclusive next-key lock on every record the search encounters. However, only an index record lock is required for statements that lock rows using a unique index to search for a unique row. 14.5.3 Locks Set by Different SQL Statements in InnoDB

粗体部分有点乱。
如您所见，我在 UPDATE 语句的 WHERE 子句中匹配主键。

搜索是否可能遇到多个记录并因此锁定此序列中的多行table？

换句话说，算法第3步的更新是只阻塞一行还是多行？

Answer 1

您没有提到您打算使用什么事务隔离级别。假设您正在使用 repeatable read（在 read committed 中不应该存在这样的问题）

来自here：

For locking reads (SELECT with FOR UPDATE or LOCK IN SHARE MODE), UPDATE, and DELETE statements, locking depends on whether the statement uses a unique index with a unique search condition, or a range-type search condition

和

For a unique index with a unique search condition, InnoDB locks only the index record found, not the gap before it

所以至少在理论上它应该只锁定一条记录并且不会使用 next-key 锁。

更多来自其他文档页面的引用来支持我的想法：

innodb-next-key-锁

link

A next-key lock is a combination of a record lock on the index record and a gap lock on the gap before the index record.

间隙锁

link

Gap locking is not needed for statements that lock rows using a unique index to search for a unique row

Answer 2

不要在主交易中抢序号；在 START TRANSCTION.
使用 autocommit=ON 在一条语句中完成任务。

两者都使它更快，更不容易阻塞。

（您的代码缺少 BEGIN/COMMIT 和 FOR UPDATE。我删除了这些而不是解释问题。）

设置测试：

mysql> CREATE TABLE so49197964 (
    ->     name VARCHAR(22) NOT NULL,
    ->     next_value INT UNSIGNED NOT NULL,
    ->     PRIMARY KEY (name)
    -> ) ENGINE=InnoDB;
Query OK, 0 rows affected (0.02 sec)

mysql> INSERT INTO so49197964 (name, next_value)
    ->     VALUES
    ->     ('first', 1), ('second', 1);
Query OK, 2 rows affected (0.00 sec)
Records: 2  Duplicates: 0  Warnings: 0

mysql> SELECT * FROM so49197964;
+--------+------------+
| name   | next_value |
+--------+------------+
| first  |          1 |
| second |          1 |
+--------+------------+
2 rows in set (0.00 sec)

从'first'中抓取20个数字并获取起始数字：

mysql> UPDATE so49197964
    ->     SET next_value = LAST_INSERT_ID(next_value) + 20
    ->     WHERE name = 'first';
Query OK, 1 row affected (0.00 sec)
Rows matched: 1  Changed: 1  Warnings: 0

mysql> SELECT LAST_INSERT_ID();
+------------------+
| LAST_INSERT_ID() |
+------------------+
|                1 |
+------------------+
1 row in set (0.00 sec)

mysql> SELECT * FROM so49197964;
+--------+------------+
| name   | next_value |
+--------+------------+
| first  |         21 |
| second |          1 |
+--------+------------+
2 rows in set (0.00 sec)

再拿20个：

mysql> UPDATE so49197964
    ->     SET next_value = LAST_INSERT_ID(next_value) + 20
    ->     WHERE name = 'first';
Query OK, 1 row affected (0.00 sec)
Rows matched: 1  Changed: 1  Warnings: 0

mysql> SELECT LAST_INSERT_ID();
+------------------+
| LAST_INSERT_ID() |
+------------------+
|               21 |
+------------------+
1 row in set (0.00 sec)

mysql> SELECT * FROM so49197964;
+--------+------------+
| name   | next_value |
+--------+------------+
| first  |         41 |
| second |          1 |
+--------+------------+
2 rows in set (0.00 sec)

`UPDATE ... WHERE ... ` InnoDB 中的多行锁定

`UPDATE ... WHERE ... ` multiple rows locking in InnoDB

mysql

innodb

innodb-next-key-锁

间隙锁