mysql数据库中默认的隔离级别为repeat-read.
innodb默认使用了next-gap算法，这种算法结合了index-row锁和gap锁。正因为这样的锁算法，innodb在可重复读这样的默认隔离级别上，可以避免幻象的产生。
innodb_locks_unsafe_for_binlog最主要的作用就是控制innodb是否对gap加锁。
注意该参数如果是enable的，则是unsafe的，此时gap不会加锁；反之，如果disable掉该参数，则gap会加锁。当然对于一些和数据完整性相关的定义，如外键和唯一索引（含主键）需要对gap进行加锁，那么innodb_locks_unsafe_for_binlog的设置并不会影响gap是否加锁。
在5.1.15的时候，innodb引入了一个概念叫做“semi-consistent”，这样会在innodb_locks_unsafe_for_binlog的状态为ennable时在一定程度上提高update并发性。
幻读（Phantom Read): 是指当用户读取某一范围的数据行时，B事务在该范围内插入了新行,当用户再读取该范围的数据行时,会发现有新的“幻影”行。InnoDB和Falcon存储引擎通 过多版本并发控制机制解决了幻读问题。
Consider the following example, beginning with this table:
CREATE TABLE t (a INT NOT NULL, b INT) ENGINE = InnoDB;
INSERT INTO t VALUES (1,2),(2,3),(3,2),(4,3),(5,2);
COMMIT;
In this case, table has no indexes, so searches and index scans use the hidden clustered index for record locking (seeSection 14.8.2.1, “Clustered and Secondary Indexes”).
Suppose that one client performs an UPDATE using these statements:
SET autocommit = 0;
UPDATE t SET b = 5 WHERE b = 3;
Suppose also that a second client performs an UPDATE by executing these statements following those of the first client:
SET autocommit = 0;
UPDATE t SET b = 4 WHERE b = 2;
As InnoDB executes each UPDATE, it first acquires an exclusive lock for each row, and then determines whether to modify it. If InnoDB does not modify the row and innodb_locks_unsafe_for_binlog is enabled, it releases the lock. Otherwise, InnoDBretains the lock until the end of the transaction. This affects transaction processing as follows.
If innodb_locks_unsafe_for_binlog is disabled, the first UPDATE acquires x-locks and does not release any of them:
x-lock(1,2); retain x-lock
x-lock(2,3); update(2,3) to (2,5); retain x-lock
x-lock(3,2); retain x-lock
x-lock(4,3); update(4,3) to (4,5); retain x-lock
x-lock(5,2); retain x-lock
The second UPDATE blocks as soon as it tries to acquire any locks (because the first update has retained locks on all rows), and does not proceed until the first UPDATE commits or rolls back:
x-lock(1,2); block and wait for first UPDATE to commit or roll back
If innodb_locks_unsafe_for_binlog is enabled, the first UPDATE acquires x-locks and releases those for rows that it does not modify:
x-lock(1,2); unlock(1,2)
x-lock(2,3); update(2,3) to (2,5); retain x-lock
x-lock(3,2); unlock(3,2)
x-lock(4,3); update(4,3) to (4,5); retain x-lock
x-lock(5,2); unlock(5,2)
For the second UPDATE, InnoDB does a “semi-consistent” read, returning the latest committed version of each row to MySQL so that MySQL can determine whether the row matches the WHERE condition of the UPDATE:
x-lock(1,2); update(1,2) to (1,4); retain x-lock x-lock(2,3); unlock(2,3) x-lock(3,2); update(3,2) to (3,4); retain x-lock x-lock(4,3); unlock(4,3) x-lock(5,2); update(5,2) to (5,4); retain x-lock
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作者：彭薄
来源：CSDN
原文：https://blog.csdn.net/cxl0921/article/details/77623439
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