数据库开启事务后，使用select语句并不会对查询到的数据加锁，当两个线程需要查询相同的数据，并且需要根据数据进行修改时，可能发生A线程和B线程都查询到数据，然后A线程根据结果对数据进行修改，而B线程还是根据旧的数据进行判断然后更新数据，将导致B线程的结果覆盖掉A线程的结果
解决问题的方式是要在事务中，对查询到的数据进行加锁（独占锁），一旦查询到数据后，在事务提交之前，不允许查询或修改该数据
Mysql/MariaDB 提供了for update来处理这种请情况
使用方式：select * from tb_recall_info where id = 45 for update;
通过增加for update 语句，mysql会对查询到的数据加独占锁
InnoDB supports row-level locking. Selected rows can be locked using LOCK IN SHARE MODE or FOR UPDATE. In both cases, a lock is acquired on the rows read by the query, and it will be released when the current transaction is committed.
The FOR UPDATE clause of SELECT applies only when autocommit is set to 0 or the SELECT is enclosed in a transaction. A lock is acquired on the rows, and other transactions are prevented from writing the rows, acquire locks, and from reading them (unless their isolation level is READ UNCOMMITTED).
If autocommit is set to 1, the LOCK IN SHARE MODE and FOR UPDATE clauses have no effect in InnoDB. For non-transactional storage engines like MyISAM and ARIA, a table level lock will be taken even if autocommit is set to 1.
If the isolation level is set to SERIALIZABLE, all plain SELECT statements are converted to SELECT … LOCK IN SHARE MODE.
更多资料参考：https://mariadb.com/kb/en/for-update/
查询的条件的字段如果是有索引，加的是行级锁，如果没有索引，加的是表级锁
在实际的使用中，需要注意避免死锁的发生，如果Mysql检查到死锁，会返回失败，此时需要回滚事务