如果一个事务已获得数据项R上的共享锁，则其他事务（ ）。
- A.可获得R上的排它锁
- B.可获得R上的共享锁
- C.不能获得R上的锁
- D.待该共享锁释放后才可获得R上的锁
正确答案及解析
正确答案
解析
（1）排它锁（简记X锁），又称写锁。若事物T对数据对象A 加上X锁，则只允许T读取和修改A，其他任何事物再不能对A加任何类型的锁，直到T释放A上的锁。
（2）共享锁（简记S锁），又称读锁。若事物T对数据对象A加上S锁。则其他事物只能对A 加S锁，不能再加X锁，直到T释放A上的S锁。
所以，此题答案为B。
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