185.User SCOTT executes the following command on the EMP table but has not issued COMMIT, ROLLBACK,

原创于 2016-08-16 11:54:31 发布 · 751 阅读

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本文探讨了在UserSCOTT执行特定SQL命令未提交或回滚的情况下，SCOTT在另一会话中执行不同操作时的等待情况。通过具体示例，解释了不同类型的表锁定如何影响并发操作。

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185.User SCOTT executes the following command on the EMP table but has not issued COMMIT,
ROLLBACK, or any data definition language (DDL) command:
SQL> SELECT ename FROM emp WHERE job='CLERK' FOR UPDATE OF empno;
SCOTT has opened another session to work with the database instance. Which three operations would
wait when issued in SCOTT's second session? (Choose three.)
A.LOCK TABLE emp IN SHARE MODE;
B.LOCK TABLE emp IN EXCLUSIVE MODE;
C.UPDATE emp SET sal=sal*1.2 WHERE job=MANAGER;
D.INSERT INTO emp(empno,ename) VALUES (1289,'Harry');
E.SELECT ename FROM emp WHERE job='CLERK' FOR UPDATE OF empno;
答案：ABE
解析：这个select ... for update 是在TX加上了S锁，在TM上增加了SX锁，
因此不能在TX上增加X锁，不能在TM上增加S、SSX、X锁
A：正确，这个相当于TMS锁
B：正确，这个是TM的X锁
C：这里只是TM的SX锁
D：和C一样
E：正确，这里是SX锁，锁的是不同的记录