OCP-1Z0-052-V8.02-58题

58. View the Exhibit and examine the parameters. 

User A executes the following command to update the TRANS table:

SQL> UPDATE B.trans SET tr_amt=tr_amt+500 WHERE c_code='C005';

Before user A issues a COMMIT or ROLLBACK command, user B executes the following command on

the TRANS table:

SQL> ALTER TABLE trans MODIFY (tr_type VARCHAR2(3));

What would happen in this scenario?

A.The ALTER TABLE command modifies the column successfully.

B.The DDL operation gets higher priority and transaction for user A is rolled back.

C.The ALTER TABLE command waits indefinitely until user A ends the transaction.

D.The ALTER TABLE command fails after waiting for 60 seconds due to the resource being busy.

Answer: D  

答案解析：

参考：http://blog.csdn.net/rlhua/article/details/12652569

每个DML 事务处理必须获得两个锁：

• 针对正在更新的一行或多行的EXCLUSIVE 行锁

• 针对包含这些行的表的ROW EXCLUSIVE (RX) 模式下的表锁(TM)。

这可避免在进行更改时另一会话锁定整个表（可能会删除或截断表）。这种模式也称为子排它表锁(SX)。

对表执行ROW EXCLUSIVE 锁定时，会禁止DDL 命令在未提交的事务处理进行到一半时更改字典元数据。这样便可在事务处理的有效期内保持字典完整性和读取一致性。

DDL_LOCK_TIMEOUT specifies a time limit for how long DDL statements will wait in a DML lock queue. The default value of zero indicates a status of NOWAIT. The maximum value of 1,000,000 seconds will result in the DDL statement waiting forever to acquire a DML lock.

If a lock is not acquired before the timeout period expires, then an error is returned.