请教关于并发性的问题

足球小子

下面几道题目是关于DB2中并发性的问题，有没有哪位高手能够讲解一下：
（1）An update lock gets released by an application using the repeatable read
isolation level during which of the following?
a) If the cursor accessing the row is closed.
b) If the transaction issues a ROLLBACK statement.
c) If the cursor accessing the row is moved to the next row.
d) If the transaction changes are made via an UPDATE statement.

（2）Which of the following isolation levels is most likely to acquire a table
level lock during an index scan?
a) Read Stability
b) Repeatable Read
c) Cursor Stability
d) Uncommitted Read

（3）Which of the following releases a lock by an application using the cursor
stability isolation level?
a) If the cursor accessing the row is moved to the next row
b) If the cursor accessing the row is used to update the row
c) If the application's current row is deleted by the application
d) If the application's current row needs to be updated by another application

（4）Which of the following processing can occur for a unit of work using an isolation level of Uncommitted Read and scanning through the table more than once within the unit of work?
a) Access uncommitted changes made by other processes
b) Update uncommitted changes made by other processes
c) Update rows of a return set and have those updates changed by other processes from one scan to the next
d) Update rows of a return set and have those updates committed by other processes from one scan to the next

（5）Given the requirement of providing a read-only database, applications accessing the database should be run with which of the following isolation levels to allow for the most read concurrency?
a) Read stability
b) Repeatable read
c) Cursor stability
d) Uncommitted read

（6）Given the following table with a primary key on empid:
Emp:
Empid Name
11 Joe Smith
23 Melanie Jones
30 Robert Bruce
49 Janice Baker
66 Mario Diaz
68 Maria Diaton

Give the following statement in an embedded SQL program bound with Repeatable
Read:
Select * from Emp where empid < 55
How many rows in the table will be locked after the statement is run?
(Select the correct response)
A. 0
B. 1
C. 4
D. 5
E. 6

cyberflying

试试
1. b
2. b
3. d
4. c
5. d
6. e

5.6比较又把握一些：），错了不准笑哦

足球小子

这两天仔细研究了一番DB2的并发性，感觉有一定的提高，下面是我对题目的理解，不知道是否正确，欢迎大家指正（第4题还是不会）

（1）An update lock gets released by an application using the repeatable read
isolation level during which of the following?
a) If the cursor accessing the row is closed.
b) If the transaction issues a ROLLBACK statement.
c) If the cursor accessing the row is moved to the next row.
d) If the transaction changes are made via an UPDATE statement.

题目的意思是“在可重复的读隔离级下，应用程序什么时候释放U锁”。
解答：按照DB2的解释，可重复的读是最严格的隔离级，在一个事务的开始之前对表加锁，直到事务结束后才释放锁。备选答案中只有（b）是表示一个事务的结束，故选（b）

（2）Which of the following isolation levels is most likely to acquire a table
level lock during an index scan?
a) Read Stability
b) Repeatable Read
c) Cursor Stability
d) Uncommitted Read

题目的意思是“在哪个隔离级下，进行基于索引的查询时，最可能获得一把表锁”。
解答：“可重复的读”要求在检索期间锁定整个表；“读稳定性”只锁定检索出来的行；“游标稳定性”只锁定游标所在的行；“未提交的读”什么也不锁定。故选择（b）

（3）Which of the following releases a lock by an application using the cursor
stability isolation level?
a) If the cursor accessing the row is moved to the next row
b) If the cursor accessing the row is used to update the row
c) If the application's current row is deleted by the application
d) If the application's current row needs to be updated by another application

题目的意思是“在游标稳定性隔离级中，应用程序什么时候释放锁”。
解答：游标稳定性隔离级中，只锁定游标所在的当前行，当游标移动到下一行时，当前行就把锁释放了（除非在当前行上面做了数据的改写），故选择（a）

（5）Given the requirement of providing a read-only database, applications accessing the database should be run with which of the following isolation levels to allow for the most read concurrency?
a) Read stability
b) Repeatable read
c) Cursor stability
d) Uncommitted read

题目的意思是“对于一个只读数据库，应用程序应该运行在什么隔离级下面才能获得最大的读取并发性”。
解答：由于是只读数据库，数据库中的数据不会有任何的更改，所以丢失更新、脏读、不可重复的读、幻像这四种不良现象永远都不会发生，我们也不需要在数据库上面加任何类型的锁，为了获得最大的都并发性，应该设置为“未提交的读”隔离级，选择（d）

（6）Given the following table with a primary key on empid:
Emp:
Empid Name
11 Joe Smith
23 Melanie Jones
30 Robert Bruce
49 Janice Baker
66 Mario Diaz
68 Maria Diaton
Give the following statement in an embedded SQL program bound with Repeatable
Read:
Select * from Emp where empid < 55
How many rows in the table will be locked after the statement is run?
(Select the correct response)
A. 0
B. 1
C. 4
D. 5
E. 6

题目的意思是“在一个处于可重复的读隔离级下的嵌入式SQL程序中，运行如下的SQL语句
Select * from Emp where empid < 55
运行完毕后有多少行数据会被锁定”
解答：“可重复的读”是最严格的隔离级，锁定整张表，而不管Select语句查询的结果是多少，故选择（e）